VBE pre-reg in plain old monolithic pre-psu

[cubastreet]

After seeing the thread on replacing the alwsr pre-reg with a vbe I got curious.

My psu was a simple affair with just two 50VA transformers followed by 2x15000 caps each and two lm317ts back to back.

The whole thing cost me less than $50NZD and gave a good sound for the money.

So I dug out a couple of dirty power FETs and followed the circuit in the thread, but had to lower the 1K resistor values to get some current flowing.

The sound is fantastic. Completely changed, it's [string of hyperboles].
My next NAC mod is definitely to replace the RC at the beginning of each stage with a vbe.

 

[maxlorenz]

Good invention, isn't it?

I will try to do it on SMPS from cheap DVD's wich carry good digital chips
(the problem is that I know nothing about SMPS; like everything else BTW )

Cheers,
M

 

[teddy_pardo]

It works very well with SMPS PSUs too.

 

[TTiamat66]

VBE is a diode with gain essentially
it can work well in the place of a poorly designed regulator

that sounds a little harsh, but its true
If you make one of Andy's regs
and a VBE improves it
You have implemented something poorly

VBE hides the symptoms, it doesn't cure the problem

Flame suit on:-} Gareth

 

[hacker]

If you make one of Andy's regs
and a VBE improves it
You have implemented something poorly

VBE hides the symptoms, it doesn't cure the problem

Flame suit on:-} Gareth

Ok, I'll bite

The thing the VBE does very well is filter the high frequency stuff that the SR (a feedback regulator) cannot effectively deal with - this is a known 'failing' of the ALWSR and other regs based on a feedback topology. The VBE is far more effective at removing this high frequency crud than a monolithic prereg such as a LM1086. No great surprise that Andy's regs sound better with a VBE prereg - all the crap is filtered, letting the regulator work more efficiently.

Flame away!

 

[martin clark]

Demonstration

I think the reason the VBE works so well is that it will reduce noise fed through the current-source that drives the output/pass transistor. This route as a limit on performance is quite clearly discussed on Andy's site and in the SR documentation (and one reason for using the tracking pre-reg option.)

The VBE is not a universal panacea though; in particular, it probably gives-up some of the line regulation at LF the 317 pre-reg adds to the Super regs (by holding the voltage drop across the Super part constant). Six of one, half a dozen of the other...

VBE is a diode with gain essentially

Yes and so VBE is the wrong description here... it's a capacitance multiplier we're discussing, which effectively is an RC low-pass filter driving an emitter follower. Subtly different.

 

[teddy_pardo]

Very true, and a nice definition Martin.

Though VBE is a so much simpler acronym

 

[PigletsDad]

We should start saying CM for Capacitance Multiplier.

 

[martin clark]

Gyrator? CM allows confusion with Common Mode.