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Old 14th December 2007, 08:22 AM   #1
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Default Help designing Class-A, tube, transformer coupled, studio grade pre-amp

I'm wondering if anyone has advice or experience on the following subjects.

First, I'd like to point out that I'd like to try building an all tube, transformer coupled, Class-A pre-amp using NO CAPACITORS in the signal path. However, this doesn't mean I won't use them, if need be. I am willing to spend extra $$ on an additional intermediate transformer stage (if necessary) in order to eliminate any coupling caps that would normally be in place with a standard pre-amp design.

One of the biggest challenges is the output transformer interfacing with the last amplification stage. Jensen transformer company says that none of their transformers can be used as configured for a direct plate driven transformer (having the plate current run through it) without a coupling capacitor--which then would be a standard, more common design. I was hoping to replace the plate resistor for the final tube stage with a transformer, thereby eliminating an output coupling capacitor.

Regardless, I would still conisder a circuit design that DOES use caps. Does anyone also know of (or have a schematic for) a high-end tube pre-amp design (like UniversalAudio (LA610 or Solo610 ?) or a Hamptone tube preamp, etc.)?

Thanks very much for any feedback or suggestions.
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Old 14th December 2007, 08:32 AM   #2
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This is my current project. It is in norwegian, but you can look at the schematics. Not shure what u mean by "studio grade", this is for my home system.
The low rolls of a bit early, and the gain is a little bit high for a preamp but I hope to get by without to much troble.
There is an inductor in the filament supply and you do not need it. I think of it as an experiment.
http://www.hifisentralen.no/forum/in...c,17478.0.html
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Old 14th December 2007, 08:59 AM   #3
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Do some poking around the prodigy-pro forum (especially the Meta-Meta sticky), and i think you will find more information then you will know what to do with.
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Old 14th December 2007, 09:17 AM   #4
Gordy is offline Gordy  United Kingdom
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Default Re: Help designing Class-A, tube, transformer coupled, studio grade pre-amp

Quote:
Originally posted by gibsonripper
I was hoping to replace the plate resistor for the final tube stage with a transformer, thereby eliminating an output coupling capacitor.
I understand your sentiment, however a transformer in the position you describe simply relies upon the last power supply capacitor for the true signal path, and that is likely to be a so-so electrolytic. Perhaps better to design it para-feed as the capacitor involved will be smaller than the power supply electrolytic and can then be a decent quality item. Also, if you go para-feed then I understand that the plate resistor can be replaced by a constant current source hence very much reducing the influence of the power supply capacitor.

As an idea, if you still want to go single ended then consider trying Kevin's (KTA-hifi.net) DHT 26 line stage plus a transformer coupled input. Here you go: http://www.kta-hifi.net/projects/pre...26dhtpre-1.pdf


(I also second the recommendation to look over at Prodigy Pro)
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Old 14th December 2007, 12:27 PM   #5
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"Studio grade" means an output imedance < 50 Ohms, only with transformers you don't reach this (you're needing feedback).

Best regards
Andreas
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Old 14th December 2007, 02:32 PM   #6
JoshK is offline JoshK  Canada
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Take a look at the Raven preamp on nutshellhifi.com. Its a preamp designed by Lynn Olson and Gary Pimm. It fits your goals and I think you might even get close to your target Zout without NFB.
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Old 14th December 2007, 02:37 PM   #7
JoshK is offline JoshK  Canada
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link

schematic
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Old 14th December 2007, 05:14 PM   #8
Jeb-D. is offline Jeb-D.  United States
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Check out Edcor interstage transformers. Their resonably priced, made in the US and I've never heard a negative thing about them. They don't have any permissable DC rating listed on their products though.
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Old 14th December 2007, 10:02 PM   #9
kevinkr is offline kevinkr  United States
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Llundahl and Sowter both make interstage and plate to line transformers intended to carry plate current.

Jack Elliano over at Electra-Print will make you whatever you want once you know what it is.

The parafeed suggestion is a good one as well, but will require a single coupling cap.
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Old 6th September 2013, 02:35 AM   #10
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Just wanted to say thanks to all of you for your feedback--after many years, I hadn't logged into diyaudio. Shame on me. So better late than never!

I did finally build that tube pre-amp (in 2009). I used a 12AX7, 12AU7 for driver and a 12BH7. I ended up using Solen fast-caps. Still working perfectly today. Just your standard input and output transformers. Very straight forward. I've recorded vocals, drums, ambients, glass breaking, door slams (for effects) and so on.

Cheers!
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