John Curl's Blowtorch preamplifier

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#16,081
john curl said:
I would like to tell my hf oscillation story that happened over 15 years ago.
When Dick Marsh developed his new caps along with Bas Lim at Reliable Capacitor, he was VERY adamant to put them in every audio product he could talk people into.
I, already, used Reliable Cap RT .1 uf devices (film and foil polystyrene) but that was not good enough for him, as he had 'improved' the cap to be better at high frequency bypassing with much better ESR.
So, without my knowledge, Michael Percy was asked to replace my REL RT caps with Dick's new REL caps of the same value for a 3'rd party customer in a VENDETTA phono stage.
Well, the caps didn't fit well in the same spot, so the leads were left slightly long.
Guess what? It oscillated, but at a frequency higher than could be seen with a 20MHz scope.
Michael Percy contacted me and I fixed it, without replacing the new high frequency caps. Without at least a 100MHz scope, I could not have seen the problem, and with my 350MHz scope, the problem was obvious.
Click to expand...

Hmm... This almost sounds as a plea for ML ceramic caps. 😉

edit: I've a Tektronix 2237 and a HP 181A storage oscilloscope.
 
#16,085
PMA said:


It will never have HF resolution as a 30MHz analog scope.

Compared to 10MHz analog, it might be questionable.
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In general, if the digital PC-type scope has high bandwidth, it has lower resolution (or, if not, becomes horribly expensive).

I have a TiePie unit which can be switched from 10 bits vertical resolution at 50MHz to 14 bits at 5MHz. So, you get good resolution at lower freqs where you probably want to look at the waveform in detail, while at higher freqs you can still determine *some* detail, but you are probably more looking at the presence or not of signals (audio use assumed).

So, there's another trade-off for you...

Jan Didden
 
#16,086
john curl said:
I find your analysis in context of the Otala amp, preposterous.
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Hi John,

Are you referring to my post in answer to Dimitri?

Actually, regardless of who's post you are referring to, you need to explain what specifically you are objecting to. Back up what you say with a specific argument, or it has no credibility and people don't learn from what you say.

Bob
 
#16,087
Edmond Stuart said:


Hi Bob,

Does this imply that (by whatever means) you cannot measure the 'sonic degradation' introduced by such caps at all?

Cheers,
Edmond.
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Hi Edmond,

I know of no way to measure the sonic degradation of a capacitor when the capacitor is supposedly degrading the sonics but it is making no measurable distortion and is causing no measurable phase or frequency response aberration.

I'm not talking about capacitors that show up distortion on Bateman's tests, for example. Obviously, capacitors like electrolytics and some ceramics that make distortion are not what I am talking about.

I'm also not talking about measuring DA or ESR of a capacitor. These are characteristics of the capcitor itself that are observable by measuring the capacitor itself, but which do not necessarily create a measurable signature (that we know how to measure) at the amplifier block level. Bear in mind, this is not to say there is no correlation between DA/ESR and sonics; its just not a measurement that can be made at the amplifier level.

I'm not saying that capacitors don't sound different. I'm just saying that I don't know of a test that can identify that difference if the capacitor is not making measurable distortion.

BTW, one time I got some expensive boutique polypropylene crossover capacitors and some of the same-value, same dielectric from Parts Express (at less than 1/10 the price). I very accurately measured DA, ESR, ESL, and they were electrically indistinguishable.

Cheers,
Bob
 
#16,088
john curl said:
I would like to tell my hf oscillation story that happened over 15 years ago.
When Dick Marsh developed his new caps along with Bas Lim at Reliable Capacitor, he was VERY adamant to put them in every audio product he could talk people into.
I, already, used Reliable Cap RT .1 uf devices (film and foil polystyrene) but that was not good enough for him, as he had 'improved' the cap to be better at high frequency bypassing with much better ESR.
So, without my knowledge, Michael Percy was asked to replace my REL RT caps with Dick's new REL caps of the same value for a 3'rd party customer in a VENDETTA phono stage.
Well, the caps didn't fit well in the same spot, so the leads were left slightly long.
Guess what? It oscillated, but at a frequency higher than could be seen with a 20MHz scope.
Michael Percy contacted me and I fixed it, without replacing the new high frequency caps. Without at least a 100MHz scope, I could not have seen the problem, and with my 350MHz scope, the problem was obvious.
Click to expand...


