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40yrs Old MC headamp

CJWonfor

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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 I was surprised to see that I had used my long forgotten Patented (now out of date) MC head amp.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?oh=316a303c9279514428d93f053df5b491&oe=5B05CE88

 

Yes there is lots of spelling errors wee = we

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Intriguing, indeed. But as far as the "noise floor" is concerned, I humbly suggest that the devil is hidden in the details.

Take a closer look at the active elements. Yes, indeed, two of them: the "Current Sources" I1 and I2.

These are active devices. It is not an easy feat to create a low noise current source.

Actually, I can give you an example of a very lousy one: consider a "one-resistor-current-source" as built on the chinese fake LM317.

This may be actually used as a noise generator :)

So, basically, if you resolve the challenge as to how to construct a LOW NOISE current source, then you are half way home.

 

But there is a much more critical flaw in this highly theoretical schematic.

You have a war going on here.

You have two competing current sources connected in SERIES with each other. And this is a route to disaster, because there is no such thing as two exactly balanced (in terms of current) Current Sources. In praxis, they will be antagonising between each other. So there will be no such thing as a zero offset output from this one. Hence the pampers / capacitors on the output.

 

So, the missing link in this schematic is some form of "arbitration" that resolves the "conflict" between the two opposing armies that are to meet on the somewhat uneven battlefield. Maybe instead of the pampers diapers thing, it would be worthy of creating a long-time-constant (1 second = 1Hz) DC-Servo that modulates one of the current sources based on the imbalance of the value of the zero-offset imbalance of the output.

 

The best coupling capacitor is the "None-At-All" capacitor. IMHO, that is.

Cheers,

Ziggy.

 

P.S. - if you do not believe me, try a similar mental experiment, but from the "other point of view". The "voltage equivalent" case.

Take two high power car batteries. Charge them up to their "12,000000000001 VDC" (humor). .

And then, once they supposedly have the "same" voltage across them .... try connecting the plus with the plus and the minus with the minus between them.

The latter connection by a guinea pig, being an Ampere current meter. That will be the last readout that the exploding device shall ever see.

Nice try, but some additional thought into that theorectical schematic should be invested, prior into converting it into praxis.

 

Spice is a nice tool, but alas, it is lacking its own intellect. Not everything that Spice tells you is actually .... true.

 

Hey, are you some incognito recruiter, or something? :)

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CJWonfor Uczeń

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Intriguing, indeed. But as far as the "noise floor" is concerned, I humbly suggest that the devil is hidden in the details.

Take a closer look at the active elements. Yes, indeed, two of them: the "Current Sources" I1 and I2.

These are active devices. It is not an easy feat to create a low noise current source.

Actually, I can give you an example of a very lousy one: consider a "one-resistor-current-source" as build on the chinese fake LM317. This may be actually used as a noise generator :)

So, basically, if you resolve the challenge of a LOW NOISE current source, then you are half way home.

 

But there is a much more critical flaw in this highly theoretical schematic.

You have a war going on here.

You have two competing current sources connected in SERIES with each other. And this is a route to disaster, because there is no such thing as two exactly balanced (in terms of current) Current Sources. In praxis, they will be antagonising between each other.

 

So, the missing link in this schematic is some form of "arbitration" that resolves the "conflict" between the two opposing armies that are to meet on the somewhat uneven battlefield.

Cheers,

Ziggy.

 

P.S. - if you do not believe me, try a similar mental experiment, but from the "other point of view". The "voltage equivalent" case.

Take two high power car batteries. Charge them up to their "12,000000000001 VDC" (humor). .

And then, once they supposedly have the "same" voltage across them .... try connecting the plus with the plus and the minus with the minus between them.

The latter connection by a guinea pig, being an Ampere current meter. That will be the last readout that the exploding device shall ever see.

Nice try, but some additional thought into that theorectical schematic should be invested, prior into converting it into praxis.

 

Spice is a nice tool, but alas, it is lacking its own intellect. Not everything that Spice tells you is actually .... true.

 

Hey, are you some incognito recruiter, or something? :)

 

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 ?

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If you say that you actually got this architecture to work, then "Chapeau Bas", and I am all eyes and ears for a more practical layout / implementation / schematic.

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CJWonfor Uczeń

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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

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zjj_wwa Stały bywalec

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The "plain stupid" pair of low noise transistors (say BC550??) strapped up with a LED and/or resistor(s) is sufficient to create such a current source?

Therefore - four transistors for the whole circuit ??

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I used a TPQ2502 quad match NPN + PNP pairs two for left and right. The nearest one seem to be the MMPQ6700

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Well looking back it could be described as a complementary common base amp .

If I can find the original rough sketch I will paste it up, but over the few year I have been doing Electronic I have so much paper work, a complete forest my wife say,s full of dead tree's,

She not found my Superman Comic's yet he he he.

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Yes now that was odd , it appeared to be zero but the loading MC resistor was still used and even large uF caps (100uF) was placed in parallel wit no ill effects, output was OK into 47K. MM input. I also did a power amp version which I used in the first Inca Tech Quatra Amps, worked well. Simple circuit with V FETS

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I used a TPQ2502 quad match NPN + PNP pairs two for left and right. The nearest one seem to be the MMPQ6700

Many kind thanks for these inputs.

 

The "gearbox" is defined by the proportion of the resistors. Kind of like 10:1 (more or less) in the presented case.

Any observations as to the "practical limit" of the gearbox ?

 

30:1 ?

50:1 ?

100:1 ?

200:1 ?

Where does it end? When do you hit a wall in terms of noise or something? Practical range?

 

... lets take it into extremes: Imagine this circuit as a voltage amplifier directly in front of w Current Power Buffer.

The two would essentially constitute a Power Amplifier, say 400W into 8 ohms, with zero feedback and a very short signal path.

Can do?

Am I missing something?

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