John Curl's Blowtorch preamplifier part II

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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 ?

You have to be precise about the term excite, an impulse excites activity at all frequencies. Dynamicly moving the operating points of an amplifier stage into a region of downright oscillation is not the same thing. The later is predominantly what is happening in many cases especially when you include amp, cable, and speaker together.

As for bypassing issues, you would probably do better looking at RF ingress. At least without eliminating that you can not make other speculations.
 
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1) higher FB designs more prone to local resonances.
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The circuit will be prone to resonance or oscillation lets assume the former - prone to resonance but not actually oscillating.
What the point ? A square wave at high frequency, fast transition time, is a perfect finger to hit the Cristal glass.
Just don't allow overshoot a any level.

[edit]Cross post with Scott, we said the same (as often)
 
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Perhaps my use of the word 'trigger' was misleading. I meant the situation where the music/test signal acts as a bias change for an RF oscillator buried in the circuit. A bypassed capacitor could of course act as an unintended RF resonator for such an oscillator.
Have-you ever noticed such a behavior, playing with square waves at various frequencies/levels ?
 
sorry I thought "excite a resonance" was precise enough.

If one hits the soundboard of a guitar one impulse comes to the face the guitar and this excites a resonance in each of the strings. The frequency is different for each string according to it's own conditions but the same impulse has excited all of the strings.

In this way in an amplifier the musical impulse comes in and the devices in the amp react according the to signal but when this signal reaches a part of the circuit which has a natural resonance that part of the circuit will add it own characteristic to the signal.

I have spent years of my life working on preventing RF ingress to audio circuits - the effect I'm talking about is independent of RF ingress. More accurate to call it RF excitation due to insufficient damping of Capacitive & Inductive Reactance
 
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Set the bandwidth of a preamp to be flat with no ringing whatever the capacitive charge with a correct serial resistance in output and no problem when short-circuit after the resistance or no charge. Set a low pass filter before the input for no overshoot at any level with the same charges... Set them for max signals and small ones... What do you expect ?

[edit] Milkem: "Soundboard of a guitar" or "Cristal glass".
 
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Wild speculation or RFI ....??

It May be that RFI causes problems that are not seen under single unit testing... but would show up if the whole system was tested (here we go again with marsh and his system testing) --- check this out: See the attachments. Equipment on the ac line produce wide bands of rfi and most power supplies in audio gear are not equiped to keep it from getting into the circuitry. It has been shown that when this happens, significant increases in THD in the amp occures.... Attached are spectrum of 1.25Mhz bandwidth of some equipment (CD player and CPU) that I measured (powered ON) on the ac line. measured with a wideband current pickup probe on the DUT equipment's ac cord. All items on the ac line (TV, lamp dimmers, etc) put a lot of RF on the ac line with thier own signature. I used this type info to create a line of ac power conditioners with outlet to outlet isolation.

This rfi on the ac line may be the real reason why people hear improvements when they use small value caps in shunt on the DC power lines. Or, better, in general with wideband low Z power supplies. A well developed ac line filter with isolated outlets often makes an audible improvment according to listeners. BTW - almost no noise of wide BW and of continuous nature comes in on the ac line. Its coming from within the home, guys. We are our own worst enemy! Thx-RNM
 

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Its coming from within the home, guys. We are our own worst enemy! Thx-RNM

The power line in my apartment is filthy now. It (whatever it is) gets into everything and renders high frequency differential measurements essential below a few tens of millivolts on the line-powered oscilloscope.

And I had thought the mandated vacuum fluorescents were bad enough!
 
Its coming from within the home, guys. We are our own worst enemy! Thx-RNM

That's a very interesting study and I completely sympathize with anyone who suggests that RFI can be injected into the mains power from audio equipment, and be injected into audio equipment from the mains and mess up the sound big time - I think this is why systems at HiFi shows often sound so bad compared with home systems.

What Scott didn't realise when he suggested RFI as a culprit regarding my original question is that in my designs I am about as fanatical as it is possible to be about preventing RF getting into my circuits.

Because switching diodes ( even schottky diodes ) and transformer secondary resonance ( when diodes switch off ) generate their own RFI I usually concentrate my filtration efforts after the bridge.

I don't think there is a power rail or earth line anywhere in my system that does not have CLC filtration. For me noise in audio circuits is public enemy No1.

Having taken all these measures I then have to make sure these efforts have not been wasted by having elements inside my "RFI free zone" that are merrily producing their own RFI due to lack of damping.

