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VCR's and LDR's
Old 18th September 2011
  #1
Gear Nut
 

VCR's and LDR's

Hi All

Just wanting to know your thoughts on resistor replacement in audio circuits when multi gang pots are not an option. Say for example you wanted to make a variable 24dB per octave high pass.

LDR's require a fair amount of additional circuitry to linearize their resistance taper and to compensate for ambient temp changes. Also matching is an issue.

JFET VCR's seem to be a better solution (ie: VCR2N) if feedback is used but is the typical distortion figures you would get acceptable?

Has anyone had experience with both?
Old 19th September 2011
  #2
Lives for gear
 
Wavebourn's Avatar
 

I did.
I used FETs in guitar effects, and LDRs in anti-clipping limiters of power amps. I would not use them vice-verse.
Old 19th September 2011
  #3
Gear Nut
 

Hi Wavebourn

I should have mentioned that I was referring to line level pro audio devices only.
Old 19th September 2011
  #4

You could use FETs and several resistors to create a stepped potentiometer.

"Digital Potentiometers" are pretty good these days, but I don't think you can use most of them to +/- 15v. http://www.analog.com/static/importe...ets/AD5222.pdf

Or, you could use a FET with a PWM gate drive to vary the effective resistance at audio frequencies.



-tINY

Old 19th September 2011
  #5
Gear Nut
 

Yeah. I guess the LM13700 would be a good candidate.
Old 20th September 2011
  #6

Can you still get OTAs? They were really hard to find in the mid 90's.



-tINY

Old 20th September 2011
  #7
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gyraf's Avatar
 

It's hard to get decent audio quality from OTA's, even the latest generation with linearizing diodes. I don't think you'll want to go there.

Why not simply use a VCA like the THAT2180 family - or even a multiplying DAC controlled from some PIC?

Jakob E.
Old 20th September 2011
  #8


I like the VCA idea - just need a control voltage....

If using it to adjust the corner frequency of a filter, the OP must remember that the output is a log funtion of the control voltage. If using one in a filter circuit, you may have to change your topology radically (as it's not really a variable resistor...). http://www.thatcorp.com/datashts/dn130.pdf





-tINY

Old 21st September 2011
  #9
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Wavebourn's Avatar
 

Quote:
Originally Posted by diezel_addict View Post
Hi Wavebourn

I should have mentioned that I was referring to line level pro audio devices only.
You should read my previous posting like for line levels I prefer LDRs because other methods like FETs and LTPs give worse distortions. On mic levels FETs may be fine in terms of distortions, but noise-wise I would not attenuate mic level signals.

But for filters (EQ/Crossover?) I would rather use stepped approach, either rotary switches, or FET switches with multiple resistors or capacitors.
Old 22nd September 2011
  #10
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