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 Forum index » DIY Hardware and Software
Amplifier circuit with logarithmic scaling?
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bigtex



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PostPosted: Wed Apr 12, 2006 12:54 pm    Post subject: Amplifier circuit with logarithmic scaling?
Subject description: How does one make an opamp get non-linear in a controllable way?
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I was just reading in the Tape Op bonus articles section about the dbx 386 preamp. What got me really interested was this part:

The Type IV system uses logarithmic scaling from -4 dB to 0 dB allowing high level transient signals to pass through without clipping. Signals below -4 dB are unaffected by the scaling. This creates a system where, because of the mapping, the 386 can accept very hot signals and still not clip. Very much like how tape can handle hot signals with out causing undesirable effects.

I like the idea of that a lot. It got me thinking that something like this, but with an ajustable amplitude point to start the log scaling, would be really really nice as a module. Possibly with a selectable "equation" that controls the scaling, too. How easy/hard is it do make a log or inverse log amplifier? Having it selectable between 1/x or x^n (with adjustable mantissa and exponent) would turn it into a nice compressor or expander. I'm sure something like this has been done before, right?

Being fairly new to this, and not really knowing how to do such a thing, I was hoping somebody here could give me a head start. How does one make an opamp get non-linear in a controllable way?
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jksuperstar



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PostPosted: Wed Apr 12, 2006 7:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

A lot of compressors have an adjustable curve, many simply have 2 to select from, such as Soft/Hard Knees.


You might want to check out this recent thread:
http://www.electro-music.com/forum/viewtopic.php?highlight=opamp+cookbook&t=10592

see MOSC's 2nd post. That should help you get started.
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bigtex



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PostPosted: Wed Apr 12, 2006 9:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, we have the Schmooze forum for general discussion on topics not related to anything else. .

Are discussions like the one I started not welcome on the DIY board? My intent is to make a module for my fledgeling modular, and I figured the people on this board would know a thing or two about circuit design and could get me started. I'm looking for an unusual effect that might not be made in just a basic compressor.

But thanks for the tip on compressors. I already kind of knew this was a sort of compression circuit. I guess I wasn't actually sure what I was looking for specifically, just kind of curious in general.
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bigtex



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PostPosted: Wed Apr 12, 2006 9:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oh... oh oh. I misunderstood. Mosc's second post, not the second post on the thread which happened to have been made by Mosc. Ooops.

Ok. That's a good basic circuit to get me started as well. Thanks!

Posted Image, might have been reduced in size. Click Image to view fullscreen.
or
Posted Image, might have been reduced in size. Click Image to view fullscreen.

I guess they are essentially the same... with one using the transistor as a diode. It's been about 5 years since electronics class, but collector to emitter can be a diode, right?... or is that only emitter to base that can be a diode? With the base grounded, there wouldn't actually be any current flow in that circuit... unless reverse biased... oh it's been too long and I just can't remember it right.

Last edited by bigtex on Wed Apr 12, 2006 9:09 pm; edited 1 time in total
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bigtex



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PostPosted: Wed Apr 12, 2006 9:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

Okay, now in this circuit, which Mosc linked to on that referenced thread, it looks like Q2 is being used backwards.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

I don't think I've ever seen the base used as the output like this. I thought the current flow from the emitter to the base determined the current flow from the emitter to the collector, making it a current amplifier.

Can there be current flow from the emitter to the collector without a voltage at the base? I am confused. I see that the collector and the base are tied together, but with the 1M resistor between them and Vcc, there couldn't really be enought current flowing to do anything, right? Where am I wrong about this?
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Macaba



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PostPosted: Thu Apr 13, 2006 8:34 am    Post subject: Reply with quote  Mark this post and the followings unread

I would point out, that, as I said in that thread, Ray Wilsons scaling circuit seems to be based on the above, but differs in some crucial aspects. The thing that clinched it for me, is that the above circuit when simulated does not work, whereas Ray Wilson's one DOES.
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mosc
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PostPosted: Thu Apr 13, 2006 12:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm not sure if the circuit I linked to works or not. It's just something I found on the internet and pointed to it FYI.

