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Old 14th Dec 2018, 5:06 pm   #1
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Default Stereo Balance Question

I am building a pre-amp that has separate left and right motorised volume controls with positional feedback to a micro-controller it will also be remote controlled. It will (with a bit of luck in writing the software) turn the knobs together for volume and in opposite directions for balance. Manually turning one knob will increase volume and the other adjust balance.

The question is do I go for the sum of both channels as voltage or power (one over root two of voltage) to keep the resultant sound level the same when adjusting the balance. It probably won't matter a jot, nice to get it right though it's only a line or two of code.

The reason is that linear or log/antilog motorised pots seem to be unobtainable. And it would be fun to see the other pot rotating all on it's own.
 
Old 14th Dec 2018, 5:43 pm   #2
GrimJosef
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Default Re: Stereo Balance Question

This is really a question about how the air pressure waves in the room combine to generate a perceived sound level at our ears. Since, in general, the sounds from the left and right channels are not the same (strictly speaking they're not 'coherent') I'd expect that you'd want to keep the sum of the two powers, one from each speaker, constant. There's some dicussion of this in the context of pro audio here https://en.wikipedia.org/wiki/Pan_law where it suggests that because rooms are not ideal there really isn't a single correct answer to your question.

I seem to recall that as the frequency falls the sounds in each channel, from 'live' music at least, become more and more similar. So in the deep bass the speaker cones will tend to move backwards and forwards together. Under those circumstances maybe the sum of the voltages is more important ?

Cheers,

GJ
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Old 14th Dec 2018, 6:18 pm   #3
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Default Re: Stereo Balance Question

I discovered that using two volume controls to provide a balance control function via software is more difficult than I'd thought when I re-wrote the code for the Beomaster 8000, which has two electronic attenuators.

In simple terms, the difference in the settings of the two controls necessary to move the sound image a certain amount off centre changes depending on the overall volume setting. Furthermore, if you set a balance offset and then reduce the volume by decrementing both controls by the same amount then one will reach zero before the other, meaning than the balance will swing all the way to the other channel. A ratiometric approach is therefore needed, but that rapidly gets complicated for such a simple (and rarely used) function.

I can't remember the exact approach I used in the end but it did end up being a compromise; other factors demanded a short programme cycle and there wasn't time for anything but simple integer mathematics. For a new design I think I'd use two separate attenuator systems, the balance one need only be coarse and so fixed differential gains selected by CMOS switches or reed relays may be the way to go.
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Old 14th Dec 2018, 6:24 pm   #4
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Default Re: Stereo Balance Question

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A ratiometric approach is therefore needed
Indeed it is in my list of things to get right (that one should be easy) but I do like a good balance control, I am not fussy about most things however the smallest step on my iPad/Pod balance is too much, I feel lopsided when it isn't smack on.
 
Old 14th Dec 2018, 10:32 pm   #5
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Default Re: Stereo Balance Question

I suspect you'll want finer steps in balance adjustment than in volume adjustment. Balance is comparative, while the sensation of volume lacks a stable reference. You'll tolerate the volume not being quite what you wanted more than you'll tolerate the image wandering. This means that your balance uses fine steps but volume steps may go in increments of several.

Using motorised pots brings in the problem that they are not really logarithmic but often a rough approximation with two linear tapers, so balance offsets will need to be different depending where up the volume range you are running, and there is tolerance in where the tapers change

It's probably easier with electronic attenuators, but they lack the twinkle factor of seening knobs move themselves.

I suspect I have the task of rewriting software for a Revox B252 around a new CPU somewhere in my future.

David
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Old 14th Dec 2018, 11:04 pm   #6
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Default Re: Stereo Balance Question

For small adjustments the two (power vs voltage) are near enough the same. 10% up (voltage) on one channel and 10% down on the other is no change in total voltage, and 102% in power. I don't think you will notice the difference.

For larger adjustments you have to decide whether you are going to stop at +3dB and no signal (equal total power), or +6dB and no signal (equal total voltage). Beyond these points you will have to go into antiphase on the quieter channel.
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Old 14th Dec 2018, 11:46 pm   #7
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Default Re: Stereo Balance Question

i don't know if its any use to you merlin but I have a spare ALPS motorised pot here that probably has 2 gangs of the 4 still ok, or you might be able to dismantle the pot and substitute a set of wafers from a linear pot as it is possible to separate the pot from the motor and gearbox with care.
Yours for the price of the postage or maybe a swapsie of some small value caps?

