John Curl's Blowtorch preamplifier part II - Page 2988 - diyAudio
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Old 25th November 2012, 12:25 AM   #29871
fas42 is offline fas42  Australia
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My rationale on the FB issue is that for that mechanism to work correctly, the circuit has to be of very high performance already; in one sense the "FB amplifier", considered for a moment as a separate structure, has to be, say, of an order of magnitude better in performance that the the underlying circuit it's attempting to always correct: follows from the old adage, two wrongs don't make a right ...

My suggestion for a test to really nail what's going on is to use two quite low level, appropriate sine signals and do an IMD test. Then, mix in a significantly lower frequency at a much higher level with frequency chosen which will make sense after reading the following: do that same IMD test, but take absolutely no notice of any IMD products derived from the presence of the 3rd sine wave - hence choice of 3rd frequency will be crucial. Bump up the level of that 3rd frequency in increments to just under that for the combined signal to clip at any point, and do those IMD measurements at each point.

My belief is that the "good" circuits will show far less variation of the IMD spectrum only resulting from the 2 test signals interacting - in other words, the 3rd frequency is stressing the circuit, including its power supply, to see if that disrupts the behaviour when only looking at the context of the low level signals ...

Frank
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Old 25th November 2012, 12:36 AM   #29872
bcarso is offline bcarso  United States
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Originally Posted by gpapag View Post
Part 5 for FETs , part 6 for BJTs.
Mind also part 1 third page .

http://www.audiofaidate.org/it/artic...p%20design.pdf



George
That's a nice series which I had heard about but never seen. Thanks!
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Old 25th November 2012, 12:47 AM   #29873
gpapag is offline gpapag  Greece
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That's a nice series which I had heard about but never seen. Thanks!
You are welcome Brad. I consider it a “must” read.


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Originally Posted by gpapag View Post
I will ask. The BH curve nonlinearity and the permeability’s dependence on H field strength are phenomena known on x-former cores.
Assuming audio x-formers (input, coupling, output) have been used, studied and optimized for decades, where is the data quantifying the harmonic distortion generation due to these nonlinearities of ferromagnetic materials? (links, reference)
Partly answering my own question, here is a good old study (theory & measurements)

http://www.alcatel-lucent.com/bstj/v...tj14-2-322.pdf

George
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Old 25th November 2012, 12:54 AM   #29874
fas42 is offline fas42  Australia
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Having said all that - even though my audio system sounds much better for all my filtration efforts - I still noticed a spike on my scope connected to my amps when I turn off a light - that was a bit depressing !
Your work is right on the money, mike, especially that related to eliminating spurious RF around the bridge rectifier.

Like you, I've spent a great deal of effort on getting rid of that nasty stuff coming in from bad behaviour in the mains, and agree that the rubbish can be tenacious, depressing as you say. A very simple RF injection device of gross proportions I use is a simple high power incandescent bulb in socket, connected to the system mains spur via an extension cord. I hold the socket and plug in the line where the cables join and deliberately pull them apart just enough so there's ferocious sparking, which I can delicately manipulate. The speakers of a normal system go mad with the sound of that interference, but it is possible to get to the point where the sound of the system remains completely mute, no matter how much you provoke it in this fasion.

Frank
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Old 25th November 2012, 01:03 AM   #29875
fas42 is offline fas42  Australia
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christophe,

You would need to parallel all the colors & all the whites then you get decent conductivity - if you think 100ohm impedance cable is a problem at audio frequencies you better look elsewhere - I think it's worth a try
Also, nothing stopping you paralleling any number of runs of the cable to get sufficiently low resistance - just more soldering required! Plus, then you can have fun with different twisting and braiding strategies ...

Frank
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Old 25th November 2012, 01:14 AM   #29876
fas42 is offline fas42  Australia
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I just had hoped that one or some you professionally trained guys could explain to me how a circuit tending towards resonance at 15Mhz for example ends up making an amp sound rough in the audio band - that's something I experienced but never really understood. If no one understands it that's fine I'll stop thinking about it.
My take would be that the FB mechanism is particularly sensitive to those instabilities; it has to react extremely fast in comparison to the signal it's attempting to correct, so would tend to provoke mild, momentary, bad behaviour ...

Frank
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Old 25th November 2012, 05:05 AM   #29877
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Fas42, that is the sort of 'abberation' that we are ignoring in our objective evaluation of many pieces of audio electronics.
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Old 25th November 2012, 05:51 AM   #29878
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Old 25th November 2012, 06:29 AM   #29879
jcx is offline jcx  United States
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My take would be that the FB mechanism is particularly sensitive to those instabilities; it has to react extremely fast in comparison to the signal it's attempting to correct, so would tend to provoke mild, momentary, bad behaviour ...
audio power amps, even with modern transistors seldom have 5 MHz unity gain intercept – in all of the Cordell circuits I've simmed only measuring inside the error correction local feedback loop managed that high an gain intercept frequency

so for a hypothetical 15 MHz “feature” the global loop of typical audio power amps probably has << 0.3 “gain”, quite a few amps don't even have 1 MHz global loop gain intercept
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Old 25th November 2012, 06:42 AM   #29880
RNMarsh is offline RNMarsh  United States
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Default The 'sound' of RF -

Oscillations - Can they be heard if above the hearing freq range. yes. Sort of... indirectly. Another one of those "its As If" things ---

RFI and oscillation - the tweeter will not behave well. Heating in the tweeter which will indirectly cause the tweeter to distort. Which then results in the audio signal sounding really, really bad just before the tweeter goes silent. It need not be 15MHz but often lower freq above hearing range is also common source of this indirect distortion. Switching amp artifacts in the output to the speaker will have similar affect if not heavily attenuated. -RNM

Last edited by RNMarsh; 25th November 2012 at 07:03 AM.
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