Amek 2500 preamp analysis

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saint gillis

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  Hello, here's the simplified differential input of the Amek 2500 and of the Green preamp (left drawing). The right drawing is the simpler version that we find on old transformless Mic preamps.
  I'd like to understand more the role of the 2 op amps UA1 and UA2, if someone has the knowledge..

https://i.ibb.co/NSbRwPQ/INPUT.jpg
 

saint gillis

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Still I don't understand this equivalence :
microphone-engineering-handbook-chapter-8-40.jpg
 

dogears

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The transistor into the op amp is two inverting stages, the second is a single noninverting gain stage.
 

JohnRoberts

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saint gillis said:
Still I don't understand this equivalence :
microphone-engineering-handbook-chapter-8-40.jpg
The transistor plus the op amp, acts like a composite op amp with the base the + input and the emitter the - input.

JR 
 

JohnRoberts

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saint gillis said:
Hello , I've changed the title of the topic, maybe it's a good place to study the Amek 2500 preamp circuit.
The documents provided by L'Andratté are awesome.

Anyone has an idea of the use of R1 and R2 10ohm input resistors?
Back in the day Paul Buff used similar low value resistors in series with his Transamp input device bases. His application notes included small caps to ground and a trim pot to provide a HF common mode rejection trim.

I can only guess that it may improve stability margin in that old design, there is probably enough input capacitance from all those devices in parallel to effect a LPF.

JR
 

abbey road d enfer

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saint gillis said:
And what is the benefit of using current sources over simple resistors to drive the emitters?
It has two major effects.
Since the current is more stable, there is less distortion from the transistors.
Since  a current source has almost infinite impedance AC-wise, all the NFB current goes to the transistors, thus further reducing distortion. In other words, the loop gain is increased.
It also increases slightly the current capability of the transistors, thus increasing somewhat the capability to handle large signals at low gain.
 

porkyc

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Dig up stuff on Paul Buff's TransAmp. The 2500 mic pre is er, .............quite similar!
 
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