Stereo Volume Knob

[eev_carl]

Hi,

I have an audio amp that's working but I have two volume knobs for the two stereo channels.  Besides a stacked pot, are there other ways to control the volume from both channels using a single knob like on most consumer applications?  I found a schematic of a MOSFET used as a variable resistor.  Would this work?

Thanks,
Carl

[Zero999]

A dual potentiometer is the way it's normally done on commercial equipment. Quite often, the potentiometer isn't stacked, but has two tracks.

This isn't an easy thing to do. Using a FET to make a voltage controlled resistor will work to some extent, but is non-linear and will distort the signal somewhat. The gate voltage vs channel resistance of  FETs is also difficult to control, so two parts with the same part number, will have different characteristics.

The easiest way to do it is to use a microcontroller, with an ADC to read the voltage on a potentiometer and use it to control the resistance of a digital potentiometer, which actually attenuates the signal. In commercial applications, the potentiometer is often replaced with a rotary encoder, but I prefer potentiometers, because they can be adjusted with the power off and the position of the dial, indicates the volume.

[josip]

The easiest way to do it is to use a microcontroller, with an ADC to read the voltage on a potentiometer and use it to control the resistance of a digital potentiometer, which actually attenuates the signal.

Easiest way is to use integrated one chip with pushbuttons solution like DS1802. With centered audio signal with -2.5V /+2.5V (TL431) supply.

[Zero999]

The easiest way to do it is to use a microcontroller, with an ADC to read the voltage on a potentiometer and use it to control the resistance of a digital potentiometer, which actually attenuates the signal.

Easiest way is to use integrated one chip with pushbuttons solution like DS1802. With centered audio signal with -2.5V /+2.5V (TL431) supply.

Yes, that would be easier, but it wouldn't replace a real potentiometer. Many people hate pushbuttons for a volume control, there's no indication of the volume, other than on a display and it can' t be adjusted with the power off.

[eev_carl]

A dual potentiometer is the way it's normally done on commercial equipment. Quite often, the potentiometer isn't stacked, but has two tracks.

Is this an example of the dual potentiometer you're writing about? https://www.mouser.com/ProductDetail/ALPS/RK1631210AX9?qs=sGAEpiMZZMtC25l1F4XBU4o6BXVFiwl9R2WRXiEL%252bfc%3d

[Zero999]

A dual potentiometer is the way it's normally done on commercial equipment. Quite often, the potentiometer isn't stacked, but has two tracks.

Is this an example of the dual potentiometer you're writing about? https://www.mouser.com/ProductDetail/ALPS/RK1631210AX9?qs=sGAEpiMZZMtC25l1F4XBU4o6BXVFiwl9R2WRXiEL%252bfc%3d

Yes, that's the kind of thing normally used, as a stereo volume control

[Benta]

For volume control you need logarithmic taper. I think the one you link to is linear taper.

[Old Printer]

I am curious as to why you are looking for a solution other than the traditional dual-pot?  Not second guessing you, just trying to learn something. To my knowledge (limited) even the highest end audio equipment uses potentiometers, though certainly very high quality ones. Thanks

[madires]

Why not replacing the volume pot of one channel with a 2 gang log one to control both channels and replacing the volume pot of the other channel with a linear one for a balance control?

Some ideas: http://sound.whsites.net/project01.htm

[drknob]

Traditional analog potentiometers are logarithmic, also known as 'audio' taper. Back in the day, there were many variations of 'audio' taper, but nowadays, choices are more limited. Stereo pots are usually two stacked units with a common shaft. The problem here is tracking, or L/R balance which will vary due to manufacturing variances. A good stereo pot that tracks +/- 1dB is VERY expensive. I think the solution offered by madires is a clever one....

[retrolefty]

Why not replacing the volume pot of one channel with a 2 gang log one to control both channels and replacing the volume pot of the other channel with a linear one for a balance control?