Andy.
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Old 15th Dec 2018, 12:05 am   #8
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Default Re: Stereo Balance Question

Found this, seemed to be others too on their site.

https://www.mouser.co.uk/ProductDeta...BcYfvQF%2fm04J

A
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Old 15th Dec 2018, 12:34 am   #9
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Default Re: Stereo Balance Question

In domestic hi-fi, it's usual to not worry about this issue. So, turning the balance control from centre will attenuate one channel while leaving the other alone. In the units I've studied, any increase in the other channel is accidental, and not intended to really produce a constant power. Usual disclaimer: there's bound to be exceptions, and I haven't looked at every amplifier ever made, but still, I've seen a lot.

But in a mixing desk, constant-power panning is essential. Not surprisingly, Douglas Self has written quite a bit about this - see chapter 17 of Small Signal Design (PDFs are available online) for lots of background.

When I built my PGA2310-based preamp, I found that 0.5dB moved the image by rather more than you might expect. Or at least, rather more than I expected at the time. About a foot IIRC (speakers about 8 foot apart).

Ideally, I'd want finer adjustment, but the PGA2310 has a 0.5dB minimum step size (more than adequate for volume adjustments). I decided to just attenuate the appropriate channel and leave the other alone, but if I wanted to approximate constant-power, then to maintain the 0.5dB step size I'd have to compromise by taking it in turns to turn one down and the other up with each successive press of the button. Hardly worth the effort for a hi-fi. Plus it would have got complicated because it was a 6 channel preamp

Luckily, that chip has a 127dB gain range, so no problem with offsets caused by balance, fading, and input trim. Before I selected that chip - which was brand new at the time - I was having to dream up all sorts of weird and wonderful ways to make it work with less control range. And deal with signal levels and headroom as most chips worked with +/-5V rather than the +/-15V of the PGA2310.

Given that the PGA2310 is a fairly pricey IC, but I can't recommend it highly enough. Or the newer PGA2320 if you don't mind SMT. You can still have the motorised pots as part of the user interface if you like, but if you were putting audio through them, I'd be worried about matching between the two tracks, especially at low levels. I didn't realise how bad stereo pots were in that regard until I built the PGA2310 pre-amp - listening at very low volumes is so much better now as there's no faffing around with a very "touchy" pot at the bottom of the range, where the tracking is usually awful. A "-20dB" button is the best way to remedy that, but not needed with a good resistor-ladder volume control IC.

Hope this helps,

Mark
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Old 15th Dec 2018, 5:42 am   #10
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Default Re: Stereo Balance Question

A proper linear solution may be the best.
Could I suggest you fix a calibrated length of strong string or line across your sound stage at the optimum listening distance from the speakers?
Then with a typists chair on casters you can pull your listening devices located on either side of your head across the stage till you achieve balance.
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Old 15th Dec 2018, 6:22 pm   #11
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Default Re: Stereo Balance Question

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Originally Posted by Radio Wrangler View Post
I suspect you'll want finer steps in balance adjustment than in volume adjustment. Balance is comparative, while the sensation of volume lacks a stable reference.
For fine adjustment you just move yourself a little bit; balance controls are intended to correct things like grossly misplaced speakers and poor recordings. Looking at some commercial designs for domestic equipment I can't find any where the balance control has anything like the number of increments that the volume control does, suggesting that I'm right about the purpose of the function and the size of the steps it will need.

Like treble, bass, middle and 'graphic equaliser' controls, a balance control on domestic audio equipment only has one function: To provide an amusing mechanical puzzle for the owner. The aim is to get it as exactly central as you can, the winner being the one who gets the line on the knob most closely aligned with its corresponding mark on the fascia. I like this new variant on the game, trying to get two knob pointers pointing in exactly the same direction.
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Old 15th Dec 2018, 6:29 pm   #12
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For fine adjustment you just move yourself a little bit
Tricky with headphones. I have decided to go for constant power as per pro mixing desks and as I am using a spare gang on the pots for position to the micro I can calibrate them.
 
Old 15th Dec 2018, 6:35 pm   #13
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Default Re: Stereo Balance Question

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Originally Posted by Studio263 View Post
... I like this new variant on the game, trying to get two knob pointers pointing in exactly the same direction ...
The balance control does allow you to compensate for the infamously bad matching between nominally identical 'log' pots. If it's two separate pots then a properly balanced system might actually have the knobs pointing in different directions .

Cheers,

GJ
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