Blown Power Amplifier: Repair or Replace? [resolved]

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iceport
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Blown Power Amplifier: Repair or Replace? [resolved]

Post by iceport » Thu May 21, 2020 9:06 am

NOTE: This question was resolved. With a little needed prodding below, the blown amp was replaced with a greatly improved new amp.

The NAD 2400 THX Power Amplifier I've been using happily for 26 years blew Tuesday afternoon :( and I'm wondering first if it's repairable, and second if it's worth it.

This might have been the first time I've run it at full volume to power both Speakers A and B simultaneously. When it failed, it just stopped working; there was no loud noise. It had been working at that volume for maybe 20 minutes before it blew. There was a very faint burnt electronics odor. When I opened it up, I found the main 125V, 7A glass cartridge fuse blown at the power source, and two 125V, 6A glass cartridge fuses also blown. The two 250V, 400mA fuses look fine. There is no other obvious sign of overheating anywhere — at least to my untrained eye.

Here is an image of the amp: http://www.audiocostruzioni.com/r_s/amp ... _small.jpg

The main fuse is under the green circuit board at the top left, and the other fuses are in the lower left, tucked among the four large black capacitors. (I only know what they are from the Service Manual.)

What are the chances that replacing the three blown fuses will fix the amp? Do these types of fuses ever blow without other damage being done, or are the blown fuses an indication that some other parts were damaged?

I've been really pleased with this amp overall, and I'm not thrilled with the prospect of replacing it. I'm using it to power a pair of Paradigm 9se Mk3 main speakers (150 Watts, 6 Ohms) and a pair of JBL J325A secondary speakers (150 Watts, 8 Ohms). The preamp is an NAD 1600 preamp/tuner. It seems this might be an outdated configuration, but it suits my needs well.

Thanks for any insight or informed opinions you could offer on either the prospects for repair or the benefits of replacement.
Last edited by iceport on Sun May 24, 2020 3:28 pm, edited 1 time in total.
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Re: Blown Power Amplifier: Repair or Replace?

Post by buhlaxtus » Thu May 21, 2020 9:23 am

Fuses are cheap...

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Re: Blown Power Amplifier: Repair or Replace?

Post by TomatoTomahto » Thu May 21, 2020 9:28 am

buhlaxtus wrote:
Thu May 21, 2020 9:23 am
Fuses are cheap...
Well worth trying if you’ve loved this amp for more than a quarter century.
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Re: Blown Power Amplifier: Repair or Replace?

Post by iceport » Thu May 21, 2020 9:30 am

buhlaxtus wrote:
Thu May 21, 2020 9:23 am
Fuses are cheap...
Unbelievably so! The 7A fuse goes for 23.6 cents each, and the 6A fuse goes for 22.5 cents each. I've got some ordered, but I'm starting to doubt whether the fuses were the only casualties.

I really enjoy music, occasionally at loud volumes :shock: , but I'm not knowledgeable about audio equipment.
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Re: Blown Power Amplifier: Repair or Replace?

Post by Jack FFR1846 » Thu May 21, 2020 9:37 am

Having revived many amps over the years (first in technician school and then on weekends in the lab of work for my engineering job), I know that you should first figure out why the fuses blew. They don't just blow for no reason. I've found in the many repairs I've done that the output stage typically has a short somewhere or a short that opened. What's the output stage? An integrated amp chip per channel? Separate FETs or transistors? At a minimum, do some testing for shorts. If you find them in circuit, you'll want to pull out whatever appears short, then test again. From the output of the power supply, you shouldn't find shorts to the return rail.

I expect replacing the fuses will simply blow all the fuses again.

While you're in there, the electrolytics are beyond their useful life. When I had a lab full of "throw this stuff away" boxes, I'd replace the 22 uF caps with 2200 uF caps. The result is some hilarity. Turn the amp up to full volume with Led Zeppelin. (Pantera will work, but come on...) Now shut off the power button. Then realize that the amp is still putting out full volume for a full 15 seconds.

Is it worth it? Only you can say. Do you know any old timey technicians who still fix TVs and stuff like this amp in their spare time? If yes, there ya go. If no, well, you'll get something on craigslist for it.
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Re: Blown Power Amplifier: Repair or Replace?