Accurate log scaling is a difficult thing, especially if you want it to track over more than one octave and be stable over time and temperature. This is why good oscillators on modular systems can be expensive. Not only are the designs difficult, but they must be made with precise and well-matched components. When DSPs came out, I personally lost interest in that level of circuit design, so I'm in no way an expert.

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jksuperstar



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PostPosted: Thu Apr 13, 2006 12:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

bigtex wrote:
Can there be current flow from the emitter to the collector without a voltage at the base? I am confused. I see that the collector and the base are tied together, but with the 1M resistor between them and Vcc, there couldn't really be enought current flowing to do anything, right? Where am I wrong about this?


Q2 is more or less acting like a variable resistor, when compared to the 1M resistor above it, more or less becoming a voltage divider. The emitter voltage is varying based on input signal, so the difference between the collector and emitter will vary. This "output" of the division feeds back into Q2, so I'd think it'd be a non-linear signal that feeds the op-amp UA1 in the end. At least, that's my best guess at the theory behind it.
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Blue Hell
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PostPosted: Thu Apr 13, 2006 1:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ok my go :-)

Q2 is used as a diode for temperature compensation of Q1.

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bigtex



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PostPosted: Thu Apr 13, 2006 1:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hmmm. I think I liked jksuperstar's answer better, bacuse I could almost understand it.

Please do elaborate.
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mosc
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PostPosted: Thu Apr 13, 2006 1:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

In the attached drawing is the transfer curve of a silicon dioed. notice that it doesn't just turn on like the think and ideal diode would, but it turns on gradually with an exponential curve. When you put that diode in the feedback loop of the op amp, the op amp will try to keep the input at ground potential. The current through the input resistor increases linearly with the input voltage and that same current must flow through the diode. The output voltage therefore increases (in the opposite direction) along the exponential curve.

These circuts only work over a relative narrow range. The point where the cruve on the right take off vertically is 0.7 V in silicon.


diode-curve.png
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bigtex



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PostPosted: Thu Apr 13, 2006 1:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ah! Now that I understand. Thanks!

So how do you adapt this to work over a higher voltage range, short of buffering and attenuating the input and amplifying the output? I bet it must be a much more complicated circuit to do that... perhaps involving lots of diodes/transistors and different value resistors, perhaps as a ladder?

I should stop speculating and just look this stuff up...
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PostPosted: Thu Apr 13, 2006 1:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

bigtex wrote:

Please do elaborate.


I'd love to .. unfortunately that would not only cost me this evening but the next weekend as well :-)

But the 2nd sentence on the page the schematic came from hints at it.

Quote:
The second transistor is used as refference and with the PTK resistor minimizes the dT/dV coeficient.


(my bold)

And it makes sense sort of, both transistors being the same type, having the same temperature coëfficient. To be sure I would have to be calcula it though, unfortunately I have some documentation that needs to be finished before next week ...

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Pehr



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PostPosted: Thu Apr 13, 2006 1:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

Dual Log/Linear VCA Confused
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bigtex



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PostPosted: Thu Apr 13, 2006 1:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

These circuts only work over a relative narrow range. The point where the cruve on the right take off vertically is 0.7 V in silicon.

And I always thought diodes just turned on and off. So much for theory!

An idea I just had, going along with my original question, would be to use a rail-to-rail opamp, and have that 0.7 volts represent the max amplitudes, so that stuff would get logarithmically attenuated before clipping. Wouldn't that work? Just have the diode be between the output and the power rail.. and then... oh I'm not sure what else.
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bigtex



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PostPosted: Thu Apr 13, 2006 1:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

Pehr wrote:
Dual Log/Linear VCA Confused


Hey, I just bought one of those! I guess I'd better shut up and just put it together Smile
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PostPosted: Thu Apr 13, 2006 1:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

bigtex wrote:
Pehr wrote:
Dual Log/Linear VCA Confused


Hey, I just bought one of those! I guess I'd better shut up and just put it together Smile


Hehe Cool

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PostPosted: Thu Apr 20, 2006 2:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Some food for thought from Bob Pease:

http://www.elecdesign.com/Articles/ArticleID/6068/6068.html
http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=11301
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