Fw: [sdiy] ETI vocoder input amps

[jhaible at t-online.de]

> It's been a while, but I believe I looked over the circuit schematics
> and noticed these errors as well, but also saw that the PCB itself 
was 
> OK.
>
> I made a small web presentation as Magnus Danielson and I discussed
> these problems off-list a while back, and I tried to "clean up" those
> errors.
> I've uploaded it again which shows how the components and wires are
> connected to the PCB and the schematic as well, where I've labelled 
the 
> various pins of the ICs etc.
>
>  http://home.c2i.net/w-392864/vocoder/new.html

I have *not* checked the layout, but:

>  http://home.c2i.net/w-392864/vocoder/amp_schematic.gif (schematic)

*This* drawing has the errors, and the handwritten pin numbers at
IC1b *confirm* the error there.

> I also have the German "Elrad" article of the same vocoder, and 
there's 
> a slight difference there. I've circled it in red:
>
>   http://home.c2i.net/w-392864/vocoder/elrad_schematic.gif

Same error here.

Maybe the confusion comes from how the Line input jack with its 
switches 
are drawn. For Mic amplification, IC1b is a non-inverting amp:
RV1 feeds the positive (not the negative!) input. The switching
contact of the line input jack (SK2) connect the feedback divider
resistor R4 to GND and the cw end of RV1 to the 1st mic amp stage
IC1a. When a Jack is inserted in SK2, RV1 just draws the positive
(not negative!) input of IC1b to GND (regardless of the position
of RV1), and IC1b works as an inverting amp with its negative
(not positive!) pin (between R4 and R5) forming a virtual GND
node.
Same for IC2a: positive input must be at GND, negative input becomes
virtual GND.

As far as I can see, IC2a could be correct in the layout. But the
picture is not clear enough to see the traces for IC1. If the
handwritten numbers in the schemos reflect the layout, then the
layout is wrong.

BTW, do yourself a favor add a resistor from the wiper of RV1
to GND. If the wiper temporarily looses contact, your inverting
line amp has no GND reference for that time. 33k would be good.

JH.