Post by iceport » Thu May 21, 2020 9:54 am

Jack FFR1846 wrote:
Thu May 21, 2020 9:37 am
Having revived many amps over the years (first in technician school and then on weekends in the lab of work for my engineering job), I know that you should first figure out why the fuses blew. They don't just blow for no reason. I've found in the many repairs I've done that the output stage typically has a short somewhere or a short that opened. What's the output stage? An integrated amp chip per channel? Separate FETs or transistors? At a minimum, do some testing for shorts. If you find them in circuit, you'll want to pull out whatever appears short, then test again. From the output of the power supply, you shouldn't find shorts to the return rail.

I expect replacing the fuses will simply blow all the fuses again.

While you're in there, the electrolytics are beyond their useful life. When I had a lab full of "throw this stuff away" boxes, I'd replace the 22 uF caps with 2200 uF caps. The result is some hilarity. Turn the amp up to full volume with Led Zeppelin. (Pantera will work, but come on...) Now shut off the power button. Then realize that the amp is still putting out full volume for a full 15 seconds.

Is it worth it? Only you can say. Do you know any old timey technicians who still fix TVs and stuff like this amp in their spare time? If yes, there ya go. If no, well, you'll get something on craigslist for it.
Ahhh, you're confirming my worst fears. But unfortunately, the testing you recommend is beyond my capabilities. The details of the circuitry are all shown in the linked service manual, but it's all gibberish to me. It helped me find all the fuses, but that's about the extent of my ability to read it. I linked to it in case someone could use it to appraise the quality of the amp, or identify any advantages to maybe replacing it with more modern technology.
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Re: Blown Power Amplifier: Repair or Replace?

Post by wfrobinette » Thu May 21, 2020 9:58 am

iceport wrote:
Thu May 21, 2020 9:06 am
The NAD 2400 THX Power Amplifier I've been using happily for 26 years blew Tuesday afternoon :( and I'm wondering first if it's repairable, and second if it's worth it.

This might have been the first time I've run it at full volume to power both Speakers A and B simultaneously. When it failed, it just stopped working; there was no loud noise. It had been working at that volume for maybe 20 minutes before it blew. There was a very faint burnt electronics odor. When I opened it up, I found the main 125V, 7A glass cartridge fuse blown at the power source, and two 125V, 6A glass cartridge fuses also blown. The two 250V, 400mA fuses look fine. There is no other obvious sign of overheating anywhere — at least to my untrained eye.

Here is an image of the amp: http://www.audiocostruzioni.com/r_s/amp ... _small.jpg

The main fuse is under the green circuit board at the top left, and the other fuses are in the lower left, tucked among the four large black capacitors. (I only know what they are from the Service Manual.)

What are the chances that replacing the three blown fuses will fix the amp? Do these types of fuses ever blow without other damage being done, or are the blown fuses an indication that some other parts were damaged?

I've been really pleased with this amp overall, and I'm not thrilled with the prospect of replacing it. I'm using it to power a pair of Paradigm 9se Mk3 main speakers (150 Watts, 6 Ohms) and a pair of JBL J325A secondary speakers (150 Watts, 8 Ohms). The preamp is an NAD 1600 preamp/tuner. It seems this might be an outdated configuration, but it suits my needs well.

Thanks for any insight or informed opinions you could offer on either the prospects for repair or the benefits of replacement.
I found this. You should have had the 4 ohm selected on the back. I think the amp has these wired in parallel meaning that the 2 sets of 8 ohm speakers together would actually be a 4 ohm load. In your case with a 8 and 6 ohm loads you were down around 3.5 ohms. Depending on the dynamics of the music you were probably clipping the amp. You probably drove the amp too hard.

Replace the fuses and don't run it that hard and see what happens. Worst case they blow again at lower work. If they do then you decide whether to repair or not. You can buy the same amp on eBay for $250 or so. Frankly, if you want to drive the speakers like that I'd move up to a higher wattage amp.

I found this too!
Posted July 3, 2001
DwK,
My NAD manual (which I forgot I had and I've actually asked about this very question elsewhere on this board) states that if two sets of speakers are hooked (A and B channels) that the 4 ohm setting should be used. NAD also recommends if you aren't sure which ohm setting to use on the amp (4 or 8) that you should use the 4 ohm setting. They state that "in many 8 ohm loudspeakers the minimum impedance is from 4 to 6 ohms, and in 4 ohm speakers the minimum is typically 3 ohms. If you connec ttwo sets of speakers to the amplifier, their combined impedance is approximately half the impedance of either."
So, when in doubt, use the 4 ohm setting. You may sacrifice some power if the speakers are truly 8 ohm but you won't damage the amp. By setting the amp at 8 ohm and loading it up with speakers with mimimum impedance of 4-6 ohm (or two sets of 8 ohm speakers) you can potentially overheat and damage the amplifier. "

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Re: Blown Power Amplifier: Repair or Replace?

