John Curl's Blowtorch preamplifier

[SY]

John indicated that it was a plain vanilla servo integrator with a one second time constant. The "trick" is to trim the circuit up to be inherently stable, then connect the servo and let it just do a bit of cleanup when things start moving around.

[forr]

QUOTED :

"the Teflon boards were mandatory because of a full open loop running, and the Line amplifier is (only ?) 350 Khz bandpass..."

A curious justification for using an expensive material :
current PCB's handle much more than 350 Khz without problems.

~~~~~ Forr

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[Panelhead]

The front panel mount switches would benefit from the use of shaft extenders to move them to the rear of the chassis. That way the amount of wiring could be minimized.
But real nice chassis. If anyone a an unused chassis from one of these, I would GLADLY pay the postage.


George

[Ultima Thule]

or why not relays...?

Cheers Michael

[grataku]

It looks to me like the 'blotorch' operation needed a fourth guy, one to tell the machine shop guy to stop the mill instead of cutting right through the real panel. I can hear the "Oh sh-t! How many did you make like that?" all the way from cali.

A fifth guy would have been helpful to come up with a better wiring scheme. Although from the looks of it it seems as though the lack of plan was the plan.

I like the idea of the hands-off servo that would interfere minimally with the audio signal, the problem I experienced with servos is when things do start to misbehave they tend to sweep the dirt under the rug instead of doing real cleanup.

All in all very "DIY"!

[forr]

QUOTED

"The Blowtorch is all open loop and does look very simple.
I'd like to know the servo arrangement."
"They always are a point of contention."

"John indicated that it was a plain vanilla servo integrator with a one second time constant. The "trick" is to trim the circuit up to be inherently stable, then connect the servo and let it just do a bit of cleanup when things start moving around."

A point of contention : a DC servo is quite the contrary of an open loop, it's the highest feedback you can find.

~~~~~~ Forr

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[Elso Kwak]

Originally posted by forr
QUOTED

"The Blowtorch is all open loop and does look very simple.
I'd like to know the servo arrangement."
"They always are a point of contention."

"John indicated that it was a plain vanilla servo integrator with a one second time constant. The "trick" is to trim the circuit up to be inherently stable, then connect the servo and let it just do a bit of cleanup when things start moving around."

A point of contention : a DC servo is quite the contrary of an open loop, it's the highest feedback you can find.

~~~~~~ Forr

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The trick is to make the circuit DC-stable and omit the servo LOL

[john curl]

Well folks, I saw your comments. It is unfortunate that you criticize without understanding my reasons for using servos, etc. You have the right to make something 'better' but I have not seen it on diyaudio at any time.

[SY]

Hi, John, glad the forum issue is resolved.

Thanks for giving me the circuit info. Can you say anything about the power supply?

[forr]

John Curl
"Well folks, I saw your comments. It is unfortunate that you criticize without understanding my reasons for using servos, etc. You have the right to make something 'better' but I have not seen it on diyaudio at any time."

It is very easy to think that there is no "better". Reasons of using servos are never been mysterious : they are intended to add gain loop at DC. Below are texts extracted from thread "To use DC servo or not?" which I have written about my knowledge and thinking about servos. They may not entirely apply to the Blowtorch preamp.

Post #19
Is servo or not servo is the real problem ?
DC feedback is to maintain the DC output of the amplifier at 0V. Two mechanisms affect this DC : the DC offset at the output of the input differential pair and the DC offset due the the following stages of the amplifier. To get the lowest and most stable distorsion from an input differential pair, it should always work with strictlly equal operating points currents in both its devices. Any DC difference due to DC feedback will affect this operating point. So capacitor or servo feedback do not make great changes. A capacitor is much easier, a bipolar type is recommmanded for ultimate performances.
A more logical way to control the offset at the output of the amplifier is a servo which will act after the input stage. See Perrot's patent US 5 635 874.
Another way would be to connect the input stage to the followingstages only at AC : an all new scheme bringing back coupling capacitors.

Post #24
I think the complementary differential input is fundamentally a wrong concept because, among other things, each input differential creates its own offset at its output and the two offsets of the upper and lower differentials can never be fully equal in value. Particularly in this case, a servo fed back to the Vas à la Perrot should do better than one fed back to the input.

Threads URLs :
To use DC servo or not?[/url]
To use DC servo or not?[/url]

~~~~~~ Forr

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