Howe Audio 701 MA...Help?

[mckayprod]

I hope my pic attaches correctly, or I'm gonna look dumber than usual!  This is a monitor amplifier module from an old Howe Audio broadcast console we scrapped back in the '90s.  Couldn't be simpler, operationally, with balanced input (line level, probably), gain adjust, speaker out (not much power judging by the connector) and 40vac power input.  I think I'll play around with it, but documentation is nonexistent and I'd like to roughly figure out a schematic.  I can trace the wires, but can anyone identify the opamp or power transistors from the case markings?  Thanks in advance.

[mckayprod]

Let's try again with more light on the opamp...

[mckayprod]

Aaaand, the photographer fails!  The chip is marked MA700, CP415, with a stylized, vertical SU or US on the left.  Means nothing to me; its a 16-pin IC so I assume its a quad opamp.

[Enzo]

Hi
Your output transistors are MOSFETs, J38 and K133.  Look up 2SJ38 and 2SK133.  They will work in a very simple amp circuit.

I don't know about your IC, but quad op amps are usually 14 pin.

I see two three pin connectors for speaker and in.  Could they be reversed?  The ones on speaker appear to be shielded cable, while the ones on input appear to be a twisted pair.  Just the opposite of what I'd expect.  I am sure those connector pins can handle 50 watts if they wanted to.

I would sit down with a pad and pencil and draw up a schematic from the board.

It is a crappy copy, but here is an example of a very simple MOSFET power amp circuit.  I suspect your circuit is similar.

http://bmamps.com/Schematics/marshall/Marshall_mosfet_100w_3210.pdf

[mckayprod]

MOSFETS!  Not what I had hoped for, but I want to find 40vac somewhere and try this thing out.  The chip has 16 pins for sure, so who knows what that is.  I may not dig too far into documenting this, but if it works, I'll use it.  Thanks, Enzo.

[Enzo]

Nothing wrong with MOSFETs, it ought to sound pretty good, actually.

I see there appear to be a couple of these inside the Howe 7000 series mixer.


By tracing out a schematic, we can probably infer what the IC was and did.


I might believe it was a dual op amp instead of a quad, just a hunch.

[mckayprod]

I'm just not very familiar with MOSFETs so I guess I'll learn some things, which is the point.  As I recall, these consoles were sold at the lower end of broadcast market, so I expect simple, durable, but not outstanding performance.  Maybe 20w of clean audio?  This should be fun!