Dual Pot Source for Passive Volume/Monitor Controller

[Matt C]

I'm planning on building a simple passive monitor controller, just some switches and a pot hooked up to my powered monitors.

I understand that for the volume control it's important to get a reasonably precise pot that won't cause L/R imbalances.  I'm not sure I'm ready to spend $100+ on a gold point stepped attenuator, so I'm looking for potentiometer options.  10k dual log and/or 2k dual linear (trying a  couple different designs).  Anyone have a source they can point me to?

Or am I better off just ordering a bunch of cheap Alpha pots and hand selecting one?

Thanks.

 

[Tekay]

We use a dual10k lin  detented P260  from BI technologies.
With a 5k1 from pin 1 (input) to wiper they are normaly with in +/-0,2dB except for the first and second step as a small resistance difference then makes  big dB differencies

 

[ruffrecords]

As a rule, linear pots are far more consistent than log pots. Their disadvantage is that the attenuation is cramped in the second half. To overcome this it is common to slug them with a resistor between wiper and ground so that the mis point is about -10dB.

There are alternatives to gold point switches., e.g.

https://www.hificollective.co.uk/switches/blue-2-pole-24-way-switch-gold-plated.html

Cheers

Ian

 

[alexc]

alps japan dual log pots are pretty good ..  around 15 .. 20 usd on ebay

I use them a lot in stereo attenuation duties and find them to be streets above most pots,  in feel and accuracy.    alpha pots I have no love for at all.

You can get the china ones at 1/3 of the price - I think they are still pretty good quality for the price

But the japan one is the go to for my best pieces for a stereo app.

 

[gyraf]

Yup, the ALPS RK27 are good pots, decent tracking. Pricing ca. 10EUR/pcs

https://www.reichelt.com/6mm-Pots-stereo/RK27112-LOG10K/3/index.html?ACTION=3&GROUPID=3136&ARTICLE=73833&OFFSET=16&

 

[tmuikku]

From eBay you'll find lots of "Uraltone" type switches with soldered resistors for different attenuator types / values for about 20$.
I don't know how well they track and they are rather bulky in size. The switch should be of good quality though.

 

[JohnRoberts]

Back when I used to buy pots from alps I recall them publishing tracking specs for dual audio taper.  IIRC they would spec an X dB tracking between sections at something like -40dB , but that was decades ago and who knows what is sold through distribution these days. The processes should have improved somewhat over the years for other than the cheapest pots.

JR 

 

[Andy Peters]

I understand that for the volume control it's important to get a reasonably precise pot that won't cause L/R imbalances. 

This is why, when I did a monitor controller, I used the TI PGA2320 as the volume-control element.

 

[Matt C]

Thanks, just ordered one of the Alps RK27 pots, we'll see how it does.  Surprisingly tough to find a source for those within the US.

 

[ruffrecords]

Back when I used to buy pots from alps I recall them publishing tracking specs for dual audio taper.  IIRC they would spec an X dB tracking between sections at something like -40dB , but that was decades ago and who knows what is sold through distribution these days. The processes should have improved somewhat over the years for other than the cheapest pots.

JR

ALPS quotes 3dB max imbalance from 0 to -60dB. Not impressed to be honest.

Cheers

Ian

 

[JohnRoberts]

ALPS quotes 3dB max imbalance from 0 to -60dB. Not impressed to be honest.

Cheers

Ian

I am impressed...

What do you think the tracking would look like for a dual linear pot (with 20% bulk resistance tolerance) using slugging resistors to approximate an audio taper?

In use as a volume control we can ASSume 10-20 dB of headroom above, so the question is can we hear a 3dB mismatch down 40-50 dB below nominal 0VU. I am not confident that I could.

JR 

 

[moamps]

In use as a volume control we can ASSume 10-20 dB of headroom above, so the question is can we hear a 3dB mismatch down 40-50 dB below nominal 0VU. I am not confident that I could.

I replaced several blue alps in some HIFI integrated amps (Exposure) because the mistracking of channels at moderate listening levels was easily noticeable. Usually there is no 10-20dB of headroom in volume controls IMO; the pot at fully cw position drives maximum power on amp-speaker.

 

[ruffrecords]

I am impressed...

