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Old 17th June 2012, 10:20 PM   #31
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that chart is in db, not volts. and that's exactly the way a linear pot "sounds".
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Old 23rd December 2012, 04:35 PM   #32
Telstar is offline Telstar  Italy
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This topic is the closest thing to what I was looking for - variable gain as attuation method. with or without GFB.

If someone has a working circuit please intervene here.
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Old 23rd December 2012, 04:40 PM   #33
JMFahey is offline JMFahey  Argentina
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Easy, but ... what do you need it for?
There's a few different configurations.
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Old 23rd December 2012, 05:55 PM   #34
Telstar is offline Telstar  Italy
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I don't use a preamp, and I believe the less impact on the signal can be achieved with this approach. I'm looking for less than unity to about 15-20x gain control in the VAS of a classic 3-stage poweramp.
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Old 23rd December 2012, 06:03 PM   #35
pergo is offline pergo  Italy
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touch the feedback ratio and you will touch the bandwidth.
Not good approach at all.
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Old 23rd December 2012, 06:14 PM   #36
sreten is offline sreten  United Kingdom
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Hi,

Seems no-one has explicitly stated :

The reason power amplifiers are driven by a passive pot if fitted with
a gain control are its gain and phase margins which cannot cope with
adjusting the gain via the feedback loop.

It will either work fine - which means its severely overcompensated
for unity gain - or it will oscillate at any gain below the nominal gain
margin for the amplifiers designed nominal gain, power amplifiers
oscillating is not good for the amplifier or expensive speakers.

The same arguments apply to line level amplifiers. One of the reasons
active tone controls got a bad name is you have to slug the line
amplifier to unity gain stability (usually) for full treble cut.

No real problem with active volume control if gain is not involved, just
attenuation, and D.Self comprehensively covers the options for that.

rgds, sreten.
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Old 23rd December 2012, 07:27 PM   #37
Telstar is offline Telstar  Italy
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I have to re-read self book on this subject.
The thread title would deny the second case I'm considering, that is an open loop amp.
I do not see instability risks there.
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Old 13th January 2013, 10:49 PM   #38
Struth is offline Struth  Canada
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Hi Guys

Self uses the Baxandall active volume circuit in his preamp, as this provides exactly the gain range requested above and uses a linear pot. It is easier to make multi-section pots track well if they have a linear taper. See the Small Signal Circuit Design handbook. There is no problem compensating this circuit as it is built around a 5532, as is usual.

Rod Elliot also shows the identical Bax circuit using a fet opamp and higher component values. This keeps the variable input impedance of the circuit from dropping too low to be easily driven by a signal source.

The circuit is inverting, which allows it to have less than unity gain. You could build it using discretes instead of ICs as someone else has done on another thread here.

Most power amp circuits are optimised for gains above a certain value, so making their gain variable can be problematic as Andrew and others have mentioned. It is possible with some circuits and certain compensation schemes to make the comp variable with the gain.

Have fun
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