I need to replace my Nad M25 Power amplifier.

[PENG]

I was in agreement with @PENG on this. I also thought that this was basically a "max voltage rating", and nothing more. How does this relate to "derating capacitors" to get a safety margin? You must also consider the charging capacity with the derating calculations???

I have never heard of this before! If this is indeed correct, then "common error" is a vast understatement! I think "proliferated error" or "ubiquitous error" would be a better description.

@TLS Guy do you happen to have any literature references for this? I do believe your technical abilities, I'm just surprised that I seem to have also made this error and never have seen any literature to the contrary!

The de-rating thing you might have heard about is not related to the electrical theory but to the physical characteristics. For example, temperature is also a factor. De-rating for higher applied voltage is indeed a factor to consider, if capacitors of high dielectric strengths are used, such as the ceramic type. We are discussing electrolytic types here, going from 35 to 50 V does not warrant any de-rating as such.

https://www.murata.com/support/faqs/products/capacitor/mlcc/char/0005
https://electronics.stackexchange.com/questions/190362/electrolytic-capacitor-voltage-rating-vs-capacitance

Even if we are talking about ceramic caps that required some sort of de-rating due to higher applied voltage, TLS Guy would still be wrong because going with a higher voltage ceramic (just as example), for the same capacitance value but with a higher voltage rating is actually one way of allowing for "de-rating". So for a 5V rail voltage, if you use a 15 V cap you will be covered, as the capacitance drop at or near 5 V will then be insignificant.

 

[PENG]

For those interested, there are many good articles on the internet, below is one of many and is downloadable from Cornell Dubilier.

http://www.cde.com/resources/catalogs/AEappGUIDE.pdf

On page 7 it mentioned how the capacitance is measured/rated:

Capacitance Measurement For aluminum electrolytic capacitors, capacitance is measured as the capacitance of the equivalent series circuit at 25 ºC in a measuring bridge supplied by a 120 Hz source free of harmonics with maximum AC signal voltage of 1 Vac and no bias voltage.

So in this case, it was based on using a measuring bridge with 1 Vac maximum, this should help clear TLG guy's doubt.

 

[TLS Guy]

This part is correct, who is arguing?? Now, Q=CV, Q is the charge in Coulomb, C is the capacitance in Farad. It should be clear from this formula that the "Charge held.." is proportional to C (unit in mfd) and the applied voltage (unit in V, i.e. volt).



No, are you mixing up "Charge" with "Capacitance" ? You don't store mfd, you store Coulombs, higher mfd value means more Coulombs you can store, at a given applied voltage.



Incorrect ! As mentioned above, you don't store 25 mfd, 25 mfd is microfarad, or uF, that is the capacitance, and is equal to Q/V. So you can see clearly that since Q=CV, so as you said, the charge is proportional to voltage, but it is also proportional to the capacitance. If the applied voltage is the same, then the charge held in the example you are using will be the same, as shown below.

In your example of 25 mfd, charged to 50 volts:

Q=CV = 25 X 10−6 X 50 = 0.00125 Coulomb.

That is so incorrect! TLS Guy, I know and do respect your knowledge. I called you out before (rarely) on certain things, usually on the power and electronic side, and you never had problem with it. For that you have my respect too. This is about electrical principles, definitions, theories etc., so it is a matter of facts, not opinions. I hope you will do your research properly, admit the error and move on, so it won't stay on record that may mislead people inadvertently.

I will look into this further, but for DC working, of a cap like smoothing, the application here this is how I have understood the relationship.





I have to leave now for an appointment. I will get back to this later.

 

[highfigh]

Last night, my Nad M25 was celebrating 4th of July early under its cover...
Upon farther inspection, I found out that it blew a capacitor.
If I need to replace it, I won’t be able to spend as much as I did back then.
I only need a 5 channel amp.
I need it to cost well under $2k
My speakers are B&W 683 front and 684 in back.
I heard good thing of the Monoprice Monolith 5x200
And another one I was recommended was the
Rotel 1555.
What do you guys think?
Will I be happy with any of those amps coming from the M25?
My preamp is a Nad M15HD2.
Thanks.

The blown cap doesn't mean it needs to be tossed- just replace the cap and make sure everything is within spec.