Some ideas: http://sound.whsites.net/project01.htm

 This is a proper solution. Audio taper ganged pots are not going to track perfectly, hence the balance
control on most hifi preamps and integrated amps.  So one would still end up using two controls, volume and balance, so I'm not sure it's worth the effort over just using your existing two volume controls.

[CatalinaWOW]

All of this discussion of tracking between gangs of a potentiometer is technically correct, but may be irrelevant.

Many of us are insensitive to a couple of dB balance shift.   So ganged pots, even lower spec ones,  work out just fine for us.  You have to take your own listening needs into account.  A college buddy of mine unfortunately had a very discerning ear.  Cost him a lot of money to get gear that sounded good to him.  And it wasn't just showing off.  On numerous occasions he pointed out that there was something wrong with a particular system, record or whatever.  And could then demonstrate the problem with measurements.  The one that impressed me the most was when he complained that a record had an off center hole.  And showed that it was nearly a millimeter off from the center of the recording grooves.  I don't even know how that can happen to a single record, though you can see how a manufacturing station could be made wrong and generate hundreds of them.  Which were apparently then sold with most people not complaining.

[Audioguru]

Is this an example of the dual potentiometer you're writing about? https://www.mouser.com/ProductDetail/ALPS/RK1631210AX9?qs=sGAEpiMZZMtC25l1F4XBU4o6BXVFiwl9R2WRXiEL%252bfc%3d

No. The details show that it is a linear taper dual pot. A volume control should have a logarithmic taper so that its matches the sensitivity of loudness of our hearing. Mouser's datasheets cannot be seen because they are "forbidden".

[tooki]

Mouser's datasheets cannot be seen because they are "forbidden".

What on earth are you talking about? Whenever possible, Mouser simply has a URL to the PDF on the manufacturer's own website. Occasionally, they host the PDF themselves. I've never encountered an issue with any of them.

[Bassman59]

All of this discussion of tracking between gangs of a potentiometer is technically correct, but may be irrelevant.

Many of us are insensitive to a couple of dB balance shift.   So ganged pots, even lower spec ones,  work out just fine for us.  You have to take your own listening needs into account. 

If you are working in a recording studio, and especially in a mastering studio, that possible balance shift matters!

For most home use, though, you are correct, a couple of dB imbalance won't be noticed. Speaker position in the room will ensure that.

[Andy Watson]

Mouser's datasheets cannot be seen because they are "forbidden".

What on earth are you talking about? Whenever possible, Mouser simply has a URL to the PDF on the manufacturer's own website. Occasionally, they host the PDF themselves. I've never encountered an issue with any of them.

Either Mouser have cocked-up the link, or Alps has changed their URL. Either way, Mouser's link leads to "Forbidden access" message from Alps' server.

Try this one:
http://www.alps.com/prod/info/E/PDF/Potentiometer/MetalShaft/RK163/RK163.PDF

[Zero999]

For volume control you need logarithmic taper. I think the one you link to is linear taper.

Frustratingly Mouser's catalogue doesn't say whether it's linear or log. You're right, the one linked to is linear. I had to Google the part number to find out.

Here's a link to the logarithmic version, on Mouser's site.
https://www.mouser.co.uk/ProductDetail/ALPS/RK16312A0BKR?qs=%2fha2pyFaduhXOdzOxd6%252bF5NUE9WpTUrqqRgWMK2XGyCUARvlCu4LJg%3d%3d

[tooki]

Mouser's datasheets cannot be seen because they are "forbidden".

What on earth are you talking about? Whenever possible, Mouser simply has a URL to the PDF on the manufacturer's own website. Occasionally, they host the PDF themselves. I've never encountered an issue with any of them.

Either Mouser have cocked-up the link, or Alps has changed their URL. Either way, Mouser's link leads to "Forbidden access" message from Alps' server.

Try this one:
http://www.alps.com/prod/info/E/PDF/Potentiometer/MetalShaft/RK163/RK163.PDF

Gotcha. The wording in Audioguru's post sounds as though all Mouser datasheets are borked, which clearly isn't the case. (Probably Alps changed its site and Mouser didn't find out. Mouser is really good about fixing bad data though, if someone reports it.)

