ADA MP-1 high voltage supply question

[Göthenlör]

Hi,

I don't understand a part of the high voltage power supply of the ADA MP-1 schematic :



It seems that the ground reference is controlled by U2, but why and how ?
Why not just connect the negative rectifier bridge side to ground directly ?
The ground reference will be separated by few kohms of the rest of the circuit ground reference. Or am I missing something ?

Can you explain the purpose of this circuitry ?

Thanks

[J M Fahey]

1) it is incomplete

2) we don´t know what it´s feeding.

3) we don´t know what´s connected through E/drive to activate it.

4) what is "Ref"?

5) where does "Out" come from?
Is it related to the visible part?

[g1]

Here are the 2 schematics that those above are part of. Complete set can be found here:
https://adadepot.com/index.php?msg=580

[g1]

2nd

[The Dude]

That is an odd circuit. Both the "REF" and "DRIVE" lines from the first schematic go to the U2 circuit on the second page schematic. U2 looks to me like it's used as a sort of comparator- monitoring the "REF" voltage with the other input grounded. The only thing I can think of is that it's some type of voltage regulation using the bridge rectifier center tap as a sort of "control". That's a SWAG.

[x-pro]

Lamps current stabilization. More accurately, current limitation

[J M Fahey]

Hi,

I don't understand a part of the high voltage power supply of the ADA MP-1 schematic
It seems that the ground reference is controlled by U2, but why and how ?
Why not just connect the negative rectifier bridge side to ground directly ?
The ground reference will be separated by few kohms of the rest of the circuit ground reference. Or am I missing something ?

Can you explain the purpose of this circuitry ?:

It switches HV supply, around +250V DC

As mentioned above, triggered by U2, apparently as soon as Preamp is turned ON, I see no switch or Microprocessor doing it, so I see no advantage compared to leaving negative side of bridge rectifier and C1 always grounded.
2 transistors in series are used just to handle half of +V each, but I see no regulation .

+V can NOT be switched ON-OFF during preamp use, time constants are relatively long and voltages would be all over the place until stabilized.

I see no practical purpose during live use.

[x-pro]

U2 LF355 ? Is that chip in there? It doesn't read well.

[g1]

I was thinking of it just as a regulator or switch but on the minus side rather than positive.
But where is the current path on the (-) side? Through the output pin of the op amp U2?

[Göthenlör]

Thanks to g1 for posting the full schematic, I forgot to do so.

The cuircuit around U2 looks like a log amplifier, which if I understand correctly, acts like an envelope follower. But I don't understand what it does exactly.

I see no practical purpose during live use.

It looks overkill to me too. With a capacitor multiplier, I suppose we could achieve a low enough PS noise in the tube circuit.
That said, the first two triodes are referenced to -15V, does it justifies the U2 circuit ?

So if I just connect the negative side of the bridge rectifier to ground, and ignore all the circuit around U2, it should work, right ?

But where is the current path on the (-) side? Through the output pin of the op amp U2?

That's what baffles me as well. And even through the output of U2, you have the 3.3k resistor on the way.

[x-pro]

But where is the current path on the (-) side? Through the output pin of the op amp U2?

Yes, and the (-15V) circuit through the PSU

[J M Fahey]

Thanks to g1 for posting the full schematic, I forgot to do so.

The circuit around U2 looks like a log amplifier, which if I understand correctly, acts like an envelope follower. But I don't understand what it does exactly.

Not clear to me either.
IT *might* be a slow start +V circuit, *or* some kind of indirect regulator.

A normal regulator would work on the +V side, but it needs a transistor standing >300V to have a little margin, probably TO220 case for dissipation, and some high voltage reference, also a driver; while this way they use 2 cheap TO92 transistors, (say standing 150V each) and their drive can be referenced to ground.

Even so, they would save a couple bucks, not more.

It looks overkill to me too. With a capacitor multiplier, I suppose we could achieve a low enough PS noise in the tube circuit.

[/QUOTE]That said, the first two triodes are referenced to -15V, does it justifies the U2 circuit ?[/QUOTE]
The -15V reference + LARGE cathode resistors is actually a constant current source, and FORCES each triode to pass 1mA idle, no matter what.
Saves on matching

So if I just connect the negative side of the bridge rectifier to ground, and ignore all the circuit around U2, it should work, right ?

It should work fine, only you will not have *exact* +250V (or whatever value they chose).
No big deal.