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Old 14th April 2012, 07:27 AM   #2391
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by Lazy Cat View Post
Hi hahfran, any progress with triple output SSA? Any practical circuit tests? Regards Andrej

P.S. Today I got p-jfets, test results soon
yes i made a breadbord amp with the EF power stage. The SSA BJT sit on a copper plate held at constant temp by thermostate, the Vas on a heatsink,
the predriver on a heatsink, and the TT Bjts on another , added some base stopper resistors. The op stability is excellent if the temp of the SSA is set to 60 C. Also the op of the power section is almost independent of temperature.
Thats ok. No listening tests yet because the attempt to have the amp inverting, that is input is the summing point of feedback, does not work.
But it should.
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Old 14th April 2012, 11:20 AM   #2392
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Originally Posted by hahfran View Post
No listening tests yet because the attempt to have the amp inverting, that is input is the summing point of feedback, does not work.
But it should.
Inverting amp should have bases (+IN) grounded and input signal connected to the feedback divider (-IN), look the pic.
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Old 14th April 2012, 11:55 AM   #2393
mikelm is offline mikelm  England
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Which for a current feedback amp can be quite a challenging load . . . .

but a headphone amp would work fine as a preamp for this.
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Old 15th April 2012, 02:20 AM   #2394
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LC,
Just to ask, is it better to have 2N5551/2N5401 to be input cascode instead of BC550/560 ?
(same question goes to input pairs ) It has higher Vce.
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Old 15th April 2012, 02:31 AM   #2395
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The BC5x0 have low noise and far better Vce characteristics at low voltages. The BC337/327 have even better Vce characteristics, but different gain specs. The BC5x0 should perform exceedingly well here I think.
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Old 15th April 2012, 03:03 AM   #2396
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LC, also with your BIBGT HP SSA, what is the heat dissipation of input & cascode, and VAS ? If its too low then the need to more thermal compensation, and vice versa ?
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Old 15th April 2012, 09:35 AM   #2397
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by Lazy Cat View Post
Inverting amp should have bases (+IN) grounded and input signal connected to the feedback divider (-IN), look the pic.
This is the objective (pic) . This positive feedback allows to set the actual parameters of the electrical representation of a speaker in a vented box ( very simplified model) . Thus one can set such parameters as mass and compliance for a target TS alignment of any speaker in any box.
Aside from harmonics, a voltage amp is assumed to deliver real voltage, which causes a complex current to flow according to complex impedance of the speaker.
This circuit does exactly the opposite it forces a complex current which causes a real voltage thus overall a real voltage on input has a real voltage in the complex load.

This does not work with the SSA . While another by design inverting amp ( commercial AVM amp) does exactly as theory predicts. Any idea?
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Old 15th April 2012, 12:25 PM   #2398
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Is there a point in seeing how a complex load behaves since you still have no idea how it sounds since you cannot simulate the transformation into moving air. All you have done is introduce even more variables.
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Old 15th April 2012, 06:40 PM   #2399
hahfran is offline hahfran  Germany
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Is there a point in seeing how a complex load behaves since you still have no idea how it sounds since you cannot simulate the transformation into moving air. All you have done is introduce even more variables.
Sure i can compute the acoustical output. of course then I need the radiation resistance but this does not create complication. No in fact I have reduced variables because of the effect, mutual cancellation.
I think Nelson Pass has writen an essay on Current source amps for speakers
but he did not get the point that the current source must force a complex current into a complex load such that the imaginary parts become zeroed out and hence no phase shifts no timing problems no frequency response problems.
Basically this is not new thinking anymore , the makers of active speakers are considering amp and speaker and box and acoustics of listening rooms as one system, not as independent separated devices.
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Old 15th April 2012, 07:27 PM   #2400
mikelm is offline mikelm  England
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About current drive I have heard that some speakers sound good using that method, other speakers sound good with voltage drive and yet other speakers sound best with a mix of both. I also remember that Nelson Pass said that for the bass driver, an amp with a low o/p impedance is essential to get a good sound.

So it sounds like you will need a custom amplifier for each speaker you build, five active elements per amp and possibly another amp for bass ? ?

If each of these active elements are not going to collectively degrade the sound, the quality will have to be very very high indeed.

You obviously have a very good command of electronics & acoustics and I would be very interested to hear such a system.

But I can't help thinking that if the same budget & resource were used to construct a much more simple, conventional design, using the finest quality components and very high quality power supplies that it might produce a sound that was more pleasing, despite being theoretically inferior.
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