Two Pot Volume Control

[scottjoplin]

Hi. Easy question, I hope it has an easy answer. I intend to use two pots to set volume level, one to limit the range of the other. Would the best way be to cascade them, or another way, such as one used as a variable resistor before a potentiometer? Also, should I consider them as a composite attenuator when it comes to impedance matching or is it more complicated than that? I sincerely hope not

 

[Monte McGuire]

Wire one as a variable resistor, as you mention, and use that in series with the track of the second pot. You could also wire the first pot normally and drive the second pot's track from the wiper of the first one.

Not sure why though - is the distortion and unreliability of one pot insufficient? ;-)

 

[scottjoplin]

I shall answer your question as though it were not loaded

It's a multi-amp system so I need to do a bit of level matching, also I have large signals to attenuate and want to be able to use all of the track on the "main" volume control

 

[Monte McGuire]

Yes, there are legitimate reasons for this - I have a distaste for pots, but they can be good if you get nice ones. Which pots do you like these days? The nice ones I know from the 80s aren't available anymore - any good rotary conductive plastic pots these days?

For your use, maybe a multi-turn trimpot for the range setting, as those are generally easier to get with reliable conductive plastic or cermet tracks?

 

[scottjoplin]

I've an Alps black beauty I bought years ago, still fine, works very smoothly, I was planning to use this to set level on the amp I use for the my widebands. I also have an alps blue satin that I'll use on the woofer amp. I recently bought a 6 channel motorised alps that will be the main control. I recently used Omeg green eco pots conductive plastic in a Doug Self preamp, if they're good enough for him they're good enough for me The Signal Transfer Company: Precision Preamplifier
As regards the impedance I presume I just consider it to be one potentiometer?

 

[AndrewT]

A vol pot used as a potentiometer has a very variable output impedance.
During use many will set the vol pot far away from the -6dB level where the output impedance is highest.
Thus most of us use the vol pot when the output impedance is quite low.

Assuming we fall into that category described above, to maintain an adequately low output impedance, a cascaded pair of pot should place the variable resistor version BEFORE the vol pot.

Let me put in some numbers to show some effects.
Vr1 = 1k or 50k
Vol pot 10k
Source impedance 100r
Receiver input impedance 50k

The source impedance seen by the Receiver will be {100r+1k+10k}/4 = 2775ohms
when the output is set to -6dB and the VR is set low. Vout = Vin*0.473 (-6.5dB)
This changes to {100r+50k+10k}/4 = 15025ohms
when the output is set to -6dB and the VR is set high Vout = Vin*0.0839 (-21.5dB)
turning the VR up from1k to 50k has reduced the output by 15dB. But with the vol pot @ =6dB the increase in source resistance is high, probably unacceptably high for most users.

Now look at comparable figures for the vol pot set to -20dB
Source impedance = 1k||10k1 = 910ohms. Vout = Vin*0.0885 (-21.06dB)
or Rs = 1k||59.1k = 983ohms. Vout = Vin*0.0163 (--35.75dB)
this time the vol attenuation is -14.7dB and the increase in Rs is only 73ohms.

The change in source impedance is acceptable when the vol pot setting are well below -6dB
The attenuation of the VR is similar for all settings of the vol pot.

Placing the VR AFTER the vol pot increases the Rs whenever the extra attenuation is required. I'll not repeat the sums.

One could use a much lower value of Vr to allow lower levels of secondary adjustment.
This could be used as a balance control, or for fine tuning of sensitivity.

 

[scottjoplin]

Thanks Andrew for the full and helpful answer, since I want to attenuate by a large margin it's an acceptable way to go. I will experiment with different settings/combinations and check by measuring. Thanks again

 

[scottjoplin]

I've been thinking about this a little more and it seems to me that cascading two potentiometers would be better than using a variable resistor before one potentiometer, very similar to what Rod Elliott is doing here Project 125

 

[Davey]

I think the scheme shown by Rod Elliot is the best, and most versatile.
Also keep in mind the taper of the potentiometers. The "preset" pot should most likely be a linear taper and the volume pot most likely a log taper......or maybe a linear taper with resistor from wiper to ground.

Dave.