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Old 20th September 2007, 07:46 PM   #11
gni is offline gni  United States
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Time for a thank-you:

Thank-you AndrewT. . . always good advice. . . maybe sometimes
harsh. . .but accurate. . . kept me from getting fried a couple of times.
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Old 20th September 2007, 09:04 PM   #12
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by AndrewT
Hi,
I think it was Cambridge audio that did this on one of their amps.
Watch for the pot wiper going open circuit.
Yes, they attempted to fo that in the preamp section of their A1 and A100 (possibly others). Probably for fear of the pot gping open the connection wasn't really well thought out and the results were sub-par.

Here is one way to do it:
Use an inverting configuration, as this can drop gain to zero with 100% feedback.
Think of the connection as op-amp based, to get the general idea. The input of the circuit is the fully clockwise end of the pot. The output of the circuit is the tully anti-clockwise end of the pot, and is also connected to the op-amp output. The (-) input of the op-amp is connected to the pot wiper. There is also a safety resistor (*) between op-amp output and (-) input. Finally, there is a resistor in series with the input side of the pot (**).

The best op-amp type to use is a FET input amp, as you ideally need input bias current to be zero. It is possible to do it with a bipolar amp, in which case a coupling cap is needed between the wiper of the pot and the rest of the circuit.

Resistor (*) is used for safety in case the wiper goes open. It's value will normally be high compared to the resistance of the pot, and it's intended to provide default 100% feedback if the wiper cuts out.

Resistor (**) sets the maximum gain, in conjunction with the resistance of the pot and safety resistor (**). Maximum gain is Av= -(Rpot || R**)/R*. The resistor also sets the minimum imput impedance, this occurs at the maximum gain setting. At the minimum gain setting the input impedance equals R** + Rpot. Sometimes choosing the values and using an input coupling cap can be used to set a variable high-pass filter, for applications where you know it would be a good idea to progressively block subsonics as the output increases, for instance to keep your speakers alive

And the clincher: The pot needs to be LINEAR. The resulting characteristic will, however, be quite close to the usual logarithmic one, but with much better tracking of stereo pots, because linear taper sections tend to have quite good tracking even for cheap pots.
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Old 20th September 2007, 09:04 PM   #13
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actually that's what's used in many preamps. it gets the potentiometer out of the signal path. i'm not a fan of having level controls or gain controls on the amp itself, since level control is the job of the preamp/mixer (IMO). amp gains in commercial equipment are usually determined by full output power at somewhere between 1 and 2 Vrms input, sometimes as high as 3Vrms. it varies from manufacturer to manufacturer, and even model to model. most manufacturers use variable level/gain controls, a few have fixed gain. i'm toying with the idea of having a variable open loop gain, since that has an effect on damping factor (unfortunately on distortion as well).
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Old 21st September 2007, 10:14 AM   #14
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The SAE 2600 power amp swaps feedback resistors to allow for '0db', '-3db', '-6db', and '-12db' to be selected from the front panel.
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Old 21st September 2007, 11:34 AM   #15
djk is online now djk
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So did the 2400, and I always disconnected the switches.

In the FB loop of an opamp the pot has to be a real good one. I did a design without a cap on the wiper, and moaned when the pots began to fail.

Maybe unclejed613 will post the volume control for the APT preamp for us (altenately I could too).
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Old 21st September 2007, 01:10 PM   #16
gni is offline gni  United States
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In the schematic included:

R637 is the feedback resistor from the output feeds the base
of Q605 (top left). R611 goes to ground.

If all I want [it's all about me. . . ;-) ] is to "mute" the amplifier
to a lower value without switching the input or output signal flow,
wouldn't some sort of smaller value for R637 do this. . .or would
an adjustment in R611 be required. . . kind of looks like a zobel. . .
passing highs to ground?

I took a look at the SEA designs. . .very complex circuit. . .couldn't
really tell what it was doing. . . but did see the resistors for changing
the gain of the amp and the level control.
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File Type: jpg rb-850-feedback.jpg (58.1 KB, 380 views)
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Old 21st September 2007, 04:49 PM   #17
AndrewT is online now AndrewT  Scotland
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C605//C607 are the DC block on the NFB return.
They increase the DC impedance to near infinity with the result that the DC gain falls to near 1times (+0db).

I would not muck about with the gain on the inverting pin to achieve switchable mute.
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Old 21st September 2007, 11:32 PM   #18
gni is offline gni  United States
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Sounds like sound advice. . . just was wondering if it could be done. .
and was secretly wanting it to work. . . a kind of magic solution
to silent muting.
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Old 22nd September 2007, 07:37 AM   #19
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by gni
. . . just was wondering if it could be done. .
and was secretly wanting it to work. . . a kind of magic solution
to silent muting.
Yes, it can be done.
But the feedback ratio ties in with the stability margin. You may find that each different gain setting requires a different optimised stabilty component. That would be difficult to arrange and also time consuming to find those optimised values.
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Old 23rd September 2007, 01:46 AM   #20
gni is offline gni  United States
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Point taken.

Gotta give the Rotel a rest. . .

Need to get the M-45 back up and happy.

Yamaha Parts "couldn't find the M-45" in
their database. . .might just order a
pile of the chips and match them.

They are cross referenced to the same
ones in the RB850!

Strange. . .one amp is 50wpc and the other
is 125wpc. . . same chips -- different company.

Sanyo Vs. Toshiba
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