- Jul 7, 2021
- San Francisco
ThanksIt appears to be part of the feedback loop, providing an inverted copy of the output signal to feed back to the second half of the input differential pair.
Thanks John.This is a variant "Cohen" topology. Peavey was already using this approach back when I started working there in the mid 80s.
The inverting op amp providing symmetrical feedback to the other input device delivers good balance using less active devices than the typical 3 op amp approach, with no performance compromise.
Note : those 2sd786 very low noise input transistors went end of life obsolete near the turn of the century a couple decades ago.
There are numerous benefits from that topology...including low noise, low distortion, flat frequency response, balanced input, etc.Thanks John.
So it's for improved CMRR?
you posted OK on this page...I love this site, but how to get permission to post on certain pages? Sorry to hijack your thread, I just don't know what to do, I feel stuck.
Actually, I was asking specifically about the function of the inverting amp, not the overall topology.There are numerous benefits from that topology...including low noise, low distortion, flat frequency response, balanced input, etc.
it is not working in isolation, it is part of the overall combined negative feedback loop. It mimics the other op amp with a symmetrical opposite polarity feedback signal. For good balance both inputs need to look the same to the outside world.Actually, I was asking specifically about the function of the inverting amp, not the overall topology.
I am familiar with the variant of that "the best capacitor is no capacitor"....As a certain (troubled) tech genius likes to say - "The best part is no part".
I was very surprised to find inside the M-Audio DMP3, that there was no interstage caps beyond the one after the INA163. Not even on the final outputs. I didn't completely trace out the circuit, but I assume that 1/2 of each NJM4560 was configured as a DC servo?I am familiar with the variant of that "the best capacitor is no capacitor"....
As a complement to what John already said. You can think of it this way. Negative feedback has the property that it opposes the input signal. That is, a copy of the output signal is feed back in such a way that it opposes the input signal. So, the output of the differential amplifier goes into one op-amp, as it is, the output of that op-amp has the same polarity or is in-phase with the input at the base of transistor Q11, so by feeding the output of U3 (non-inverting) to the emmiter of Q11, when the input at the base of Q11 raises it voltage, the signal feed back at the emmiter also raises its voltage opposing any difference between the base-emmiter of Q11. If you only use that op-amp without the added inverting op-amp, then the 'cold' input at transistor Q10 does not get this same treatment, since the input of Q10 has the opposite polarity than the signal at the base of Q11, thus, the feedback is not balanced or symmetrical, that is, the output of the non-inverting op-amp will not oppose the signal at the base of Q10, this will increase distortion. By adding the inverting op-amp, it feeds a signal at the emmiter of Q10 with the same polarity of the cold signal at the input of Q10, hence opposing it. This way you have balanced feedback, that is, both transistors are getting the same treatment.Actually, I was asking specifically about the function of the inverting amp, not the overall topology.
I'd seen that sketch of the input, but not the others - that's the entire siganl path; thanks for that!re: DMP3 schematic, here's a page with part of it (the input, HPF, and output) :
Actualizar componentes Previo M-Audio DMP3 - Pics insideHola. He estado entrometiendome un poco en esto del DIY, realmente es algo que envicia bastante. Empece modificando mi SE2200 (Primera Version) (No mas sibilancia, sonido mas claro y natural) y el MXL v67 (Sonido mas oscuro y natural, menos sibilante). A lo que vinimos. Mi inquietud vino traswww.hispasonic.com
Enter your email address to join:
Register today and take advantage of membership benefits.
Enter your email address to join: