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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