DO NOT go nuts if you read comments about replacing all of the caps and hearing angels sing- it's BS and very common advice from people who don't understand what caps do, when they need replacement and why. If the others are within spec, leave them.

 

[PENG]

The blown cap doesn't mean it needs to be tossed- just replace the cap and make sure everything is within spec.

DO NOT go nuts if you read comments about replacing all of the caps and hearing angels sing- it's BS and very common advice from people who don't understand what caps do, when they need replacement and why. If the others are within spec, leave them.

No one mentioned the possibility of angels sing..., yet... That particular rectifier bridge won't affect sound quality anyway. It's about minimizing chance of losing another cap like the failed one a few years later, potentially. It is not uncommon for electrolytic caps to deteriorate over time even if there is no visible damage, so while at it, it is generally advisable to spend a few more dollars on parts. A 3300 uF 35 V electrolytic cap isn't going to cost much.

 

[slipperybidness]

The de-rating thing you might have heard about is not related to the electrical theory but to the physical characteristics. For example, temperature is also a factor. De-rating for higher applied voltage is indeed a factor to consider, if capacitors of high dielectric strengths are used, such as the ceramic type. We are discussing electrolytic types here, going from 35 to 50 V does not warrant any de-rating as such.

https://www.murata.com/support/faqs/products/capacitor/mlcc/char/0005
https://electronics.stackexchange.com/questions/190362/electrolytic-capacitor-voltage-rating-vs-capacitance

Even if we are talking about ceramic caps that required some sort of de-rating due to higher applied voltage, TLS Guy would still be wrong because going with a higher voltage ceramic (just as example),for the same capacitance value but with a higher voltage rating is actually one way of allowing for "de-rating". So for a 5V rail voltage, if you use a 15 V cap you will be covered, as the capacitance drop at or near 5 V will then be insignificant.

I was referring to derating caps during the design stages. In other words, using a higher Voltage rating Cap for a safety margin. Going by memory, I think it's pretty standard practice to derate a Cap by ~25% (i.e. use a cap in the design that is rated for "peak voltage + 25%").

Going through your math and info, I'm in agreement with your side of the argument here, but granted I have not run through the calculations myself. Taking that side of the argument, my derating question is no longer a valid question.

 

[highfigh]

No one mentioned the possibility of angels sing..., yet... That particular rectifier bridge won't affect sound quality anyway. It's about minimizing chance of losing another cap like the failed one a few years later, potentially. It is not uncommon for electrolytic caps to deteriorate over time even if there is no visible damage, so while at it, it is generally advisable to spend a few more dollars on parts. A 3300 uF 35 V electrolytic cap isn't going to cost much.

The rectifier shouldn't affect sound quality at all, unless the original was completely under-sized.

If the filter section had blown caps, I would probably replace them all if they're really old, but still, they should be tested and probably upgraded for voltage. In a solid state amp, they're usually good unless they have been stressed or the working voltage is close to operating voltage- that would just be a cheap thing to do.

Angels singing in 3,2,1.....

 

[PENG]

The rectifier shouldn't affect sound quality at all, unless the original was completely under-sized.

If the filter section had blown caps, I would probably replace them all if they're really old, but still, they should be tested and probably upgraded for voltage. In a solid state amp, they're usually good unless they have been stressed or the working voltage is close to operating voltage- that would just be a cheap thing to do.

Angels singing in 3,2,1.....

Agreed. That rectifier appears to be for standby power only, so 101% won't affect SQ.

 

[Pablo2k]

Thank you for all this information guys.
I ordered the caps and ofcurse they sent the wrong ones, 33000uf instead of the 3000uf I need!!
Also after cleaning the area, I noticed that the cap did some damage to the board:

http://imgur.com/a/KeppYUt

It will have to be jumped.
I think I’ll have to send it in for repair...
How much do you guys think it could cost?
More than $500? $600? Or less?! I have no idea.
I’m not going to send it to Nad, too far from me and too expensive to ship.
I will use an electronic repair shop that has many positive reviews on line
Davidsonelectronics here in Long Island.
Thanks again.

