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Old 31st January 2010, 08:26 PM   #1
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Join Date: Jan 2010
Location: Germany
Default Symetrical out low Zout

Hello,

I would like to include a symetrical out to my preamp project. I know the easiest way is to use an output transformer driven from an catode follower. Unfortunately a reliable transformer is very expensive ( jensen transformer, 10K611M ).
So using tubes should be cheaper ans with less distorion but lost the benifit od galvanic decoupling. But how to do it?
Theoretical a long-tailed-pair phase inverter designed to high balanced output followed by a DC coupled cathode follower per phase should do the job.

Make this sense and is this design similar to an output transformer ( excluding the galvanic decoupling )

Thanks for your input an suggestions...

Best regards
Karsten
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Old 31st January 2010, 09:34 PM   #2
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Join Date: Apr 2004
Location: Monroe Township, NJ
Along the lines you're thinking, use an ECC99 as in the LTP. Don't forget the CCS in the splitter's tail. IMO, it's much easier to use a quality enhancement mode MOSFET as the DC coupled buffer. The ECC99 triode is quite "muscular" and will have no difficulty in feeding an IRFBC20. An ID of 15 mA. will get the O/P impedance under 100 Ω.

I agree that shipping "iron" from California to Deutschland makes little sense. However, I think that the transformer option is still viable. Sowter, England, much closer to home offers products specifically for this task. Check their site out.
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Old 1st February 2010, 07:33 AM   #3
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Default DC coupled MOSFET

Hey Eli,

Thanks a lot for your answer So I see the idea to use a LTP followed by an buffer seemed a good one.
Sorry, It's a little bit difficult for me to understand all these nice shortcuts like LTP/CCS ... But now I understand them, reading around the Forum. The CCS for LTP is discussed just in an other therad ..

Well, to use a MOSFET as DC out buffer as you proposed looks very smart for me. Please be so kind and describe it a lite bit more detailed how to wire it. The ECC99 is a great tip! Thanks a lot for that!

So a full puristic tube solution can also be to use two ECC82 valves. Two triodes wired as LTP and two as cathode follower. Zout will be in a range of 600 Ohms which is similar to transformer.

Additional thanks a lot for the link to Swoter!

Have a nice day..
Regards
Karsten
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Old 1st February 2010, 12:18 PM   #4
Gordy is offline Gordy  United Kingdom
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You need to buy the tubes, buy the power supply components, buy the heater components, buy the auxilliary components (capacitors, sockets, etc.)... then you realise that a transformer is not so expensive!
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Old 1st February 2010, 02:27 PM   #5
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Default Costs and feasibility

Hey Gordy,

well I already bought tubes, Power transformer, Power supply parts .... So the Project is in an ongoing state
Nonetheless I would like to have an option. The costs are not only the big issue: I also would like to know if my consideration to use a LPT with an buffer can provide me a sufficient symmetrical out.
Costs are simply to calculate:
Two tube for LPT and Buffer costs ~ 20 EUR
One tube plus an audio transformer will cost ~ 100 EUR

Best regards
Karsten
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Old 1st February 2010, 10:19 PM   #6
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Join Date: Apr 2004
Location: Monroe Township, NJ
Quote:
Originally Posted by karsten21 View Post
Hey Eli,

Thanks a lot for your answer So I see the idea to use a LTP followed by an buffer seemed a good one.
Sorry, It's a little bit difficult for me to understand all these nice shortcuts like LTP/CCS ... But now I understand them, reading around the Forum. The CCS for LTP is discussed just in an other therad ..

Well, to use a MOSFET as DC out buffer as you proposed looks very smart for me. Please be so kind and describe it a lite bit more detailed how to wire it. The ECC99 is a great tip! Thanks a lot for that!

So a full puristic tube solution can also be to use two ECC82 valves. Two triodes wired as LTP and two as cathode follower. Zout will be in a range of 600 Ohms which is similar to transformer.

Additional thanks a lot for the link to Swoter!

Have a nice day..
Regards
Karsten

Karsten,

Lesen Sie MOSFET Follies! Then, look at the phono preamp schematic I've uploaded. That design shows a small MOSFET DC coupled to a 12AX7/ECC83 section. You do the same thing with the big FET. Don't forget the gate stopper.

IMO, the 12AU7/ECC81 is a poor voltage amplifier, due to non-linearity. I would not use it in a LTP. OTOH, it's OK as a cathode follower.
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