Shaka Express - One Last Question

Paul Marossy · August 14, 2004, 10:54:11 AM

I've wanted to build a Shaka Express for a long time now, so I am finally actually doing it. I'm using the PCB layout from GEO, but I noticed a few discrepancies/omissions between Aron's schematic and the GEO layout.

1. GEO layout shows two "C13" caps. I assume the round cap labeled C13 should be C14? And, what size that cap should be isn't specified. I intuitively used a 100uF.

2. Schematic shows V+ as 18V, GEO layout shows 9V connection. Is the GEO layout designed to be operated on only 9V?

3. Value of R22 is not specified, nor is "Ed Rembold's mod" described. What is this mod supposed to do, and how do you determine what size this resistor should be?

4. There are two jumpers required that aren't shown on the PCB layout.

Any help on this one would be appreciated very much. :wink:

R.G. · August 14, 2004, 12:13:02 PM

Man, it's been a long time since that one.

I'll take a look.

Paul Marossy · August 14, 2004, 01:04:56 PM

R.G.

Looking at the layout at again, I think there should be a C14 (cap on Vref) and a C15 (power filter cap).

Paul Marossy · August 15, 2004, 10:26:07 AM

Has anyone else here built this circuit? I need a little clarification here...

Torchy · August 15, 2004, 11:01:33 AM

Quote from: Paul MarossyR.G.

Looking at the layout at again, I think there should be a C14 (cap on Vref) and a C15 (power filter cap).

C14 is below the two 10k resistors - the label is there but obscured.
C15 (power filter cap) would be the one to the right of C13.

Paul Marossy · August 15, 2004, 11:33:05 AM

Thanks Torchy, I figured out that part. What I really need to know is if that GEO layout is meant for 9V or 18V. The circuit was originally designed for 18V, but the GEO layout shows a 9V connection...

RDV · August 15, 2004, 01:39:13 PM

I think that's a 9v version for sure. I don't see why it wouldn't work with 18v though.

RDV

Paul Marossy · August 15, 2004, 08:08:07 PM

Yeah, I'm sure it would operate on 18V just fine, but what I'm wondering is it would sound good with 9V. I guess I'll find out tonight, got the PCB all populated and everything wired up...

Ed Rembold · August 15, 2004, 09:35:36 PM

Paul M,

As R.G. said, that was a long time ago- I breadboarded it mostly because I wanted to see how that "last section" tone control worked.

I thought it was really cool, and told Aron so, he said he lifted it from the Ibanez "super tube screamer".

I'd have to pull my notes to fill you in on the "mod ".....but I do remember this- the opamp sections clip hard, at max gain, running on 9 volts- I don't like opamp clipping. 18v lets you hear only the fets clipping.

Aron can tell you more.....

Thanks, Ed R.

Paul Marossy · August 15, 2004, 11:40:13 PM

Thanks Ed. It looks that resistor marked R22 is in series with C9, at the last FET. I would guess that it changes the tone a little bit, maybe smooths it out a little? I guess I'll have to try it at 9V and 18V and soon as I shut off the computer for the day.

Paul Marossy · August 16, 2004, 01:43:40 PM

OK, I tried it with 9V last night, got it to where it sounds pretty good. I didn't get a chance to try it with 18V yet, but I suspect that it probably sounds better at the higher voltage. I also like the way that the "bite" control works, too. Pretty cool!

Paul Marossy · August 16, 2004, 11:47:39 PM

OK, I looked at the schematic again http://www.diystompboxes.com/pedals/ShakaExpress.pdf and noticed there was a second page at the bottom... :oops:
I printed out the schematic a couple of years ago, and didn't print the other page! :x

Anyhow, I think I have my answer on what R22 is supposed to do. It looks as though the GEO layout only allows for one of the two required resistors for Ed Rembold's mod...

Anyhow, it's going into a box this week and is going to get tested for live use this weekend.

Paul Marossy · August 22, 2004, 02:44:02 AM

OK, one last question: Those trimpots on the the three FETs, I know those are for adjusting the voltage on the drain, but what should they be set to? I have them at half the supply voltage right now. Was that the idea? There is something on the schematic that refers to starting voltages, 15V, etc. I am not sure if I follow what that means...

BTW, I have mine running on 12V. I like the way it sounds at that voltage. To get 12V, I am using the 120VAC/12VAC adapter that was powering my Shaka Tube, which is getting pulled of the board for a little while. I built a rectifier module with an adjustable regulated DC output that goes in between the AC adapter and the Shaka Express. Works well and I don't have to mess with my pedalboard at all.

syzygy · June 02, 2005, 12:51:42 AM

I have just finished this circuit also, and noticed the same discrepancies as Paul did. So, assuming the C13 listed at the input cap is really C15 (because there is another C13 across tone and volume pots) what are the values for C14 and C15? I don't see them listed anywhere? Also, has anyone been able to determine what the "starting voltages" are referring to in the original instructions? ("Starting voltages: 15V, 9.5V, 5.4V") Is this by measuring DC voltage from ground to drain on the FETs?

With my (most likely) incorrect cap values for C14 and C15, and a 12V DC power supply, I am getting a decent sound but want to make the circuit "correct".

Any help would be great! Thanks.

syzygy · June 02, 2005, 09:55:14 PM

Actually I see in the top right hand corner of the Shaka Express schem dated Rev2 3/9/2001 that a 47uF cap is used. :oops: So this maps to C14 on the R.G. Keen layout. I still need to find out what power filter cap to use for C15.

Paul Marossy · June 03, 2005, 12:08:07 AM

If C15 is across the battery, the a 47uF or 100uF would be fine. I don't remember what I used... it's been a while since I built mine!

syzygy · June 03, 2005, 09:27:43 AM

Thanks for the info! The circuit sounded a lot better with those larger sized caps than the smaller ones I was using. Are you still playing your Shaka? I think it sounds great!

Paul Marossy · June 03, 2005, 11:08:00 AM

I haven't played it for a while. I was thinking about putting it back on my pedal board the last week or so. There's just so many different pedals to rotate! :wink:

WGTP · June 03, 2005, 02:02:10 PM

Just a thought. Joe Davidsons Diode Compression Jfet stage would fit in there, hmmmm.

syzygy · June 03, 2005, 02:30:37 PM

Quote from: WGTPJust a thought. Joe Davidsons Diode Compression Jfet stage would fit in there, hmmmm.

Looks like you are a fan of the Vulcan... are you referring to the home made opamp on Joe's web site?

How does it compare with regular opamps, and what other characteristics does it bring to the party?

Thanks!

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Shaka Express - One Last Question