Balance pot for WAD CF Pre

[colin.hepburn]

Ho dear
Why is it you can never find that component you know you have?
I Seem to have mislaid the mini 50k Lin pot but found the 50k log
Sorry Project on hold damit

[pre65]

My old QED A240cd amp has a twin concentric volume control.

[colin.hepburn]

Right found the article to go with the balance pot schematic
pretty much as you described Richard
The Ear+ models refers to a 12B4A headphone amp input volume control
By Dr Lloyd Peppard of mapletree Audio
the balance controls in the early Ear+models were typically implemented by a single 50K or 100K linear
potentiometer with two series resistors as shown below. At mid position, the R1= R2= RB/2, the voltage
division produces a 6 dB overall gain loss. At the extreme right and left positions, the active channel gain is
only attenuated by 2.5 dB. For most line input source voltages, there is sufficient overall gain in the Ear+ circuit
to accommodate the 6 dB loss without causing a problem. If the potentiometer is not perfectly linear, the zero
balance condition can be set by ear or measurement and the knob adjusted to be at exactly mid position.
The input signals are typically fed from the input jacks and the output
signals feed to the left and right halves of the volume control
potentiometer. With a 100K volume control, and R1= R2= 50K, RB=
100K, the input resistance is reduced from 100K to 50K with the balance
control near the mid position.

[bikerhifinut]

Forgive me if this seems a bit barmy.
If you are going to the effort of fitting an extra pot, why not just use a couple of single potentiometers?
A bit of a bind when adjusting Volume I'll grant you. But you wouldn't be introducing any extra components into the signal path. And you could try some of the more interesting conductive plastic film potentiometers that seem to be only available as singles? As far as I am aware anyway.
How many inputs do you use? A couple of subminiature toggle switches could work for 4 inputs assuming you use ones with a centre stop detent and have the relay input board. This would free up the rotary selector switch hole for the second pot. Unless there's room to fit a small rotary switch where the LED sits with a tiny knob?
Sorry if that seems a bit off the wall.

Andy.

[VantheMan]

It would seem I have a similar hearing problem to Colin although unconfirmed so I fitted another Alps 50k manual pot to my Sugden A21SE in the place of the input selector ( I only need 1 input which I left permanently connected ), so my amp looks exactly the same. I fix the right hand channel volume manually, then after sitting down, using the remote I bring the left hand channel volume up to the level needed. Sounds weird but very easy in use.

[colin.hepburn]

Hi Andy/All
Yes well I did this two volume pots on the amp when I first built it and to be honesty it’s a pain in the ass to use as you say it’s a pain setting volume levels and even worse setting channel balance you are forever tweaking the pots to get it right I just do not like this idea
Having used the single pot balance control before on a mission Cyrus one amp it is better as it only moves the image from L/R unlike the standard balance pot which turns the L/R speaker down to off the single pot is a more real way in shifting the image in my view
My WAD CF pre has no room to add this anyway and inputs are all relay operated by the selector switch see image of my pre amp


PS i think DNM audio were the ones to promote the two single pot idea with the Series 2 Pre Amplifier but i could well be wrong

[bob orbell]

Hi all, Colin go to ebay and put in (dual concentric potentiometer) there are a few for sale, they are cheap, but worth a look ,if you need knobs I can make you two from stainless steal and polish them. BOB

[Richard]

Yes the Pioneer A400 had a concentric volume pot with a friction coupled split knob. Croft also used 2 separate pots for L and R. There was another who coupled 2 pots with a rubber band too I think.

[colin.hepburn]

Originally Posted by bob orbell

Hi all, Colin go to ebay and put in (dual concentric potentiometer) there are a few for sale, they are cheap, but worth a look ,if you need knobs I can make you two from stainless steal and polish them. BOB

Hi All
BOB I have ordered a couple of the 50k single lin pots so will give his idea a try
falling that I will go for a 100k concentric pot and may take you up on the offer to make the knobs for me thanks
PS I think the Hoover must have got the Lin pot I had

[colin.hepburn]

HI All
Well went and got a 50k single pot for balance control experiment for the WAD CF Pre
So knocked up a test prototype Board
Using 10/15k in Series got 24.5k
Pot set at mid point gives
L. Channel 23.2k R channel 22.7k ish
So now, circuit checks out ok fitting the pot to WAD CF I then notes that the CF Alps volume pot is a 50k not a 100k like I thought bugger