Power supply for a MAS Marquis Preamp

[Peredur]

I recently acquired a MAS (Metaxas Audio Systems ?) Marquis pre amp, but without the accompanying power supply unit, and other than it requires dual power in (confirmed by their being two three pin connectors on the back, with it looking like each powers a separate channel) - I am struggling to find any information on the design input voltages.
I believe it is a Mk I, although purely on the basis that it doesn't have a Mk number !

It looks nicely put together, and from the limited amount I can find online, is a tempermental, but potentially very capable, unit. I am therefore keen to try and build a power supply for it, and at least get it up and running.

Any information anyone has on the DIP switches used for the phono section loading would also be appreciated.

Thanks in advance for any help provided

 

[rayma]

Manual and schematic:

 

[Mark Johnson]

It looks like "JP1" on page 18, is a 6-pin connector that allows the owner to plug in an umbilical cord. The cord appears to connect to a dual-secondary transformer in another chassis, far away. The six pins are SEC1-A, SEC1-B, SEC2-A, SEC2-B, GND, and GND. It appears the rectifier diodes are contained within the preamp chassis (DE5, DE6, DE7, DE8), suggesting that the umbilical cable carries unrectified AC.

 

[Peredur]

Thanks.

I have an earlier version, but from I can find online, the core design hasn't changed very much.

Despite several pages of the manual promoting the virtues of the power supply regualtion, it doesn't actually say what the power supply voltages are, other than a reference in the Testing section :
1. Connect the + and - supply lines [at the FUSEHOLDERS or RECTIFIER DIODES] to an
external current limiting Power Supply with maximum +/- 30VDC output voltage (or use
2 x 30VDC supplies).

And in the absence of anything else, I will go with this.

 

[rayma]

I think +/-15VDC was mentioned somewhere in the testing sections.
Seems the +/-30VDC is the input to the regulators.

 

[Peredur]

+/- 15v seems more sensible for a pre-amp.

A google search yielded :

Power Supply: 4 X 4,700uF/40V using high speed diodes. This gives you the +/-35-40V DC voltages which preceed the voltage regulators which drop it down to +/- ...

so annoying stops just at the information I need, and this links to a French L3a forum that has restricted access (apparently you need an invite).

The external power connections go direclty to the gold capacitors, so it is defintely dual rail (as well as dual supply) dc in.

Thanks again for the valuable input,

 

[rayma]

Here's the specific info on page 18.
Output of regulator ~20V
Audio circuit supply voltages +/-15VDC.

 

[Peredur]

Thanks again.

They are LF353's on mine, but the input power rails go to pins 4 and 7 via some resistors, so this confirms the +/- 15v, and that the voltage regulation was in the external power supply.

The labelling of the front panel LED's is a little mis-leading as one is labelled 'AC', which actually is connected to one of the DC power in (so I suppose indirectly confirms the presence of AC at the external power supply) and the one labelled 'DC" is linked to the DC offset servo section, so presumably shows either the presence or absence of a DC offset fault (I'll do a bit more digging through the circuit on that one.

Thanks for being patient with my slowness on the uptake of where you were guiding me.

 

[rayma]

Those LF353s are 36V devices, so up to +/-18VDC. But you wouldn't want to run them at max.
I guess AC means "AC is present" and DC means "DC offset is present". Not obvious though.