Post by David Jay » Thu May 21, 2020 10:10 am

wfrobinette wrote:
Thu May 21, 2020 9:58 am
I found this.
Wow, are you a resource.

I was an Audio tech in another life (over 40 years ago), I agree that you should first replace the fuses, then switch to 4 Ohm and turn it on. If the fuses blow immediately then you have to make a decision.

I would lean towards replacement. Too many components get old (w.f. suggested the electrolytics), a quarter century is pretty good return on investment.
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Re: Blown Power Amplifier: Repair or Replace?

Post by iceport » Thu May 21, 2020 10:18 am

^^^^ Yeah, I'm positive the 4 ohm setting was selected. The main speakers say "Nominal: 6 ohms Minimum: 4 ohms" so that settled the selection, even without the second pair of speakers hooked up.

There's an overload indicator, and when I want maximum volume, I typically turn it up until the overload indicator light starts blinking, and then back down so that it stays off.

There's also some sort of built-in circuit protection for the output transistors. I think it was engaged once or twice a couple of decades ago if memory serves, but not since. When the protection was triggered, the amp shut off the output automatically. Then I'd just turn it off, let the amp cool down for a while, and it would work just fine when turned on again. Maybe the protection failed? As if running a test, the protection light always lights up for half a second when the amp is turned on, and then it goes of with a click. So I had always presumed the circuit protection feature was still working.
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Re: Blown Power Amplifier: Repair or Replace?

Post by illumination » Thu May 21, 2020 10:32 am

Unless it has extreme sentimental value, replace it. It's going to cost close to half of its value to just have a tech open it up and diagnose. I occasionally do small repairs like solder in new capacitors, but my feeling on most modern electronics is unless its an easy repair I can make myself or it's some serious high end equipment, you're better off just throwing it away and buying another one than paying someone to repair it. And I could easily see a repair being made and then something else goes out immediately afterwards.

Used high end audio equipment can be dirt cheap if you want to roll the dice, I've seen flagship receivers that were like $2,000-$4,000 new go for like $50 because it doesn't have some modern features like an HDMI port.

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Re: Blown Power Amplifier: Repair or Replace?

Post by nisiprius » Thu May 21, 2020 10:42 am

I would certainly take a shot at replacing the fuses. As you describe the situation, there's a chance that they could be functioning as designed. The odor is certainly a concern but sometimes hot electronics will have a smell if they haven't been up to full temperature recently.
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Re: Blown Power Amplifier: Repair or Replace?

Post by adamthesmythe » Thu May 21, 2020 11:41 am

iceport wrote:
Thu May 21, 2020 10:18 am
There's an overload indicator, and when I want maximum volume, I typically turn it up until the overload indicator light starts blinking, and then back down so that it stays off.
!

Maybe you need a new amplifier with a megawatt rating.

Seriously- try the suggestion about using the 4 ohm switch setting after replacing the fuses. If that doesn't work, then replacement is the only reasonable option given that you cannot troubleshoot it yourself.

There does seem to be a market for vintage audio gear, so maybe you could recover a few bucks.

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Re: Blown Power Amplifier: Repair or Replace?

Post by iceport » Thu May 21, 2020 12:01 pm

adamthesmythe wrote:
Thu May 21, 2020 11:41 am
iceport wrote:
Thu May 21, 2020 10:18 am
There's an overload indicator, and when I want maximum volume, I typically turn it up until the overload indicator light starts blinking, and then back down so that it stays off.
!

Maybe you need a new amplifier with a megawatt rating.
Heh, heh... Yeah, seems excessive doesn't it? I call these occasions my moments of "irrational exuberance." :wink:

The JBL speakers are used either in the basement or outdoors when I'm working in the yard. Outdoors, sometimes the volume is just needed. At other times, I'm usually only interested in hearing a particular tune or two in its full glory. I also listen from distant rooms when doing housework.
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Re: Blown Power Amplifier: Repair or Replace?

Post by CRG » Thu May 21, 2020 12:25 pm

It might be worth exploring the options for service in your area, as there are still techs out there that won't charge a fee for an initial diagnosis. I live in the Cleveland area and recently dropped off a couple of vintage pieces and there was no charge until I agreed upon the quoted repair.

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Re: Blown Power Amplifier: Repair or Replace?

