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Old 18th April 2005, 01:48 PM   #1
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Default Inverting GC variant

This is a little variation to the usual circuits from datasheet. Compensate the output Voltage offset problems and adapts the phase and the impedance of the input signal. If desires, it may even inserts a potentiometer in the U1b net for active volume control (R11).

Ciao

Mauro Penasa
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Old 18th April 2005, 02:31 PM   #2
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Very clever, Mauro .
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Old 18th April 2005, 03:56 PM   #3
Hugh M is offline Hugh M  Canada
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I think I get most of this(the easy part).
U1a is unity gain buffer no phase shift.
U1b (unity gain) and u2 provide 360deg of shift.
Active volume in the feedback loop of U1b.

Questions
What is the purpose of the loop from o/p to +i/p of u1b with the clipping cct(4148)?
Does C2 form an i/p lowpass filter?
Does the Lin pot give a fake log taper in conjuntion with the feedback res of u1b?

Interesting cct.
Thanks for sharing
Hugh
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Old 18th April 2005, 04:17 PM   #4
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R2 and C2 define the high freq. low pass ( about -3dB@140Khz )
R5 and C4 polarize on DC U1b ( and nulling the offset voltage of U2 )
the pot. forms a simple Baxandall config., and the result is similar to a logarithmic function using a linear pot.

ciao

Mauro
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Old 18th April 2005, 04:54 PM   #5
Hugh M is offline Hugh M  Canada
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Thanks Mauro
Have you built this yet or just simulated.
I'd like to build it up and try it. It looks like a better implementation for a project I'm working on. I really like the active vol control.

Hugh
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Old 19th April 2005, 07:45 AM   #6
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The ideas that propose are always tested in laboratory with "real" conditions.

Mauro
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Old 19th April 2005, 08:14 AM   #7
digi01 is offline digi01  China
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looks a good idea,I like that U1a and U1b parts.

have you consider to set up 3886 work on unity gain,as a current buffer?

trying modify R8=1K and remove R5,maybe should do some others modification

rgds
Z
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Old 19th April 2005, 08:25 AM   #8
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unfortunately LM3886 a unit gain becomes unstable, and the techniques to stabilize it is nonelementary and degrade the performances. R5 cannot be removed, because polarizes U1b ( and U2 ) . Eventually has to connect to 0V.

Mauro
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Old 19th April 2005, 12:15 PM   #9
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Default giving it a go

Is the Burr Brown OPA2227 a suitable part for U1? If not I also have OPA627, which would you think preferable?

I have a few dozen of the above on hand, but no Fairchild pieces
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Old 19th April 2005, 12:46 PM   #10
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Generally, are able use on U1 all the OPamp compensated to works at unit gain. Nearly all the duals OPamp used in audio have this characteristic, but be necessary see the datasheet case by chance. The circuit is calibrated for a direct coupling among U1a-U1b and U2. If U1 has diverged input corrent , this don't come compensated by the R5-C4 net, then be necessary change the R5 value among 15K min and 47K max. If uses the pot. with other chips it may compensate adding in parallel R4 a capacitor from 10pF to 100pF depending on the circumstances.

Mauro
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