Symetrical out low Zout

[karsten21]

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

[Eli Duttman]

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 I_D 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.

[karsten21]

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

[Gordy]

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!

[karsten21]

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

[Eli Duttman]

Originally Posted by karsten21

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.