Variable power supply to control gainclone volume?

[MWP]

Hi all,

Im looking at making a 6 channel amp.
Its going to be difficult to make a volume control/preamp for such a beast.

With the talk on regulated gainclones latley im starting to think about using a variable voltage supply to the gainclone ICs using LM338s or similar.

Can you guys think of any problems i may have with setting up such a system?

Thanks.

 

[mateo88]

In my opinion (take it with a grain of salt, I'm not an expert), if it does work, it seems to me that it would be very difficult to control the volume, especially at low outputs.

 

[jan.didden]

It will not work. The supply has no influence on the volume, until you get so low that the amp output hits the supply voltage, which is called clipping. It is like the tops of the sine waves are sheared off, and it sounds VERY unpleasantly.

The volume is set by the level from your source (say the CD player), the gain of the amp, and the sensitivity of the speakers. Assuming that you cannot change the output level of the CD player and the sensitivity of the speakers, that only leaves the amp gain.

By manipulation of the amp feedback factor, you can vary the gain, but only within small limits. The lower you make the gain (min is one), the more the amp becomes prone to oscillations. The higher you make the gain (although you wouldn't want that probably, it's too high as it is normally) you get more distortion, because at higher gains you have less feedback to linearize the amp.

So the only real possibility to control volume is to vary the attenuation of the signal. That's why everyone does it that way. Because it is easiest to vary a small signal, it is mostly done before the power amp. In theory, you could put a heavy power pot between power amp and speaker, but the waste of power and the (un)availability of good quality power pots makes that less attractive.

Jan Didden

 

[millwood]

you can use some solid state pots to adjust volume. Or alternatively, you can use a gain trim opamp (either digital or analog).

changing the rail voltage on the chip does not work.

 

[mrlots2do]

So the only real possibility to control volume is to vary the attenuation of the signal. That's why everyone does it that way. Because it is easiest to vary a small signal, it is mostly done before the power amp.
Jan Didden

Jan,
Using BrianGT's boards how would I attenuate the signal to control volume. Could you show a schematic?

Phil

 

[Sandy H.]

Jan,
Using BrianGT's boards how would I attenuate the signal to control volume. Could you show a schematic?

Phil

Check the current revision of the manual on Brian's site. On page 22, the simple pot diagram is included and may do just what you want. You might also be able to configure 3 stereo pots (like the Noble pictured) with gearing (as Cowrang did) to use 3 controls for one knob.

Let me know if that is not clear a clear enugh explanation. I think people are using 100k audio taper pots, but you might want to search to be sure.

Sandy.

 

[mrlots2do]

Check the current revision of the manual on Brian's site. On page 22, the simple pot diagram is included and may do just what you want.

Thanks for the heads up. Would you advise connecting R3 as an unmounted resistor to board (two leads from board to unmounted resistor), this would allow an easy modification should I decide to add volume control after board assembly?

You might also be able to configure 3 stereo pots (like the Noble pictured) with gearing (as Cowrang did) to use 3 controls for one knob.

What did Cowang do???
How do I use 3 controls for one knob?

Let me know if that is not clear a clear enugh explanation. I think people are using 100k audio taper pots, but you might want to search to be sure.

Could someone give me a digikey part# for 100K audio taper pot and Noble stereo potentiometer

Thanks Phil

 

[MWP]

It will not work. The supply has no influence on the volume, until you get so low that the amp output hits the supply voltage, which is called clipping. It is like the tops of the sine waves are sheared off, and it sounds VERY unpleasantly.

Yeh, thanks.
A few hours after posting this i realised this would happen.
It shows ive been thinking about class D amps too much latley

 

[Sandy H.]

Thanks for the heads up. Would you advise connecting R3 as an unmounted resistor to board (two leads from board to unmounted resistor), this would allow an easy modification should I decide to add volume control after board assembly?

What did Cowang do???
How do I use 3 controls for one knob?

Could someone give me a digikey part# for 100K audio taper pot and Noble stereo potentiometer

Thanks Phil

Sorry, I misspelled his ID, its Cowanrg and his site is here. Click on his BOSOZ link to get the volume control arrangement he used. http://cowanrg.mesanetworks.net/projects/

You should be able to mount everything to the board as is and just use the volume control prior to the board. Signal in from source -> pot -> signal in on board.

I got my cheap stereo pots from mouser and some Noble pots from a Bill Fitzpatrick in the trading post a while back. I think percy audio also has high end pots and there are probably other good vendors.

Good luck.

Sandy.

 

[mrlots2do]

I think people are using 100k audio taper pots, but you might want to search to be sure.

Sandy,

Regarding the 100K audio taper pot....
R3 on BrianGT's board is 680 ohm, is there a reason I need the 100K pot. Seems too large, maybe should be 50K but than I don't know nothing. Besides I haven't found them with more than 47K taper pot.

If I find I need greater than 47K taper pot could I put a 50K resistor before the 47K taper pot.

BrianGT if you read this.... The manual Rev B (page 5) component names R3, Rf, and so on, doesn't match schematic (page 3).

 

[jan.didden]

I agree, 100k is too much. There's no need for such a large value, and it messes up your S/N ratio.
Some people might have tube preamps and since these often cannot drive as low loads as SS (unless they have a cathode follower or something), for such a case 100k might be required. But then the input to the amp should be at least of comparable input impedance or the pot control law will be upset. The GC's I've seen on the forum cannot handle 100k.
I would go for no more than 20k.

Jan Didden

 

[Fenris]

The GC's I've seen on the forum cannot handle 100k.
I would go for no more than 20k.

Jan Didden

I'm using a 100K Noble with no problems. I would have preferred a 20-50K instead, but that's what was available.