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Old 15th May 2011, 02:49 PM   #1
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Default Advantages of separate PS for input and output stages

Are there advantages to using separate transformers and rectifiers for the input/driver and output stages in a tube amp? If so, what are they? Thanks
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Old 15th May 2011, 03:23 PM   #2
DF96 is offline DF96  England
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The main effect would be to swap one set of grounding issues for another set of grounding issues, and increase cost, weight etc.
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Old 15th May 2011, 07:30 PM   #3
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I do this, simply because I optimised a preamp supply then put all 3 stages in one amp so had the preamp supply anyway. Advantage is that the parts are smaller in the small tube HT supply - small chokes - and you have more options for rectifiers, so I use an AZ1 mesh. Really nice choke input supply with all polypropylene caps.

This allows you to use a more basic supply for the final stage - like cap input. So you don't need such a high secondary voltage on the mains transformer as you would with choke input.

It is bigger, but it can be better unless you make a monster choke input supply for the whole amp. Then you lose the AZ1 mesh unless you use multiple rectifiers in parallel.

andy
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Old 15th May 2011, 10:18 PM   #4
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I've done it so the input and drive stage can be operated from a higher supply voltage. I also regulate the input/drive stage supply.

Triode output stages typically need a pretty wide (peak to peak) drive voltage. Increasing the input stage supply voltage allowed me to design an input stage with very good linearity.

I don't think it will present you with grounding issues, you just need to connect them at one (and only one) point.

Best of luck with your project.
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Old 15th May 2011, 11:28 PM   #5
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I have found with experiments, that the best sound comes from a PSU that has a single B+ feed for all the stages (per channel). Preferably thru a tube regulator, so there are no electrolytics in the signal path.

The sound improvement is surprisingly big, although 90% of it is only audible thru headphones. (At least in my cheap system.)

I wouldn't separate my B+ feeds anymore for any reason.
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Old 16th May 2011, 11:05 AM   #6
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I believe SY's RLD amp used such arrangements, i see no reason not to use that if you can......The Red Light District - another PP EL84 amp
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Old 16th May 2011, 11:19 AM   #7
SY is offline SY  United States
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As has been mentioned, in a power amp, a separate supply for the driver stage can be run at a higher voltage which can usefully increase swing and lower distortion. It's also amenable to simple regulation. In AB amps, there's an extra advantage- when the output stage's current increases with signal, the inevitable power supply droop is not transmitted to the driver stage, which can enhance stability.

I have not yet had any grounding issues with this arrangement when following basic rules.
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Old 16th May 2011, 12:51 PM   #8
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Quote:
In AB amps, there's an extra advantage- when the output stage's current increases with signal, the inevitable power supply droop is not transmitted to the driver stage, which can enhance stability.
now this is the part i like the best....
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Old 16th May 2011, 01:09 PM   #9
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Quote:
Originally Posted by john dozier View Post
Are there advantages to using separate transformers and rectifiers for the input/driver and output stages in a tube amp?

I have been doing this in all my power amps, both SS or tube and will not have it any other way. It is cheaper and preferable soundwise to monoblocks. At least to my ears.
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Old 16th May 2011, 01:15 PM   #10
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Its generally accepted that the final stages have much better PSRR (Power Supply Rejection Ratio) than the driver stages. As it is the final stages that put the biggest load on the PSU, it is them that cause problems in preceding stages. Far better to have the driver stages being fed form a well regulated supply which is separate from the power hungry output stage. This is true for valves, Bi-Fets, Mos-Fets and any other device that you care to mention.
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