B+ power supply

[00940]

I'm going to rebuild my headphones amp and I'd need some advice on the power supply.

Currently, the amp is using a very simple supply : a 250VAC xformer and a 220uF cap. That's it, and that's for both channel. No need to say that ripple measures are awful in psud.

The amp is class A and its total consumption is 40W. B+ is around 320VDC. Am I right assuming I've to work on psud with a current of 150mA (both channels, 150mA * 320 = 48W) ?

As it is class A, is it necessary to use seperate power supplies, for each channel ?

Here are the two options giving me the best results in psud :

For a single supply :








For seperate supllies :





If someone has better ideas, I'll gladly take them Just don't ask for more iron please. Costs are already high enough .

[SY]

A couple of comments:

First, and most important, you have to consider the circuit being driven as well as the supply. What's the PSRR of the amplifier? Can that be improved? Remember, every 20dB increase in PSRR is like reducing the ripple by an order of magnitude.

Second, the supply can be reconfigured to give you a cap input, then a pi section. This should smooth things out quite well. Careful bypassing at the output will take care of higher frequency crosstalk and coupling between stages.

Third, if you really want the supply rail to be DC and nothing but DC, an active regulator is absolutely the best way to go.

[00940]

Thx for the comments.

The amp is an old kit, which evolved into a commercial product, the Audiovalve RKV mkII. According to the website, the circuit is strictly the same. Evolutions are at first sight : use of a toroidal xformer, dual mono power supply and a bunch of bypass caps. However, they still use a basic power supply, with only one big cap and perhaps a resistor. So I suppose the PSRR is quite good, considering the amp is 1000$ and is among the best of its category.

A pi filter is ruled out : no money for a big choke sadly.

Is a 0.006V ripple as I got in the first case not good enough? Would the advantages of a dual supply justify the increase to a 0.26V ripple ?

[SY]

First question: do you currently hear hum?

[00940]

there's a low background noise but in an amp with : a shitty old pot, 20years olds big output caps and a noisy xformer located at 2 inch of the circuit, without shielded wires, it'd be a miracle if I didn't have noise Actually, the background noise is really low.

All of the above issues will be fixed of course.

[SY]

Honestly, it sounds like you've got some other issues to address before you go crazy with ripple reduction.

Nonetheless, I'd strongly consider some regulation- you can use a simple three-pin adjustable floated with a HV pass transistor in this application. If you're nervous about using a chip, use the reg as a preregulator to get rid of the ripple, then use a simple R->C to your circuit.

[00940]

Originally posted by SY
Honestly, it sounds like you've got some other issues to address before you go crazy with ripple reduction.

More than you think

but I've already ordered a stepped attenuator, shielded wires, I've a nice big transformer and lots of caps to change.

In fact, I've now to order metal sheets to make a new box (I didn't tell you that it was needed too ? ) and the layout and thus the size of the box depends a bit of what I take for PS. As i have those 4 big caps, I'd just want to know the best way to do it.

It's not just a lifting, more like a rebuild as you can see.

[richwalters]

Sorry to put cold water over ideas.....OF Using all those caps.
Why not dispense with most except one/two and use a cap multiplier or basic mosfet stabiliser which will give you easily 30dB ripple rejection, a far lower output impedance and noise and you can connect A+B together without intermodulation effects between channels. I use it in my amps.
Look under Steve Bench's pages in Triode electronics site to get a crack what's involved.. There is a whole section about stabilised PSu's. It's a big site and the simplest circuit using SS works wonders on tube circuits.

You shouldn't have to use sep supplies for each channel If you you a SS regie-.

As SY mentions.......it's all up to your ability.

rich

[00940]

using a floating lm317 wouldn't be a problem for me, beside the fact that I read the worse things you could imagine on chip regulation. I found some schematics of mosfet regulators but then it becomes clearly more complicated.

[richwalters]

It isn't more complicated........The thing to remember about using chip reggies on H.V applications above their intended application voltage i.e lifting the reference term way higher than ground potentials is to put plenty of protection between adj and o/p and IN terminals....With MOSFEts, a15V zener between gate and source and feed... gate via resistor of a few hundred ohms. Often I fit a low ohm in the drain as well.
We aren't looking for perfect regulation as these SS devices have a much faster slew rate when compared to the old circuits using OA2's etc.
Linear technology appls note AN2 <<performance enhancing techniques for three terminal regulators>> gives a practical overview of using 317 types with lifted adj term.

I regulary use IRF840's in 450V supplies, but don't ignore gate protection w.r.t source. No protection = No working device.

hope this helps

rich