John Curl's Blowtorch preamplifier

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#1,941
I would like to remind everyone that the line amp(s) of the Levinson JC-2, the JC-80 (discussed here) and the CTC Blowtorch preamp are NOT op-amps, they are transconductance amps. This is important, and variations that use op amps or add caps in series with the audio path, are not consistent to the Blowtorch approach, which is the title of this thread.
 
#1,943
Hi lumanauw,

Thanks.

I'm the guy who asked you where to buy 2SK389/2SJ109 last month. Remember ? 🙂

Lucky me, I found the last 4 sets. 2 sets will be JC3 clone. 😎

About the design, I'll be happy if you help me optimizing this poor version of BT !

You can visit me anytime you want at Jl.Junjunan Dalam III/4, but please call me before at 0818206039.

See you !!
 
#1,945
Toshiba duals

Early this week I contacted a North American distributor that still has the 2SK389BL / 2SJ109BL in stock (a few hundreds of each)
But the good news stops here: the downturn is the price, about 11$ each when bought in quantities of 20 or even in multiples of 20. Apparently that guy knows what he's selling... 😉

This calls for small group buy, if someone volunteers to organize it. (as we have a project deadline at work and imagine... lately we've been working even on Saturdays :bawling: )
I wouldn't mind getting just a few of each.

If it sounds like a good plan I will provide the contact info to anyone willing to undertake this effort...
 
#1,946
john curl said:
I would like to remind everyone that the line amp(s) of the Levinson JC-2, the JC-80 (discussed here) and the CTC Blowtorch preamp are NOT op-amps, they are transconductance amps. This is important, and variations that use op amps or add caps in series with the audio path, are not consistent to the Blowtorch approach, which is the title of this thread.
Click to expand...


Right. The output 2N4401/2N4403 (Levinson JC-2 line stage) acts as current sources to each other. Input voltage is converted to output current. You could get output voltage over a resistor to ground. But instead voltage is developed over the feedback resistor back to the negative input. So the passives around this transconductance (op)amp are connected as they would around e.g. a NE5532 conventional opamp.

RK
 
#1,949
There are four feedback dividers in the input stage and one feedback through coupling resistor between its + and - side. Do not mystify people that BT has no feedback. There is nothing like no feedback in amplifiers. If so, Blowtorch would have some 20% distortion. You taught me what is opamp, I can teach you what is feedback.
 
#1,951
The semantics of what does and what does not constitute negative feedback have been done to death in the past. If you want to go back into that for the hundredth time, please dredge up one of those threads. Don't waste twenty or thirty pages of this one going over the same ground.
By and large, feedback that is unavoidable (e.g. a follower) doesn't count. Only explicit feedback loops put there by the designer count.

Grey
 
#1,952
PMA said:
There are four feedback dividers in the input stage and one feedback through coupling resistor between its + and - side. Do not mystify people that BT has no feedback. There is nothing like no feedback in amplifiers. If so, Blowtorch would have some 20% distortion.
Click to expand...

It logically follows that if there is nothing like no feedback in
amplifiers, then you cannot cite an example of an amplifier
without feedback.

I suppose the argument runs that feedback occurs as degeneration
and as loop feedback. Loop feedback encompasses one or more
gain stages.

Even a bare transistor has some internal degeneration as Emitter
or Source resistance, and you can interpret its gain dependency
vs Vds or Vce and the values of Cdg or Ccb as loop feedback.

If you want to be precise in describing the feedback used in
a circuit, you must explicitly state all the instances of degeneration
and loops. It will get quite complicated and probably will not
bring much clarity to the table.

Does anybody here have any good ideas on a coherent way to
simply describe these things?

😎
 
#1,954
Nelson Pass said:
Does anybody here have any good ideas on a coherent way to
simply describe these things?
Click to expand...

How 'bout something like this?

Global Feedback: A feedback loop which encompasses two or more devices or stages and extends from the circuit's output to its input.

Local Feedback: A feedback loop which encompasses one or more device(s) or stage(s) in a circuit which has one or more device(s) or stage(s) than is encompassed by the local feedback loop.

Degenerative Feedback: External resistance applied to a device's cathode, emitter, or source.

se
 
#1,956
For marketing purposes, manufacturers want to be able to use
the word NO FEEDBACK. I don't blame them, this being a
competitive arena. I doubt that Charles Hansen would be
eager to use the word DEGENERATIVE in his product description.

😎

Correct me if I'm wrong, Charles...
 
#1,957
Nelson Pass said:
For marketing purposes, manufacturers want to be able to use
the word NO FEEDBACK. I don't blame them, this being a
competitive arena. I doubt that Charles Hansen would be
eager to use the word DEGENERATIVE in his product description.
Click to expand...

He wouldn't have to use the word "DEGENERATIVE." He'd just have to say "NO GLOBAL FEEDBACK" and/or "NO LOCAL FEEDBACK" if it's applicable. To make the blanket statement "NO FEEDBACK" when the output stage relies heavily on it, is being rather disingenuous in my opinion.

se
 
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