Re: Heater Supply for SRPP / Mu Stage

Mike D wrote:

> Patrick,
>
> I just looked at your "10 tube Preamp" on your website. I noticed both mu
> stage and SRPP. That's interesting to see both topologies in the same
> schematic. Usually a designer is in one camp or the other.

Well I don't see why one has to be one eyed in one's attitude to design.

I will soon include the upgraded form of this amp,
with a 2SK369 j-fet in cascode with a 6EJ7 in triode,
for MC capable front end, and then with mu follower stages, rather than SRPP.

SRPP usually refers to a series arrangement with the top Rk nearly equal to
the
bottom Rk, then as the top Rk is increased, the item approaches a mu
follower,
since gain of the bottom tube goes up to equal the U of the tube.
Some folks use a fet CCS between the tubes.
I have tried with say 10k between the top and bottom tubes,
so its more like a "bootstrapped follower", a more correct and old fashioned
term.

> I've read the
> TubeCad article "SRPP Deconstructed" and the Alan Kimmel article "Mu
> Stage Philosophy" and they both look pretty good to me in theory anyway.
> I've never built or even heard anything with either topology, but I can
> see the objective advantages.

Low output impedance, and low thd.

>
>
> Subjectively, as always, opinions are all over the place with some folks
> claiming neither of these topologies sound good and others saying SRPP is
> best and others Mu stage. Well I certainly don't have the "golden ears",
> I can barely tell the difference between a Telefunken 12AX7 and a Sovtek
> and this is supposed to be a "night and day" thing.

I used to think that until I tried 3 different types of 6CG7 in
a line preamp in an otherwise blameless system.
Telefunken was best, most accurate and dynamic, clear vocals,
then Mullard, sweet vocals, slightly vague bass, easy treble,
and EH unfortunately was poorer in all departments.
4 of us came to the same conclusion in an afternoon of listening,
with the same music and levels.
Its probably more obvious where there are no feedback loops used.

>
> So I have to go with more of an objective approach. Which one has lower
> output impedance?

The bootstrapped follower using twin triodes is my fav,
because the Ro is low, and thd about 0.1% at 10 vrms output,
instead of much higher thd you get from plain R loading.
The top tube can be biased from a fixed +150v, with a 1.5M,
and its grid cap coupled to the bottom tube anode which has say
a 5k to 10k R to the top tube cathode.
I like the sound I hear, and thd which measures very low at ordinary levels.


> Which one has lower distortion or a more favorable
> distortion spectrum? Which one has better noise figures? The reason I ask
> is I'm considering a Preamp project and I have lots of 12AX7s and 12AT7s
> (I have built several guitar amps). The SRPP and/or Mu stage looks like a
> good way to get better performance out of these rather pedestrian tubes.

As it is, my 10 tube pre has low enogh noise to suit cartridge inputs
where the rated signal from the cart is at least 1.5mV.

Best low noise results with 12AX7 come from careful selection of tubes.
The MM amp should have noise lower than the vinyl track noise
from a quiet unmodulated groove.

For MC carts, like my Denon 103, with only 0.3 mV output,
the 12AX7 isn't quiet enough, and a step up tranny or
a fet input stage is needed.
I have a couple of 6C45pi, which I ain't tried yet.

>
> My present preamp is a stock Dynaco PAS3 and I feel it is the weak link
> in my system. My power amp is a modified Mullard 5-20 topology using
> Dynaco transformers, no complaints there.

Feel free to try any of the schematics at my webpages.
I think 6CG7 throughout for a driver amp are superior to
the original EF86 and 12AX7 of the 520.

> Thanks for your reply.
>
> Regards,
> Mike D.

Patrick Turner.