freaky new phonostage

[Vinylsavor]

Hi Kevin,

I used Lundahl chokes and transformers. They are very well insulated and rated for 2kV coils to core and 4kV coil to coil. And that is conservatively rated. No issue with insulation problems there at all! Such a phonostage is already in operation almost 10 years

Best regards

Thomas

[mashaffer]

Interesting. I have often wondered why inductors weren't used in phono preamps more often. I have a couple of questions. Why did you choose to use step down IST? Are the ISTs just providing the coupling function or is their reactance a part of the filtering functions.

[chris661]

Lovely and simple design, though I expect the PSU would be something to behold. I'm looking for a phono pre-amp, but please don't let me build this one, 1200v is far too many for someone as inexperienced as me.

[Vinylsavor]

Hi!

The thread title is a bit misleading now. 'nichoch46' resurrected this thread from 2002.

Originally Posted by mashaffer

Interesting. I have often wondered why inductors weren't used in phono preamps more often. I have a couple of questions. Why did you choose to use step down IST? Are the ISTs just providing the coupling function or is their reactance a part of the filtering functions.

The first IT drives the 600 Ohm LCR RIAA. The second transformer provides a low Zout.

All the RIAA is done in the LCR network.

Best regards

Thomas

[Vinylsavor]

Hi!

Originally Posted by chris661

1200v is far too many for someone as inexperienced as me.

I absolutely agree. This design is not recommended for inexperienced builders. The same principle can be implemented with other tubes with minor adaptions. I've built similar phono stages with EC8020, WE437A and also more cheaply available tubes like 6AM4. All those operate around 200V B+

best regards

Thomas

[kevinkr]

Originally Posted by Vinylsavor

Hi Kevin,

I used Lundahl chokes and transformers. They are very well insulated and rated for 2kV coils to core and 4kV coil to coil. And that is conservatively rated. No issue with insulation problems there at all! Such a phonostage is already in operation almost 10 years

Best regards

Thomas

This is good, I put much more credence in the fact that you have run one of these for nearly 10yrs without problems than in the voltage breakdown specifications because my experience with high voltage apps in semiconductor ATE and elsewhere has taught me to be very suspicious. We had transformers that were rated to 4kV core to winding and winding to winding that failed fairly quickly due to continuous electro-static stress, and in applications where the supply slewed rapidly insulation failures occurred below 1.2kV. I suspect moderate hours of usage and low dv/dt in this application probably reduce the stress on the insulation. I know Lundahl makes good transformers, and I use them in some of my own designs.

Still IMHO, a very cool and original design..

[Vinylsavor]

Hi Kevin,

Per Lundahl made a presentation at last years ETF and specifically talked about the insulation measures they take. I can assure you their rating is very conservative. That's the least I'd woory about. Special care needs to be taken in the wiring and socket connections of such a design. I drilled out the neighbour pins besides the plates of the sockets of the 6HS5s to provide more creepage distances there.

I did implement a differential version of this design which I even ran at up to 2kV.

best regards

Thomas

[mashaffer]

Thanks Thomas. I am also glad to here that other more pedestrian tubes are possible. I may try one of these when funds are more available but my first one will have to be RC passive.

[nigelwright7557]

Originally Posted by planet10

Thomas,

Bravo... that circuit will make people think for awhile (me included). 1250V is beyond my self-imposed B+ safety limit (subject to change as i get more practise).

dave

I remember my first go at making a valve pre amp.
Of course I switched it off and grabbed it and got a shock.

So the next time i got smart and discharged the capacitor first and touched it and got a shock again, I had forgot to switch it off !

I now stick to sub 200 volts systems so hopefully i will live to a ripe old age.

[Vinylsavor]

Hi!

Originally Posted by mashaffer

I may try one of these when funds are more available but my first one will have to be RC passive.

A while back I shared a design of a full function preamp with RC RIAA phonosection on AA. I call it the Octal preamp. It uses 6SL7/6N7/6SN7 tubes.
If there is interest, I'll open a thread about it and post the schematics

Best regards

Thomas