Post by Nestegg_User » Thu May 21, 2020 12:55 pm

or if you had my older Electrovoice speakers that are very efficient, you wouldn't have blown your amp in the first place.


(your ears would have gone before the amp; I couldn't even get to 80% in the amp before it was way too loud (easily hear it three plus floors down in a fairly acoustically dampered dorm))

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Re: Blown Power Amplifier: Repair or Replace?

Post by TomatoTomahto » Thu May 21, 2020 1:09 pm

^ The previous owner of our house had a pair of Klipschorns in a small room. 1 watt could have driven them to ear drum shattering levels.
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Re: Blown Power Amplifier: Repair or Replace?

Post by iceport » Thu May 21, 2020 1:16 pm

Nestegg_User wrote:
Thu May 21, 2020 12:55 pm
or if you had my older Electrovoice speakers that are very efficient, you wouldn't have blown your amp in the first place.
Oh, now you're bringing back some really fond memories... 8-) I actually owned a wonderful pair of Electrovoice speakers I bought mail order at maybe 15 years old in the mid-70s from an outfit in Cherry Hill, NJ, if I recall correctly. Lawn mowing money. (Hi-fi Stereo Warehouse or something like that? Very nice folks there.) I actually picked out the speakers with their help through letter correspondence back and forth! Unfortunately, the foam rubber surrounding the woofers disintegrated and I just ended up replacing them when I bought a decent system. Those Electrovoices had a remarkably clear, clean natural sound, though.
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Re: Blown Power Amplifier: Repair or Replace?

Post by Nestegg_User » Thu May 21, 2020 1:20 pm

TomatoTomahto wrote:
Thu May 21, 2020 1:09 pm
^ The previous owner of our house had a pair of Klipschorns in a small room. 1 watt could have driven them to ear drum shattering levels.
yeah, can appreciate

the 80% was a "turn it on and run" type thing... it could easily be louder than a jet taking off if you weren't careful. They're Interface 2's...(but alas, like many of the vintage, have the foam fronts coming off, and I believe that they would probably have to be totally remounted... not exactly worth it)... they were probably one of the best of the less expensive (not exactly cheap in the day) but high efficiency good speakers.

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Re: Blown Power Amplifier: Repair or Replace?

Post by Nestegg_User » Thu May 21, 2020 1:21 pm

yeah, Iceport, some of the cleanest sound of the times

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Re: Blown Power Amplifier: Repair or Replace?

Post by baconavocado » Thu May 21, 2020 1:34 pm

Best of luck with this. I recently lost a 35-year old Yamaha amplifier. Turned it on and smoke and the smell of burned electrical parts came from the top. I did some research and decided it really wasn't worth repairing. It was very difficult to discard it.

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Re: Blown Power Amplifier: Repair or Replace?

Post by wfrobinette » Thu May 21, 2020 2:02 pm

illumination wrote:
Thu May 21, 2020 10:32 am
Unless it has extreme sentimental value, replace it. It's going to cost close to half of its value to just have a tech open it up and diagnose. I occasionally do small repairs like solder in new capacitors, but my feeling on most modern electronics is unless its an easy repair I can make myself or it's some serious high end equipment, you're better off just throwing it away and buying another one than paying someone to repair it. And I could easily see a repair being made and then something else goes out immediately afterwards.

Used high end audio equipment can be dirt cheap if you want to roll the dice, I've seen flagship receivers that were like $2,000-$4,000 new go for like $50 because it doesn't have some modern features like an HDMI port.
No need to roll the dice. There are a lot of great resellers out there that back there stuff. Plus if he is happy with the specs he has now.

OP should keep his preamp too and just go for a power amp.

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Re: Blown Power Amplifier: Repair or Replace?

Post by wilked » Thu May 21, 2020 2:09 pm

I would buy this
https://www.ebay.com/itm/NAD-2400-2-Cha ... Sw5QVexXym

$235 shipped. If you get another 26 years that's less than $10/yr

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Re: Blown Power Amplifier: Repair or Replace?

Post by jharkin » Thu May 21, 2020 2:13 pm

+1 on not replacing the fuses until you figure out what caused them to blow.

Its a solid state amp, the output stage consists of a bunch of high power transistors (they would be under that large finned heatsink that takes up most of the top in your image. Odds of one of them failing is pretty low, unless something happened like a speaker wire came loose and the amp wasn't designed with protection against running unloaded- but that's rare for solid state.

