| kubeek |
I just listed through a complex 480 pages long book about op-amps, but I didnīt find this circuit.It is shown in few Fender amp schematics, itīs not connected in signal chain, but on the path to CA3080 in compressor circuit.
Can anyone tell me what is it good for, and why is it connected the way it is?
I have also plotted the in(green)/out(blue) curve. |
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| FastEddy |
Unity gain, inverting Op-Amp circuit with + & - terminals reversed top to bottom from "common practice".
(Everything you wanted to know about Op-Amps but was 'fraid to ask = http://www.tinaja.com/ ... When he was a lad of 20, he began writing Op-Amp Cookbooks circa 1970 ... eventually becoming an engineer for National ... now quite independant and full of crazy ideas about "perfect" sign waves, etc. ... :eek: ) |
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| kubeek |
I donīt understand, you want to say it has unity gain? No it doesnīt.
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| FastEddy |
OK, so what is the gain? (I assumed that with 560 ohms on both the top and bottom, inverting and non-inverting feedback loops, the gain of The Amp would be unity ... granted there is resistance of 5.6K on both inputs, but The Amp is set to unity = 1 = gain. ... Overall circuit might produce something like a negative "gain" of -1.8 to -2.x or more (= -less).
:cannotbe: |
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| kubeek |
You see the graph, the ouput changes 24volts for cca 1volt input change, thatīs a "gain" of 24 (+/- somehting).
I donīt understand this circuit, but I think the ouput product highly depends on input offset voltage. |
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| kubeek |
| The output doesnīt depend on offset voltage, nor offset current. Changing R2 changes the slope, other resistors just move the ouput "left/right". |
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| FastEddy |
Yes, yes ... yes, now I see what you mean.
This thing is set up like a Schmidt Trigger .. as soon as the input voltage crosses ~= +0.6 volts (input transistor threshold), it flips over to the positive rail voltage ... and vise versa. A voltage follower with high gain = Schmidt Trigger = a common "flip flop" circuit since the invention of TTL logic ala TRW (Dr. Ramo = the R in TRW).
(I am totally wrong about the circuit gain and the Op-Amp gain ... without the input resistors this would probably oscilate or latch to one rail or the other = gain >>= 10X.) |
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| pinkmouse |
| Where in the circuit is it? What is it connected to? It looks to me that it is amplifying a lot and driving the output to the rail limits with a small input. Almost like some sort of square wave generator. |
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| FastEddy |
" ... It looks to me that it is amplifying a lot and driving the output to the rail limits with a small input. Almost like some sort of square wave generator. ..."
Exactly. |
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| pinkmouse |
| quote: | Originally posted by FastEddy
This thing is set up like a Schmidt Trigger .. |
Doh! :) |
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| kubeek |
http://www.fender.com/support/amp_s...0_Schematic.pdf
its on the first page, connected through diode to CA3080.
And x axis on the graph is in miliseconds, the amp does not have so small slew rate, the ouput is a function of input, not time. If you want I can do the analysis slower, like tens of seconds. |
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| FastEddy |
kubeek, help my tired old eyes: got a longitude / lattitude for diode CA3080 ... B x 2 ... C x 3 ??
The top and left of drawing perimeter has numbers & letters A thru D and 1 thru 7 or so ...
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| kubeek |
Now I understand, in the 200.pdf itīs between B and C, 6 and 5.
In the 400.pdf itīs B5. |
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| FastEddy |
Ah, yes!!
A simple latch to "capture" the state of the Mute circuit. ...
Probable: the "compressor" circuit Op-Amp (U14A) "conditions" & isolates the audio signal / information for this Op-Amp (U8B) ... thus The Op-Amp (U8B) "reacts" to the "compressor", latching to either the +VCC rail or the -VDD rail, driving subsequent circuits (U9A & U13A) through diode D27 into a state of "sample and hold" ... for the purposes of controlling whether to mute the whole pre-amp or not ... (Note that U9A is a programmable TTL chip A3080, not a true Op-Amp [or diode], pin 5 is the programming input.)
This is not a Fender tube amp ... :D |
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| FastEddy |
Esoteric reference about TTL circuits: http://en.wikipedia.org/wiki/Simon_Ramo
I met him once, at TRW Space Park in Redando Beach Ca. ... he was putting the finishing touches on the Voyager One space craft... The TTL circuits in that puppy are still operating and it has left the Solar System after photographing several planets and is now looking for Black Holes (which most folks don't know was one of its original missions).
Dr. Ramo also is credited with inventing the photographic process that makes integrated circuits (chips) possible. |
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| kubeek |
So I added the model of pin 5, and things got much more understandable.
This "amp", when provided input lower than 0v, creates adequte current through the diode in bias part of CA3080. On the picture the diode in CA3080 is D3. V4 is supply voltage for CA3080, which is +15 and -10 volts.
I donīt know exactly how, but at least I understand what it does. Thank you both for your assistance. |
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| FastEddy |
correct.
" ... CA3080, which is transconductance amplifier - changes gain through pin 5. ..." and is thus, for this circuit, either full on (not muted) or full off (muted) ... and, as you note, this is based on the "Overlimit" or not output from that descrete JFET gain stage (Overlimit Inhibit = ~ B6 ~ B7 area of the diagram).
The CA3080 is a normal, "programmable" Op-Amp with a digital (TTL) muting feature, yes, but in this case, the pass through analog audio signal is either allowed or disallowed (muted) = pin 5 passive/normal/"don't care" or activated/muted ... "transconductance" being defined here as "audio pass through" or not. (Latin derivitave: Trans = across, -conductance. ... Almost as bad as German chemical names. ;) )
" ... at least I understand what it does. ..." And that's the important thing.
BTW: what made you suspect the Op-Amp (U8A) as being troublesome?? Fender not working??
(Reference: http://www.intersil.com/cda/devicei...7,CA3080,0.html ... an out of date part, but not forgotten.) |
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| kubeek |
In this case itīs not gated, it is lineary controlled - page 1 from the datasheet I mentioned:| quote: | The CA3080 and CA3080A types have differential input and a
single-ended, push-pull, class A output. In addition, these types
have an amplifier bias input which may be used either for gating
or for linear gain control.
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The overlimit inhibitor is just the zener, which makes sure it doesnīt compress the signal too much.
I am about to buld this preamp, so I am first trying to know how to troubleshoot it - what to measure, and also to know how it works. |
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| FastEddy |
" ...The overlimit inhibitor is just the zener, which makes sure it doesnīt compress the signal too much. I am about to buld this preamp, so I am first trying to know how to troubleshoot it - what to measure, and also to know how it works. ..."
That Schmidt Trigger (U8A) is definately serving a "digital" function, shutting down the CA3080 in a "snap action", latching methodology. ( http://en.wikipedia.org/wiki/Schmidt_trigger )
I might have chosen a different pre-amp design ... but ... to each his own purpose. Go for it!
:smash: |
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| kubeek |
But it is not a schmitt trigger..., If you have some simulation program then use it and you will see the output of the amp moves slowly, and the output to the CA3080 has of course 10-2*V_forvard of the two diodes, and the current changes slowly from 1uA to 2.1mA with input form -10mV to -12V.
The graphs before were incorrect, because I didnīt simulate it with the load connected- two diodes in series. |
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