Basic Audio - Scarab Deluxe  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
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toneman
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Post by toneman »

hey cub,
that's a good link that i havnt seen b4!!
thanks!
:thumbsup
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Post by Cub »

Any time ! A thread is never too old to be bumped if the information is relevant.
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Post by roseblood11 »

Who has tested transistors with different gain? I wonder if 2n5088 with hfe close to 600 or bc547b with hfe around 350 would be better?
I had forgotten this project, but today I found a layout and a half populated board. So, after three years, this will be finished soon.

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Post by HamishR »

I had an original Basic Audio Scarab Deluxe. I have a few others of John's pedals too - he makes great pedals. Not just clever circuits but beautifully built too. The Alter Destiny is amazing.

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Post by marshmellow »

Image

Built it this afternoon. I like the general tone, more overdrivey and less all-out fuzz. With 1µ the Fat control isn't doing that much for me, but I think that is due to the tone control, which cuts a lot of bass in general. I think I would have preferred a BMP style control or a simple treble cut here. Transistors are all BC550C. I'll also try a bigger feedback resistor for higher maximum gain, right now it's just a little bit lacking.

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Post by roseblood11 »

After three years, I finally finished my build...
- Using bc547b transistors with hfe around 350 is no problem, I even liked them better than 2n5088 with higher gain
- I tried both variants of the tone circuit that were discussed in this thread. Both are useful, so I put them on a switch as shown in the vero board layout.

Wiring five 9mm pcb-mount pots in a 1590b enclosure is slave labor! Does anybody sell a pcb for this circuit?

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Post by roseblood11 »

...but the bias isn't perfect in my build. Maybe I'll use transistors with higher hfe or increase the feedback resistor?
The original pedals sounds thicker and creamier in some of the videos.

And I have a strange problem with hiss in my pedal:
- hiss increases, if I turn up the bias pot, but after the 2 o'clock position, it's totally gone. Maybe this is normal, because that's the point where the sound gets gated.
- there's hiss on the fat pot, but only at two small areas, around 10 o'clock and 2 o'clock, and only if the fuzz put is fully clockwise. Maybe caused by creeping current on the veroboard? I used some foam as isolation, that I've never used before...

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Post by roseblood11 »

The wires to the fat pot and the 10µF cap at the input are very touch sensitive. Maybe a leaky cap? Could this explain the hiss and strange behaviour?

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Post by roseblood11 »

I doubt that the fat pot is wired correctly in the schematics.

Wouldn't it be better to connect the 10n cap directly to the input and add the 10µF via the pot wired as variable resistor?
Like this:
http://music.codydeschenes.com/wp-conte ... ansfer.pdf

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Post by roseblood11 »

Some people here and at tagboardeffects reported problems with hiss, oscillation, touch sensitive parts and some power adapers...
All of that might be caused by the fat pot, which is definitely wired wrong in the existing schematics and layouts. It should look like this:
Scarab Deluxe Fat .jpg
Scarab Deluxe Fat .jpg (10.09 KiB) Viewed 4278 times
Quick fix: Add a jumper between lugs 2&3 of the pot or on the board. The pot is still wired backwards, but that doesn't matter as it's linear taper.

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Post by destro »

Toss a .0022 between the base and collector of Q2 for the 5th cap that all the schems and layouts are missing. The difference is small, but noticeable.

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Post by Shango »

Came across this schematic through a google image search. The two differences I noticed are the 22uf cap coming off the wiper of the Fuzz pot and the extra resistor and cap to ground on the tone control. Is this a version 2 by Basic Audio? Or just someone modding the original design?


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Post by destro »

What you have there is the assumed schematic applied from a circuit design mentioned earlier in this thread.

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Post by ggbb »

~arph wrote:Yes, you can influence taper, but shouldn't the 10k be connected to the wiper instead of the low side of the pot to simulate a reverse log action? The way it's connected now just makes the 50kB pot a 8.3kB pot.
I think the tone pot probably needs to have pins 2 and 3 joined. That makes the 10k in parallel with the pot a true 0-8.3k C-taper variable resistor, and it makes the tone control a true shelf control with the 100n in series with varying resistance from 10.5k to 2.2k.