At 25+ years old, the most common thing is that one of the electrolytic filter caps in the power supply has dried up and failed. The power supply is on the left in your image, the gold object is the main power transformer, and the black cans below it are the filter caps. Those are the first thing I would check.

Since you are not experienced working on electronics I would take it to a repair shop. NAD is a quality, high end brand and worth repairing.

I found this repair shop link on the NAD website: https://support.nadelectronics.com/hc/e ... e-near-me-

If you want to ask more detailed questions - audiokarma.org is an enthusiast forum for all things audio and video and their vendor listing may be another source of good repair shop leads. I could throw out some names but more of the ones I am familiar with deal with older vintage equipment...

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Re: Blown Power Amplifier: Repair or Replace?

Post by gmaynardkrebs » Thu May 21, 2020 3:20 pm

You were running two sets at high volume...replace the fuses, and don't do what you did last time.
Fuses are protective devices, and they probably were just doing their job.

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Re: Blown Power Amplifier: Repair or Replace?

Post by oldcomputerguy » Thu May 21, 2020 3:28 pm

I'm gonna agree with Jack FFR1846 here, if something happened after all these years of normal use to blow all those fuses at once, that's pretty catastrophic. My guess is something is shorted, and if you replace the fuses, you'll just blow them again.
:(
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Re: Blown Power Amplifier: Repair or Replace?

Post by arcticpineapplecorp. » Thu May 21, 2020 3:36 pm

Marty McFly?

https://www.google.com/search?client=fi ... +amplifier

I heard they've been talking about a sequel for Back to the Future, but boy, that's some audition.

In the meantime, I suggest you spend some time social distancing with a reunion of the cast (mostly) from Back to the Future with Josh Gad:
https://www.youtube.com/watch?v=crdYIUdUOhc
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Re: Blown Power Amplifier: Repair or Replace?

Post by iceport » Thu May 21, 2020 4:29 pm

Thanks everyone for your comments and insight! I appreciate them all.

Lots of varied opinions, and I think that's a reflection of the fact that different modes of failure are real distinct possibilities.

I contacted the nearest NAD factory service provider, identified from their own published list. He's only a 40 minute drive away. The guy said a simple fuse replacement could go either way: it could either solve the only problem, or only address the symptom of other damage. He asked if the fuses were burnt looking. The 7A fuse was burnt, but the other two 6A fuses just had broken filaments. He said the burnt appearance was not a good sign. :( He wants $50 just to open the hood and provide a repair estimate.

In the meantime, I also contacted the small independent audio dealer that sold me the system 26 years ago, and that sold me a replacement CD player when that failed 8 years ago. Like he had done 8 years ago, he stressed how much better modern components are now than they were 26 years ago. (He was absolutely right about the new 2012 CD player being far better than the 1994 version — much to my surprise!) Then, as now, he stressed how much better speakers sound now, and cautioned me not to consider a better quality amp than the 26 year old speakers can benefit from. After saying I'm open to a speaker upgrade in the future, and considering my affinity for NAD, he recommended a new NAD C 388 Hybrid Digital DAC Amplifier if the simple fuse replacement doesn't work. It's an integrated preamp/amp, but he said it would produce far better sound than the NAD 2400. I trust this guy, because he has been nothing but consistent in his message and recommendations over the years. His price for the C 388 at $1600 is about $100 lower than I found it elsewhere online. (Compared to the rest of his inventory, this is considered a good value and high quality option, but still an "entry level" component by his standards.) This amp would be a bit more powerful than the NAD 2400, so I would welcome the upgrade. The wireless connection capabilities would also be welcome.

The suspense is killing me by now to see what a simple fuse replacement will do. The fuses are due to arrive by the USPS on Saturday. At less than a quarter a pop, it's worth trying the simplest solution first.

I feel lost without a functioning quality stereo system...
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Re: Blown Power Amplifier: Repair or Replace?

Post by Northern Flicker » Thu May 21, 2020 5:03 pm

Jack FFR1846 wrote:
Thu May 21, 2020 9:37 am
Having revived many amps over the years (first in technician school and then on weekends in the lab of work for my engineering job), I know that you should first figure out why the fuses blew. They don't just blow for no reason. I've found in the many repairs I've done that the output stage typically has a short somewhere or a short that opened. What's the output stage? An integrated amp chip per channel? Separate FETs or transistors? At a minimum, do some testing for shorts. If you find them in circuit, you'll want to pull out whatever appears short, then test again. From the output of the power supply, you shouldn't find shorts to the return rail.

I expect replacing the fuses will simply blow all the fuses again.

