John Curl's Blowtorch preamplifier part II - Page 30 - diyAudio
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Old 8th July 2009, 06:58 PM   #291
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Quote:
Originally posted by john curl
Rick, the best that we have found is high speed, high frequency linear open loop, low or no global feedback designs. Then, a little RF in the loop is not too bad. Kind of like designing a video amplifier.
IF it is an entirely open loop design, like the Ayre, it is relatively easy, because the high speed low open loop Z output stage, absorbs the RF coming in, just like an attenuator. If you use global feedback, then a very linear, high speed design is necessary. This is one place where a 'lead' cap in the feedback might be questionable. Think about it, everyone.

Hi John,

It is usually not going to be the case that the high-frequency open-loop output impedance of a non-feedback amplifier will be lower than that of a feedback amplifier.

While it may be true that the open-loop output impedance of a no FB amplifier is lower at audio frequencies, the usual Miller compensation will typically make the open loop output impedance of the feedback amplifier at RF just as low as the amplifier without NFB (maybe even arguably lower).

Cheers,
Bob
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Old 8th July 2009, 07:13 PM   #292
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Quote:
Originally posted by Bob Cordell



Hi John,

I agree. I have always avoided the temptation of using the lead capacitor in the feedback path for this very reason. I seem to recall someone else pointed this concern out a long time ago. It might have been Neville Theil.

Cheers,
Bob

Not only that. Leach states that lead compensation may be unreliable.

"[Lead compensation] is normally applied in the feedback network to correct for phase lag in the output stage. Because this lag is a function of the load impedance, lead compensation can be unreliable if the load impedance changes."
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Old 8th July 2009, 07:35 PM   #293
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Originally posted by PMA
The case is, that this is a Blowtorch thread, i.e. the best possible solution. You could improve RFI suppression considerably, and maybe get even better sonic result then you have ever obtained.

Any ideas as to how it may be done?
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Old 8th July 2009, 07:45 PM   #294
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Quote:
Originally posted by pooge



Not only that. Leach states that lead compensation may be unreliable.

"[Lead compensation] is normally applied in the feedback network to correct for phase lag in the output stage. Because this lag is a function of the load impedance, lead compensation can be unreliable if the load impedance changes."

...and it changes every time the gear is moved from the laboratory for it's ordinary duty.
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Old 8th July 2009, 08:03 PM   #295
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Quote:
Originally posted by Joshua_G



Any ideas as to how it may be done?
Not only ideas.
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Old 8th July 2009, 08:11 PM   #296
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Good question, Joshua. I have some idea, in hindsight how, IF I made the BLOWTORCH again, how I could reduce the RFI input. However, I doubt that I would add either the expense (metal vs Plastic power connector) or extensive internal shielding, because I have never found the need for it.
You know, when I added shielding, I was criticized for being wasteful. When I avoided shielding (to protect the audio quality, or to lower cost) I was scoffed at for not adding extra shielding.
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Old 8th July 2009, 08:18 PM   #297
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Originally posted by PMA


Not only ideas.

I'd love see your suggested solutions.
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Old 8th July 2009, 08:23 PM   #298
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You might see it later, in another thread. As you probably know, I not only theorize.
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Old 8th July 2009, 08:26 PM   #299
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Oh, going to criticize and run? Sounds like what they accuse me of.
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Old 8th July 2009, 08:33 PM   #300
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