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Jorge 18th August 2005 02:55 PM

Low noise head amp (MC)
2 Attachment(s)

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.

peranders 18th August 2005 03:01 PM

Are you sure about this design? Shouldn't the transistors be in parallel?

Jorge 18th August 2005 04:38 PM

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.

analog_sa 18th August 2005 04:50 PM

15uF at MC levels :crazy:

Jorge 18th August 2005 04:56 PM

Yes, as said low input impedance...

forr 18th August 2005 05:02 PM

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 18th August 2005 05:10 PM

2 Attachment(s)
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:

Jorge 18th August 2005 05:11 PM

2 Attachment(s)
And here's the noise:

forr 18th August 2005 05:17 PM

If I understand the curve well, distorsion is better than 80 dB, I think, for 1 mV input.

~~~~~~~ Forr

Jorge 18th August 2005 10:26 PM

Yes, in the sims distortion is -87 dB, but since this is just a simulation and there's no feedback, I wanted to play safe - too much, perhaps...

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