While you're in there, the electrolytics are beyond their useful life. When I had a lab full of "throw this stuff away" boxes, I'd replace the 22 uF caps with 2200 uF caps. The result is some hilarity. Turn the amp up to full volume with Led Zeppelin. (Pantera will work, but come on...) Now shut off the power button. Then realize that the amp is still putting out full volume for a full 15 seconds.

Is it worth it? Only you can say. Do you know any old timey technicians who still fix TVs and stuff like this amp in their spare time? If yes, there ya go. If no, well, you'll get something on craigslist for it.
Running the amp at full power to drive two pairs of speakers in parallel? I think it is likely that the parallel load presented a low impedance to the amp which led to high current draw that overdpec’d some components. If a fuse is all that is damaged, replacing it may lead to a functional amp.

The OP is lucky that it did not clip and blow out a tweeter in one or more of the speakers (which may have actually happened as well if they have not been tested properly).

This was operator error, not the fault of the amp. Solid state amps are not generally designed to be operated at max volume, although some NAD amps do have anti-clipping filters.

Also, when running two pairs of speakers in parallel, the impedance may be too low and over spec the amp. Even if within spec, load impedance loads are hard on an amp. If the speakers were each 8 ohm average loads and the parallel load of about 4 ohms was within spec, it is in the boundary of what the amp can deliver in terms of current and running it at full power this is a very challenging load for the amp to deliver.
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Re: Blown Power Amplifier: Repair or Replace?

Post by iceport » Thu May 21, 2020 6:47 pm

Northern Flicker wrote:
Thu May 21, 2020 5:03 pm
Jack FFR1846 wrote:
Thu May 21, 2020 9:37 am
Having revived many amps over the years (first in technician school and then on weekends in the lab of work for my engineering job), I know that you should first figure out why the fuses blew. They don't just blow for no reason. I've found in the many repairs I've done that the output stage typically has a short somewhere or a short that opened. What's the output stage? An integrated amp chip per channel? Separate FETs or transistors? At a minimum, do some testing for shorts. If you find them in circuit, you'll want to pull out whatever appears short, then test again. From the output of the power supply, you shouldn't find shorts to the return rail.

I expect replacing the fuses will simply blow all the fuses again.

While you're in there, the electrolytics are beyond their useful life. When I had a lab full of "throw this stuff away" boxes, I'd replace the 22 uF caps with 2200 uF caps. The result is some hilarity. Turn the amp up to full volume with Led Zeppelin. (Pantera will work, but come on...) Now shut off the power button. Then realize that the amp is still putting out full volume for a full 15 seconds.

Is it worth it? Only you can say. Do you know any old timey technicians who still fix TVs and stuff like this amp in their spare time? If yes, there ya go. If no, well, you'll get something on craigslist for it.
Running the amp at full power to drive two pairs of speakers in parallel? I think it is likely that the parallel load presented a low impedance to the amp which led to high current draw that overdpec’d some components. If a fuse is all that is damaged, replacing it may lead to a functional amp.

The OP is lucky that it did not clip and blow out a tweeter in one or more of the speakers (which may have actually happened as well if they have not been tested properly).

This was operator error, not the fault of the amp. Solid state amps are not generally designed to be operated at max volume, although some NAD amps do have anti-clipping filters.

Also, when running two pairs of speakers in parallel, the impedance may be too low and over spec the amp. Even if within spec, load impedance loads are hard on an amp. If the speakers were each 8 ohm average loads and the parallel load of about 4 ohms was within spec, it is in the boundary of what the amp can deliver in terms of current and running it at full power this is a very challenging load for the amp to deliver.
Thanks for a lot of good information here.

Just to clarify, though, when I say full volume I don't mean turning up the volume up as far as it will go. The NAD volume controls range from no volume at about the 7 o'clock position to maximum volume at about the 5 o'clock position. The amp itself has individual volume knobs for each of the left and right channels. Those stay at about the 11 o'clock position, or about 40% of full amplifier volume and I never touch them. The preamp is what I use to modulate the system output volume. That was set at about the 1 o'clock or maybe the 2 o'clock position, or about 60% to 70% of maximum volume. And as noted above, the amp has an overload indicator light, and as far as I turned it up, I was careful not to let the overload indicator flicker at all. And I wasn't hearing any distortion. That doesn't mean I didn't screw up by driving both pairs of speakers hard at the same time, but I have been operating this thing for 26 years before this happened. :oops: I would have thought the amp protection system would have shut itself down before the fuses would blow. That has happened maybe a couple of times before.
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Re: Blown Power Amplifier: Repair or Replace?

