Basic Audio - Scarab Deluxe  [traced]

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Re: REQ: Basic Audio Scarab Deluxe

Postby storyboardist » 02 Jun 2014, 21:41

IvIark wrote:Yes that looks like the way John described it, or at least that's how I understood it.


Cool. Maybe John will chime back in and confirm. Thanks for you input lvlark. Oh, and thanks for your blog. Without it I would not be building pedals and would have zero soldering skills. :thumbsup
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Re: REQ: Basic Audio Scarab Deluxe

Postby IvIark » 02 Jun 2014, 22:06

:thumbsup
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Re: REQ: Basic Audio Scarab Deluxe

Postby John Lyons » 02 Jun 2014, 22:22

I meant Q4, correct.
Remove C8, put R13 between C7 and ground.
R7 should be 100 ohms.
100 ohms between C5 and ground.

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Re: REQ: Basic Audio Scarab Deluxe

Postby storyboardist » 03 Jun 2014, 00:48

Thanks John! Here's the final schematic.

Image

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Re: REQ: Basic Audio Scarab Deluxe

Postby roseblood11 » 03 Jun 2014, 01:45

Why is C1 so big? Wouldn't 1µF sound exactly the same? I don't like electrolytics in the signal path...
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Re: REQ: Basic Audio Scarab Deluxe

Postby John Lyons » 03 Jun 2014, 02:58

1uf might be enough for you. The cap stores energy as well as passes signal at all frequencies.
I like the sloppiness of 10uf. My goal is to make it as tight and responsive with the .01 and then sloppy
with the 10uf. So you set the FAT knob as you like. 10uf has a good bit of thump to it...

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Re: REQ: Basic Audio Scarab Deluxe

Postby IvIark » 03 Jun 2014, 08:53

So does Q4 base and collector connect to the bias resistor? That looks peculiar to me.
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Re: REQ: Basic Audio Scarab Deluxe

Postby ~arph » 03 Jun 2014, 09:29

Why not replace the tone pot with a 10k and leave out R13? (or am I missing something? it's still early)
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Re: REQ: Basic Audio Scarab Deluxe

Postby IvIark » 03 Jun 2014, 09:46

roseblood11 wrote:Why is C1 so big? Wouldn't 1µF sound exactly the same? I don't like electrolytics in the signal path...

Get some of the 106 multilayer ceramics like these.
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Re: REQ: Basic Audio Scarab Deluxe

Postby IvIark » 03 Jun 2014, 12:39

IvIark wrote:So does Q4 base and collector connect to the bias resistor? That looks peculiar to me.

Just going back to this, I'm assuming the Bias pot just connects to Q3 collector, and Q4 collector actually connects straight to the supply. That would then be in keeping with the Hot Silicon.
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Re: REQ: Basic Audio Scarab Deluxe

Postby Nocentelli » 03 Jun 2014, 13:12

IvIark wrote:
IvIark wrote:So does Q4 base and collector connect to the bias resistor? That looks peculiar to me.

Just going back to this, I'm assuming the Bias pot just connects to Q3 collector, and Q4 collector actually connects straight to the supply. That would then be in keeping with the Hot Silicon.


I thought it looked odd, and also assume the Q4 collector is only connected to +9v. If the collector and base were shorted, wouldn't it just be a diode?
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Re: REQ: Basic Audio Scarab Deluxe

Postby ~arph » 03 Jun 2014, 14:17

storyboardist wrote:Thanks John! Here's the final schematic.

http://i756.photobucket.com/albums/xx20 ... 906ef3.jpg


Please add your name to the image, so it doesn't look like it was just me :wink:
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Re: REQ: Basic Audio Scarab Deluxe

Postby storyboardist » 03 Jun 2014, 14:45

~arph wrote:Please add your name to the image, so it doesn't look like it was just me :wink:


I did. Team effort! 8)

Here's the schem again, hopefully in its final form, with the Bias control moved to effect only Q3.

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Re: REQ: Basic Audio Scarab Deluxe

Postby IvIark » 03 Jun 2014, 15:10

Image
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Re: REQ: Basic Audio Scarab Deluxe

Postby John Lyons » 03 Jun 2014, 16:13

~arph wrote:Why not replace the tone pot with a 10k and leave out R13? (or am I missing something? it's still early)


The 10k/.1/2k2 is the low pass filter. The 50k pot pans between a low pass and tone network bypass (for the most part)
Imagine the big muff tone network with only the low pass side.
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Re: REQ: Basic Audio Scarab Deluxe

Postby IvIark » 03 Jun 2014, 16:34

I think what ~arph was saying is that because the 10K is in parallel with the tone pot, it's just like having an 8.3K tone pot, so why not just omit the 10K resistor and use a 10K for the pot.
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Re: REQ: Basic Audio Scarab Deluxe

Postby ~arph » 03 Jun 2014, 19:10

IvIark wrote:I think what ~arph was saying is that because the 10K is in parallel with the tone pot, it's just like having an 8.3K tone pot, so why not just omit the 10K resistor and use a 10K for the pot.


Yup
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Re: REQ: Basic Audio Scarab Deluxe

Postby John Lyons » 03 Jun 2014, 23:36

Ok, I see it now. Paralleling the pot changes the taper as well.
Cheaper to use a 50KB pot rather than a 10KC...
The 10k resistor is only a couple cents. A 10KC pot is $1.50 or so.
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Re: REQ: Basic Audio Scarab Deluxe

Postby ~arph » 04 Jun 2014, 08:53

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.
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Re: REQ: Basic Audio Scarab Deluxe

Postby storyboardist » 04 Jun 2014, 20:37

Drew up a perf and pcb layout.

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