John Curl's Blowtorch preamplifier

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anatech

anatech

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Hi John,
. I could NOT understand how a preamp with gain could sound better than a 20 turn quality dual wirewound pot
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I imagine there are times when BJT differential pairs must be driven with a low impedance source. That is something a control can not give you. Then it is simply the least of the less than perfect choices.

Did you try this with one amp or many? Were they tube, BJT or J-FET input types? I am assuming the possible inductance of that pot was negligible (have to ask ;) ).

-Chris
 
PMA

PMA

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anatech said:
Hi John,

I imagine there are times when BJT differential pairs must be driven with a low impedance source.
-Chris
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Exactly!! In my experience, excellent active preamp always sounds better than any passive, just for the reason you mentioned.

Only in case the passive divider is LOW IMPEDANCE, the resulting sound is neutral. Then you need buffered output to drive it.
 
rdf

rdf

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Unless the active preamp's output stage is fronted by BJT differential pairs? As long as a potentiometer is in the circuit at some point a 'passive' drives an active stage. To meet the criteria appears to require a low Zout input buffer > low impedance pot > output buffer.
 
anatech

anatech

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Hi John,
This is not important. I use fet input stages
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Thanks, that answers that question.

I wonder. I don't like the sound of complimentary differential pairs. I have a Luxman C-05 that was a disappointment to me. My thoughts turn to repeating your experiment with a straight J-FET diff pair type circuit. The complimentary types seem to sound a touch harsh, but not as bad as a BJT equivalent.

Hi Pavel,
I'm glad to see I'm not the only one who has noticed this. :)

-Chris
 
john curl

john curl

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Please good people: Those of you who do NOT like or appreciate complementary differential fet input designs should refrain from considering this thread or any effort by Charles Hansen as potentially useful to you, since that is what we make, and I have done so for more than 1/3 of a century. My experience will not be useful to you.
 
anatech

anatech

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Hi John,
Sorry. Accept my apologies then. In no way did I intend to attack your products. I can only comment from experience (personally, over 35 years).

My own experiments lead me to believe that high impedance sources work better with tube and FET inputs and not BJT types. I'll leave it to that.

Do what you want. Please don't waste time in asking me to do it.
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No sweat. Since you had experimented I thought you might have been interested in this possible variable. A simple "no thank you" would have sufficed.

My experience will not be useful to you.
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Actually, it is. Even if I don't agree with you all the time.

You know what John? I learned a long time ago that I should listen to people. Even those who don't know as much because there are those experiences and tidbits of information that I may not know. You have far more experience than I do, so I do listen to what you have to say. That does not mean I accept everything, it does mean I may rethink things though.

-Chris
 
john curl

john curl

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Are you still confused? I am. I haven't use a complementary differential BIPOLAR TRANSISTOR input since 1972. I USED the complementary differential bipolar input for 4 years, from 1968 to 1972. In 1972, I SWITCHED over to complementary differential JFET inputs, except when it is better just to use a simple differential JFET input. Sometimes it is slightly better, but not usually. Charles does the same. Is this really a problem?
Now the question is: What is the characteristic difference between a complementary differential JFET input and a simple diff JFET input?
 
V

Vogue

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john curl said:
Rdf, your input is right on. We don't really have much problem with the circuitry itself, BUT we do have a big problem with the Shallco switches and TKD pots as they are not well shielded and are very large in size.
Now why do we use these, rather than some teeny-tiny pot or switch, or relays, or solid state switches on the board itself?
Over the decades, and even today, I have used relays with cheaper products, and we board mount virtually everything that we can. Same designer, layout expert, and tweaker, (3 different persons). It just isn't as good as Shallco switches, but it's OK.
When we use these big, open frame switches, we get the best switching that we know about, but they are virtually perfect receiving antennas for RFI. That is why we try to make a sealed, thick walled aluminum enclosure. We don't like to use iron. Our major tweaker, Bob Crump, would never allow such a thing,
This is our choice, and we have found it to be a good one.
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Hi John,

"Sealed, thick walled aluminum enclosure". Is the sealed part very important here ... thick walled i understand, and of cause the walls should be well connected - but sealed!.

Is this why you use teflon for the RCA Jack panel in the beck (at least it looks like teflon).

Vogue
 
anatech

anatech

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Hi John,
Are you still confused?
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Nope. I answered and commented in a general sense. Nothing was actually directed at you or your design choices.
I am.
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Sorry, do my comments above help clarify?

I SWITCHED over to complementary differential JFET inputs, except when it is better just to use a simple differential JFET input. Sometimes it is slightly better, but not usually. Charles does the same. Is this really a problem?
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No. Those are your design choices and are completely valid. I am simply curious about your findings and how they may have differed when the input stage was a single J-FET differential type.

Now the question is: What is the characteristic difference between a complementary differential JFET input and a simple diff JFET input?
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Yes!
Exactly. I hear a difference. Whether the reason for the difference is known or not is another question. You may not hear the difference either. This may depend a great deal on the equipment you use in your systems.

Basically, the question popped into my head once you mentioned the bit about the control alone being inferior to an active stage. I agree with you on that and am simply looking for an answer. I have noticed this often. Most members here know I am not a fan of "passive preamps". I even strongly dislike the term.

-Chris
 
john curl

john curl

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Joined 2003
I can't tell YOU why you hear a difference. I designed BOTH topologies in the original Levinson JC-2 pre amp in 1973. Just LOOK at the schematic.
Usually, a single differential input can be slightly quieter for a given open loop capacitance, BUT the overall distortion is about twice as much and the dynamic input current available (important in power amps), is much less.
I know what I heard, your hearing may vary.
 
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