Help me understand this Volume Control circuit (Counterpoint Solid 8 pre)

[dogwan]

I've been staring at this on my bench for a couple weeks. Not sure I understand why it's done like this, and what will be the implications of modifying it.

It's a Counterpoint Solid Line 8 Control Amplifier (preamp).
When I bought it a while ago I was disappointed to find it didn't have the Alps Black Velvet balance and volume pots seen in the rare pics online. Instead if has pretty standard commodity Alps pots, though it does look factory wired and there's no sign on the original pads for the BV pots that they were removed. Suspect this was a later run after supply issues? Though PCB board does say "Rev. A".

Anyway, here are the issues that caused me to think about replacing the volume pot.

1. There's an intermittent buzz when you touch the volume knob (kinda slight, but annoying. Sounds like a capacitance induced buzz?)
   Also note: I did jumper the case of the pot to ground and it does eliminate the issue, so could easily solder a ground wire to the case.
   
   
2. The shaft has some loose play.
   
   
3. There's an annoying surge (not a jump) in the Volume control at about 11:00 on the dial (right where the loudness tap is).
   
   
4. Not a quality pot as the originally designed for "Black Beauty".

SO, I have a spare BB pull that matches the 100k spec of the existing pot, but the BB doesn't have a Loudness tap.

I took some measurements and also lifted the "RTaper" resistors that goes from the Loudness tap to ground and here's what I found.



With "Rtaper" resistors in place...

Measurements taken "in ciruit" and obviously kinda rough, but you get the idea.

A-C, 120 ohm, no change
A-B, 1.6 ohm (min) -> 2.8k (10:00) -> 1.5k (11:00) -> 6.5k (12:00) -> 22k (3:00) -> 121 ohm (max)
A-D, 624 ohm, no change
B-C, 120 ohm (min) -> 2.8k (10:00) -> 750 ohm (11:00) -> 21k (3:00) -> 1.9 ohm (max)
B-D, 625 ohm (min) -> 2.2k (10:00) -> 5 ohm (11:00) - > 21k (3:00) -> 743 ohm (max)
C-D, 742 ohm no change

With "Rtaper" resistors lifted...

A-C, 3.5k, no change
A-B, 1.5 ohm (min) -> 8k (10:00) -> 7.5k (11:00) -> 25k (3:00) -> 3.5k (max)
A-D, 7.17k, no change
B-C, 3.5k (min) -> 11k (10:00) -> 10.5k (11:00) -> 25k (3:00) -> 2.0 ohm (max)
B-D, 8k (min) -> 2 ohm (11:00) - > 25k (3:00) -> 10.5k (max) [didn't seem to change much between 10-11:00]
C-D, 10.5k, no change

So basically I don't understand why the resistance climbs, then drops from about 10:00 to 11:00, then climbs again until about 3:00, then drops again.
Across these sections the volume during use still climbs, with the exception of the volume surge* I experience around 11:00 on the dial.

*when I say surge, I mean the taper of the volume increase from about 11:00 and 12:00 is significantly steeper in this range than the rest of the dial.

Also, with the "Rtaper" resistors lifted the gain was significantly louder for the same input level.
So I set up the right channel only playing pink noise and took the following rough measurements using a sound pressure level app on my phone mounted approx 1m from the speaker. Again, fairly unscientific and rough, but still insightful.

With the preamp internal jumpers in the 24db gain position....

Position. w/ Rtaper w/o Rtaper
9:00 27db 41db
12:00 57db 63db
3:00 71db 71db


With the preamp internal jumpers in the 18db gain position....

Position. w/ Rtaper w/o Rtaper
9:00 27db 36db
12:00 49db 56db
3:00 67db 67db


With the preamp internal jumpers in the 12db gain position....

Position. w/ Rtaper w/o Rtaper
9:00 27db 31db
12:00 42db 49db
3:00 62db 62db

 

[dogwan]

Here's a couple screenshots of the relevant portions of the schematics. I don't have a full schematic showing everything all together. But you can see the volume control in the block diagram inputting straight into the gain block.





Gain module:

 

[dogwan]

So, help me understand this circuit.

Is it just some clever engineering to make the taper below the 11:00 loudness tap flatter, or more usable at low volume?

Is it really necessary?

With the Rtaper resistors lifted the curve seem to follow a much more traditional audio curve. Frankly the steep section at around 11:00 is mostly just annoying when using my TT and external Phono. Of course it does change with different cartridges and Phono stages. But with digital I find about 90^% of the time I'm between 9-10:00. With Analog (vinyl, cassette, vintage tuner) I am more often fiddling in the steep curve area and beyond with the preamp in the 24db gain setting.

Now, I could lower the gain, but that pushes the digital components into the less desirable positions, and with Analog I'm running in the 1:00-3:00 position. Which doesn't hurt anything. It just feels wrong.

Which leads me to my ultimate question. Does anyone see anything wrong with swapping in my 100k Black Beauty pot sans loudness tap?
I would out of necessity have to run in the 12db gain setting because of the increase experienced taking the loudness tap out of the equation.

Whew, I know that's long. But like I said, I've been staring at it for a while now.

 

[elnaldo]

It looks ok to me. A regular potentiometer will work fine, just a little less control at lower levels, but that's all I see from that schematic.

 

[dogwan]

It looks ok to me. A regular potentiometer will work fine, just a little less control at lower levels, but that's all I see from that schematic.

Thanks!

As it turns out the pads didn't quite line up with my BB pot, so I had to abandon that idea.

I did add a ground wire from the pot shell to ground and that eliminated the "capacitive" buzz when touching the knob. It's weird, because there's a ground wire on the balance pot. And, I found some old solder on the volume pot shell but no wire. Hmmm...

I also added a drop of two of Phil's Tenacious Oil to the shaft of the volume pot and that eliminated the loose wiggly feeling.

I'm going to look for a better quality volume pot eventually.

While I was in there I replaced all the local power supply decoupling caps with Panasonic FC's (were Nichicon VX). A couple of them measured iffy so I replaced all 10. Didn't have the correct caps to replace the 3 power supply filtering caps, but did make note on an ongoing shopping list.

Also took the opportunity to install an IEC power inlet.

Another thing I noticed was that the schematics spec'd AD744 Fet input opamps (in sockets in most pics online), and my unit has TL081's direct soldered to board ($16 vs $0.50 at Mouser). So obviously I have a later model where beancounting was happening. Don't know if I would notice a difference, but I'm willing to throw $30 at it.

 

[gadget73]

Looks to me that they aren't actually using it as a loudness tap, but to mess with the taper of the control.

The odd resistance readings may just be that its in circuit. I suspect if you pulled it you'd get a more normal increase. Unless the pot is just damaged somehow. I honestly can't work out what the taper is because of circuit loading but its possible they used a linear pot and stuck that resistor in there to make it a pseudo-log thing, or just used the resistor to make it a shallower curve below the point where the tap is.

There are also a couple different "audio taper" pots apparently. News to me, but see here for some more details. I'll admit to never having studied the datasheets so others probably already knew this.

Potentiometer Log Tapers

I recently completed a very complex tube preamp project. I say "completed," but in fact any project of this magnitude needs a few tweaks when all the basic work is done. One of the problems I encountered is that the volume control fills the room with sound even when it's just nudged off the...

audiokarma.org

If its sensitive on the bottom end you might want one of the shallower taper pots. That will give more travel before it really starts ramping up the volume.