EPFM Pre-amp

[rompler808]

Found an old pre-amp kit from Electronic Projects for Musicians book. Probably about 20 years old. I have some questions for you all about the kit:

• The bag containing the components has been stapled shut, but I'm guessing some of the parts have oxidized wires. Do I need to apply some sandpaper to the the component wires (e.g. resistors, caps etc)?
• The PCB is 2.5" x 5.5". Given the size of the PCB, what size enclosure would you put it in, maybe a 1590bb?
• Would this be suitable for use as an acoustic guitar pre-amplifier? I'm thinking of using it with an acoustic guitar that uses a McIntyre feather pickup. If its not suitable as-is, could it be modded or put additional pedals in line (thinking EQ).

Thanks!

[vigilante397]

1) Case-by-case basis. If you can get the solder to whet on them they're fine. Obviously sanding down all to clean them (steel wool would work) would be the safe answer, but likely a little time-consuming.

2) That's too big even for a 1590BB. It looks like it should fit a 1590X though.

3) It's hard to say what it would be good for without a schematic or any information on the circuit

[rompler808]

Hi vigilante397,

Thanks for your reply. Find attached the schematic and pcb layout.

[Mark Hammer]

Actually, that EPFM project is closer to 35-40 years old.  You will note that, like almost all the projects in EPFM 1 and 2, it requires a bipolar supply.

The size of the board is a function of a) the use of much larger components of that era (e.g., the 4739 dual op-amps are 14-pin rather than the 8-pin we are accustomed to), and Craig's overall plan to have these modules mounted in a "card" chassis, similar to current Eurorack format.  JD Sleep of General Guitar Gadgets used to have board masks on his site for most of the EPFM projects, redrawn to suit 8-pin chips, that take up less space, but they are still predicated on bipolar supplies.  And besides, I'm not seeing them there anymore.

Would it be suitable for acoustic guitar?  It would appear to be quiet enough (which is partly why he preferred 4739 chips at the time), and it has a maximum gain of 101x, which should be sufficient.  Is the input impedance high enough?  That would depend on your specific guitar electronics, I suppose.

The circuit, and board are also amenable to being a dual input/output device.  If you connect point/pad F to point/pad N, instead of connecting N to a meter of some sort and F to the Master volume pot, you have a second channel and output.  As shown, that output has a gain of 1x.  That's fine if the purpose of IC1B is simply to provide an invert equal-level signal for running a balanced output.  But it doesn't need to be unity gain if that's not your ultimate purpose

For example, if your guitar was one of those with a piezo bridge pickup plus an internal gooseneck mic or mag pickup, that second channel could be used to amplify that other signal source.  Replace R4 with a higher value fixed resistor, or pot, or pot in series with a fixed resistor, and you can achieve other higher gains than unity, which would permit you to achieve some degree of preferred balance between the two signals.

[willienillie]

Looks like the PCB could be trimmed down considerably, almost as small as 2"x4", then should fit in a 1590BB easily.  Maybe even a 125B, with the jacks under the board, but no battery.

I wonder if the electrolytics are still viable.

[Marcos - Munky]

I wonder if the electrolytics are still viable.

Probably not. I'd just buy new ones.