Another transformerless mic preamp

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chrissugar

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Joined
Jun 4, 2004
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I was looking for some studio equipment recall sheets at Barry Rudolph's site and accidentaly discovered the schematic of a current product.
I saw before this product, but was never interested because it looks like cheap/bad gear trying to catch your attention with the look (tube in a window) personally I hate these kind of products, but 1300$ per channel and the good reviews it received made me curious.
If you want to look at the product go to the manufacturers site.

http://www.barryrudolph.com/recall/manuals/robbieschematic.pdf
http://www.barryrudolph.com/recall/manuals/robbieblock.pdf
http://www.barryrudolph.com/recall/manuals/robbie_manual.pdf

Transformerless, bipolars and tube and no chips.
What I find interesting is that diode across the capacitor in the gain circuit. Also the balanced out Jfet DI circuit.

I posted it here because it fits very well in the latest mic preamp topology discussions. What do you think?

chrissugar

P.S. Save the file, I have the feeling that it was uploaded accidentally and it will disapear soon.
 
[quote author="chrissugar"]I was looking for some studio equipment recall sheets at Barry Rudolph's site and accidentaly discovered the schematic of a current product.
I saw before this product, but was never interested because it looks like cheap/bad gear trying to catch your attention with the look (tube in a window) personally I hate these kind of products, but 1300$ per channel and the good reviews it received made me curious.
If you want to look at the product go to the manufacturers site.

http://www.barryrudolph.com/recall/manuals/robbieschematic.pdf
http://www.barryrudolph.com/recall/manuals/robbieblock.pdf
http://www.barryrudolph.com/recall/manuals/robbie_manual.pdf

Transformerless, bipolars and tube and no chips.
What I find interesting is that diode across the capacitor in the gain circuit. Also the balanced out Jfet DI circuit.

I posted it here because it fits very well in the latest mic preamp topology discussions. What do you think?

chrissugar

P.S. Save the file, I have the feeling that it was uploaded accidentally and it will disapear soon.[/quote]

Interesting, as in "that's a little different".

An important part of circuit design is selecting the right part for the job. In this case the low noise bipolar input transistors happen to be my personal favorites (2SB737). AFAIK these parts are no longer made but probably available in small quantities for the right price. Likewise discrete bipolar for output devices should be adequately low source impedance and work well. The use of a vacuum tube for the interior gain stage may be a marketing concession as that is about the only place a tube could be used without some obvious performance compromise.

Since the tube is inside the overall negative feedback loop it's sonic contribution should be reduced by that NF. While I am not very confident in predicting what this would sound like, my suspicion or guess is that at low to moderate gains it would be clean/neutral sounding, and at high gain it could take on some "tube" character depending upon actual loop gain margin (which I'm not competent to estimate).

I understand the arguments for completely balanced audio paths within signal blocks but remain apprehensive about not performing any common mode cancellation early around low level input stages that are subject to contamination by interference. This would add undesirable complexity to an all discrete design. When you rely upon a later differential stage to remove common mode noise it is important that both + and - paths be well matched in phase and amplitude response.

Regarding input LP filtering there seems to be plenty of input capacitance but the only series R or L, without the pad engaged, is the phantom blocking capacitors' ESL and ESR.

Probably a good sounding unit with perhaps a little extra "personality" at high gain settings.

JR
 
Different strokes....

I'd have used a current source load for the triode plates for lowest open-loop distortion, but to each his own.

The cascaded JFETs might work a bit better if the top one were of a higher pichoff voltage family, but perhaps he selects for that.
 
[quote author="Samuel Groner"]That's simply to prevent C10 from exploding when reverse-bias should happen.[/quote]

Yes, I know, but it is one of the rare circuits where it is implemented.

chrissugar
 
I like the industrial design better than the circuit design, though both are quite creative. John is probably spot on about the tube being a marketing concession, since it's being displayed so prominently. :wink:
 

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