[fiddler]

Alright,

I've got a MINT amp here that works apart from noise that happens when I turn the pot past 3 o clock. It starts with a "click" sound around 3 o clock, then when I turn it more, I hear swishing sounds, the kind you hear when you're trying to tune a radio.

I've changed the pot.
I've swapped out the power caps, opamps, buffers, bypassed the input caps...

nothing.

The noise is much louder when the amp is being powered by a 18V switching supply. When running of a single 9V, you hardly hear the radio noises, but there definitely is a click around 3 o clock.

I'm completely lost. Any ideas?

 

[BPRJam]

I had the same problem with at least 2 of the Mint amps that I have built. Changing out the pot helped on one of them. On the other I found that the sounds were coming from my source, which was harddrive based. Even if the source wasn't hooked up, if I was near the active harddrive, I would get RF-sounding interference after about 3 o'clock. If I turned off the source, I would get a little bit of a swooshing sound as I turned the pot, but the "tuning in a radio" noise went away.

I never was able to solve the source problem, but luckily my gain was high enough that I never have to turn the pot that high.

Also, there have been a series of complaints here about the pana pots. It is possible that you have 2 marginal ones.

Hope this leads you down the right path.

BPRJam

 

[tangent]

Try measuring the current draw of the amp as you fiddle with the volume. If the current draw spikes when you hear the click, it has begun to oscillate. You might also check DC offset under the same conditions.

 

[fiddler]

Tangent, you da man!

Current draw doubles from 10 to 20 mA precisely where it clicks, and it stays that way anywhere above 3 o'clock.

Ok... so any ideas on what's causing the oscillation?

 

[fiddler]

Just tested DC offset, no source connected:

At minimum volume:
R= +3.4 mA
L= +2.6 mA

Where the click occurs:
R= -21.5 mA (note the negative)
L= +10.3 mA

Just a wee bit past the click:
R= -25 mA
L= -19 mA

At max:
R= -27.1 mA
L= -20.7 mA

 

[tangent]

Quote:

so any ideas on what's causing the oscillation?

Post your amp's configuration details: chips used, R values, whether D2 is present or not, etc.

 

[fiddler]

right,

AD8620
D2 present
R1 1K
R2 499K
R3 2x 499 piggybacked to parallel them (I needed to raise the gain but didn't have the right resistor value handy... I suspected this area first so I remove the extra resistor but this didn't solve the problem)
R4 4.99K
R5 4.99K
R6 499K
R7 100
R8 jumpered
All caps are the recommended Panasonic ones..

 

[tangent]

Lower R5 to about 2K, and check all the joints, especially those around the op-amps' +INs.

 

[fiddler]

Quote:

Originally posted by tangent Lower R5 to about 2K, and check all the joints, especially those around the op-amps' +INs.

Ok, I moved the piggybacked resistors to R5 effectively lowering R5 to 2.5K, no result. Reflowed all the joints on the opamp, nothing.

Argh.

 

[fiddler]

Forgot to mention:

I even tried rebuilding on a brand new board, and the symptoms remained. I guess this narrows down the problem to a bad part.

So far I've checked that it's not the caps, it's not the chips, the railsplitter is fine, ...

A bad CRD?

BRB!

*comes back*

[edit, i'm an idiot]

Nope, not the CRD's.....