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]

modify the original A. Loesch schematic ?

Hi,

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

http://www.fortunecity.com/rivendell/xentar/1179/projects/toccata/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]

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:

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

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]

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".

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)

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..

 

[SY]

The A.W. schematic is quite different IMO, why I have to look at it?

Because he does the first stage correctly. Your friend's proposal won't.

The change in source impedance from the first stage will upset any interstage passive RIAA, not just a split one.

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

"Dammit, I want SQUARE wheels for my car, but it has to ride smoothly!!!"

 

[inertial]

SY,

Ok, in short you are saying that my proposal modify is technically impossible, am I right?

P.S.: I am a newbie on tubes, no need of sarcasm!

Thanks,
Paolo

 

[SY]

I don't mean to be sarcastic, it's just my sense of humor. Mostly at your friend's expense.

Your proposal isn't impossible, it's just much more involved than your first cut at it. That's why I thought it would be a good thing to understand how Allen did it and what the issues are. I used a FET-tube cascode phono stage for many years and have only the deepest respect for someone who can do it properly.

 

[SY]

One other thing- when you convert to a cascode, the power supply rejection goes to about zero. That means the power supply may need to be reworked to make it as quiet as possible.

 

[inertial]

I don't mean to be sarcastic, it's just my sense of humor. Mostly at your friend's expense.

No problem kind SY, I like humor!
And about mine friend, it is a "human-case" ! (ironic)

Your proposal isn't impossible, it's just much more involved than your first cut at it. That's why I thought it would be a good thing to understand how Allen did it and what the issues are. I used a FET-tube cascode phono stage for many years and have only the deepest respect for someone who can do it properly. [/B]

Ok, I understand and respect your position.
Similar problem I have when newbies ask me " is it possible take this tweeter and add to mine speaker that is a modified-clone...."
No one born "expert".......
Back to my problem, I have this A.Loesch builded. And I hope we can save a lot of it.
If modifications are Possible, I am "open ears". When we will have understood exactly how much we have to "touch-soubstituite", we 'll be at half of the opera,no?
Can you please help we ?

Thanks,
Paolo

 

[inertial]

One other thing- when you convert to a cascode, the power supply rejection goes to about zero. That means the power supply may need to be reworked to make it as quiet as possible.

Of course. No problem here.

Cheers,
Paolo

 

[Alex Kitic]

PHONO MC

Hi to all!

Frankly, without reading everything that has been said in the above thread, I can only say that there is no reason why one cannot use tubes to build phono MC stages and obtain perfectly good noise levels.

I have personally designed and built several, and the one I currently use can be viewed on my website www.tubeaudio.8m.com

The phono section illustrated has been later change to a configuration where ECC83 is replaced by E180CC giving a little less gain, but a very dynamic sound (MC amplification levels + MM cartridge = CD player volume and incredible phono dynamics, particularly with excellent recordings/records).

The point here is to have the least possible number of stages, and adopt noise-free (as much as possible, sometimes selecting) tubes in the first stage (because the noise of the tube will be amplified in the later stages as well). You must also take care of the microphonic properties of the tube: usually high transconductance tubes that are low-noise tend to be microphonic... but that is a mechanical problem relatively easily solved in the building stages.

Noise has never been a problem in my designs: what I had to contend with was hum -- but with a little care in designing and building, as well as a very well filtered power supply, hum will be inaudible as well.

Regards,
Alex

 

[inertial]

Re: PHONO MC

Hi to all!

Frankly, without reading everything that has been said in the above thread, I can only say that there is no reason why one cannot use tubes to build phono MC stages and obtain perfectly good noise levels.

I have personally designed and built several, and the one I currently use can be viewed on my website www.tubeaudio.8m.com

The phono section illustrated has been later change to a configuration where ECC83 is replaced by E180CC giving a little less gain, but a very dynamic sound (MC amplification levels + MM cartridge = CD player volume and incredible phono dynamics, particularly with excellent recordings/records).

The point here is to have the least possible number of stages, and adopt noise-free (as much as possible, sometimes selecting) tubes in the first stage (because the noise of the tube will be amplified in the later stages as well). You must also take care of the microphonic properties of the tube: usually high transconductance tubes that are low-noise tend to be microphonic... but that is a mechanical problem relatively easily solved in the building stages.

Noise has never been a problem in my designs: what I had to contend with was hum -- but with a little care in designing and building, as well as a very well filtered power supply, hum will be inaudible as well.

Regards,
Alex

HI Alex,
WOW, nice preamp!! Chocke input PSU !
I have 0,25mV only that become half when loaded with the "correct"
resistor ( 5 ohm !)

Unfortunately I disagree about your statement about microphonic:
You can even to souspend the only raw triode at 1 Hz but it still be a real issue. Been there, done that . Ok I 'm obsessed by this aspect
but IMO people undervalutate this phenomena!

Anyway the important thing for me is to see a 100% tube MC preamp can be done ( with some limits of course)

Thanks for your schematic and experience, Alex!

Cheers,
Paolo

 

[Alex Kitic]

LINE STAGE GAIN

Hi, Paolo!

While I'm online, just another word or two:

Less than 0.5mV is not a problem, if your line stage has more than 20dB gain. In practice, as you might see by looking at the photos of my preamp carefully, the line stage tubes are not ECC82 but 5670. The gain is higher and thus a Supex Ruby works fine indeed with this preamp.

PS: you can always make a PS with solid state regulation (LM217) and achieve excellent filtration (I posted a similar design a long time ago at www.vt52.com - and it can still be seen there) but the sound is way better if you invest in chokes and caps

Regards,
Alex

 

[inertial]

Thanks for your suggestions, Alex!

To be honest I hope some kind tube-guru ( ironic,eh) here can make for me the "conversion" from the original A.Loesch schematic to the
Jfet-hybrid cascode version! ( with his intrisecal limits, of course)

Mine friend on his A. Loesch has 6 (six) separates mosfet-regulated PSU ( tube rectification).

Cheers,
Paolo