[fortney]

Good afternoon,

I built a CMoy (OPA 2227, gain of 4) without a volume control and adjusted volume with the control on the Panasonic 32 ohm headphones.

Subsequently I added a 10K (ALPS RK097) potentiometer to the audio input in the usual fashion and it appears sound quality has suffered.

Are there any audio amp/electronic issues with putting a pot at the output instead of at the input?

I realize there are 2 variables here-pot location and the use of different volume controls-but I thought I would ask before I put the 10K pot at the output.

Thanks

F

 

[AndrewFischer]

10K pot at output won't work.

 

[fortney]

Thanks. Ohm's Law...

F

 

[heatmizer]

10k is the size pot on the inline volume control on koss ksc75

 

[fortney]

I wondered about the volume control on my phones...
The headphones are lo-Z and sensitive and I have never lacked for audio volume with the CMoy.
I'll check it out on a plug board and re-post if the results are wonderful.

F

 

[AndrewFischer]

Quote:

Originally Posted by heatmizer /img/forum/go_quote.gif 10k is the size pot on the inline volume control on koss ksc75

But fortney is using 32Ω headphones. It will be nearly impossible to set a 10K pot to a useful voltage division.

 

[fortney]

I moved the above pot to the output side of the amp and the sound quality is much improved. I can't say if it is as good as it was without any pot until I wire up another jack.

I was listening to Chopin and then Baez CD's.

The OPA is cool and there are no sparks or smoke.

Turning the volume control now results in only slight increase in audio from silent until near the 2/3 point then starts to increase rapidly, but at a rate that is safe for hearing for reasonable people.

I don't hear distortion until near max volume. This is the way it was regardless of potentiometers.

I have no hi-Z and/or low sensitivity headphones to compare.

I guess I will observe the situation a while before I box it up.

F

 

[AndrewFischer]

Would someone please explain why I was wrong here.

I guess with an audio taper that last third may be useful. Still I wouldn't want to be adding series resistance at the output...


Edit: I just looked at the resistance taper curve for that part. In 25% of rotation you only get about 10% of the total resistance -- ding! now I understand.

 

[fortney]

I took a couple resistance measurements from the output jack with the audio volume set at two comfortable listening levels and the results were 240 and 580 ohms.

With the pot at the output one can really appreciate the log taper of the potentiomete.

F

 

[fortney]

Does anyone have any further comments on this CMoy that sounded "tinny" with the pot in the normal position but very good with the pot absent or at the output jack?

The unit is set up on plug board.

It appears that this latter arrangement would work best with a linear taper pot regarding changing the volume.

F

 

[fortney]

With the amp actually assembled it sounds very good with the pot in the conventional position. I think there is slightly more bass and depth without the pot or with it attached to the output, but not much.

F