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Old 3rd Aug 2020, 10:41 pm   #1
vinrads
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Default Home brew amplifier balance control

Best position in the circuit to fit a balance control,on my valve amp . Mick.
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Old 3rd Aug 2020, 11:31 pm   #2
bikerhifinut
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Default Re: Home brew amplifier balance control

What sort of control Mick? a single pot with wiper to ground, or a dual pot wired as an extra potentiometer.
Both types incur losses but the twin pot solution will lose you 6dB of signal at the mid position and you may not wish to lose that amount of signal.
A lot of circuits use the balance either before or after the Volume control, I'm not sure there's any difference either way.
Some circuits use the balance pot separated by at least a stage in a preamp so that any impedance variations as pots are rotated are buffered by a stage.
The Leak valve preamps had the balance control at the output with the wiper of each balance pot connected to the top of the volume control track, which then led direct to the output and into the power amp via umbilical or separate lead.
100k lin for the balance pot, and 100k log for the volume pot. It worked so there you have it.
Myself if I was having to buy the pots and didn't have a suitable one in stock, I'd go for the single linear pot approach with the wiper connected to earth. You'd also use buffer resistors on each side of the track in most circuits.
Its simpler and effective.

One thing to note Mick, if its possible to have the attenuators for balance and volume further downstream than at the inputs you do get a benefit from lower noise at partial volume settings. This of course is not always possible, and I think that was partly why leak had theirs at the output rather than inputs of the preamps.

Andy.
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Old 4th Aug 2020, 7:55 am   #3
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Default Re: Home brew amplifier balance control

Did/has anyone do a job with just two mono pots and a differential gearbox?
 
Old 4th Aug 2020, 8:45 am   #4
vinrads
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Default Re: Home brew amplifier balance control

Thank's for that Andy, I have fitted a 500k lin as you suggested ,on the vol con't out to the main amp seems to work ok ,a slight loss of sig won't be a problem, it could do with loosing some gain ,
The finished amp with oak casing. I was going to let the control panel panel into the front ,but ah! I had already glued the thing together so I cut a thin bit of oak and fitted it around the front panel, I am pleased with the result , Mick.
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Old 4th Aug 2020, 10:57 am   #5
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Default Re: Home brew amplifier balance control

On the Wadar stereo amp I built I used a stereo linear pot just after or just before (can't remember) the volume control, but wired one in reverse so as one side increases the other side decreases.

Ian
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Old 4th Aug 2020, 2:13 pm   #6
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Default Re: Home brew amplifier balance control

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Originally Posted by vinrads View Post
Thank's for that Andy, I have fitted a 500k lin as you suggested ,on the vol con't out to the main amp seems to work ok ,a slight loss of sig won't be a problem, it could do with loosing some gain ,
The finished amp with oak casing. I was going to let the control panel panel into the front ,but ah! I had already glued the thing together so I cut a thin bit of oak and fitted it around the front panel, I am pleased with the result , Mick.
OOOh! That does look nice Mick. Does you credit and I wish I wasn't so cack handed at metal and woodwork.
Pleased your solution works, there's plenty of decent info on the net if you wanted to read further into it. Just don't look on some of the more extreme DIY audio forums for daring to think a subtle balance control just might be a bit useful!

Andy.
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Old 4th Aug 2020, 2:34 pm   #7
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Default Re: Home brew amplifier balance control

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Originally Posted by Ian - G4JQT View Post
On the Wadar stereo amp I built I used a stereo linear pot just after or just before (can't remember) the volume control, but wired one in reverse so as one side increases the other side decreases.

Ian
That's exactly how you'd do it Ian, standard practice.
It's precisely how Leak did it if you have a look at the circuits for their Valve stereo preamps.
Sacrificed 6dB of gain for the feature but that's not always a problem.

I've got a modern (as in my idea of modern!) take on the Leak point1 stereo preamp on the stocks once the current spate of op amp RIAA experiments is done. It's because I have an empty Leak case complete with all original pots in the pile of bits. So on this one after a bit of head scratching and because I can, I have decided to put the balance control at the input which is a dual 100k pot wired antiphase (is that the right word?). This will feed into a relatively low gain stage using either ECC82 or 6CG7 cos I have some. that will give me the necessary buffering to the Tone controls which will follow the original Leak circuit with a triode connected EF86, although I shall probably use EF40 as I have a stash needing a purpose in life, they're identical. The tone control will then follow the leak idea in so far as the volume pot 100k log will be situated on the output of the baxandall tone control. If it was simply feeding a 1 meg ohm input of a valve amp then I could leave it there, but as I would like it to be able to work with solid state stuff with an input Z perhaps as low as 10k, then I'll put a Bootstrapped ECC82 cathode follower on the output for a lower output impedance which with the cathode stopper resistor will be around 1k.
I'll put it on the DIY thread once I am up and running with it it'll have a few other refinements for an easier life from the sofa.
Andy.
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Old 4th Aug 2020, 2:43 pm   #8
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Default Re: Home brew amplifier balance control

