Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › New amp......holy S*hit what's this....
New Posts  All Forums:Forum Nav:

New amp......holy S*hit what's this....

post #1 of 12
Thread Starter 
Hey guys,
Just built a new amp - its a buffered CMOY using 1 OPA637 and 1 EL2001 per channel with a BUF634 in the power supply. I'm using 2200uF rail capacitance and have decoupled all PS pins with a 0.1uF Ceramic bypassed with a 100uF Electrolytic. I'm using a 24V ac-dc wallwart supply to power the beast.
However thing is guys I get this really crappy humm some of the time when using the amp. It's a low level humm which seems to fade as the amp is left on and it is loudest when first turned on. However when turning off the amp because I have so much rail capacitance the amp continues to run. As it is doing so the humm dissapears altogether and the amp works perfectly for about a second or two!

However once the humm has dissapeared after the amp being left on for a while it won't reappear for some time even if I turn the thing on and then off.

I suspect it's the power supply, wallwart one that I'm using. It's somehow takeing time to stabilise I think, that could account for the amp being perfect when turned off???

I tried another wallwart too, even though it was a bit of a dodgy one too, and it gave the same humm but this time even more deeper.

What do you guys thing it could be? I'm certain the amp isn't oscillating since I've tried OPA627s and OPA132s in there too.

You help would be much appreciated!
Thanks
post #2 of 12
What is your grounding scheme like for amp, power supply, and chasis?
post #3 of 12
Thread Starter 
basically everything is hooked to the virtual ground, decoupling caps, jacks etc. Plastic case also.
post #4 of 12
Is the humm the same tone all the time, or does it vary?

What pot are you using?
post #5 of 12
Try hooking it up using batteries. If that fixes it you might want to add some more caps across the incoming voltage rails. Also a bad cap could cause something like that, but i would try adding the caps first.

EDIT: I mean more, smaller caps across the incoming. 2200uF caps arn't noted for their speed, and it simply may not be able to filter out the higher frequency noise.
post #6 of 12
Thread Starter 
thing is guys I built this amp as a replacement for my other amp, which suffered from the same problem!! all thats happned is that the problems followed me, lol!

the humm is the same tone throughout and is more noticeable with low impedence cans.

ALPS pot by the way, i recycled it from my last amp.
post #7 of 12
Thread Starter 
after adding a 22uF capacitor bypassing one of the resitors in the resitor divider in the PS, I now get a turn on pzzt/bzzt after which the buzzing continues without stopping.

Also, the buzzing can be altered by rotating the POT, i can change it from a low buzz to a high buzz by rotating from min to max on the pot (thats maximum resistance to minimum resistance).

Please help me guys, I'm becoming deperate, this ain't fair!

here's a schematic:

<img src="http://communities.msn.co.uk/_Secure/0OgAKG2EUbk02TM6aSeu!CAusYx64BZAuZxXpnCsMAom4Y5j!Z FeReAQsD2ZgXQyoxZkO30OtNbt8GQQZcuRTYZJvFbQCOL6p/Untitled-20.jpg">
post #8 of 12
Low buzz to high buzz meaning the volume of the buzz, or the frequency of the buzz? If you turn the pot to completely full volume, is the buzz still there?
post #9 of 12
Your problem sounds like it could be oscilation. the OPA-637 has a bandwidth of 80 MHz. and the EL-2001 has a bandwidth of 70 MHz. if the Buffer is placed in the feed back loop of the OPA-637 then the amount of phase shift through the Buffer can cause the entire Amp to be unstable. Try an OPA-627 and see if this cures the Hum. Also remember that the bandwidth of the Output stage is lower under Low impedance Loads than it is at higher Impedance Loads. if installing the OPA-627 cures the Problem great. Also if after poser down and just prior to turn off you hear a burst of white noise this is also indicative of instability but its cause is slightly different but still inter related. for the OPA-627 use the EL-2002 180 MHz. buffer and operate the OPA-627 at a gain of 11 or more. also use a 200-400 ohm resistor between the opamp's output and the Buffer's input. Like mentioned before a grounding Problem can and will cause the exact same thing.
post #10 of 12
Thread Starter 
Just to retierate, initially there is a turn on pzzt after which the humming/buzzing continues faintly indefinitely.

Yes I have tried different more stable OP Amps, none seem to help.
post #11 of 12
Thread Starter 
i got a feeling it may be my wallwart, since like i mentioned this happened with my last amp too!
post #12 of 12
Thread Starter 
IT WORKS!!! It's a dodgy wallwart! Running 2 9V batteries in series runs it as clean as a baby's bottom!
New Posts  All Forums:Forum Nav:
  Return Home
Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › New amp......holy S*hit what's this....