Low noise head amp (MC)

Jorge

Member
2001-04-30 7:32 pm
Brazil
Hello,

Here's a reincarnation of a head amp I've made for a Ortophon MC-20 in the mid 70's.
At that time I've used BF200 RF transistors.

S/N RIAA equalized shall be better than 80dB and distortion at 1mV input is about 0.1%.

Note: Nowadays I've not rebuilt the circuit - just simulated it.
 

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Jorge

Member
2001-04-30 7:32 pm
Brazil
I'm sure...

Very unusual, but it works.
For the lowest noise with a low impedance source, one has to use a high emitter current and a transistor with a low bulk and parasitc resistences.

Parallel transistors are used exactly to lower these resistences, and any feedback resistor will increase noise.

Using a large die transitor will have the same effect as palleling smaller ones. A RF one is even better (low rbb).

The series connected transistors used as diodes are the load, their exponential V/I curve will compensate the exponential V/I Vbe curve, cancelling distortion (at least most of it).
This is not true for a high impedance source.
 

forr

Member
2004-12-01 6:46 pm
Next door
JORGE
I mentioned this circuit a few weeks ago as being recently published in Electronics World. I think you told me you have used it a long time before. The circuit is fascinating as it is a very rare one with really no feedback at all and it still has low distorsion. Do you remember as it sounded ? I would be interested to know the results of your simulation.
It should be good as a mike preamp too.

~~~~~ Forr

§§§
 

Jorge

Member
2001-04-30 7:32 pm
Brazil
hello, forr

Honest, after all these yrs, and taking in consideration the amps & speakers of the time, I wouldn't give any opinion on sound quality.
A friend I regarded as having very good ears liked the Ortofon trafo better :bawling:
But then the input cap was an electrolytic, who knows.

Here's the disto curve:
 

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forr

Member
2004-12-01 6:46 pm
Next door
Hi Jorge,


S. Chekcheyev (from Tirapsol in Moldavia) in his circuit idea of Electronics World, July 2003, p 33, gives the following results with a load of four BC547A connected as diodes :
input 1 mV 0.00062%
input 10 mV 0.0056%
He says the circuit is insensible to variations of supply or temperature.
I've never heard any of the lovers of negative-feedback-less circuits saying he has tried it.


An other very linear and very simple circuit published in EDN Ideas by the same author :
http://www.edn.com/contents/images/91604di.pdf


An third circuit idea from Checkcheyev was pubished in Electronics World in decembre 2004. It's a zero resistance analogue of a zener.
Very simple, too.


~~~~~~~ Forr


§§§
 

Jorge

Member
2001-04-30 7:32 pm
Brazil
forr said:

An other very linear and very simple circuit published in EDN Ideas by the same author :
http://www.edn.com/contents/images/91604di.pdf

~~~~~~~ Forr


§§§


That's a most unusual idea - the two caps 'invert' the diode for AC currents!

And I'm glad to know the distortion of a real life circuit compares quite well with the simulations above!

Merci,

Jorge