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Originally Posted by merlinmaxwell View Post
Did/has anyone do a job with just two mono pots and a differential gearbox?
Not sure what you mean by that Merlin? Is it like the planetary gears on a vehicle whereby you can rotate one shaft and the other rotates with it in the opposite direction or you can hold one shaft stationary and rotate the other to get a balance?

If you want individual pot control why complicate things, Blore Edwards still make dual concentric potentiometers, there may be others making them. Saves on a hole and space on the panel too and gets you the control you desire on each channel.

A.
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Old 4th Aug 2020, 2:58 pm   #9
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Default Re: Home brew amplifier balance control

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Originally Posted by merlinmaxwell View Post
Did/has anyone do a job with just two mono pots and a differential gearbox?
It my be a nod to my idea of ganging two 4 pole switches together to make an 8 pole 3 way ,in the end I didn't use the Brimar pre amp cct , I had all sorts of trouble with it ,it may have been down to not using the specified valves . Mick.
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Old 4th Aug 2020, 3:32 pm   #10
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Default Re: Home brew amplifier balance control

Ah alles klaar.
I think I suggested using subminiature relays for that? I'm afraid I have a bit of a peccadillo for relays in signal switching especially if it means I don't have to make extended shafts etc...……...……..

A.
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Old 4th Aug 2020, 3:58 pm   #11
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Default Re: Home brew amplifier balance control

You can add a resistor to each track on a balance control - this will reduce the attenuation at the mid position, while still giving good control. The resistor goes between the input and the wiper, and is perhaps 25% of the value of the track, reducing the loss to just under 3dB, depending on the impedance of what follows.

One thing to watch out for: when set to anywhere other than centre, the input impedance of the balance control will be different on each channel.

If you place the balance control after the volume pot, this unequal impedance will cause the balance to shift with volume setting, requiring adjustments to the balance control at different settings. The effect might not be ruinous, but it's something to bear it in mind and check for.

With that in mind, it's probably better to place the volume control after the balance control if possible.
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Old 4th Aug 2020, 5:39 pm   #12
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Default Re: Home brew amplifier balance control

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Originally Posted by mhennessy View Post
You can add a resistor to each track on a balance control - this will reduce the attenuation at the mid position, while still giving good control. The resistor goes between the input and the wiper, and is perhaps 25% of the value of the track, reducing the loss to just under 3dB, depending on the impedance of what follows.

One thing to watch out for: when set to anywhere other than centre, the input impedance of the balance control will be different on each channel.

If you place the balance control after the volume pot, this unequal impedance will cause the balance to shift with volume setting, requiring adjustments to the balance control at different settings. The effect might not be ruinous, but it's something to bear it in mind and check for.

With that in mind, it's probably better to place the volume control after the balance control if possible.
That would explain why Harold Leak did just that on the point1 stereo and varislope stereo preamps.

As an aside Mark, My draft design for a preamplifier will have the balance control at the input and buffered by the first stage. I plan to put the Volume control after the tone controls and before the output buffer. This seemed to make more sense to me than Volume control into the first stage but I am willing to be educated here. Especially as I am tempted to reinstate the preset 100k log pots on 3 of the preamps inputs. Which I always felt was a nice touch on the leak preamps so you could get the sources nicely equal in level when switching between them.
Andy.
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Old 9th Aug 2020, 9:44 pm   #13
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Default Re: Home brew amplifier balance control

Very nice build! assuming it’s integrated, what is the amp and what are the other controls?

Edit: is that the one with the EL506s?

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Old 9th Aug 2020, 10:32 pm   #14
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Default Re: Home brew amplifier balance control

The main amp is a Brimar design ,using EL506 In push pull , which worked fine but there pre amp I had trouble with ,so I use a different design and the mm pre amp was an RCA circuit, the controls are as in most hi fi amps vol, bass, treble , balance ,input selector, on off switch .
The picture was before I fitted the balance pot.,and tidied up the wiring ,Mick.
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