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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 24th November 2012, 02:54 PM   #29811
mikelm is offline mikelm  England
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Also I think there are more opportunities for localized resonances in high FB & complex designs.

These will have no harmonic relation to the applied signal even though they may be excited by it, but I guess their demodulated derivatives could still show up in a fourier analysis . . . and ruin the sound of the amplifier.
 
Old 24th November 2012, 02:56 PM   #29812
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Interesting concept, mikelm. I don't know, one way or another.
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Old 24th November 2012, 02:58 PM   #29813
scott wurcer is online now scott wurcer  United States
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Quote:
Originally Posted by john curl View Post
Still, there must be SOMETHING that will measure the differences in two line stages (for example) and give me direct evidence of why one sounds better than another. I'm open to serious suggestions.
That was my point John, if a device does not register what is the use of the test one way or the other? In my opinion the Hirata test is just a special case of the multi-tone power ratio test where you can mathematically create any crest factor test signal you want (within physical limits).

We're still left with negative indicators, beloved amplifiers measure big numbers.
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Old 24th November 2012, 03:03 PM   #29814
scott wurcer is online now scott wurcer  United States
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Quote:
Originally Posted by mikelm View Post
Also I think there are more opportunities for localized resonances in high FB & complex designs.

These will have no harmonic relation to the applied signal even though they may be excited by it, but I guess their demodulated derivatives could still show up in a fourier analysis . . . and ruin the sound of the amplifier.
You're on dangerous ground here, highly speculative and not based on much physical reality.
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Old 24th November 2012, 03:12 PM   #29815
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Originally Posted by john curl View Post
Interesting concept, mikelm. I don't know, one way or another.
Hello John,
I would appreciate your opinion about line amp with no GNFB shown here Current conveyor as a voltage amplifier
Thanks Damir
 
Old 24th November 2012, 03:28 PM   #29816
mikelm is offline mikelm  England
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Quote:
Originally Posted by scott wurcer View Post
You're on dangerous ground here, highly speculative and not based on much physical reality.
There are two elements to my argument.

1) higher FB designs more prone to local resonances.

2) HF resonances on audio amplifiers somehow get demodulated and ruin the audible sound of the amp.

I admit that point 1) is intuitive and not proven and I also admit that there are plenty of opportunities for local resonances in low gain amps . . . but I think point 2 can be substantiated with simple logic.

If we imagine an audio circuit with a mosfet present that is in need of a gate stopper resistor but the gate stopper is either too low value or simply not present. The circuit will be prone to resonance or oscillation lets assume the former - prone to resonance but not actually oscillating.

Do we not all know from experience that the above description is an almost guaranteed formula for an amp that will sound awful ? I thought this was common knowledge. I know I have heard this effect.

Somehow that resonance, which will typically be in the 10 - 20Mhz range, manages to ruin the audible sound of the amp - if it isn't somehow getting demodulated into the audible range and thus appearing in the H2 - H20 spectrum can someone explain to me how this effect can be heard.

I would welcome a fuller understanding of this phenomena.

cheers

mike

Last edited by mikelm; 24th November 2012 at 03:35 PM.
 
Old 24th November 2012, 03:52 PM   #29817
DF96 is offline DF96  England
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An oscillating transistor is usually non-linear, which should show up in harmonic or IM tests provided the test signal triggers the oscillation. Some oscillations only occur at certain signal levels or frequencies, so won't always show up in a test but if present are almost bound to be triggered by some music signals.
 
Old 24th November 2012, 03:56 PM   #29818
john curl is offline john curl  United States
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Dadod, I would think that the Current conveyor is a good concept to try. That is about all that I can say, because I mostly design with fets, and they don't apply.
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Old 24th November 2012, 04:01 PM   #29819
mikelm is offline mikelm  England
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OK

So do we think that this audible effect comes from the fact that when this resonance is excited by the music signal and the some oscillation begins the transistor has been transformed from a reasonably linear device to a non-linear device and this causes audible distortion ?

Is it that simple ?
 
Old 24th November 2012, 04:12 PM   #29820
mikelm is offline mikelm  England
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If the above is true then why is it that the tendency to resonance caused by many capacitor bypass arrangements cause a similar roughness to the sound of an amp . . .

Personally, I'm not yet convinced that the above explanation is the full story
 

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