Post by Northern Flicker » Thu May 21, 2020 9:51 pm

Your amp is rated to drive loads down to 2 ohms. Your speakers are 6 ohms and 8 ohms so the combined impedance of them in parallel is 1/(1/6 + 1/8) or about 3.4 ohms. It is not exact because impedances vary with frequency so the ratings of the speakers are averages. Still, this should not pose a problem for a high current design amp spec’d to drive loads as low as 2 ohms.

This is a 1980’s NAD amp assembled in Taiwan. NAD amps of that era are known for sometimes developing weak solder joint problems due to the soldering techniques used.
Last edited by Northern Flicker on Thu May 21, 2020 11:45 pm, edited 2 times in total.
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Re: Blown Power Amplifier: Repair or Replace?

Post by whodidntante » Thu May 21, 2020 10:09 pm

I get that people get attached to stuff like this. It's been with you for a long time and you've gotten enjoyment out of it. That said, it's old and has very little value. Electronics don't exactly improve with age. So, I would try to repair it myself since I have a relevant educational background. But if I could not, I wouldn't pay a tech to try. I would get rid of it and buy something new.

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Re: Blown Power Amplifier: Repair or Replace?

Post by iceport » Fri May 22, 2020 2:57 pm

Well, it looks like that's it for the trusty old NAD 2400. :( :( :oops:

The fuses came a day early in the mail. I replaced the three blown fuses, plugged it in and pressed the power button. Nothing. The main 7A fuse had blown again.

Looks like I'll be getting a replacement. :(
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Re: Blown Power Amplifier: Repair or Replace?

Post by nisiprius » Fri May 22, 2020 4:06 pm

iceport wrote:
Fri May 22, 2020 2:57 pm
Well, it looks like that's it for the trusty old NAD 2400. :( :( :oops:

The fuses came a day early in the mail. I replaced the three blown fuses, plugged it in and pressed the power button. Nothing. The main 7A fuse had blown again.

Looks like I'll be getting a replacement. :(
Bummer. "I feel your pain."
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Re: Blown Power Amplifier: Repair or Replace?

Post by midareff » Fri May 22, 2020 4:26 pm

Replacing the three fuses is rally inexpensive. They generally use auto type fuses available at almost every auto parts store. While the NAD is a good amplifier it is consumer grade electronics, and easily replaceable with like quality for a couple hundred bucks.

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Re: Blown Power Amplifier: Repair or Replace?

Post by Bogle7 » Fri May 22, 2020 5:13 pm

Replace the fuses with [real copper] pennies and see what happens.
Then go buy the new integrated amp.

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Re: Blown Power Amplifier: Repair or Replace?

Post by Dave55 » Fri May 22, 2020 5:26 pm

iceport wrote:
Thu May 21, 2020 9:06 am
The NAD 2400 THX Power Amplifier I've been using happily for 26 years blew Tuesday afternoon :( and I'm wondering first if it's repairable, and second if it's worth it.

This might have been the first time I've run it at full volume to power both Speakers A and B simultaneously. When it failed, it just stopped working; there was no loud noise. It had been working at that volume for maybe 20 minutes before it blew. There was a very faint burnt electronics odor. When I opened it up, I found the main 125V, 7A glass cartridge fuse blown at the power source, and two 125V, 6A glass cartridge fuses also blown. The two 250V, 400mA fuses look fine. There is no other obvious sign of overheating anywhere — at least to my untrained eye.

Here is an image of the amp: http://www.audiocostruzioni.com/r_s/amp ... _small.jpg

The main fuse is under the green circuit board at the top left, and the other fuses are in the lower left, tucked among the four large black capacitors. (I only know what they are from the Service Manual.)

What are the chances that replacing the three blown fuses will fix the amp? Do these types of fuses ever blow without other damage being done, or are the blown fuses an indication that some other parts were damaged?

I've been really pleased with this amp overall, and I'm not thrilled with the prospect of replacing it. I'm using it to power a pair of Paradigm 9se Mk3 main speakers (150 Watts, 6 Ohms) and a pair of JBL J325A secondary speakers (150 Watts, 8 Ohms). The preamp is an NAD 1600 preamp/tuner. It seems this might be an outdated configuration, but it suits my needs well.

Thanks for any insight or informed opinions you could offer on either the prospects for repair or the benefits of replacement.
iceport, I purchased a NAD C368 Amp a few years ago, great amp - 80 watts/channel. There is so much great gear out there, have fun exploring!

