Audio Research MCP 33

[reinhard]

Can anybody help with information, schematics etc.
of thr ARC MCP33. This unit is an MC Prepreamp.

Reinhard

[reinhard]

Hello,

have got the signal Path from Pat´s tubes schematic, now I need the power supply.

12 AT 7 as error amp and 7044 as pass element
and the two pre regulators.

Can anyone help.

Reinhard

[fdegrove]

Hello,

have got the signal Path from Pat´s tubes schematic

I can't really help you on this but is there a link to where you found the circuit diagram?

Thanks,

[reinhard]

Hello Frank,

answer comes late, but comes.

Have found the schematic of the MCP33 in the ARC database
at
www.arcdb.ws

Here under phono and then MCP 33 and scrolling to the end of the document.

Reinhard

[Brian Beck]

I have found the MCP33 to be one of the more interesting designs coming from ARC. It is a true “grounded grid” design (unlike Rosenblitz’s so called GG, which is really better described as a diff amp), even though there is some feedback to the first stage grids. But the real point is that signal is input to the cathode (through some very large film caps) rather than to the grid, which then allows for some degree of impedance matching to the source, as well as for non-inverting operation. The upper triode section is really only a partial cascode since it tracks an attenuated version of the output signal via feedback resistors. This follows William Johnson's patent very closely. I am leery of ARC's common use of diodes or FETs to protect the grids of cathode followers, because of my concern about voltage-dependent (non-linear) capacitance smearing higher frequencies. Granted, the boot-strapping effect of the cathode keeps voltage swings across the FET to a minimum, but I'd be tempted to avoid it altogether if the power-supply start-up sequencing can be arranged to avoid stressing the cathode follower. This design represents trade-offs between the two feedback paths to optimize the compromises among gain, input impedance and noise performance. The design is pretty clever overall, although I’m not sure that it would perform all that much better than a well-conceived common cathode design. Power supply rejection is not all that good, so the power supply would have to be quite special (and ARC went to great pains to make sure that it was).

Reinhard, please keep us posted on how this project works out for you.

[reinhard]

I think I don´t try the MCP. I´m not really happy with that big caps in such a sensitive position at the input.

Reinhard

[Brian Beck]

That's a very valid concern, Rienhard. A common cathode design with a "cheap red led" bias circuit per Morgan Jones might be a better option. You could choose from any of a number of higher transconductance triodes, or even just parallel 6DJ8s. There are plenty of designs posted about.

[reinhard]

Brian,

is this LED bias in Morgan Jones third Edition. Have not seen that, have first and second edition.

Victor from BAT uses paralled 6922, in the Aesthetix IO are two paralled 12AX7`s (4 systems) running from 550 Volt with a 100k
anode load and a 274 Ohm cathode resistor, bypassed with a big cap.

CAT uses cascoded 6922, the MFA Refrence a 6922 cascoded with a AX7 system, special Cascode with extra anode load for the 6922

Do you have a scan of the Jones circuit?

Further i am looking for the Carver in annother thread

Regards from Germany,
Reinhard

[SY]

The bias circuit is just a forward-biased red LED in place of the cathode bias resistor. A red LED (not the high efficiency type!) will give you about 1.7V. Once caution is that this is only suitable for current draws over 5-10mA or so, otherwise the LED impedance goes too high. For lower current circuits, there's a work-around, using a CCS from the B+ to run some extra current through the LED without upsetting the tube's operating point.

If you don't have MJ's Third Edition, you need to run right out. It's at least 50% larger than the Second. In it, he goes through LED and other reference options in gory detail.

[Brian Beck]

I agree with SY that you really ought to buy Morgan Jones' latest "Valve Amplifiers". It's a great book and and has plenty of design information on RIAA stages. I would not feel comfortable scanning anything from his book since he deserves your purchase, and there are copyright laws after all.