I hope this clarifies why I am interested in local resonant elements within circuits and my question about how it is that we can hear the effects of a 15Mhz resonance remains.

thanks

mike
 
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diyAudio Member RIP
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Because switching diodes ( even schottky diodes ) and transformer secondary resonance ( when diodes switch off ) generate their own RFI I usually concentrate my filtration efforts after the bridge.

Although I sympathize with anyone coming in late to this nearly-interminable thread, some of the early schematics dealing with power supplies did show some carefully-positioned common-mode chokes in "unconventional" locations in the power supply.
 
Kgrlee, I think you are only making 'trouble'. For example, do you have any idea how the Hirata test works, or what might set it off to read something significant? Have you seen examples or done the test, yourself? If not, please read up, first, before demanding satisfaction.
Your PIM argument is more 'reasonable', but again there is no clear mechanism to create PIM. The input capacitance is LOW, and the input is cascoded. Where oh where can we get PIM from this circuit?

Hi John,

PIM most certainly can occur in the absence of negative feedback, and I have measured it and shown it in theory. This has been pointed out by myself and others many, many times, including on this thread. Any situation where the signal will cause phase to change as a function of signal will create PIM. There are numerous such mechanisms in most amplifiers, some stronger than others. Any amplifier in which the bandwidth is a function of signal creates PIM. Video designers have been worried about differential gain and phase for over 60 years, and those amplifiers generally did not employ negative feedback.

Cheers,
Bob
 
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AC Line filtering - where is RFI coming from??

Although I sympathize with anyone coming in late to this nearly-interminable thread, some of the early schematics dealing with power supplies did show some carefully-positioned common-mode chokes in "unconventional" locations in the power supply.

Some of the better audio designers keep on top of everything they can. I am not surprised to see that here -- the thread started in--what? 2006? Those two line noise photo's are from my archive and are dated 4/2006! Other data goes back 10+ years before that. What wasn't wideley known and still isnt appreciated is the source of all this line RFI.... it is not from the incoming ac lines and thus you need each noisy product to have a sepeerate filter on its line cord/power input (output). That means Isolation of each noisy product.... which is just about all products. Now that we have everything powered with switching supplies (and on-line wall-wart battery chargers), the RFI from equipment sharing a common line/outlets is worse than ever..... forget about living in an apartment -- the poor audiophile apartment dweller has everyone's RFI producing equipment to contend with. In their case incoming filtering IS a problem as well as isolating their own noise generators. Circuits with widebandwidth are often an outcome of great designs but have a down side, too. Naturally. never a free lunch.
[note the max number and level of unwanted frequencies are generated within the bandwidth of audio circuits. Above those freqs are filtered as required by UL, FCC et al] Thx-RNMarsh
 
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thx barsco,

I'm just about to set off on a trip to India and instead of taking a book to read in my spare time I decided to read these blowtorch threads from start to finish - I hope I have enough time !

I have experimented with common mode chokes - not sure if my use was conventional or not - my understanding was that they would help with noise on the earth and I think they did help significantly . . . . . but nowhere near as significantly as full differential chokes on all power lines after the bridge - that was a completely different sonic story and my default arrangement on all my supplies at present.
 
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and on signal and speaker cables.
I was looking yesterday for shielded speakers cables (i always use them). Not easy to find, and more at a reasonable price ! I wonder why.
I thing i will shied my power cords as well.
Mr March: good idea, your "line of ac power conditioners with outlet to outlet isolation". Not to be obliged to modify each part of his equipment. Power supplies are a real nightmare, and, somehow, the key of everything.

Any amplifier in which the bandwidth is a function of signal creates PIM.
Reason for my preference for negative feedback, less sensible to this problem ?
 
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This rfi on the ac line may be the real reason why people hear improvements when they use small value caps in shunt on the DC power lines. Or, better, in general with wideband low Z power supplies. A well developed ac line filter with isolated outlets often makes an audible improvment according to listeners. BTW - almost no noise of wide BW and of continuous nature comes in on the ac line. Its coming from within the home, guys. We are our own worst enemy!

RFI on the AC power lines comes both from outside and inside the house.

On my stereo setup there are 4 isolation transformers, with balanced output and Pi capacitors filter. They make marked improvement on the sound quality of my setup.

Also, I have a dedicated in-wall power line to the stereo setup, fed from one of the three power phases which go to circuits which are inactive while I listen to music (bedroom and boiler).
 
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