When a circuit oscillates with an extra quarter inch or so on each lead of a bypass (or because the new bypass has a lower ESR), this is usually a sign of a poorly designed power supply rail that was probably on the verge of oscillation even in its original arrangement. Many designers don't realize that High-Q capacitors and stray inductance love to work together to form resonances, sometimes with a high Q.

You have to do something to get the Q down. One approach, used regularly in industry, is to have at least one electrolytic bypass capacitor on each power rail net. The higher ESR of the electrolytic damps out the resonances.

For those in audio who prefer not to have electrolytics even on supply rails, the proper approach is to add a Zobel network to ground. In other words, take one of your good film capacitors and deliberately degrade its ESR by putting parhaps a one-ohm resistor in series with it. That will kill the Q on the rail.

Cheers,
Bob
 
#16,090
Hi PMA, Jan,
From what I have seen, you may always need an analog scope. Most DSOs are 8 bit, with a few with better resolution, 12 bits being common. I'm sure that those very expensive Agilent, Tek, LeCroy .... DSOs with sampling rates in the 10s of gigahertz probably look a lot like a 100 MHz analog scope for waveform detail. But see how much money is required to equal the display quality of our old "clunkers"? Still, I'd love a 500 MHz MSO from Agilent. In my dreams!

Analog 30MHz scope will show you even 100MHz oscillations, with lower amplitude of course (thick trace).
Click to expand...
Yes, that's very true PMA. Still, a 100 MHz scope has generally nicer features, like cursors and readouts.

-Chris
 
#16,094
Hi PMA,
I know, I do work with both.
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I expected that from both yourself and Nelson.

The digital 'scopes do have a number of features that are very useful. That's why I picked up a USB 'scope until I can afford a "real" DSO. That's the broken one. 🙁

I do have an HP 1722A I'd like to repair. My Philips PM 3070 emits so much hash that I can't align an FM tuner if it's even running! I wish I had known that when I bought it.

-Chris
 
#16,095
Bob Cordell said:



You have to do something to get the Q down. One approach, used regularly in industry, is to have at least one electrolytic bypass capacitor on each power rail net. The higher ESR of the electrolytic damps out the resonances.

For those in audio who prefer not to have electrolytics even on supply rails, the proper approach is to add a Zobel network to ground. In other words, take one of your good film capacitors and deliberately degrade its ESR by putting parhaps a one-ohm resistor in series with it. That will kill the Q on the rail.

Cheers,
Bob
Click to expand...


So true, so practical.
 
#16,099
john curl said:
Fast source followers can oscillate, BUT the real cause of oscillation, Bob Cordell has not figured out yet.
Click to expand...

You can impress audio hobbyists with that, but never people who have
learned s-parameters and such. The capacitively loaded emitter/source
follower is the standard workhorse of UHF / microwave oscillators.
Creates a negative impedance at the base. Oh, --- that was hard!

regards, Gerhard
 
#16,100
scott wurcer said:


I personally hate digital scopes. Low rez (often 8bits) and sampling/aliasing artifacts make some measurements difficult. People forget how much the brain can fill in with analog and how much needs to be filtered out with digital. Yes one can learn to do it, that's not the point.
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DSO's are particularly useless for a lot of radio servicing. Any form of audio modulated carrier gets garbled beyond recognition.
The worst digital scopes ever made (IMO) would have to be the “revolutionary” Tek DPO (digital phosphor) series. When they came out the sales guff sound reasonable so we had one delivered for evaluation. I’m not enamoured with a blurry trace ¼ to ½ a division thick.
 
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