audio-talk

I’m going to build a big MoFo and plan on using a pair of LRS-200-24 https://docs.rs-online.com/6f52/0900766b814e4214.pdf
Some builders on DIYA have added a cap Mx such as this https://allo.com/sparky/capacitance-multiplier.html and a CRC.

I understand the MoFo has very good PSRR, so my question is, am I likely to reap much improvement by adding these extra bits to the PS ?

Hi Ian

these are the smps I plan to use. They are the ones I used in the last mofo and there were no problems I could detect. They are marked
S-120-24 24v 5A
but I imagine they are exactly the same as the ones you linked excepting yours are 200va and mine 120va.

I didn't add any filtering last time and hadn't intended to add any...I don't suppose it would do any harm.

the frame transformer in the pic is 2 x 12 12va for the opamps.

YMMV, but in the past the problems I have seen with SMPS is common mode noise, which that "cap multiplier" won't have much effect on. Its noise between the supply rails and ground generated across the capacitance in the front end of the power supply.

I quoted the cap multiplier as I have never understood why its called that, its just a series regulator working as a emitter follower with a constant voltage on the base. Its output terminal has nothing in common with a cap in any real fashion other that a low impedance. But that just me being picky. Now a gyrator, that I would call a cap multiplier.

Ironically (In this case) is there is almost no difference between "cap multiplier" and the MoFo that its feeding, and they both have the same sort of PSRR as each other based on the transconductance (or β in the case of a bipolar) of the active device. I wouldn't call the MoFo's PSRR as very good, maybe good would be closer, but it may be enough depending on the spectrum of the noise.

So Nick my takeaway from your post is; don’t bother with the cap Mx, just use the SMPS, OR, use a linear PS. I’m guessing you’d favour the latter . . .

Sort of, yes, I would always prefer a linear supply in this case, especially if you are going to the trouble of getting some chokes wound. I have nothing against multipliers, I use something similar in amps myself, and it may be worth trying, its simple and cheap and may have some benefit. But having not built one of these amps its all speculation on my side. Ed has built one so I would take more notice of his opinion.

My understanding is with 100% feedback the amp should cancel any power supply noise.

My mofo at owston has a pair of £10 pc power supplies for each side.

The one I'm now building has a pair of the power supplies that Ed has pictured, so going up market ! .., but I dont expect any sound improvement by using them

If I were starting MoFo now, I'd probably go for a switch mode DC supply like Ed.

But, when I built mine I went linear, mostly using parts I already had so cost was not a problem, just weight.

Using switch mode power supply and much smaller fan cooled heat sinks (which I have) I reckon I could half the weight.

Would it sound any different ? No idea.

I built mine with cpu fans and SMPS to minimise cost as I didn't really know whether I would like it. If I box it up I think I might go for a linear supply - big trafo and caps, just in case it does make a difference.

I think I'd stick with the cpu fans though - £20 instead of c£120 for suitable large heatsinks from Farnell, Mouser etc. The fan at around 7V keeps the heatsink entirely cool and I can't see that the expensive sink option offers any audible benefit.

My understanding is with 100% feedback the amp should cancel any power supply noise.

I don't know if I should say anything as you have not asked a question. But do you want to buy a bridge?

I'm trying to cut down on the stuff I buy nick, but I'm sure lesley would surprised if I brought a bridge home, but one could make a nice garden feature ?

It would give you somewhere the park the caravan under.

It's turning into a must have !