Passac EC-100 Acoustic Preamp [schematic]

[Manfred]

The 2 trimpots are marked 103B - so 10K linear.
The main control pots from left to right are 105k 103b and 103b - I assume !Meg linear, 10k linear and 10K linear.
The cap values are in posts above - C8 (red WIMA cap near the ribbon cable) 0.1uF
C6 (red cap in the middle) 0.15
C2 (red can next to the pots) is hard to read because the value is printed on the side right next to the pot but it looks like 0.1uF
the 2 electro caps are 100uF. The blue wima cap is marked 1500, so I assume 1500pF

Thanks!

[cspar]

Here's the schematic tidied up a bit, just lacking the sockets and 3 jumper wires.

The impedance jumper is the switch over the 2M2 resistor.

Passac-EC-100-block.sch

(134.85 KiB) Downloaded 20 times

The LED is designed to illuminate for a few seconds when the input jack is inserted and then goes out.

According to the manual the removable jumper changes the input impedance from 1 MOhm (installed) to 3 MOhm (removed)

The resonance and balance controls are about tuning the preamp to remove dead spots. The process is to set balance to zero and play up and down on the bass strings. When you hear one note that sounds sounds dead or lacks timbre you then set the resonance control to the frequency for that note - 9 o clock is 110Hz, 12 o clock is 131 Hz, 3 o clock is 160Hz. Increase the balance level until the dead note takes on the same timbre as the others. A/B the acoustic sound with the amplified sound and if there is a noticeable boost on either adjacent fret then re-tune the resonance.

[mediy]

Looks mature enough that I am going to box it back up. Doesn't look like you will need anything more. Don't be afraid to ask if you do need it though. My acoustic doesn't usually get amplified these days so this doesn't get masses of use - can always pull it all out again. It wasn't super hard

[mediy]

This is a bit embarrassing. When I put it back together I have lost one of the knobs Hopefully it will turn up, but if not, does any recognise them and know where I might get a replacement?

[Manfred]

Hi Cspar, thanks for redrawing the schematic.
I retraced it using my Layout and find two faults.

[cspar]

Thanks for catching that Manfred. I had quite the whirl going on deleting connections and repatching them working on getting the proper signal flow outlined. No excuse for the resistor value though

I've updated the schematic with the fixes and added the jacks. Cheers.

Passac-EC-100-block1v2.sch

(143.44 KiB) Downloaded 17 times

[mauman]

This is a bit embarrassing. When I put it back together I have lost one of the knobs Hopefully it will turn up, but if not, does any recognise them and know where I might get a replacement?

As old as they are, you probably won't find an exact replacement, sorry. However, they're 6mm shaft, 18 tooth knurled, so you can get 3 new ones all alike that are similar, and they'll look fine. Possible US sources include those below. Measure the diameter of your current knobs to ensure there's room for the new ones:
Small Bear (select the 18 tooth version): https://smallbear-electronics.mybigcomm ... ck-indent/
Love My Switches: https://lovemyswitches.com/knobs/?_bc_f ... 8T+Knurled

[mauman]

Thanks cspar and Manfred for your work, it's an interesting circuit.

Can either of you enlighten me on the function of Q2? I've not seen anything quite like that. Thanks, Mike

[ppluis0]

Hi mauman,

The purpose of Q2 is to mute the output jack during startup by an amount of time determined by R23 and C11

Cheers,
Jose

[Manfred]

Finally, the completed layouts the component designators correspond to those of the schematic.
Ready-to-print PDF files are not possible, because sometimes the tracks are too thin and the distances between the tracks or the solder pads are too close.

For this, the Sprint Layout 6.0 file to continue processing.

Passac EC-100 Acoustic Preamp.zip

(38.48 KiB) Downloaded 25 times

[mediy]

This is a bit embarrassing. When I put it back together I have lost one of the knobs Hopefully it will turn up, but if not, does any recognise them and know where I might get a replacement?

Found it It had obviously rolled into the multimeter case, so when I put the multimeter away it vanished. When I got it out again, there was the knob.

[mauman]

This is a bit embarrassing. When I put it back together I have lost one of the knobs Hopefully it will turn up, but if not, does any recognise them and know where I might get a replacement?

Found it It had obviously rolled into the multimeter case, so when I put the multimeter away it vanished. When I got it out again, there was the knob.

I'm glad this ended well!