Passive and active power supplies for phonos

What people are working on at the moment
simon
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#16

Post by simon »

I was close :D. I'll give it a whirl when my iron arrives.

Which reminds me of something James D once recommended, and Dave Dove liked. Doesn't always work apparently...

Image

I tried it on an Aikido headphone amp and thought it made a difference, but then forgot all about it. Must give it another go. Presumably it does a similar sort of thing, but on the primary rather than the secondary?
Will
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#17

Post by Will »

There is also the use of heater tx the primary is connected as the choke and a resistor-cap across the 6.3v
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izzy wizzy
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#18

Post by izzy wizzy »

Hi Simon,

Looking first at the HT supply in your first post, that looks a good place to start.
I actually also added 4 No. 1R 3.3uF RC sections which aren't shown; two per stage, one per channel.
This part got me a bit confused. If you mean you fed each stage/channel with these RC combos, then I would think something odd might happen but I'd have to check and look a bit harder. It would seem on first look, not to offer enough stage/channel isolation if I read you right.

If you look here
http://www.izzy-wizzy.com/audio/preampnew.html the links at the top show how the power supplies for my phono developed.

When I used voltage regulators, I put small RC combos after them to filter out the HF rubbish like this http://www.izzy-wizzy.com/audio/vw_heatcirc.html Where I started from is described here http://www.izzy-wizzy.com/audio/preamp.html

Some time ago, I believed in mucho regulation. If one was good, then more was better was my maxim in all things audio. However regulators are noisy things and inject a noise signature of their own into the audio stage. I asked myself, do I want to be doing that to each stage of my delicate audio circuit? So I started removing them but they have to be replaced by something of similar performance. I once modelled all this but it was in an old DOS sim so now can't show the results. I'll try to show what I mean about noise, impedance etc in LT SPICE but it might take a while.

When I meant each circuit would have to be slightly different for comparisons sake, it doesn't have to be the best of each as each has it's own sorta flavour. The one thing I'm not clear on is the build of the phono and how each stage is decoupled from one another and how each stage AC loop is configured. Looking at each stage, the anode resistor should be connected in AC terms to ground for that stage http://www.nutshellhifi.com/library/ETF.html - check the first pic. The B+ cap closes the AC loop and should be local to that stage. Now if you are using one reg for the amplifier then it could look something like this http://www.vacuumstate.com/schematics/fvp5a_s.gif where the B+ cap is replaced by the regulator.

As the reg has very low impedance at low frequencies, the stages don't need to be decoupled from one another as by brute force, the reg holds them steady. As the frequency climbs it does this to a lesser degree but that's another story.

For a passive supply it is normal to either daisy chain the 1st stage off the second like this http://www.hagtech.com/pdf/cornetoctal.pdf or this http://www.kandkaudio.com/images/phono. ... ematic.pdf; both being a good examples of a passive supply. Each stage therefore must have a B+ decoupling cap of sufficient value so the stage doesn't roll of too early going down in freq.

IMHO, over the long term, a passive is more satisfying but initially may be less impressive sounding. Everyone's taste is different though :-)

You ask about Wkz being comparable or not to pure film caps. IMHO, if they are metallised caps, the the Wkz IME are better. Film/foil might be different but they would also be huge and expensive. I sold all my large ANSARs on after deciding on BGs for power supplies. I use Wkz and VK throughout.

cheers,

Stephen
simon
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#19

Post by simon »

Hi Stephen,

Thanks for taking the time to reply. There's plenty there for me to read and digest; I'll have a think about things and no doubt come back with some rather simple questions.

I think you understand correctly about 4 caps - 1 each per channel and section. I'll sketch it tonight if I get chance - can't help it, I'm an Engineer and a picture tells a thousand words, as Will said. I modelled the 1R 3.3uF in PSUD as best I could, but it didn't look particularly good, but it's what I had so went with it. Interesting that you used a little larger R.

Simon.
simon
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#20

Post by simon »

Updated PSU attached, heaters left out for clarity. I'v had a play with PSUD and 4R upwards makes the plot look a lot better. I'm never quite sure how accurate PSUD is when messing around with so many/much current taps but the results look pretty conclusive...

My belief was that these 4 No. RC combinations were closing the AC loop. When you say:
izzy wizzy wrote:The B+ cap closes the AC loop and should be local to that stage.
do you mean that the cap should be physically next to the audio circuit? I had mine remote in the standalone PSU. Maybe they should be in the same box as the audio circuit?
izzy wizzy wrote:If you look here
http://www.izzy-wizzy.com/audio/preampnew.html the links at the top show how the power supplies for my phono developed.
I can see there's some toil gone into your preamp :D. It would be fascinating to hear it at one of our meets... The dc heater arrangement is interesting and something else for me to try one day. Obviously you prefer it to a reg supply, but how would you characterise it's sound/benefit?

Okay, here comes the simple question.
izzy wizzy wrote:Each stage therefore must have a B+ decoupling cap of sufficient value so the stage doesn't roll of too early going down in freq.
How do I work that out? I can play about with the few bits I have but it would be interesting to try to predict what I should be aiming for.

Cheers,
Simon.
Attachments
C3gAikido PSU rev B.jpg
C3gAikido PSU rev B.jpg (24.38 KiB) Viewed 4439 times
simon
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#21

Post by simon »

Thought I'd bring this up to date. Went over to Nick's a couple of weekends ago and he debugged the Steve Bench series reg. The principal problem was there wasn't enough voltage across the 6080 - adding a first cap before the choke sorted this out. We used 6uF at Nick's but I've got 20uF there as it's the smallest I had to hand. The circuit's attached, but there are also two additional 100R resistors, one on each cathode to balance the halves of the 6080. It's working properly now and hum is negligible. :D

What's interesting is that adding decoupling between the C3g and Aikido had no effect on the hum that I could hear, so the reg is doing it's job. All in all, I'm pretty pleased with it - it's a pretty compact and lightweight solution, and relatively cheap. Next to try is lots of L and C and see how that sounds. Unlikely to have that built by Owston though.
Attachments
HT reg 1.jpg
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