| Jorge |
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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| peranders |
| Are you sure about this design? Shouldn't the transistors be in parallel? |
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| Jorge |
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. |
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| analog_sa |
| 15uF at MC levels :crazy: |
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| Jorge |
| Yes, as said low input impedance... |
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| forr |
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
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| Jorge |
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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| Jorge |
| And here's the noise: |
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| forr |
If I understand the curve well, distorsion is better than 80 dB, I think, for 1 mV input.
~~~~~~~ Forr
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| Jorge |
| 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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| forr |
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
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| Jorge |
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 |
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