advertisement


40yrs old MC head amp

C

CJ14

O.H.
The last few days I have been testing and repairing a Magnum Pre-Amp and Power amp I designed for Etude in Belgium now nearly 40yrs ago. I was surprised to see that I had used my long forgotten Patented (now out of date) MC head amp. Even more surprised when testing it that the noise flour was a few dB lower than I expected when one considerer the new device we have today. I thought some of you Techy Buffs would like to see the circuit so here is the basic concept, Have fun.

28059241_1976108312707240_4148314589042545804_n.jpg


OK yes there is spelling error wee = we.
 
I did using a transistor and a green LED but the transistor had a low Rbe which helps so the limiting factor was the HFE, two of the circuit was used and the ref voltage the green LED was common giving me a random noise and a 3dB cut. There are 1000s of these circuit in Magnum's working well, at the time LM741 was the class act op-amp and NE5532 series was to be honest shit.
The worst source of noise was the PSU, but since that is my design skill set and I had to work on PSU with 10nV noise floor at lees than 1Hz no real problem. And some at 55V 100,000Amps (battery charger not for mobile phones) using transistors and thyristors and transformers at 7 English tons, tiny 1mA PSU's are easy.

Fun days and big bits no real SMD and pebble glasses to wear, now I am 65 they seem to be getting thicker every month, or are they cheating and making them smaller ?

Oh I used spice in this case only to act a simply drawing package, in real life there are 2 NPN and 2 PNP transistors in each stereo pair of constant current source on the same substrate and the design has been working for 40yrs, oh when I took it to Rotterdam I had the response by the Prof this can't work, oh dear I took the prototype out of my briefcase and guess what it works.

So Spice was not used and at the time there was no Spice only in Curry. he he
 
Even the beautifully simple W Marshall Leach two transistor in common base circuit achieves excellent noise with a 2N4401/2N4403 pair
There is no real reason to use some of the over complicated modern designs
 
"The prof" was right. It can't work. just to be sure I tried your simulation.... it doesn't work either with perfect virtual current sources or real ones (real virtual ones... you know what I mean...). You can't amplify with resistors!
 
Arkless Electronics said:
"The prof" was right. It can't work. just to be sure I tried your simulation.... it doesn't work either with perfect virtual current sources or real ones (real virtual ones... you know what I mean...). You can't amplify with resistors!
Click to expand...


Maybe the 'fun' is to get people to spot that the circuit - as shown when I just looked - has constant current sources in series with R2 and R3. If these *are* constant current sources it follows that the currents in R2 and R3 can't change as you waggle the "zero point".

A constant current symbol has an infinite dynamic resistance. So all that happens is that you vary the voltage on, and current though, R5. R1 and R4 will continue to see 1 mA and thus their voltages won't change.
 
Jim Audiomisc said:
Maybe the 'fun' is to get people to spot that the circuit - as shown when I just looked - has constant current sources in series with R2 and R3. If these *are* constant current sources it follows that the currents in R2 and R3 can't change as you waggle the "zero point".

A constant current symbol has an infinite dynamic resistance. So all that happens is that you vary the voltage on, and current though, R5. R1 and R4 will continue to see 1 mA and thus their voltages won't change.
Click to expand...

Precisely yes. a 5 second glance at it told me it couldn't possibly work but just in case... I simulated it too...
The only way this could "work" is if it inadvertently modulates the current sources... giving us a normal amplifier!
 
I guess the principle might be to run a constant current source into a large resistor, to obtain a voltage. Steer some current way (due to an input signal current) and you produce a corresponding voltage change across the resistor.

The circuit above seems to be trying to do a differential version of that. But I don't think it works as drawn (at least with theoretical perfect constant current sources).
 
davidsrsb said:
Even the beautifully simple W Marshall Leach two transistor in common base circuit achieves excellent noise with a 2N4401/2N4403 pair
There is no real reason to use some of the over complicated modern designs
Click to expand...

Yes nice circuit common Base but they do tend to be noisy at LF frequencies. But I did my first with Germanium AF114.
 
davidsrsb said:
The Diodes Inc (was Zetex) parts would be better at a reasonable price
Click to expand...

Maybe I have tried lots include OA Germanium series in the past but the Green LED's at low current were best, but saying that when I first did it things like TL431 were not yet here so If I went back to design maybe I could try them.
The phase shift was less than a degree (0.1Deg) at 100KHz not many OP-amp could do that then let alone have that BW.
 
You must log in or register to reply here.


advertisement


Back
Top