Balanced Fet phono?

[tubee]

I now have a tubed phono pre-amp, but am not completely satisfied by the sound. Very warm sounding, but also sometimes blurry around tones, and distorted compared to other sources due to a lesser harmonic distorsion, eg tuner and CD.
This could be because the phono is a single ended design (2 ECC83 halves with passive Riaa inbetween) The line amp is an 6N1P SRPP, the poweramp E88CC SRPP and cap coupled to two good old & big Hitachi TO-3 powerfets per channel.

All Power supplies are single and with a choke, only the fets have a balanced PS, so in all stages the second harmonic is emphasised, except in the last Fet-stage, wherein the uneven harmonics are amplified due to the balanced design. There is no overall feedback whatsover, only local feedback on some places, a decoupling cap over cathode resistor.

So now i have 3 "even" stages in my ampl. chain and 1 "uneven"

I am thinking of a balanced fet phono MC\MM phono stage to replace the tubed phono. This to combine "uneven harmonic" amplifier stages with "even harmonic stages" to reduce overall distorsion. Even harmonics are better to listen to, but it still is distorsion.

What do you here thinks of this?

[tubee]

Well, has somebody a thought on this?
(combine single-ended stages with balanced ones, to reduce overall distorsion in an amplifying chain)

[cicero32]

Dear friend,
this is the classic monotube preamp.
Some times ago there was a beautiful article on Audiophile written by Hiraga whrw you should understand how much difficult is build a phono pre with a single tube.
The distorsion is probably due to need to have maximun gain from tubes.Much gain mean much distorsion and consider tou throw gain with passive eq.Simply add another tube e retouch the gain in the stages.
You should have a fast confirmation of this removing the passive riaa eq and use a feedback riaa.
Ciao

cicero32

[tubee]

Ok thanks Cicero32. This is an other approuch to the theory. The designers of the phono stage had its reasons to choose a passive riaa. Will search out what reasons.

[cicero32]

Dear tubee,
If you want use passive riaa it's better have a coumpound stage and not a single tube.The first stage must have high gain, high acceptance and low noise.All request are conflicting! In your design you should have 30- 35 dB in first stage and you throw it away with passive Riaa (-30dB).Try the feedback riaa, or feedback tout court. If pre sound better this is reason of your muddy sound and you can consider what configuration you want.
cascode or SRPP - passive RIAA - cascode or SRPP.
ciao cicero

P.S. I apologize for my poor english linguage

[Audiofanatic]

Hi Tubee,


Why don't you try a Pearl stage.
This is one of the best Phono stage I've ever listen to for the $$$


Best regards,



Audiofanatic

[emw]

Look at the Borbelyaudio.com site.
Very very good balanced phono kit.
Best...EMW

[cicero32]

In my opinion Borbely sound bad, too much feedback.
The tubee preamp sound bad cause it's a wrong design.
You should split Riaa passive network with another stage.
(and correct phasing).

cicero32

[tubee]

Hello all

Nice to have so much reactions!

The design-goals of the phono was to build it as simple as possible. They wanted a srpp first, but choosed a single stage. The first stage, 1st halve of ECC83 (Old Philips tubes) amplifies about 35dB, then a cap + resistor and one cap for RIAA (thats all!) and then 2nd halve which does about 25-30 dB to amplify the riaa loss. ( i fitted a decoupling cap over both cathode resistors to get some extra dB's, i have a Benz Gold with moderate output. The designer advised a cap for first stage only, a cap for 2nd stage only with low output carts.
I think i fitted Elna cerafines, not sure.

The line stage is SRPP, as the first stage of the hybrid poweramp. This is the reason why i don't want another srpp for the phono. A common cathode would be better i think, if this is possible for a phonostage.
Another fact: i have no feedback at all, not in phono, pre stage nor hybrid amp. I could try a small feedback in the phono stage and see what happens. Also a Sovtek 12AX7 could give some extra, but tried Haltron's no improvement, so the Sovtek won't either i think.

[EC8010]

I think the problem with your existing phono stage is not that it is unbalanced or that it is valve or that it uses passive equalisation (I use all three of these things), the problem is the ECC83. ECC83 has high ra (typically about 80k or so) so it has to be included in the design of the passive RIAA. Trouble is, it forms a rather large proportion of the series resistance in the RIAA network but it varies. It is most likely that you have inaccurate RIAA and that is why it sounds lush.

It may also be that the RIAA stage is distorting due to inadequate HT voltage or poor gain structure, but without a diagram it's not possible to comment fully.