Logarithmic Amplifier Design
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 25th March 2010, 06:14 PM #1 diyAudio Member   Join Date: Nov 2009 Logarithmic Amplifier Design Hello people! I already posted a question similar to this back in January, however now I have more understanding on the subject I think my question might be a bit clearer! I'm currently designing a sidechain to feed a VCA which will act as a compressor for an audio input. The problem I have is that the sidechain signal is linear, and the VCA attenuates at 33mV/dB which is obviously logarithmic. I've done all of the maths and graphs in MS Excel and calculated that I need a logarithmic amplifier that does the following function: Vout = ln(Vin)*0.393 Where Vin = the linear input signal, and 0.393 is the gain required to make 33mV per step. (I do also realise that I could simply use a log amp followed by a standard amp of gain 0.393 if that is simpler!) I've done plenty of research on logarithmic amplifiers, and although there is an abundance of information on the theory behind them, there is no information on how to implement this theory and design one. To sum up, my question is: how do I design an amplifier that has a logarithmic output and if possible, a gain of 0.393? Thanks in advance!
 25th March 2010, 06:16 PM #2 Electrons are yellow and more is better! diyAudio Member     Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4 Have you tried Google using the key words "logarithmic amplifier"? __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work |Thread
diyAudio Member

Join Date: Nov 2009
Yes, I have. As well as actually making the effort to read numerous text books on operational amplifiers. Hence:

Quote:
 I've done plenty of research on logarithmic amplifiers, and although there is an abundance of information on the theory behind them, there is no information on how to implement this theory and design one.
Thanks anyway...

 25th March 2010, 06:26 PM #4 diyAudio Member   Join Date: Apr 2006 Location: Minnesota Please explain what a mV/db is.
 25th March 2010, 06:30 PM #5 diyAudio Member     Join Date: Jan 2004 Location: cambridge ma You could search for Dave Blackmer's DBX patents. They (IIRC) had pretty clear disclosure of all the circuit blocks for this family of VCA's. __________________ Clay is embedded in our subconscious. It has been there for at least 50,000 years. Last edited by scott wurcer; 25th March 2010 at 06:34 PM.
diyAudio Member

Join Date: Nov 2009
Quote:
 Originally Posted by sawreyrw Please explain what a mV/db is.
Sorry this isn't a unit - it's 33mV per dB. I.e when 33mV is applied to the control pin on the VCA, the audio input is attenuated by 1dB.

Quote:
 You could search for Dave Blackmer's DBX patents. They (IIIRC) had pretty clear disclosure of all the circuit blocks for this family of VCA's.
Thanks, I'll look into that

 25th March 2010, 06:53 PM #7 Electrons are yellow and more is better! diyAudio Member     Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4 Why can't you simply adjust the gain? LOG104 from TI has 500mV/ per decade current. Put a V/I converter in front of the LOG104. A decade is 20 dB. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work |Thread
diyAudio Moderator

Join Date: Sep 2007
Many moons ago I came up with this as an alternative to a special function IC.

This used the CA3046 matched transistor array.
The function I needed was V1 divided by 1/V2 for Cyril Batemans "Tan Theta meter" but the log amps may be of interest.

It was all a long time ago.
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 25th March 2010, 07:05 PM #9 Banned   Join Date: Jan 2008 Blog Entries: 2 There's a logarithmic converter (amplifier) on p212 of my copy (1994) of Horovitz & Hill with values. As constituted it offers -1V/decade of input current. There's loads of good stuff in fact. ...read up to about p214. w Last edited by wakibaki; 25th March 2010 at 07:09 PM.
 25th March 2010, 07:10 PM #10 diyAudio Member     Join Date: May 2005 Location: Californication Are you designing your own chip? see log detectors here and some problems with discrete designs. "The AD640 uses five cascaded limiting amplifiers to approximate a logarithmic response to an input signal of wide dynamic range and wide bandwidth. This type of logarithmic amplifier has traditionally been assembled from several small scale ICs and numerous external components. The performance of these semidiscrete circuits is often unsatisfactory. In particular, thelogarithmic slope and intercept" more reading here __________________ like four million tons of hydrogen exploding on the sun like the whisper of the termites building castles in the dust

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