Regulated supply rails for amp - as big as you want.

[Circlotron]

I'm getting this itch to build another multi-channel gainclone; something that will drive eight 10 inch woofers as subs. Now, my collection of transformers never seems to have something that will give + / - 35 volts, more like 43 volts is what will be produced by those I have a few of laying around. Ones like the one that blew the gizzard out of my ETI-480 sub amp a while ago So.... what to do? Why not bite the bullet and build up a regulated supply that 1/ brings the DC voltage down to something more suitable, 2/ is totally hum-free, and 3/ annoys the "1000uF is best" crowd all at the same time? A hat-trick says I.

To cut a long story short I simply turned one of my more unusual class A amp circuits into a regulated supply. The performance is really quite good. The output voltage is always nominally 1.25 volts less than the zener voltage. The output voltage stability and regulation depends almost entirely on the zener itself. As an example, I measured the voltage between the top of the zener and the fet source with 17mA preload (the 2k resistor) and the source was 1.23059 volts lower than the zener. Now stick a 24 ohm load across the output for 1.4 amps and the voltage difference rises to 1.23065, i.e. the output sags by 0.00006 volts. 60 microvolts.

That means if we had a perfect zener or other reference source the output voltage would drop from say 35 volts to 35.99994
volts. That's 0.00017%. I could live with that. In practice the zener current and therefore voltage varies a little with load because zener current drops with increasing load. The cap across it minimises this effect for sudden load changes though. This same cap also gives the supply a soft start as it charges up.

One of the things about this voltage regulator cct is it's scalability. All you need is a suitably rated N-channel mosfet and heatsink and caps to make the current or voltage or both as high as you want. This is due in part to the floating 12v supply that feeds the gate pullup resistor. I only prototyped it with 50 volts going in but there is nothing that I can see that would stop it even being used for vacuum tube psu's.

The first drawing is the basic circuit.

[Circlotron]

This second drawing is how to use it for split rails like in an amplifier. Input 1 and input 2 *must* each be fed from their own =separate= transformer winding / rectifier / filter. Either 2 completely separate windings on the same transformer, or 2 transformers. Trying to run them both from one winding will only make a big fat short circuit.

Now, just gotta make the amp to use all that juice!

[Circlotron]

Originally posted by Circlotron
the output voltage would drop from say 35 volts to 35.99994
volts.

D'oh! That should be 34.99994 volts.

/Circlotron - caught out being vain, reading his own posts.

[iCebReakEr409]

how much power loss do you get due to those regulators ?

i have exactly the same problem...only 2x30V transformers

[AKSA]

Graham,

Very, very clever.

However, the speed of the LM317 is a tad limited over 20KHz I believe. Have you considered using a simple diff pair with a voltage reference (could be LED, bandgap or zener) one side and a voltage divider from the output? You can use the second transistor's collector to drive the gate of the source follower.

I built one; blistering speed, a little overshoot, but really good rejection characteristics, and not too much feedback.

Cheers,

Hugh

[mrfeedback]

Does the regulator need high speed, and is this actually an advantage if there is sufficient decoupling capacitance on the output side of the regulator ?.
By this I mean, if the regulator response is relatively slow (only fast enough to respond to envelope average demand) and feeding relatively large capacitors, then one could efffectively have a power supply with the characteristics of a standard transformer/bridge/caps supply but controlled average voltage and without ringing/overshoot problems.
The PSRR of these gainclone chip-amps is best at lower frequencies of psu noise/ripple, so why not keep possibility of high frequency noise/ripple/overshoot right out of the equation ?.

Eric.

[Circlotron]

Hi iCebReakEr409. The losses on the main mosfet in watts would be the volts dropped across it times the amps through it. For an amplifier as a load, the current and so the actual watts loss would be going up and down all over the place so the heat loss would depend on the "duty cycle" of the music.

Gidday Hugh. Yeah, I just built it that way because I have this thing about LM317's. Very docile they are. I also tried a TL431 and it just wanted to oscillate and I didn't really feel like sussing it out. As far as speed is concerned, if the output cap is big enough then it's hardly an issue IMHO. Always keen to have a listen to your thoughts though! Gotta get over and see you again sometime soon too.

I did another measurement on it this morning, this time from 1.4 -> 2.8 amps and under these conditions the voltage only drops by 15 microvolts. Seems that with higher current the 32 amp fet I am using gets to the steeper part of it's transconductance curve where it starts to boogie.

Eric, yep, I'm with you.

[djk]

" "1000uF is best" crowd "

I was at the NS site the other day reading some of the app notes for the various 'clone' chips.

The 470µF~1000µF caps recommended are for local bypass, they are not the main filter caps.

Where did those guys ever come up with the idea that those were the main filters?

As regards full regulation, do we really need it?

Maybe for the 'clones', because they are all on one set of supply pins, but for an output stage? I find regulation adequate for just the voltage gain stages, say 50mA, and a no feedback design to be adequate in most cases as most voltage gain stages are biased class A. Electrocompaniet uses a simple series pass transistor with a zener in the base, to me they sound better than most three terminal regulators.

Something disgusting to think about:

Virtually no filter cap ahead of the pass transistor. I was able to use only 220µF to keep the regulator from dropping out with a 60hz line. The pass transistor needed less heatsink too.

[mrfeedback]

I think it comes down to sonics/dynamics tuning regarding the 1000 uF idea.
The 1000 uF supplies cause throttling down of these GC chip-amps, and the PSRR (or lack of) interacts with light speakers to give a 'bouncy' bass, that seems well regarded by some, and is probably reasonably appropriate at the power levels of these chip-amps.

Grahams regulator idea is mainly to enable usage of whatever power transformers he has lying around.
There had been talk of GC amps changing sound according to supply voltage, with a small range regarded as optimum.
As I see it, Grahams regulator could be used as a means of tuning the amplifier sound by varying the supply voltages - also useful to reduce supply voltage when driving 4 ohm etc loads.
For a practice guitar amp it could be used to set max power/distortion characteristic.

Eric.

[AKSA]

Eric, Graham,

Thanks for your thoughts on the use of a filter cap to scotch any speed issues. I'll certainly think on this POV more; it makes a lot of sense.

Recently I was faced with designing a +5V power supply for a DAC. Looking at the clock speed, and the on/off nature of digital current demand, I felt speed was necessary, if only to lessen the load on the output cap, since electros probably won't be adequate. That was the thinking behind this; analog was far from my mind.

Graham, I recently built a 1.5W SET using the 6EM7 out of simple curiousity. I'd encourage you to have a listen; it certainly doesn't do it all well, and the AKSA clearly beats it on dynamics and detail, but there is something beguiling about the female vocals in particular which typifies just about all SETs, and I'd love your opinion.

I've got a couple of SV811-3 Svetlanas lying around unusued, and am considering taking the plunge, as the lovely vocals and sound stage has me entranced. Tube lineup will be 6BQ7A SRPP, 12HG7 (pentode!!), and SV811-3, cap coupled all the way, fixed bias, running 600V and 75mA. I will incorporate the secondary into the cathode circuit, should get around 15W, should really boogie....

I'm also working on a tube/PP SS hybrid which might interest you, very trick.

Cheers,

Hugh