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Post by Boba7 »

destro wrote:Toss a .0022 between the base and collector of Q2 for the 5th cap that all the schems and layouts are missing. The difference is small, but noticeable.

Do you mean 2n2 or 220pf? 2n2 seems a lot

I agree with roseblood11 about the fat pot wiring.

So the circuit is still not exactly traced right? The tone section is still a mystery, and so is the 5th film cap.
And the latest schematic doesn’t include the 4k7 resistor

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Post by elbeeuk »

roseblood11 wrote:After three years, I finally finished my build...
- Using bc547b transistors with hfe around 350 is no problem, I even liked them better than 2n5088 with higher gain
- I tried both variants of the tone circuit that were discussed in this thread. Both are useful, so I put them on a switch as shown in the vero board layout.

Wiring five 9mm pcb-mount pots in a 1590b enclosure is slave labor! Does anybody sell a pcb for this circuit?
They sell a PCB over at PedalPCB :)
https://www.pedalpcb.com/product/dungbeetle/

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Post by benmarshall »

Hi All!

I've got one of these bread boarded at the moment, and it's a great fuzz. Really enjoying the versatility. I've been experimenting with some of the tweaks suggested here and elsewhere for the Hot Silicon type circuits and would like to put a question to someone with more experience than myself with this circuit.

I've seen Hot Silicon designs where R11 (on the schematics in this thread - 750R from Fuzz Pot) can be replaced with a trimmer. I've read this is useful to control the level of gating, however in the Scarab circuit we have the bias control which gives a nice control of the gating and splatty type fuzz. I've been playing with some different fixed values for R11 and to my ear increasing this (I've gone through a few values between 500R and 3.2K) seems to reduce the overall gain of the circuit. Does anyone know if that is correct, either from experience with the Hot Silicon, or from electronic theory? The circuit seems to have quite a lot of gain with stock values, more than I was expecting even with fuzz pot on minimum. It does still clean up pretty nicely with guitar volume but I'd like to see if I can get a slightly broader range of lower gain tones out of the thing.

Any guidance/experience that you can provide is much appreciated!

Edit:
I think I'm going to answer my own question here. After thinking about this some more and trying out a few things whilst checking with a multimeter I can say that R11 definitely is changing the bias of Q3, which I probably should have expected. The "lower gain" sound I was hearing as a result of increasing R11 was a similar effect to turning up the bias knob on the Scarab towards the "overdrive" end which smooths out the fuzz and towards the very end sounds to me like it rolls off the gain a little too. I also realised I should have known the best way to change the overall gain range, without playing with different transistors, is by changing R8 (47k feedback resistor). I had played with this before, went down a little, then up as far as 470k, but wasn't convinced I liked the change. What I've realised this time though is that changing R8 also affects the bias of Q3. Increasing this resistor a lot brings down the bias voltage and puts you down into the splatty, choked out, velcro fuzz type region. When I first tried this it was too extreme and i quickly went back, but this time I adjusted the bias to compensate and found some really awesome tones! I find that at 470k, with pretty high output humbuckers even the "ideal" bias of ~4.5v (which I find is about spot on for normal fuzz sounds with other feedback values) gets a little bit of gating and splat which sounds different, but good. Turn up the bias a little though and this starts to fade and you just get mad amounts of almost distortion like fuzz and loads of sustain. Throw some extreme tone shaping in with the amp or an EQ and your getting into metal muff type territory. I think this all makes sense from reading about the Hot Silicon circuit, and probably isn't new to most, but I thought I would add these notes in case anyone else finds this useful.

I'm still trying to decide what exactly to box up. I'm certainly going to at least socket R8 so I can change it easily. I may even add a toggle switch and have two values available. Or maybe I should stick with stock Scarab values here and build up a different "Hot Silicon" and play with different transistors and feedback values to get a really high gain fuzz? I did say above I want to be able to roll back the gain on the scarab even more sometimes, too many options!

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