Power Supply Design Strategy--Solid State for Tube Amp, From Morgan Jones - diyAudio
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Old 14th December 2006, 09:18 PM   #1
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Default Power Supply Design Strategy--Solid State for Tube Amp, From Morgan Jones

Hi All,

I posted this on the World Designs forum, but thought I'd add it on this side of the ocean, in case someone is interested.

I've been studying the power supply design chapter in Morgan Jones' Valve Amplifiers. It's quite mathematical, and I certainly don't understand it all, but I hope I've managed to derive a useful design strategy from it. (Note I didn't say the *best* design strategy.) I'm post it here, for your consideration and comments.

Any pointers to better online info on PS design would be great!

Here's what I've come up with--anything good should be credited to Jones. Regarding anything stupid, well, the source is obvious:

0. Create a filter consisting of either an initial LC filter, or a CLC filter, followed by as many RC stages as necessary. In other words LCRCRC...
or CLCRCRC... (this is not Jone's prescription, but my interpretation of it for this discussion--of course there are myriad ways to make a filter--this just seems a reasonable pair of strategies)

1. Select transformer VAC and current rating

Current rating should be at least as high as the anticipated load. VAC should be such that the DC component after the first filter element (either L or C) is a bit higher than the desired B+, to allow for additional voltage loss after filtering. I decided on 20V over B+

Initial inductors supply DC of approximately 0.9VAC. Large initial capacitors supply DC of approximately 1.4VAC, down to 0.9VAC for small caps--this allows tuning of the DC voltage.

2. Select either an inductor or capacitor as the initial filter element.

As seen above, inductors supply less voltage, but are less subject to voltage changes in response to load change. They also are more finicky, and require a minimum load (current flow) in order to function correctly. With too little load the voltage also drifts up towards 1.4VAC.

Capacitors supply more voltage and do not require a minimum current draw to filter correctly, but are more prone to changing voltage in response to changes from the nominal load. I selected a capacitor input, and the following steps follow that path.

3. If going capacitor input, size the initial cap so ripple is 5% of DC.

I'm guessing/hoping this keeps inrush current to a reasonable level , along with the corresponding RFI. My manipulation of Jones' equations for this results in:

C1 = I_Load/(5 * V_B+)

4. To stabilize the filter, add a resistance in series with the inductance in the the LC filter to give it a Q of 0.5

My manipulations give the equation:

R_additional = 2*Sqrt(L/C) - R_inductor

5. Determine the R necessary to drop the voltage at the LC filter to the desired B+

6. Decide how many RC stages are necessary to drop the ripple after the LC filter to an acceptable level, divide Step 5's R by that number, and calculate the value of the C's in each of these RC stages.

Unfortunately you'll have to buy the book to do this. It's easy once understood, but it involves consulting a table. It wouldn't be fair to Jones to reproduce it here. Jone's strategy seems elegant, for it allows iterative RC filters to do the work of a single RC filter, with smaller individual capacitors and less total capacitance.

Example:

I'm building an Aikido Headphone Amp based on John Broskie's Aikido Headphone Amplifier Recipe on TubeCad.com. It' requires a B+ of 250V, and I'm guessing that it has constant average current draw of 50mA, so I went with a capacitor input filter. I also want to incorporate a 10H 270R inductor I have, along with a 100uF ASC oil filled capacitor for the final cap.

Using the above strategy I came up with this as the filter sequence:

C(L+R)CRCRCRC.

Specifically 225VAC, 40uF, 10H/270R + 424R (for Q = 0.5), 83uF, 147R, 90uF, 147R, 90uF, 147R, 90uF.

PSUDII gives me a B+ of 250V and ripple of 13uV, and I do not see any ringing in response to stepped load changes, just a smooth rise/fall to a new voltage.

Whew! I hope folks find this useful in some fashion, and invite your comments.

Best Regards,

George
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Old 16th April 2015, 05:07 PM   #2
Bigun is offline Bigun  Canada
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A pity this thread stopped - it looks like some useful psu guidelines being developed.
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Old 16th April 2015, 07:12 PM   #3
DF96 is offline DF96  England
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One of the classic newbie mistakes is to assume that supply rail ripple is the main source of hum so they add lots of RC sections. Beyond a certain point (often just a few RC sections) most of the hum comes from poor grounding or poor layout. Meanwhile, the excessive resistance introduced by all the RC sections plays havoc with subsonic stability.

However, the OP has noted one important point raised by MJ: remember that LC circuits can resonate, so may need damping.
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Old 16th April 2015, 07:44 PM   #4
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Old 16th April 2015, 09:16 PM   #5
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Now everybody starts with simulation like PSUD2 free from Duncan amps. SO EASY! Shows up unanticipated resonances, rise rates, etc. I just wish that it supported more details and options like 'snubbers' on the rectifiers and 'dampers' on the inductors. And I don't understand some of the specs of the components in the simulation as well as I should. And it doesn't support simulation of how it reacts to a varying load, varying wall voltage etc.

And in my area of interest, large tube bass guitar amps, they're starting to use light compact powerful high-voltage switching power supplies for the B+. Check out the Peavey VB-3. Now if they only had an OTL option. The VB-3 is a lot lighter than an SVT but a bass output transformer is still enormous and heavy.
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Old 18th April 2015, 04:40 PM   #6
Jaffrie is offline Jaffrie  Indonesia
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This old school approach has many good attributes. Sound is grainless & effortless if properly implemented. One can even tune slam factor by using different value of caps before & aft of the inductor.
Though cumbersome & taking up lots of space, I always prefer the sound of CLCLCLC.

CHEERS
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Old 18th April 2015, 05:21 PM   #7
wwenze is offline wwenze  Singapore
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How much filtering is needed actually? I started asking this question after realizing a -80dB 100Hz hum won't actually be heard on bookshelves speakers unless your ear is capable of hearing negative dB SPL. Earphones are more sensitive (say 120dB/V for sensitive ones) but it's also easier to build a headphone amp to the require standard of noise.

If a single RC stage can provide enough filter I would rather dump all the remaining C to the last C for lower impedance.
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Old 18th April 2015, 09:18 PM   #8
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by wwenze View Post
How much filtering is needed actually? I started asking this question after realizing a -80dB 100Hz hum won't actually be heard on bookshelves speakers unless your ear is capable of hearing negative dB SPL. Earphones are more sensitive (say 120dB/V for sensitive ones) but it's also easier to build a headphone amp to the require standard of noise.

If a single RC stage can provide enough filter I would rather dump all the remaining C to the last C for lower impedance.
That attitude is certainly not worthy of a true believer: the more R, the more L and the more C is always better, as a matter of principle.
It is not even allowed to question or discuss the truth or otherwise of such dogma's. That is the final and definitive truth, period.
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Old 20th April 2015, 08:51 AM   #9
DF96 is offline DF96  England
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True believers don't own calculators, so their only way of ensuring enough L and C is to use lots of L and C. The resultant sound is deemed to be 'good'; anything smaller than the resultant design is dismissed as 'puny'.
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