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Re: Blown Power Amplifier: Repair or Replace?

Post by iceport » Fri May 22, 2020 5:57 pm

nisiprius wrote:
Fri May 22, 2020 4:06 pm
Bummer. "I feel your pain."
Thanks nisiprius.
Dave55 wrote:
Fri May 22, 2020 5:26 pm
I purchased a NAD C368 Amp a few years ago, great amp - 80 watts/channel. There is so much great gear out there, have fun exploring!

Dave
Thanks Dave. I actually would have picked that one, except the amp I blew up was rated for 100W, so I figured I could get myself into trouble again... :| As it happens, I ran down to the same little audio shop I bought the original system from 26 years ago and came home with the NAD C388. :) I heard one set up in the showroom with a modern pair of Paradigm speakers. Wow. It felt like swimming in a pool of sound. New technology seems so far advanced from "just" 26 years ago. Don't know how my speakers will sound with it, but I can't wait to set it up. Three days without a hi-fi system and I was going through withdrawals. I never realized how much I took the stereo system for granted. I'd say the priority of appliances for me would go something like: 1) fridge; 2) stove; and 3) stereo system. 8-)

Thanks again to everyone for all your comments. I don't know why I get all sentimental about a household appliance like this. I think I needed to get some kind of permission to replace the old amp. So thanks for that!
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Re: Blown Power Amplifier: Repair or Replace?

Post by Kenkat » Fri May 22, 2020 6:44 pm

iceport wrote:
Fri May 22, 2020 5:57 pm
I don't know why I get all sentimental about a household appliance like this.
Because there is just something inherently cool about vintage electronics / audio equipment; I completely get it.

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Re: Blown Power Amplifier: Repair or Replace?

Post by MathIsMyWayr » Fri May 22, 2020 7:26 pm

iceport wrote:
Fri May 22, 2020 5:57 pm
I don't know why I get all sentimental about a household appliance like this. I think I needed to get some kind of permission to replace the old amp. So thanks for that!
I still remember an old RCA radio. It was a transformerless 5-tube superheterodyne AM receiver with 12BE6, 12BA6, 12AV6, 50C5, and 35W4(?) miniature vacuum tubes. It also had a large loop antenna on the rear panel.

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Re: Blown Power Amplifier: Repair or Replace?

Post by iceport » Sun May 24, 2020 3:24 pm

Dave55 wrote:
Fri May 22, 2020 5:26 pm
iceport, I purchased a NAD C368 Amp a few years ago, great amp - 80 watts/channel. There is so much great gear out there, have fun exploring!

Dave
Hey Dave,

I just wanted to say how amazing I think these NAD amps are. (Yours is the same design.) What a good call replacing the old 2400 turned out to be!

The new amp is so far superior to its 26 year old predecessor, there's just no comparison. I always thought the Paradigm speakers were a little muddy on the low end, and they might be. But this new amp really cleans up the bass — they sound like different speakers. The rest of the audible range is also so much cleaner and crisper, with much higher definition. I never realized how critical an amp could be for sound quality, not just quantity. The older music I play sounds better, sure. But the difference it makes in reproducing the music produced more recently is truly astounding. It's almost like I'm hearing new versions of songs or albums, I can hear so much more detail than I ever knew was there!

In terms of operation, it's a little bit of a mixed bag. There are some great new conveniences, and a couple of bothers. It took me most of the afternoon yesterday to set it up and figure out how to customize it to fit the system configuration. I ended up maintaining the use of the old tuner as a preamp, not just a tuner. When I bring the music outside, I bring only one speaker outside at a time, so I need a mono setting. Neither my PC audio software nor the new amp has a mono setting. :annoyed But it turns out by using the "fixed" volume setting (set at a lower level) for the line in from the tuner I can keep using the tuner preamp output (instead of just a line level output), and the new integrated amp functions as a de facto dedicated power amplifier. That provides the critical mono setting for me, as well as the convenience of a loudness button and knobs for tone control, which I only use rarely for very low volume playing, but I doubt I'd ever navigate through the menu to use on the new amp.

I still did use direct digital connections to the new amp from the CD player (optical) and the PC (Bluetooth), but I duplicated those digital connections with analog patch cords to the preamp, which I'll use when I need the mono setting. (So far I really can't hear the difference between the digital and analog connections, even with the CD player.)

Anyway, hope you're still enjoying your NAD amp. It's a beauty! :beer
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