Vandal - a 2-channel preamp inspired by the Randall RG100ES

sixthfloor · Post by **sixthfloor** » 20 Mar 2018, 14:12

I've been playing with the schematics of the Randall RG80/100 series for some time now, and I noticed both green and red channels have very few differences. After some time spent with the layout, I managed to fit both in a single stompbox thanks to the use of a 4PDT footswitch for channel change.

Here's the layout and schematic :

The red components in the schematic are exclusive to the red channel, all other components are common to both channels.

The circuit is meant to be powered at 18v in order to have enough headroom. Lowering the voltage is possible, 12v worked quite well on my build - but not with active pickups. Lower voltages don't yield enough headroom for it to sound good. The Randall circuits this is based upon used 24v, which I tried at first, but the voltage regulator I used for powering the LED indicator didn't tolerate it and fried

Transistors have to be 2n5484 as far as I can tell. I tried 2n5485, J202, 2n5457 and 2SK117 without success. No idea about J201 as I have no legit ones.

marcao_cfh · Post by **marcao_cfh** » 20 Mar 2018, 21:22

Very nice! A long time ago I've started a project to clone the RG100 amp, but never got it done. Maybe I'll restart it someday.

roseblood11 · Post by **roseblood11** » 21 Mar 2018, 21:04

I don't understand why you need a 4pdt? Can't it be done with a 3pdt?
And the capacitor might cause a crackling noise when it's switched, because one end is floating... Put a high value resistor in series with the cap and just short the resistor.

Question to the pros: Could it be done with a 2pdt?
- hardwire the 470n and the clipping diodes.
- connect the side that is "closer to gnd" of that cap and the diodes
- add a 100k (or larger) resistor to gnd and short only that resistor with a switch

Would that work? Or would it form some kind of feedback path, when the switch is open?

marcao_cfh · Post by **marcao_cfh** » 21 Mar 2018, 23:11

Sounds like a good solution. Unless it causes some feedback path by one of the diodes. It may worth a try. Swapping the 4pdt to a 3pdt (adding one pole for the leds) will make this one more interesting.

The way sixthfloor did the switching really needs a 4pdt. In the schematic, you have a pole for the cap, a pole for the resistor and 2 poles for the diodes - this one can be done with just one pole. But quickly looking at the layout, you can see there's a pole used to switch between a red and a green led (actually it's a bicolor led), so the diodes switching was actually done using just one pole.

sixthfloor · Post by **sixthfloor** » 22 Mar 2018, 16:32

Using a 3PDT instead of the 4PDT is indeed possible if one doesn't care about having a LED indication of which channel is active, Marcao describes perfectly the way this is done.

The 100k resistor idea sounds good, however I fear it might introduce some tonal change or distortion by creating feedback for the high frequencies the 470n wouldn't block. I've drawn this idea as I understand it, I hope it is what Roseblood meant :

marcao_cfh · Post by **marcao_cfh** » 24 Mar 2018, 14:01

Yeah that's what I got from roseblood11's description too. Thinking about it after seeing a schematic, maybe the 100K+ resistor isn't necessary - that considering there isn't any feedback loop by the diode and cap.

sixthfloor · Post by **sixthfloor** » 25 Mar 2018, 08:36

Thinking about it, I'm pretty sure a portion of the high frequencies would pass through the diodes and 470n then go to ground via the 2k2 resistor. I don't know if it qualifies as feedback, but it would probably alter the EQ of the signal.

rowelej · Post by **rowelej** » 27 Dec 2018, 11:38

Hi all. Would it be correct to use 1N4731 as 4V Zenner diodes?

sixthfloor · Post by **sixthfloor** » 27 Dec 2018, 18:15

According to the datasheet, 1N4731 is 4.3V so it should work. 1N4730 (3.9V) would be a closer match though; I used BZX55C3V9 (3.9V).

Welcome, BTW

rowelej · Post by **rowelej** » 27 Dec 2018, 21:10

sixthfloor wrote:According to the datasheet, 1N4731 is 4.3V so it should work. 1N4730 (3.9V) would be a closer match though; I used BZX55C3V9 (3.9V).

Welcome, BTW

Thank you!

rowelej · Post by **rowelej** » 22 Mar 2019, 12:24

BTW, I hope I'll finish it in a week

UPD. Cr*p! I found that 4PDT switches are absent in my local store, so it could take more time

thetragichero · Post by **thetragichero** » 22 Mar 2019, 19:05

I'm mad that you have a local store... the closest radio shack is over an hour away

rowelej · Post by **rowelej** » 28 Mar 2019, 10:46

Also, could I replace 20n capacitor with 18n or 22n?

rowelej · Post by **rowelej** » 28 Mar 2019, 14:03

thetragichero wrote:I'm mad that you have a local store... the closest radio shack is over an hour away

Don't be angry. Usually, I have to order parts in stores which are 500 and 1000 kilometers away from me)

HENK · Post by **HENK** » 28 Mar 2019, 14:06

rowelej wrote:Also, could I replace 20n capacitor with 18n or 22n?

YEPP!

sixthfloor · Post by **sixthfloor** » 29 Mar 2019, 14:17

As Henk says. I probably put a 22n cap there myself as I'm pretty sure not having any 20n around ^^

rowelej · Post by **rowelej** » 29 Mar 2019, 14:33

sixthfloor wrote:As Henk says. I probably put a 22n cap there myself as I'm pretty sure not having any 20n around ^^

I found one in trusted store, but its size wasn't specified. So I was surprised to see it: around 0.5x0.5 inch and pins don't fit in board's holes

rowelej · Post by **rowelej** » 02 Apr 2019, 17:04

Hope it will work)
https://ibb.co/345KD7t
https://ibb.co/LPYDQX8
BTW, What resistor could I use to connect bypass switch led to 18v? 5 kOhm will work?
And where should I wire "Channel 3" from LED panel?

thetragichero · Post by **thetragichero** » 02 Apr 2019, 22:25

http://ledcalc.com/

I use that to figure out how low I can go
but yes 5k should be plenty large

sixthfloor · Post by **sixthfloor** » 03 Apr 2019, 08:28

rowelej wrote:Hope it will work)
https://ibb.co/345KD7t
https://ibb.co/LPYDQX8
BTW, What resistor could I use to connect bypass switch led to 18v? 5 kOhm will work?
And where should I wire "Channel 3" from LED panel?

Nice, I hope you'll like it

Channel 3 from LED should go to pin 3 (green LED ground) of the 4PDT. It's kind of symetrical to red LED and channel 1.