Help identifying the preamp section of a Univox EC-80 Delay

[pete_g]

I got a request for a preamp that resembles this tape delay, so I looked at it, and have never sliced a section out of such a schematic, and need to know where to chop it and which components to add. He wants it as a single volume with no eq.
I can see where the 8.2k and 27k resistors meet after Q2 and am assuming you treat it like any 2 transistor boost/fuzz but don't know enough about which value caps and resistors to change or add, and where. Can anyone help? It's obviously a one input pedal, so which input should I omit?
Schematic is here: https://estecho.com/univox-ec-80-schematics-cover/

[antonis]

Q1/Q2 implement dual feedback  (via 27k and 220k) so I don't think it shouldn't sound like a Fuzz..

[anotherjim]

From the collector of Q2, follow the 4.7uF to 22k (ignore the 56k path) to the 10k (ignore the path into Q6) down to the 50kB pot. The 22k and 10k can be a single 33k. From the pot to the output jack tip.
The 50kB pot is the output volume control.
This is the way some audio controls worked in that the volume control works to ground out the signal at minimum volume requiring some fixed series resistance to protect against the grounding. It should be better to keep the 22k and go direct to a conventional potentiometer volume control with a 50kA pot with the wiper to the jack tip. Now the 22k only works to reduce the maximum signal level which might be too hot without it.

[antonis]

What lazy Jim said..

[pete_g]

Thanks guys. So I ignore everything past the output jack? Should be obvious eh? I'm wanting to build this as a pedal and not to run with a transformer. Do I place the 9volt in the usual section - the 100k and 8.2k? Also, as there are 2 inputs do I ignore the one with the 47k resistor and place the input top directly onto the 4.7uf cap?

[anotherjim]

If the 7.4v is an expected reading for the preamp +supply then to suit 9v power, the 3.9k feeding in the power is kept but needs reducing in value to 1k by my reckoning.

The input circuit will have a huge bearing on how it sounds with a guitar plugged in. The lower input direct to the 5k pot would probably be intended for a low impedance microphone and the upper input with the 47k for most other things. I suspect a guitarist would pick the upper input but there is uncertainty depending on what was liked or if anything was plugged in between guitar and recorder.
I think the easiest way forward would be to copy the 47k input but put a toggle switch in that shorts across the 47k for "Lo" with the switch closed and "Hi" with the switch open. If you prefer, copy the original with the two jacks.

[antonis]

If the 7.4v is an expected reading for the preamp +supply then to suit 9v power, the 3.9k feeding in the power is kept but needs reducing in value to 1k by my reckoning.

I think OP is further puzzled (voltages in purple were just left there in the copied schematic) so it should be better to make 3k9 resistor a 47R to 100R one and place a 100μF to 470μF shunt cap..

[anotherjim]

There is a 100uF supply bypass on the scheme right by the input.
7.4v implies less than 1.5mA draw given the rectified supply is 12.5v as shown.

[antonis]

7.4v implies less than 1.5mA draw given the rectified supply is 12.5v as shown.

I think current draw of modified circuit should be less than half of 1.5mA (736μA)
(10.27 - 7.40) / 3900 = 0.000736

So, a 100R resistor in place of 3k9 one should cause negligible voltage drop while forming a LPF of 16Hz corner frequency with already existed 100μF cap..

[anotherjim]

Oops! My bad. I was looking at your reduced schematic, and completely forgot about the extra load in the fully loaded original!

[pete_g]

Once again, thanks a lot for all this info. Hopefully someone else will want to make this too, now. I'm gonna build it this weekend. And if anyone feels like drawing up a layout in DIY creator, go right ahead. Any layout that uses this 2 transistor circuit should be easy enough anyway without it

[pete_g]

Just a heads up, I built this with the 100r / 100uf cap instead of the 3.9k resistor and also the toggle with the 47k resistor in front and it sounds great. It's very transparent. I've built the EP-3 boost, the mini bomb, lpb1, and this is even more uncoloured compared to those. There's a slight amount of hair on the top end (especially with humbuckers) when both the pre and master are on full, and it depends on how hard you hit the strings to hear that small amount of hair. Sounds kinda like a multitrack tape unit boost. Hear it here, and thanks again. Someone definitely needs to make a layout on DIY creator as I think it's worthy of it. I'll attempt making one at some point. You can beat me to it if u want.
https://youtu.be/IT8OFRwlkds