How to tell if a suspected faulty amp will blow your speaker... without plugging it into your speaker?

[kingforaday]

I know i'm opening myself up to "if you don't know what you're doing, leave it to the professionals" type comments but hopefully someone can offer some advice...

I bought an old Peavey Century amp head out of curiosity and was surprised how good it sounded, I started googling information/opinion on them. I found a lot of threads on other forums in which people mentioned that although they're good, doing an inexpensive mod, that involved adding capacitors because "The additional capacitors make the bottom end bigger and the top end more open." would really improve the amp.

 

I messaged a guy who had done the mod and he talked me through what I needed and how to do it, assuring me it was very easy etc etc

I got the parts, followed the instructions and all seemed to go well until i put it back together and turned it on and it made a crackling noise and smelt of burnt out electrics! So i turned it off. 

I checked my speaker cab with another amp and it had blown the speaker. I messaged the guy and he's gone very quiet all of a sudden (!) to be fair to him, it's not really his problem. 

I've now replaced the speaker and its back up and running on a different amp but that leaves me unsure how to proceed with this one. I could go through it again and check for issues, I could try putting it back as it was, I could muck about with it to my hearts content but ultimately, I'll have to plug it into a cab again to audio test it. 

So at that point, or hopefully before, is there a way to test the output to the cab to know it won't do the same thing? I have a multi meter, I'm pretty good with a soldering iron/tools etc but sadly this is the first experience of the power/amp side of things, so my knowledge is zero. 

Thanks 

[bartelby]

Can you post the information on what the mod was, along with some decent photographs. Someone might be able to help beyond saying "You should have taken it to a professional".

 

[Woodinblack]

The thing that will blow your speaker is a large DC offset which is just putting power into your speaker and burning it out. You can measure that. If it is a solid state amplifier, it doesn't need a speaker connected when you turn it on, so turn it on and measure the DC offset on the speaker terminals. There should be very little or none.

Find a little rubbishy speaker - something from on old radio or something, and connect that, at low volume. Does it go bang?

As said above, what was the mod and if you were surprised at how good it was, did it really need it?

[kingforaday]

2 hours ago, Woodinblack said:

The thing that will blow your speaker is a large DC offset which is just putting power into your speaker and burning it out. You can measure that. If it is a solid state amplifier, it doesn't need a speaker connected when you turn it on, so turn it on and measure the DC offset on the speaker terminals. There should be very little or none.

Find a little rubbishy speaker - something from on old radio or something, and connect that, at low volume. Does it go bang?

As said above, what was the mod and if you were surprised at how good it was, did it really need it?

Thanks, this is what I was thinking of... the amp tech I used to have at work mentioned it can happen to me once and it always stuck in my brain but not enough to understand it! 
 

It is indeed solid state so is it as simple as, set the multi meter to dc, put the probes on the socket, if I get a reading then it’s going to damage speakers? 

[kingforaday]

2 hours ago, Woodinblack said:

As said above, what was the mod and if you were surprised at how good it was, did it really need it?

Well, in hind sight no... but if someone told you, you could take your gear and with just a few quids worth of parts, improve it massively would you not at least want to try!?

i’ll get some pics and details 

[kingforaday]

Ok, so here’s what was done, I’ll find the text from people explaining what is done and why in a sec

removed existing cap and replaced with a stack of caps and a film cap 

[kingforaday]

I've owned a couple of these amps and they are surprisingly fantastic-sounding.

I got into a discussion of it with Bobby Baldwin (Peavey amp engineer) and he made some great suggestions about the output coupling capacitor that were easy to implement:
 

"Basically, capacitors pass higher frequencies with ease, but lower frequencies have a harder time passing through. This means that your power amp puts out a little less in the low frequency range than it does at 1K. The other thing that happens is that when you start to clip the power amp with low frequencies, instead of a pleasant growl, you get a nasty buzz sound. The way to fix that is to use a larger capacitor, but...

The other thing is that capacitors can be inductive, especially electrotytic caps, because the plates are rolled into a coil. This means that your power amp also puts out a little less in the upper audio range because the inductance starts to factor in a little. Inductors pass lower frequencies freely, but tend to block higher frequencies. Just using a larger capacitor can increase the inductance because the plates can be longer, thus more inductance.

Ultimately, the output response of your power amp is sort of a bell curve. 

This capacitor is also subject to high ripple current (rapid charging and discharging) because there is pure, high AC current passing through it all the time. This can dramatically shorten the life of a capacitor.

The way to get around all of this is to use multiple capacitors in parallel, one of them being a film capacitor so there is no high frequency degradation. The inductance will go down because inductors in parallel divide. The ripple current will also divide so you won't be stressing a single cap."

Implementing this idea cost me less than $5 in parts and made a HUGE improvement in the sound of the amp -- much bigger down low and more open on top.

See the attached pic; the coupling cap is located in the lower right hand corner of the power amp board. For reference, in the attached pic, the speaker jacks are in the upper right and the mounting tab for the filter cap is in the lower right.

[Woodinblack]

3 hours ago, kingforaday said:

Thanks, this is what I was thinking of... the amp tech I used to have at work mentioned it can happen to me once and it always stuck in my brain but not enough to understand it! 
 

It is indeed solid state so is it as simple as, set the multi meter to dc, put the probes on the socket, if I get a reading then it’s going to damage speakers? 

Pretty much. There will always be a really small reading, nothing is ideal, plus some AC voltage might register with the general noise of being on, but any significant (ie, volts rather than fractions of millivolts) is going to melt a speaker eventually

3 hours ago, kingforaday said:

Well, in hind sight no... but if someone told you, you could take your gear and with just a few quids worth of parts, improve it massively would you not at least want to try!?

Well, I would but I was an electronic engineer for many years, so it might be a bit different!

But yes, hindsight is a wonderful thing.

[Woodinblack]

Assuming that nothing is touching anything, I cant see that that itself would create much of an issue, although I wouldn't have done it myself. What blew up / smoked?

[kingforaday]

7 hours ago, Woodinblack said:

Assuming that nothing is touching anything, I cant see that that itself would create much of an issue, although I wouldn't have done it myself. What blew up / smoked?

Unfortunately I’m not entirely sure, it just sounded like something was crackling and a bit of a hum and then I turned it off... could’ve been the speaker making the noise. There was the smell of burning electronics but no actual smoke as far as I could tell. I haven’t touched it since. Put a new speaker in the cab though so that’s ok now. 
 

it’s annoying as although I have mainly steered clear of power sections on amps through lack of knowledge... I did mod a guitar amp I have, there was way more to it but it went fine. Although to be fair it was supplied in kit form with really comprehensive instructions. 
 

i guess with this I just figured as it seemed so straightforward, It wouldn’t be an issue