[Born2bwire]

I built myself a CMOY amp for my HD580's and Sony PCDP. I've got a Brown Burr OPA2132PA opamp and I currently have the amp set at a gain of 11 by using a 1Kohm resistor for R3 (going off of what's up on Tangent's website). I'm using a Clarostat 10K pot and right now the gain too high. This wouldn't be too much of a problem if the pot didn't have a skewed balance at very low levels (which I listen at due to the high gain...you know, I don't seem to have that much luck with getting balanced pots of late). Anyway, I only have one day before I'm out for a week to run by the ECE store and pick up new resistors. So, I really do not have the time to experiment before I can show the amp off to my dad when I get home for Thanksgiving. I was wondering what gain setting y'all would recommend for the HD580's?

 

[andrzejpw]

I used a gain of approximately 8 on my meta42. Works great!

 

[tangent]

Dropping gain from 11 to 8 will only give a 2.8 dB change. That's less than 3 dB, which is the minimum difference you should make to get a noticeable change. Personally, I always change gain by 2x at minimum.

I find that a gain of around 5 is sufficient with HD580s. But remember, the strength of your source makes a difference. A gain of 5 is a lot with a full line-level source, but with a weak portable it may be just barely enough.

 

[Dreamslacker]

Quote:

Originally posted by Born2bwire I built myself a CMOY amp for my HD580's and Sony PCDP. I've got a Brown Burr OPA2132PA opamp and I currently have the amp set at a gain of 11 by using a 1Kohm resistor for R3 (going off of what's up on Tangent's website). I'm using a Clarostat 10K pot and right now the gain too high. This wouldn't be too much of a problem if the pot didn't have a skewed balance at very low levels (which I listen at due to the high gain...you know, I don't seem to have that much luck with getting balanced pots of late). Anyway, I only have one day before I'm out for a week to run by the ECE store and pick up new resistors. So, I really do not have the time to experiment before I can show the amp off to my dad when I get home for Thanksgiving. I was wondering what gain setting y'all would recommend for the HD580's?

You can try putting a 10K ohm or 20K ohm resistor inline with the pot on the input side. That will allow you to turn the pot up a little more. Having a total larger resistance also reduces the proportion of the difference between the channels.
BTW, I hope the load resistor is reasonably large if you attempt this.

OTOH, paying a little more for a nice ALPS pot. would be a good choice. It'll last you through upgrades as well.

 

[tangent]

If you put the resistor between the pot and the R2 input impedance resistor, you form a voltage divider, so what you're really doing is adding attenuation to the front end of the amp, not adding "load". This will raise your noise floor, so it's not the best solution. Lowering the gain will actually lower the noise floor, while still accomplishing the stated goal.

 

[Dreamslacker]

Quote:

Originally posted by tangent If you put the resistor between the pot and the R2 input impedance resistor, you form a voltage divider, so what you're really doing is adding attenuation to the front end of the amp, not adding "load". This will raise your noise floor, so it's not the best solution. Lowering the gain will actually lower the noise floor, while still accomplishing the stated goal.

Actually, on the other side of the pot. than R2.

But lowering the gain does affect the opamp stability, doesn't it?

 

[tangent]

Quote:

lowering the gain does affect the opamp stability, doesn't it?

Lowering the gain increases the op-amps susceptibility to instability. As long as the op-amp is stable in your circuit on that board with your chosen gain, that's all that matters. And a gain of 5 isn't exactly risky with op-amps used in your average CMoy.

Besides, raising the gain back from ~5 to 11 is easy if you find you need to: just solder a second 2K resistor over the top of your R3 so that they're in parallel. Voila, R3 is now effectively 1K again.

 

[Born2bwire]

Well, I put in a pair of 2.5 Kohm resistors in place of the 1Kohm and it's still way too loud. I don't know, I'm out of time to play with it for today and I plan to finish the enclosure and install the amp into over the week so I guess I'm keeping it like this for a while.

 

[Dreamslacker]

Quote:

Originally posted by Born2bwire Well, I put in a pair of 2.5 Kohm resistors in place of the 1Kohm and it's still way too loud. I don't know, I'm out of time to play with it for today and I plan to finish the enclosure and install the amp into over the week so I guess I'm keeping it like this for a while.

Did you put the 2.5K ohm pair in series or in parallel?

Or was it just replacing a pair of the 1K ohm resistors with a pair of 2.5K ohm resistors?
If so, that will bring the gain down to 5X. Which didn't make much of a difference in volume when I tried it myself.

You could try what I told you or buy a better pot. I usually buy 2 or 3 pots if I get generic ones. That way, I can choose the best from the lot.

Adding a resistor would be Like this:
Quote:

Your impedance resistor should be relatively larger value though. At least 10X the total resistance of the pot & added resistor.

 

[Born2bwire]

I replaced R3 with the new resistors. I messed around with the amp a little more on a long car drive back home to Virginia and I think the problem is that the pot may increase its resistance using the wrong scale (I guess the one I don't want would be linear...). It seems to grow loud very quickly and then the volume does not increase too much from that point. I also added a volume knob now and the larger turning radius helps make it less sensitive. I think I'll keep it this way, I've got it to work so I don't feel too much compelled to toy with it at this junction.

 

[tangent]

If that's a log taper pot (a.k.a. audio taper) then you've probably hooked it up wrong if it's changing volume rapidly even with a knob. It should change volume steadily -- the same amount of turn should give the same perceived jump in volume level, no matter where you are in the turn radius. But if you hook the pot up wrong, it can sort of work but with the smooth volume taper defeated.