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Old 19th March 2009, 10:49 PM   #16101
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Well Gerhard, you are right and Bob is partially right. I easily fixed it, without adding resistors in the gates, how?
 
Old 19th March 2009, 11:12 PM   #16102
gerhard is offline gerhard  Germany
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Quote:
Originally posted by john curl
Well Gerhard, you are right and Bob is partially right. I easily fixed it, without adding resistors in the gates, how?
Make the capacitive load at the emitter smaller, or the emitter resistor.
A ferrite bead on the base would help, too, but this is frowned upon
by audio types. Shifting the serial resonance frequency of the base circuit upwards
to a frequency where the transistor runs out of gain will help, too.
(decoupling for RF with a small & very close capacitor).
But if you are unlucky and have a fast transistor, this will only move
the problem to a higher frequency.
I have formulated this in
<http://www.hoffmann-hochfrequenz.de/...oren_Teil2.pdf>

part 1 is interesting, too, but the articles are about VHF oscillators.
Text is both german & english, intertwined.



Gerhard
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Old 19th March 2009, 11:23 PM   #16103
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Quote:
Originally posted by john curl
Fast source followers can oscillate, BUT the real cause of oscillation, Bob Cordell has not figured out yet.
Hi John.

Well then why don't you share your secret faux pas with us?

It did occur to me that the different caps they put on the rail might have had higher winding inductance than the original, but the conclusion would still be that the rail was designed without enough regard for damping.

Cheers,
Bob
 
Old 20th March 2009, 12:28 AM   #16104
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Originally posted by john curl


I find HP equipment hard to fix and easy to replace. You were just cheated, Grey. I bought 2 HP 3580's over the years and 2 HP 3581's and 1 HP 3563 without any problem. They should be cheap on E-bay, now.


I've had more good transactions on e-bay than bad. I haven't bought anything in a while, partly because I can't justify/afford the time to cruise the joint looking for things and partly because I'd prefer not to start a PayPal account...which seems to be required these days. (My wife has one. I grumble about it, but could use hers if push came to shove.)
Unless Demian pops up with a really groovy idea, I'll probably go back to my fallback plan, which is to track down an AP System 1. Maybe Nelson will sell me one of his. As I understand it, the man owns a small herd of the confounded things. The money, per se, doesn't bother me nearly so much as the idea that I will get so little use out of it. If I could fiddle every day or even every week, I'd feel better about buying something, but plunking a couple of grand down for a toy I only get to play with once per blue moon curdles the Scottish portions of my genetic makeup.
You can either have money or time, but not both at once. The seeming exception to this...isn't. Assuming you retire with enough money to live comfortably, your health blindsides you and gobbles both your time and your money. Currently, I'm in the best financial condition I've ever been in, but have virtually zero time. My health is (fingers crossed) fine. I fully anticipate that my circumstances will change once I retire--like The One Hoss Shay, I'll find that everything falls apart at once:

The poor old chaise in a heap or mound,
As if it had been to the mill and ground!
You see, of course, if you’re not a dunce,
How it went to pieces all at once, –
All at once, and nothing first, –
Just as bubbles do when they burst.

(Oliver Wendell Holmes)

Grey
 
Old 20th March 2009, 01:50 AM   #16105
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Quote:
Originally posted by Bob Cordell



Hi John,

If you increase the impedance at the VAS collector node, and do not change the gain crossover frequency, you increase the amount of low-frequency negative feedback, and decrease the dominant pole frequency, but do not decrease slew rate or increase TIM.

That is what you don't know.

Cheers,
Bob
Agreed Bob.

Even though the dominant pole might be at a very LF the amp can
still have extremely low HF (20kHz) distortion characteristics and
be characterised as 'fast'.

Now, what if we were to get such an amp and resistively load the
VAS so the pole shifts nearer to or above 20kHz. This should
bring the distortion flatter over the audio band and less tilted up
at high frequencies.

One thing I notice about the Dart Zeel and the Ayre MXR open loop
amps that both got rave class A reviews - is their distortion,
although comparatively high, is almost entirely independent of
frequency.

Very few amplifiers exhibit this characteristic.

cheers

Terry
 
Old 20th March 2009, 02:22 AM   #16106
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Another common element between the DartZeel and MX-R is that they both blew up in test. Maybe this means that in order to get a class A rating, the amp must blow up in test?

I found it quite entertaining to read the "my dog ate my homework" manufacturer's comments in both cases.
 
Old 20th March 2009, 02:24 AM   #16107
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Quote:
Originally posted by Terry Demol


One thing I notice about the Dart Zeel and the Ayre MXR open loop
amps that both got rave class A reviews - is their distortion,
although comparatively high, is almost entirely independent of
frequency.

Very few amplifiers exhibit this characteristic.


Indirectly related: For years I've disliked current sources. I had a gut sense that there were frequency-related compliance problems, just based on the sound. In many posts over the years, I have recommended against current sources; use a simple resistor if at all possible. And, no, differentials do not always require a CCS, regardless of what some say. Through trial and error, I had homed in on a current source that was least offensive and tended to use it in cases where I absolutely needed a CCS.
Imagine my gratitude and relief a year, perhaps two ago when someone (Walt Jung, I think) went and methodically tested a dozen CCS topologies against each other. Now, he was looking for something a little different, but he published graphs and--mirabile dictu--the ones that showed the flattest response regarding compliance or noise or whatever it was he was looking for, were the same ones that I had found to be best sounding. The flattest of them all was the one I'd decided was the best of the group, sound-wise.
He did no listening tests, just bench tests.
I'll have to see if I can dredge that up so as to refresh my memory exactly what it was he was testing for. My point being that sometimes "flat" whatever (distortion, noise, etc.) is better sounding than "lower" that then rises at one frequency extreme or the other.
Just another case where "better" specifications do not necessarily mean better sound. And resistors are pretty bloody flat when it comes to response.

Grey
 
Old 20th March 2009, 02:50 AM   #16108
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Gerhard, if you were available and I needed someone to troubleshoot a high frequency circuit. I would hire you.
 
Old 20th March 2009, 02:55 AM   #16109
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Quote:
Originally posted by GRollins



use a simple resistor if at all possible.
Grey
I do in my phono stage purely for "zen" reasons. I would never bother with the controls and rigor necessary to actually try and convince myself that there really was an audible difference. In casual listening I can never repeatably tell any differences in most things, including those some consider obvious.
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Old 20th March 2009, 02:59 AM   #16110
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Quote:
Originally posted by andy_c
Another common element between the DartZeel and MX-R is that they both blew up in test. Maybe this means that in order to get a class A rating, the amp must blow up in test?

I found it quite entertaining to read the "my dog ate my homework" manufacturer's comments in both cases.

Same thing happened when I visited a friend reviewing Quicksilver monoblocks for TAS.
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