need suggestion about MC pre-pre

[inertial]

Hi,

I have a strange idea: make a pre - preamplifier with a triode ( instead the normal step-up or solidstate stage).
The phono is an Arthur Loesch DIY clone.
Cartridge is a Linn Asaka ( 0,25 mV).
My target is a linear gain of about 3-5 times with the less noise as possible.
Thus I need a lower-mu triode, I believe ,with high transconductance, am I right?
The circuit will be a basic common cathode. Output impedence is unimportant, microphonics tubes has to be avoided of course.
With these carachteristic I have no idea of what tube I can considerate
to make a little experimentation.........

Every suggestion will be very appreciated!

Thanks,
Paolo

[Eli Duttman]

Sorry, it's a BAD idea. S/N ratio rules at these extremely low signal levels. No "hollow state" device, triode or pentode, has a good enough noise factor for this job.

IMO, the way to avoid both a SUT and "sand" sound is Allen Wright's JFET cascode. AW's operating conditions are such that the FET is "completely invisible".

[inertial]

Thanks Eli for frankly speaking!

You are right, S/N is the problem.
Just I am quite tolerant about noise, but do not like place a transistor in front of my all-triode electronics...
Is it not possible found a decent compromise ? ( apart the step-up transformer or ( maybe better) a different cartridges?

Cheers,
Paolo

[inertial]

Hi,

In order to the suggested compromise, I am looking at this:

http://www.fortunecity.com/rivendell...a/Toccata.html

It is interesting, no?
But I have a problem:
I have tried to e-mail to Mr.Thoersten Loesch, but the address is off.
So I have to ask to you, kind guys...

Mine friend, who is the owner of the turntable, phono, etc would be
add ONLY the jfet to the cathode of the 5842 without modifing other things.
ie he wants leave the second and third stage and PSU as like the original Arthur Loesch design: 5842 , 6922 , 6922. B+ = 150V
(if possible) because his phono is exactly so: the original A. Loesch schema.
I understand so little about electronics, but I have fear that the different B+ can be a problem , am I right?
What can you suggest to me?

Thanks, Paolo

[SY]

It will take somewhat more than just chunking a FET in there. Besides getting the operating points right, which will require quite a bit of first stage rework, this will also upset the RIAA because of the change in sourse impedance from the first stage. So we're talking about a fairly complete redesign effort.

I think that the easiest choice would be a good stepup transformer. I use one from Sowter and it works superbly well. A well-balanced input transformer and rewiring the turntable and interconnects to provide a balanced output will do wonders for noise.

[inertial]

Originally posted by SY
It will take somewhat more than just chunking a FET in there. Besides getting the operating points right, which will require quite a bit of first stage rework, this will also upset the RIAA because of the change in sourse impedance from the first stage. So we're talking about a fairly complete redesign effort.

thanks SY!
Please, can you explain me better, before I start to illuse my friend (he do not want absoluteli transformers )?
Are you saying that the different B+ change the WP of the Jfet so we have to "compensate"? Do you have the original A.Loesch schema or is it better I post it?

Paolo

[inertial]

Ok, this is the schematic mine friend have build:




Note the different B+ , only 150 V respect the "toccata" 250 V.
Why he has increased so high?

Cheers,
Paolo

[SY]

The source impedance of a triode stage with bypassed cathode (or in this case, a very small cathode resistor) is about equal to the plate resistance in parallel with the plate load resistor. The cathode resistance increases it a bit, but it's still considerably less than the plate load resistor. The source resistance of a cascode is the plate load resistor alone. Big difference. That will kill the accuracy of the RIAA.

The FET will need 5-7V or so across it to operate linearly. That means the tube's grid needs to be raised a few volts- and done with a VERY quiet supply. See Allen Wright's schematics for details.

A transformer brings so many advantages to the table that it's sheer irrationality to avoid it.

[audio-kraut]

A transformer brings so many advantages to the table that it's sheer irrationality to avoid it.

agreed. I have converted a couple of older MM studio phono preamps (logitek) to accommodate MC this way.

[inertial]

Originally posted by SY
The source impedance of a triode stage with bypassed cathode (or in this case, a very small cathode resistor) is about equal to the plate resistance in parallel with the plate load resistor. The cathode resistance increases it a bit, but it's still considerably less than the plate load resistor. The source resistance of a cascode is the plate load resistor alone. Big difference. That will kill the accuracy of the RIAA.

Ok ,I believe to understand. Split RIAA need "correction".

[i]
The FET will need 5-7V or so across it to operate linearly. That means the tube's grid needs to be raised a few volts- and done with a VERY quiet supply. See Allen Wright's schematics for details.[/B]

Excuse me, in short terms, what modify I have to make? just on the 5842, right?
(The A.W. schematic is quite different IMO, why I have to look at it?
Excuse I am confused)

[i]
A transformer brings so many advantages to the table that it's sheer irrationality to avoid it. [/B]

Sorry, I have tried to persuade it about a good transformer, but he is near 70 and a lot ostinate..