[floobydust]

Another option is relay-based resistor-attentuators, they are binary-weighted relays i.e. 7 DPDT relays gives 64-steps volume control. Lots of kits with IR remote control from china.

Dual potentiometers suck, they are poorly matched. You can easily hear -1dB mismatch, it shifts the stereo image over, if you are an audiophile.

[Audioguru]

Yeah, this one datasheet is not available on Mouser's link as shown here:

[josip]

To my knowledge (limited) even the highest end audio equipment uses potentiometers, though certainly very high quality ones.

This is more or less off topic...

There is hi-end audio that is using digital pots (IC solution). Right now, I don't have links on my hand, but I saved all of this, 15 years ago when I was deep in DIY audio. Pedja Rogic boosted popularity of DS1802 (other DS pots can be found also in some hi-end devices)...

https://www.audialonline.com/topics/model-a-volume-control/

Still, we want the best. And when I say this, I consider classic passive potentiometers of all kinds, different stepped attenuators, transformer based volume controls, and less conventional stuff such as LDRs, with or without different active stages, as a competition. In my view, the best chances here still have digitally controlled stepped attenuators (or so-called “digital potentiometers”, please search around for simple DS1802 solution I published years ago), but this would be the topic for the other day.

https://www.elektronika.ba/forum/viewtopic.php?f=3&t=12984#p147054

The circuit is simple, put it together, listen to and compare to some expensive audiophile volume control solution, you might be nicely surprised. Blue ALPS can not equal that precise soundstage and that female voices. Dynamics is excellent. One might find ALPS more natural but for me this silicon part had more good sides. Even if you take into account 2 regulators and caps (assuming you can steal raw voltages somewhere in the unit they are used in), price stays quite modest. High input impedance of the next stage is recommended. In my case the DS1802 drove the buffer whose input impedance was set to 50k. At the pot's input side, as expected, my EF86 anode follower failed to drive it (dynamics collapsed, 45kOhm and wiper loaded with 50k is too hard for this tube) but the diamond output stage of the AD844 did it happily. Cheap high end. Recommended.

[Zero999]

Another option is relay-based resistor-attentuators, they are binary-weighted relays i.e. 7 DPDT relays gives 64-steps volume control. Lots of kits with IR remote control from china.

But won't that introduce a clicking and popping noise, when the volume is adjusted?

Dual potentiometers suck, they are poorly matched. You can easily hear -1dB mismatch, it shifts the stereo image over, if you are an audiophile.

I haven't done any tests or seen any evidence of double blind trials, but I imagine that is only noticeable through headphones. If one listens through speakers, the room acoustics will dominate.

[paulca]

I was looking for dual pots the other day and found one with an offset adjust tab thing so you can at least balance them at one point on the tracks.  If I come across it again I'll link it.

[Svgeesus]

Yet another option is to use a couple of Voltage Controlled Amplifiers (VCAs). For example, the THAT 2181 has plenty of headroom, low distortion, a logarithmic response to control voltage, and a pair of them would track much better than a stereo audio taper pot.

You could drive them from a DAC, or from a (single, linear) pot dividing down a the output of voltage reference chip.

Edited to add: actually the pre-trimmed 2180 would be easier in this case, and just as good in practice in terms of distortion.

[Bassman59]

Yet another option is to use a couple of Voltage Controlled Amplifiers (VCAs). For example, the THAT 2181 has plenty of headroom, low distortion, a logarithmic response to control voltage, and a pair of them would track much better than a stereo audio taper pot.

You could drive them from a DAC, or from a (single, linear) pot dividing down a the output of voltage reference chip.

Edited to add: actually the pre-trimmed 2180 would be easier in this case, and just as good in practice in terms of distortion.

If you have a micro available, the TI PGA2320 and its friends works well. Sample your pot, and send a control word over SPI to the 2320. Quiet, repeatable settings, all good.