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Old 19th April 2011, 02:30 PM   #1
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Default Need help with volume control

I am making an integrated stereo amp using a three band tone control(Purist please forgive!) using TDA2320A. The signal will be fed to a pair of mosfet poweramps. My question: Should the volumecontrol be a) after the tone control or b) before the tone control? What value? Linear or Log? Any buffer circuit required? If so how to design?
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Old 19th April 2011, 03:26 PM   #2
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volume control should be after tone.
Depending on your source you might need a buffer BEFORE tone if your tone load and volume control is too much for your source to drive. If its a good dac/cdplayer/pc you should be good without a buffer.
Volume control should be log.
Volume control value depends on the amp/source/input filter. You should be fine with 50k depending on your decoupling cap (too small of a volume control could mean no bass if you have a very small input cap). You might even be able to go down to 25k if you have a buffered source, or as high as 100k if your power amps "like" it.

I cant really help you on the buffer design as thats a whole other subject. Generaly I just cheat and use a low gain headphone amp pcb in the box since they are small, for a buffer.
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Old 19th April 2011, 05:44 PM   #3
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Thank you! This is so useful.
The input shunt capacitor for the poweramp is 100uF and the series capacitor which comes after is 100nF... What do you suggest?

Last edited by junjuvava; 19th April 2011 at 05:56 PM.
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Old 19th April 2011, 05:57 PM   #4
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The volume control, reduces the signal level, and brings it closer to the noise floor, I.e. anything you do after the pot has a reduced range before it becomes audiable...
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Old 19th April 2011, 06:00 PM   #5
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Its a very strange choice of opamp

However it will drive any reasonable load so I would suggest a 10k log pot. If the opamp is used on a single supply rail rather than split supplies then the pot must be fed via a cap to minimise noise as it is turned. Use at least 22uF. The wiper of the pot will feed the power amps and the value of cap used here depends on the input impedance of the power amp.
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Old 19th April 2011, 06:14 PM   #6
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Quote:
Originally Posted by junjuvava View Post
Thank you! This is so useful.
The input capacitor for the poweramp is 100nF... What do you suggest?
Guitar Pedals: R-C Filter Calculator

You want your corner to be at least 1.5 octaves below your lowest note, if not 2-3 octaves. An octave is half or double the note incase you didnt know. So lets say your lowest note is 40hz. one octave lower is 20hz, 2 octaves is 10hz.

http://www.rs-met.com/documents/tuto...cyAndPitch.pdf

so you want to shoot for 10hz, with a 100nf (.1uf) cap. that means the volume control should be about 150k.

Now you see the problem. if you use a 50k pot you get a -3db of 30hz and your music will have phase problems up to 100hz or more.

So if you increase your input cap to say .5uf then you would have a nice corner of 7hz.
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Old 19th April 2011, 06:14 PM   #7
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Thank you
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Old 19th April 2011, 06:23 PM   #8
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So you suggest a 50K log pot and changing the cap to 500nF? Your understanding of the interplay of elements is amazing. thank you.
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Old 19th April 2011, 06:31 PM   #9
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Quote:
Originally Posted by digits View Post
The volume control, reduces the signal level, and brings it closer to the noise floor, I.e. anything you do after the pot has a reduced range before it becomes audiable...
Tha's one way of looking at it. Another is that if you boost the bass/treble before the volume control you could clip before the volume control. Not likely but possible.

FWIW the Hafler DH-101 preamp has tone controls after the volume/balance group. It's generally regarded as a decent preamp.

G≤
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Old 19th April 2011, 07:35 PM   #10
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Looks like you edited your post later. If your input cap is a 100uf bypassed(parallel) with a .1uf box/foil cap ( a common practice) then you should be good with any value pot and you could go low like 10k-25k.

I dont like tone controls... But on all the tone stacks on instrument amps, its buffered, then toned, then volume control.

This is probably due to the instrument having a low output (say 200mv) that cant drive anything( high source impedance). I could see where if it was a real source that put out 2vrms, the tone stack could clip the buffer if the buffer didnt have wide enough rails. But a standard buffer that can take +-15v or +-12v shouldnt have any problem running 6-9db hot. And thats if you even need a buffer...
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