 

[Winkleswizard]

Thank you for all this information guys.
I ordered the caps and ofcurse they sent the wrong ones, 33000uf instead of the 3000uf I need!!
Also after cleaning the area, I noticed that the cap did some damage to the board:

http://imgur.com/a/KeppYUt

It will have to be jumped.
I think I’ll have to send it in for repair...
How much do you guys think it could cost?
More than $500? $600? Or less?! I have no idea.
I’m not going to send it to Nad, too far from me and too expensive to ship.
I will use an electronic repair shop that has many positive reviews on line
Davidsonelectronics here in Long Island.
Thanks again.

Yes, the circuit trace will have to be jumped.

Should not be a major issue.

If the shop is reputable then they should quote cost to diagnose and then the cost to repair. If the damage is limited to the power supply, then could be $100 to $200. If it is a lot more, then it really boils down to how much you are willing to put into it before your money would be better put towards a replacement...

Since it is a modular design, you might be able to sell it for salvage. In that case, it is still worthwhile to get a professional diagnosis. If they can state the amplifier modules are likely good, it will drive up the salvage value. Many shops sell used equipment, so they may be willing to buy it from you too.

Another factor is the repair warranty. If not an authorized repair center, you might not get much than a 90 day warranty. This may play into your decision to fix it or not. I have had mixed repair experiences and have heard others have too. Have had better outcomes when the problem was clear cut. However, when signal quality was an issue, have had few positive outcomes. If the warranty really is fairly short, would only spend on repair when the outcome was high confidence. In your case, if is only the power supply, I would get it repaired. Otherwise, would be “exploring other options”...

Ww

 

[TLS Guy]

For some reason the picture would not hold on my previous post.

My point is there is a huge difference between AC and DC working.

Sure the total charge able to be stored does not change as you increase the voltage spec of a cap. But it does take a higher voltage to store the charge. Otherwise the charge stored would be independent of voltage which is totally contrary to common sense.

All I can tell you is that if you install smoothing caps of a higher voltage than specked in a power supply you are very likely to end up with deficient smoothing.

However if the cap is a DC blocking cap, then you can increase the voltage spec.

Clearly the blown caps in that NAD unit are smoothing caps.

 

[PENG]

For some reason the picture would not hold on my previous post.

My point is there is a huge difference between AC and DC working.

Sure the total charge able to be stored does not change as you increase the voltage spec of a cap. But it does take a higher voltage to store the charge. Otherwise the charge stored would be independent of voltage which is totally contrary to common sense.

Look at your own graph, Q=CV, the x-axis is "applied voltage", not rated voltage. The charge is not independent of the applied voltage, but it is independent of the rated voltage (as long as the applied voltage is <= the rated voltage). That's exactly what you graph shows.

The rated, or maximum working voltage is just the voltage that you can expose the capacitor to safely without sustaining damage.

There are exceptions, such as caps of very high dielectric strength material, but that's not deviation to the formula Q=CV, but a matter of physical characteristics. It is irrelevant in this case, I just want to mention it in advance.

 

[PENG]

Thank you for all this information guys.
I ordered the caps and ofcurse they sent the wrong ones, 33000uf instead of the 3000uf I need!!
Also after cleaning the area, I noticed that the cap did some damage to the board:

http://imgur.com/a/KeppYUt

It will have to be jumped.
I think I’ll have to send it in for repair...
How much do you guys think it could cost?
More than $500? $600? Or less?! I have no idea.
I’m not going to send it to Nad, too far from me and too expensive to ship.
I will use an electronic repair shop that has many positive reviews on line
Davidsonelectronics here in Long Island.
Thanks again.

I would solder the new caps on using temporary jumpers to find out if that's the only issue. Then you can decide on sending it to an authorized, or known reputable shop or not.

If you send it out as is now, and if the shop is a good one, they will charge you probably $75-$125 for troubleshooting and estimating. If they recommend just replacing the two caps and repairing the board, it may cost you a total of $250-$350 depending on their labor rate. Obviously, if they find, or claim to have found something else is wrong, the cost would be higher. Or they may recommend replacing the caps of the main power supply as well, then the cost will increase exponentially.

IMO, 7 channel amps for home use is rarely the way to go for several for practical and technical reasons. AVR plus a 3 channel amp should do the trick for most. I do like the Monolith 5X200 regardless though. I am mentioning it now in case.., hopefully it is just a straight forward repair for <$200. Labor rate is the killer, typically..