What do you think the tracking would look like for a dual linear pot (with 20% bulk resistance tolerance) using slugging resistors to approximate an audio taper?

Pretty good with a little effort. All you need do is measure the actual end to end resistance and scale the slugging resistor appropriately for each track. Not something I would want t do in volume production but fine for DIY.

In use as a volume control we can ASSume 10-20 dB of headroom above, so the question is can we hear a 3dB mismatch down 40-50 dB below nominal 0VU. I am not confident that I could.

JR

The spec does not say where in the range the 3dB difference may occur. It just says it will be no more than 3dB in that range. Could easily happen in the first 20dB. You would certainly hear that. You could see it on a VU meter.

Cheers

Ian

 

[alexc]

In my last sum bus unit, I used the alps japan dual 50K log taper as the main 'level' control.

I didn't measure anything approaching 3dB mismatch from left-right decks over the range of pot travel in my configuration.

More like  0.5dB  max variation over the 25dB of trimmable gain I am running.

ie. the inverting sum make up amp has fixed resistor for around 25dB of gain with the alps dual deck providing the variable resistance for the remaining 25 dB or so.

It mostly 'lives' at the 12 o'clock position in my setup and there, the variation is around 0.3 dB or so, which I further trim out with independent left/right 'fine gain' pots

To round it all off, in the case where I want easy 'full off' level, I have independent left/right 'mute' switches also.   

As usual, your mileage may vary. 

 

[JohnRoberts]

Pretty good with a little effort. All you need do is measure the actual end to end resistance and scale the slugging resistor appropriately for each track. Not something I would want t do in volume production but fine for DIY.The spec does not say where in the range the 3dB difference may occur. It just says it will be no more than 3dB in that range. Could easily happen in the first 20dB. You would certainly hear that. You could see it on a VU meter.

Cheers

Ian

Actually I think it does say..  <3dB @ -60dB <2 dB for less attenuation than that. (I saw a spec today while searching around).  They stopped making the slick stepped attenuator variant.

opinions vary... I have always had good luck with alps pots.

JR 

 

[ruffrecords]

Here is the data sheet. Check out the third page.

http://cdn-reichelt.de/documents/datenblatt/B400/ALPS-RK271.pdf

Cheers

Ian

 

[JohnRoberts]

Here is the data sheet. Check out the third page.

http://cdn-reichelt.de/documents/datenblatt/B400/ALPS-RK271.pdf

Cheers

Ian

or... http://www.alps.com/prod/info/E/HTML/Potentiometer/RotaryPotentiometers/RK271/RK27112A00CC.html
that says    for  specific 100k 15A dual

:Gang error -70dB to 60dB 3dB max.
                                -60dB to 0dB 2dB max.
===================================

－60dB to 0dB
within 3dB －80dB to －60dB
within 5dB
possible depending on total
resistance and taper


I've found both specs on the WWW  pick the one you like better....

JR

 

[Matt C]

Got another question related to this project:

The inputs to this monitor controller are balanced, and I attached a simple schematic of the volume pot itself below.  My problem is this - I have two sets of speakers I want to switch between, one has balanced inputs, the other is an unbalanced power amp.  I can't figure out how to easily adapt this volume pot arrangement to work with both balanced and unbalanced destinations.  I can't just ground the cold signal because one of the sources is active balanced and probably doesn't want to have its output shorted to ground. 

Any ideas on how to go about this without totally reworking the whole thing?

 

[Tubetec]

I dont think that schematic is going to work ,at the full volume position your going to get phase positive max volume, and in the middle it will cancel out completely ,Your phase negative output is connected directly to ground in the case of unbalanced connection. Usually when you use a pot on a balanced output you have padding resistors ,and the pot is wired across the phases like a rheostat, this only allows a certain gain range depending on the magnitude of the padding resistor in relation to the pot value . Maybe a four gang pot might allow the switching arrangment your looking for . Try googling 'Building Valve Amplifiers ' from Morgan Jones ,I think its downloadable in Pdf for free ,theres definately some info in that to help out .

 

[Tubetec]

I think the easiest solution is just to keep the neg phase disconnected and go unbalanced ,not ideal ,but simple at least