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-   -   Balance pot for WAD CF Pre (http://www.world-designs.co.uk/forum/showthread.php?t=6992)

colin.hepburn 25th February 2014 02:09 PM

Balance pot for WAD CF Pre
 
1 Attachment(s)
Well the WAD CF is up and running and I like it
However, there is a problem with me not the pre and this is due to my odd ear canal differences, which is a pain in the **** for correct imaging :(

As it pools the image to the right of centre so will need to fit a balance pot to the CF, which needs to be a small pot to mount on front panel
There for a single pot would be best something like this see image but would it work correctly

The CF Alps pot is a 100k I have a single pot 47k and 50k one is a liner the other problem is the resistor values so as not to lose too much gain

bob orbell 25th February 2014 03:27 PM

Re: Balance pot for WAD CF Pre
 
That may well work Colin, some time ago I added a two gang linear pot to mine, as in the FAQ, just to see if it would be useful, but I lost far too much gain, so never fitted it in. BOB

Richard 25th February 2014 05:46 PM

Re: Balance pot for WAD CF Pre
 
Yes it looks elegant but I think you have to add on the vol control following to see what will happen.
The balance pot adds a parallel load to earth so lowers the input impedance usually set by the volume pot.
It also adds series resistance before the volume pot attenuating the output.
We need to keep R1 R2 RB high in order to keep input impedance reasonable but the higher we make them the more they will attenuate the volume pot following.
Example, using Col's 100K volume pot and the 50K linear balance pot with R1 R2 at 25K to keep attenuation as low as we dare go considering input impedance would be,
loss at balance pot mid position 25K/(25K + 25K) = X0.5 = -6dB
loss at volume pot max position 100K/(100K + 25K) = x0.8 = -2dB
total loss x0.4 = -8dB and input impedance 50K//100K = 33K fine for CD but less so/short cables for Phono 3.
It would probably work fine but (as Bob found and is mentioned in the FAQ) you'd need some gain in the pre unless the power amp is very sensitive.


http://homepage.ntlworld.com/richard...VolControl.gif

Guy Pettigrew 25th February 2014 06:52 PM

Re: Balance pot for WAD CF Pre
 
Apologies for ignorance, but what's a WAD CF, please?

Thanks.

Guy

A Stuart 25th February 2014 06:55 PM

Re: Balance pot for WAD CF Pre
 
Does no-one make coaxial-shafted separate potentiometers any more?
I mean, as found on old radios where you might have on the left volume on one control ring and tone on the other ring; and similarly on the right waveband selection and tuning as another paired control.
A very brief look on Google did not show anything.
I do not think I am inventing my recollection that Mission Cyrus amps around 1990 had a split volume knob, for L & R channels, so a cleaner path than a separate balance control followed by a volume control.
Note the earlier 1987 Cyrus1 only had a weedy miniature size Alps pot which rapidly wore out in my case, hugely improved by replacing with full size Alps Blue. So caution if looking for an old Cyrus to cannibalise.
Alastair

colin.hepburn 25th February 2014 07:32 PM

Re: Balance pot for WAD CF Pre
 
BOB/Richard
Thanks for the details Richard :thumbsup:
I will try it later and report back:eek:

Alastair

No the mission amp was not slit L/R it was a single pot balance control on the volume pot shaft same as the above schematic

A Stuart 25th February 2014 07:36 PM

Re: Balance pot for WAD CF Pre
 
They do exist!
I should have looked up the word "concentric" potentiometer.
Apologies, I stand corrected on the Cyrus I just assumed that was how they worked from the pictures.
Alastair

Greg. 25th February 2014 08:00 PM

Re: Balance pot for WAD CF Pre
 
Quote:

Originally Posted by Guy Pettigrew (Post 81444)
Apologies for ignorance, but what's a WAD CF, please?

Thanks.

Guy

Hi Guy,

CF refers to the Cathode Follower alternative to Pre2/3 some of us prefer to use. See the FAQ.

http://www.world-designs.co.uk/forum...read.php?t=144

Greg

bob orbell 26th February 2014 07:39 AM

Re: Balance pot for WAD CF Pre
 
Quote:

Originally Posted by A Stuart (Post 81445)
Does no-one make coaxial-shafted separate potentiometers any more?
I mean, as found on old radios where you might have on the left volume on one control ring and tone on the other ring; and similarly on the right waveband selection and tuning as another paired control.
A very brief look on Google did not show anything.
I do not think I am inventing my recollection that Mission Cyrus amps around 1990 had a split volume knob, for L & R channels, so a cleaner path than a separate balance control followed by a volume control.
Note the earlier 1987 Cyrus1 only had a weedy miniature size Alps pot which rapidly wore out in my case, hugely improved by replacing with full size Alps Blue. So caution if looking for an old Cyrus to cannibalise.
Alastair


This is a very good idea, my Stax head phone amp has just this arrangement, all you need to do Colin is find such pot. BOB

colin.hepburn 26th February 2014 02:08 PM

Re: Balance pot for WAD CF Pre
 
Quote:

Originally Posted by bob orbell (Post 81451)
This is a very good idea, my Stax head phone amp has just this arrangement, all you need to do Colin is find such pot. BOB

BOB
Yes, it is a possibility to fit a concentric pot would save any drilling of front panel
My plan was/is to use the mini pot if it works ok to mounted it above the led not the easiest of tasks thou with just a hand drill or black in Decker:eek:


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