Some times ago, I posted a message on this forum about a reverb problem with a CMOY amp.

Shortly, I have build a CMOY following tangent's tutorial. Only difference is that I use an OPA2134PA instead of the 2132PA, which is B.O.

Problem is that the sound show a lot of reverb, as if it was coming out of a big steel cave. At this point, I get suggestion to check out the solders and if evrything was at the right place. I've done this, but haven't seen anything to looks bad. Redone some solder, without any results.

So, I decided to desolder everything and start up again from scratch on a new board. Result: exactly the same, I have reverb!!!

Even if I made some bad solders, it's hard to think that I have made exactly the same bad solders on two different boards to the point of getting the same results... Adding to that, I've checked out every solders and everything that was possibe to verify with a multimeter, and everything seems fine. Voltages are OK, resistance value are OK, without any obvious short circuit (not obviuos enough to be detected by the multimeter).

Also, since the problem is similar on both channel, it pointed to something that is common to both. The only thing I could see is the 2134PA amp. All others components coming in pair, one for each channel, means that if one of those was defective, I should hear the problem in only one channel... Am I right to think that ?

So, do you think it may be worthwile to try an other opamp ? The 2132 isn't available at this time, but what about trying an OPA2227 ? Could it help solve the problem ?

Thanks for your help! Any comments will be appreciated !

Perhaps your opamp is defective? Try another one. [OPA2134].

Have you triple checked against tangent's diagram?

Just exactly how much "reverb" are we talking about here?

If it's not the board, the only things common to both channels are the output jack, the op-amp and the pot. Another thing is, make sure all the ground connections are made: one to each jack, one to each input jack, one to each channel on the pot, etc...

CarlJF,

You are right in that it is highly unlikely you made the exact same mistake(s) twice, producing reverb. Try the following in this order:

1) place 0.1uF capacitors (polyester or polypropylene preferred; ceramic will do in a pinch) as close as possible to the op-amp from pins 4 and 8 to ground.

2) place a 10 ohm resistor in between each output and the headphones.

3) replace the op-amp with something dreadfully slow like a LM1458 (just to see if the reverb stops, but without spending a bunch of money).

4. Place 10pF NPO ceramic capacitors across the feedback resistor on each channel.

I strongly suspect your op-amp is oscillating at a low frequency because of unintentional feeback through the power rails (hence suggestion #1).

A votre sante!

Quote:



Problem solved!!! My mistake was that I hadn't properly grounded the RCA input jacks. Kind of a very newbie mistake... Now, at least, I know it, the RCA plugs also have to be grounded to the virtual ground.

Now, the amp really sound great. When first starting this project, my goal was only to get more powerful amplification since my CD player wasn't strong enough to drive my headphones. But now, not only the amplification is stronger, but everything just sounds better!!!

By the way, in the meantime, I was able able to find an OPA2132 at a local store. Even if evrything now works correctly with the 2134, I will make a test with the 2132 to check if it make any difference. Anyway, I will certainly try to build other amps so the extra chip will find some use in the near future.

Thanks all for your help!

And just one last newbie question about grounding, on a DC adapter plug with 3 pins, does the third pin also has to be grounded ?