[BradJudy]

I've had a PPA for a while now that I built (1.x board with Glassman buffers) and have enjoyed with my HD-600s. Recently, I picked up some SR-225s to have my choice of sounds. When using the 225s with the PPA, I get an odd buzzing noise. The noise doesn't get louder with the volume, is present without a source connected and isn't a constant buzz. In fact, I can usually change the buzz by touching the headphone cord or moving it around. It seems to calm down a bit after 10 minutes or so, but I haven't tested that a lot.

I have used the 225s on other setups and they work perfectly fine, so they don't appear to be the problem.

Any ideas?

Thanks in advance.

 

[MisterX]

Which glassman buffers?

You can try preloading the outputs.
Got a pair of 10 ohm resistors handy?
If not you can go as high as 50 ohms....solder each of them right to the headphone jack between the left channel and ground and the right channel and ground.
You may also want to install a ferrite bead on the wires to the jack.

 

[BradJudy]

Quote:

Originally Posted by MisterX Which glassman buffers? You can try preloading the outputs. Got a pair of 10 ohm resistors handy?

My buffers are from the last round that Glassman did (I don't remember the version number, but I could find it if I dig up the old thread). The SMD work was pre-done, but I did the soldering on the through-hole components.

I have some 10 ohm 1/4 watt resistors (I bought one of the Radioshack 500 count variety packs a while ago to have things around when I need something) so I'll give that a try.

I don't have any ferrite beads sitting around, but I can pick one up to try.

 

[MisterX]

Quote:

My buffers are from the last round that Glassman did (I don't remember the version number, but I could find it if I dig up the old thread). The SMD work was pre-done, but I did the soldering on the through-hole components.

Cool that helps...

Do you know what value of your C6 is?
And did you install the compensation caps?

 

[BradJudy]

I'll double-check, but I believe C6 is 10pF

By 'compensation caps' do you mean the 100pF caps for R12 with the Glassman buffers? If so, yes on left and right, no on ground.

 

[BradJudy]

I double-checked and C6 is 10pF.

 

[MisterX]

Yup those are the caps I meant.

 

[BradJudy]

Pre-loading with 10 ohm resistors has silenced the buzz with my Grados. Thanks.

Perhaps now I'll have to be an audio snob and get some fancy resistors to use in place of the RadioShack ones.

 

[Glassman]

which opamps are you using btw.? loading outputs is not the right solution.. the behavior you are describing is known to me, it has a solution..

 

[BradJudy]

Quote:

Originally Posted by Glassman have you installed 100pF across R14? which opamps are you using btw.? loading outputs is not the right solution..

Yes, on R12L/R (which I believe is correct for my version of the PPA board). I am using 8610 opamps at the moment, but I think I have some 627s around that I could swap in.

 

[Glassman]

had no particular probs with ad8610, should be stable with them.. try more/less than 100pF, these are phase compensation caps, I picked 100pF because it made it stable under all conditions I tried, but I haven't tried Grados, maybe other value will work.. R12 are 1k, right?

 

[BradJudy]

Ummm...R12 were removed and replaced with the caps as per your instructions: http://www6.head-fi.org/forums/showp...&postcount=568

 

[Glassman]

forget the numbering, there should be 1k resistor with 100pF cap across it between the output of the opamp and input of the buffer..

 

[BradJudy]

Yes, because R12 was replaced with a 100pF cap and it's slot was bridged to the adjacent R14 which contains a 1k ohm resistor, there is a 1k resistor with a 100pF cap in parallel between the opamp and the buffer.

 

[Glassman]

if so, experiment with the capacity, try increasing/decreasing those 100pF caps.. also try OPA627, just out of curiosity.. unfortunately I'm not able to reproduce this problem not having Grados, but it was the same story with Porta Pro and PX100.. you might also consider using AD744 in special configuration - I found it to be rock stable even w/o those caps I believe.. and the sound is the most transparent by far, skipping the whole internal output stage, makes an almost-discrete amp out of PPA..