# Balance on a small stereo hi-fiamp.

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#### grrr_arrghh

##### New Member
Hi

I am in the process of constructing a small, basic amplyfiying stereo mixer (see diagram below, which was taken from an earlier thread: https://www.electro-tech-online.com...-for-our-industrial-electronics-subject.6594/), however, i have (as an afterthought), decided to include a 'balance' control. I wondered what the best way to do this would be? The smaller diagram below shows my idea. A dual pot, with one of the pots connected in reverse to the other, so that when you turn it, one volume increases, and the other decreases (one of the two pots would be connected to the left channel, and the other to the right channel). Would this work? where I have written 'from Op-Amp', would infact be the output from the pot that controls the volume of that channel. Bearing in mind that I would then have two pots on each channel, would i need another amplifying circuit to bring the volume back up to an acceptable level? If i did need another amplyfying circuit after the mixing circuit, what would be the easiest way to do this? (I will probably use NE5532N op-amps all round) I know it would be easier to have to separate pots, one for each channel, so that you could manualy increase the volume on each channel as you wished, but i would rather have separate volume and balance controls. All suggestions welcome Thanks Tim

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• 1balance.JPG
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#### Nigel Goodwin

##### Super Moderator
grrr_arrghh said:
I wondered what the best way to do this would be? The smaller diagram below shows my idea. A dual pot, with one of the pots connected in reverse to the other, so that when you turn it, one volume increases, and the other decreases (one of the two pots would be connected to the left channel, and the other to the right channel).
Would this work?

Yes it would, but you really need a log/anti-log dual gang pot, these used to be available (but hard to find) - they are probably even harder to find now. Another way is to do it with a single linear pot like this:

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• Balance.GIF
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#### grrr_arrghh

##### New Member
oh, ok

are the two connections where you have written 'input' conected to each channel?

also, i'm sure i have read that you need to put amplyfiers in aluminium boxes to avoid interference from radio signals etc. Is this true? if so, should I ground the box (bearing in mind that I am using a dc supply, so ground is just 0v, not earth)?

another quick question - as I have the two resistors and the cap connected to the non-inverting input, could I connect the same three components to the non inverting inputs on both op-amps? (i am trying to the lower the area of PCB I have to use)

thanks

Tim

#### Nigel Goodwin

##### Super Moderator
grrr_arrghh said:
oh, ok

are the two connections where you have written 'input' conected to each channel?

Yes, as are the two outs as well - either left at the top, and right at the bottom. Or right at the top, and left at the bottom - whichever you want.

also, i'm sure i have read that you need to put amplyfiers in aluminium boxes to avoid interference from radio signals etc. Is this true? if so, should I ground the box (bearing in mind that I am using a dc supply, so ground is just 0v, not earth)?

Yes, amplifiers are best in metal boxes, connect the box to signal ground (where the ground connection of the sockets is) in order to screen the circuit.

another quick question - as I have the two resistors and the cap connected to the non-inverting input, could I connect the same three components to the non inverting inputs on both op-amps? (i am trying to the lower the area of PCB I have to use)

Yes, you can do that, it's simply generating a midpoint HT rail.

#### grrr_arrghh

##### New Member
ok thanks

IT WORKS!! not that i didn't expect it to or anything, but I have only made it up on a breadboard with a 741!!

Thanks for everything, you've been a great help.

Tim

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