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Preamp Power Supply Design Input (6SN7 SRPP)
Preamp Power Supply Design Input (6SN7 SRPP)
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Old 17th April 2018, 08:36 AM   #1
blueglow is offline blueglow  United States
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Default Preamp Power Supply Design Input (6SN7 SRPP)

Hi everyone,
I'm looking for some thoughts and ideas from the collective minds here. I've been working on a 6SN7 tube preamp design for little bit now and have it performing very well using a HV bench supply.
Where I've found myself deliberating so much is on the PS design as I'd like a really quite preamp but trying to keep it somewhat simple at the same time.

First a few things about the preamp to set context, not that I'm doing anything too off the beaten path:
- uses 4 6SN7 tubes (2 per channel)
- has cathode follower input buffer
- then SRPP stage
- then output buffer stage
- need to float the heater voltage for SRPP stage (or all)
- need 250-300v B+ (would like to split rail)

One big factor is that I have quite a few 325-0-325v@70ma, 5v@3A, 6.3v CT@2.25A transformers at hand. While a bit high on the HV, it would be great to use since I have them. I don't mind burning off a little HV as heat :-)

The transformers were leading me down a simple tube rectified path but I've also been pondering more complex solid state ideas such as UF style diodes, all poly cap design, buddha snubbers, etc. Guessing I'm really pondering the age old debate of SS vs Tube for this amp.

I'd love to get your thoughts and input and/or suggested designs/schematics you have had success with. Also, if I went with these transformers, any wisdom on dropping the voltage a bit other than traditional power resistors in the chain and where is best to place them in the PS design?

Thanks in advance for reading and your thoughts.
Mark
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Old 17th April 2018, 01:55 PM   #2
kodabmx is offline kodabmx  Canada
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The 6SN7 has a -100V+200V HK limit, no need to elevate heaters.
If you use a choke input instead of a cap input your voltage should be good to go using the transformer you have.


I think you can put the choke in between centre tap and ground to use one choke for both rails. Correct me if I'm wrong.


Personally I use a 12V SMPS to power heaters and a HV boost converter for the B+.
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Old 17th April 2018, 02:16 PM   #3
Lingwendil is offline Lingwendil  United States
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Preamp Power Supply Design Input (6SN7 SRPP)
With the transformers you have on hand the extra voltage won't hurt at all, and if anything will increase gain and linearity. Choke input isn't my style but it should be pretty easy to knock something up in PSUD2 using easily available parts. Use a fancy tube rectifier if you must, at such currents their downsides are negligible. Otherwise I would look to a simple Mosfet ripple filter, with a decent size cap from gate to ground to give you a nice soft start. UF4007 diodes for rectification, nice and cheap, while still providing formidable performance rivaling some of the more costly options.

Personally, I would elevate the heaters, just to be safe. It'll give you a little better noise rejection on the lower cathode of your SRPP stage anyway, and only costs a few resistors and a cap to make the reference (you need a bleeder in your PSU anyway, so it can to double duty as one) and then a couple across the heater winding. Easy peasy.


I try not to nitpick or get too critical of other people's designs, but I find an SRPP inside a buffer sandwich a bit odd for a linestage. I'm not a big fan of SRPP (due to their want of a specific load to perform their best) but it shouldn't really need a buffer on either side for most same loads. But, I'm not trying to derail your thread! Do your thing man. Post some diagrams and pics. Always neat to see different builds.
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Last edited by Lingwendil; 17th April 2018 at 02:22 PM.
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Old 17th April 2018, 02:46 PM   #4
popilin is offline popilin  Argentina
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Quote:
Originally Posted by blueglow View Post
I'm looking for some thoughts and ideas from the collective minds here. <snip>
SRPP is the best topology IMHO, it sounds really transparent, I use to use about 350V with an ECC82, do not forget that distortion decreases with an increase of +B.

Elevating the heaters to about +B/4 is always a good habit.

Poor PSRR demands a very clean +B, I always use a regulated PSU.
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Old 17th April 2018, 03:02 PM   #5
blueglow is offline blueglow  United States
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Got an example of the PS schematic you use w/ SRPP?
mark
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Old 17th April 2018, 03:44 PM   #6
popilin is offline popilin  Argentina
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Now I use shunt regulators, but mine is too complicated, Salas HV shunt regulator should be OK.

A long ago I did use this
ScreenShot001.PNG

Valve rectification with EZ80, D3 and D4 are only for the simulation, I did use a noisy LM385 1.2 (reverse the polarity!), almost any power MOSFET should be OK, I did use STP6N60 from TV PSUs.

R9 is a trimpot of about 1Kohm.

This circuit was intended to replace a TL783C, unobtainable here at that time.

All Op Amp sucks, so use your favourite one.
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Last edited by popilin; 17th April 2018 at 03:48 PM.
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Old 17th April 2018, 04:30 PM   #7
popilin is offline popilin  Argentina
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As far as I know, unbypassed SRPP has a PSRR of about 6dB.

Yet another ECC82/12AU7 Line Preamp
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Old 17th April 2018, 04:33 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by blueglow View Post
I have quite a few 325-0-325v@70ma, 5v@3A, 6.3v CT@2.25A transformers at hand.
While a bit high on the HV, it would be great to use since I have them.
You could also use the 5V winding to buck the primary down by 5%.
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Old 17th April 2018, 05:06 PM   #9
Lingwendil is offline Lingwendil  United States
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Preamp Power Supply Design Input (6SN7 SRPP)
Quote:
Originally Posted by popilin View Post
SRPP is the best topology IMHO, it sounds really transparent, I use to use about 350V with an ECC82, do not forget that distortion decreases with an increase of +B.

Elevating the heaters to about +B/4 is always a good habit.

Poor PSRR demands a very clean +B, I always use a regulated PSU.
A totem pole, mu follower, or a beta follower (see Morgan Jones for that one) give lower distortion into most of the loads you see in audio when compared to the SRPP, and in my own testing, give much more favorable harmonics than SRPP when set up with the same tubes, and operating points. I'm not overly fond of SRPP with the better alternatives that are just as elegant and simple. People love SRPP either way, and just plain like using it. Nothing wrong there.

(Climbing down off my soapbox)

I really like lifting the supply for the heater for these circuits, it can often help with noise, and any free lunch method of noise reduction is fine by me. Less heater-to-cathode stress also gives me a warm and fuzzy too.

For those that aren't allergic to solid state, a mosfet ripple filter would be perfect for the supply voltage. Since the whole thing will not vary in current draw that much it would be a good place to use a stack of zeners or VR tubes as a gate reference, and a small heatsink will be more than adequate to allow it to eat some voltage. Run the reference voltage to the gate via a largish resistor, and run a cap from gate to ground for soft start. It will be stabilized, but open-loop so not truly regulated, but for a circuit like this it's still massively overkill, and will be dead silent.

Hell, even a pair of 0D3 VR tubes per channel on their own would probably work well, depending on total draw. Feed them with a nice current source, and it should be plenty quiet.
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Last edited by Lingwendil; 17th April 2018 at 05:10 PM.
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Old 17th April 2018, 05:13 PM   #10
blueglow is offline blueglow  United States
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Hadn't thought of that but you are right, good idea.

Quote:
Originally Posted by rayma View Post
You could also use the 5V winding to buck the primary down by 5%.
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