 

[Pablo2k]

I have had a look at the service manual and the circuit.

By the value of the cap, the cap has to be C 202 or 203. They are the same value and paired.

I suspect that the primary culprit is Bridge rectifier BR 201 DB 104G.That is switch diode 1N4148 . I would replace that and C202 and 203 both the same value. I would also replace R 2H1 Metal film resistor 100Ω, 1/6W, 5% and C 204 and 206 both Polyester caps 10nF, 10%, 100V . I would suspect fuses 201 and or fuse 203 are blown. They are T1.6AL/250V .

After replacing those components it should be started up on a variable voltage auto transformer, know as a Variac and observed closely.

This should be repairable at modest cost.

Thanks again, I ordered all those parts.
Fuses did blow, I wonder why.
The capacitor burn through the board, I had to jump wire it, I hope it works.
Here are some photos:

http://imgur.com/a/E8paHFJ

 

[Pablo2k]

The Monolith would be my choice of those mentioned, but personally would likely just buy three Crown XLS1502s....

A friend of mine has one Crown XTI 4002 and he will sell it to me for $300. Do you know how it compares to the XLS? I know they are a lot more power, but I was wondering sound quality for movies. Thanks again.

 

[lovinthehd]

A friend of mine has one Crown XTI 4002 and he will sell it to me for $300. Do you know how it compares to the XLS? I know they are a lot more power, but I was wondering sound quality for movies. Thanks again.

XTi 4002 has not only more power but also a bit more in the way of dsp options. $300 is a great deal, get it!

 

[TLS Guy]

Thanks again, I ordered all those parts.
Fuses did blow, I wonder why.
The capacitor burn through the board, I had to jump wire it, I hope it works.
Here are some photos:

http://imgur.com/a/E8paHFJ

That board really had a fry up. I would bet that the bridge rectifier failed and caused this. The blown fuses indicate massive increase in current. I think the rectifier failed and sent AC to the caps, in which case massive current would pass through them.

I think if you just replace the components I mentioned, I would bet it will be fine. Jumping the damaged part of the printed circuit was the correct thing to do. Just check your work and make sure there are no shorts to adjacent conductors and you have no dry joints.

I don't think a shop will charge a lot to do this. If it is as I suspect a good tech should have this done and dusted in about an hour.

Before the unit can be tested the repair needs to be made. It won't take long to find out if it is working correctly. If things are not right after the components have been replaced, then charges will rise.

 

[Pablo2k]

That board really had a fry up. I would bet that the bridge rectifier failed and caused this. The blown fuses indicate massive increase in current. I think the rectifier failed and sent AC to the caps, in which case massive current would pass through them.

I think if you just replace the components I mentioned, I would bet it will be fine. Jumping the damaged part of the printed circuit was the correct thing to do. Just check your work and make sure there are no shorts to adjacent conductors and you have no dry joints.

I don't think a shop will charge a lot to do this. If it is as I suspect a good tech should have this done and dusted in about an hour.

Before the unit can be tested the repair needs to be made. It won't take long to find out if it is working correctly. If things are not right after the components have been replaced, then charges will rise.

I’m such an idiot!!!

Fuses DID NOT BLOW I meant to say!!!
Fuses, all four, tested fine.

 

[TLS Guy]

I’m such an idiot!!!

Fuses DID NOT BLOW I meant to say!!!
Fuses, all four, tested fine.

Well that changes the picture. Usually with a fry up like that a fuse blows someplace.
In that case this might well be aging caps. Since the fuses did not blow, the it would be reasonable just to replace the caps and see what happens. Since the fuses did not blow that would indicate you are unlikely to make matters worse with this course of actions.

 

[Pablo2k]

Well that changes the picture. Usually with a fry up like that a fuse blows someplace.
In that case this might well be aging caps. Since the fuses did not blow, the it would be reasonable just to replace the caps and see what happens. Since the fuses did not blow that would indicate you are unlikely to make matters worse with this course of actions.

You have given me such great information TLS Guy! I really appreciate your help.
I can’t wait for the parts to get here and see what happens.
Thank you!