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Topics - RobA

Open Discussion / Ge transistos in a muff
March 28, 2015, 10:42:14 PM
I've been doing some playing with Ge transistors in a Big Muff like circuit on breadboard. I like it. I just flipped the middle two transistors to PNP configuration and it works well. In the schematic below, I've added some switching in the clipping sections and changed the tone control to be what I used in the Scramjet tone control and then followed by what's essentially a Rat high cut control. The standard Muff TC works perfectly well too, I just wanted to do some playing with this idea.

The diode switching is fairly critical though and needs an on/off/on switch so that you can get a fuzz purely out of the Ge transistors overdriving. I've been playing with some AC125 transistors in the approximately 100 hfe range.

Before I start laying this out for a PCB, I was wondering if anyone had suggestions for improvement or had seen something similar that I should check out first.

Open Discussion / 2SK596S and J201
February 13, 2015, 04:52:20 PM
I'm getting ready to place a Mouser order, so I'm doing my standard run through the transistors to see if they have anything interesting and came across a couple of JFETs. The first is the 2SK596S from On Semi. It's an audio JFET. The only other thing I came across is that it looks to be a Sanyo part that On has now because they purchased Sanyo semiconductor. Anyone know anything about this one? The data sheet is here,

The second is that it looks like there is a new manufacturer of J201's. Totally overpriced especially compared to using SMD with an adapter board, but I figured I put up the link here anyway in case anyone really needed a PTH J201 and didn't want to risk the scammers.
Open Discussion / Arp Odyssey
January 15, 2015, 01:44:05 PM
Seems it's confirmed that Korg is bringing out the Arp Odyssey this month. I've wanted one of these for years.
Open Discussion / Morley Sapphire Flanger Box
December 28, 2014, 08:46:08 PM
While wandering around the web looking for other schematics, I happened on one for the Morley Sapphire Flanger Box -- never heard of it before. It uses an MN3207 with a CD4007 and CD4047 as the logic glue between the LFO and BBD with no weird IC's. So, I'm wondering if anyone's heard the thing.

Here's a link to the schematic,

I've been playing with the expression control of the Flintlock and the Stage Fright and I've been having a blast. So, I'm looking for pointers to other DIY projects with expression pedal control.
Open Discussion / NERF Darts
December 15, 2014, 04:17:38 PM
I was glancing around the house a couple of nights ago for housing material for roll-your-own LED/LDR combos when I spotted a NERF dart. Actually, that's not too hard around here as HvZ is currently a big deal in our house. So, I asked if I could chop one up and was allowed to, but only if I used one of the crappy originals that came with the blaster. End result, the fit is nearly perfect.

I was trying to get a bit of housing that would let me separate the LED from the LDR a bit and the dart let me do that pretty nicely. I just popped the leads through a bit of electrical tape so I could hold them where I wanted, pushed the LED in one end of the tube and the LDR in the other and then wrapped that tape up and put another bit of tape around the cylinder to hold it all together. I think the end result works better than what I was getting with using heat shrink.

It does only work well with 5mm LED's and the larger LDR's. I'm still looking for a cheap and easy housing for the smaller types. Any ideas?
Open Discussion / Heads up on more EoL transistors
December 04, 2014, 06:48:59 PM
Got an email alert from Mouser this morning about On Semi moving the J310 and 2n5089 PTH transistors to end-of-life. I checked a few others, including the 2n3904 from On Semi and they are listed as EoL too. Things must be getting serious if they are killing off the 2n3904.
Open Discussion / New course related to electronics
November 15, 2014, 02:58:57 AM
Not directly audio/music related, but it does look interesting. The focus of the course is on the interface between analog and digital -- sensors, transducers, actuators, etc. So there's some crossover to a lot of the stuff that's beginning to be more prevalent in effects.

The project for the course is supposed to be on building a robot. It's based on the MPS430 microcontroller which is the processor in the EHX expression pedal I just picked up. I think they said the parts kit for the robot would be about $50 US.

I'm thinking about using the processor in a non-audio project, so there's a pretty good chance that I'll try to do the class.

The pre-reqs for it are supposed to high school level algebra.

The link is at
Open Discussion / NTD
November 07, 2014, 11:36:08 PM
T is either for tool or for toy -- not sure which is more true at this point.

I finally got an oscilloscope. After waffling back and forth for months about which DSO to get, I went with analog  ;D. I got a GW-Instek GOS-630FC. It's relatively cheap, has a built in frequency counter in the LCD display, and reminds me of being in physics labs in school.

In the foreground of the pic is the Flintlock, which was the first patient.

Build Reports / Li-Po power supply for active pickups.
October 21, 2014, 06:54:31 PM
Well, it's not a pedal, but I'm happy with it, so here's a report anyway.

I've been working on this for some time now and I'm finally really happy with the results. This is a circuit I built to take the input from a Li-Po battery and outputs 9.6V of really clean power. Actually, according to my measurements, the output power is cleaner than a straight 9V battery. The other main advantage is that the power sits at 9.6V for the entire life of the charge -- and the charge lasts months.

The photo below is of the battery and the boost installed in my Kramer "American" Forum II bass. The bass has active SD pickups in it and active electronics (bass & treble TC). (The "American" is in quotes because according to every thin I can find, the bass was made in Japan.) I've got the Li-Po circuit installed with EMG and Active Blackouts as well.

The boost PCB is sitting on the original battery clip tray and clipped in using a battery snap. The only other connection to the bass is an additional ground wire.

Note that in the photo the charge port isn't connected. I haven't decided yet if I should just drill the mounting hole in the bass' cover plate or not. I can't find any good info yet on if the bass actually has any value or not. I'm guessing no, but if it does, I'll cut a new plate out of plastic to keep the original intact.

Open Discussion / Red Pitaya Oscilloscope
October 17, 2014, 01:55:49 PM
I've been looking around trying to figure out which scope to buy and I came across this thing.

It looks really interesting. The software is open source and there's a dev environment for adding new programs. It comes with an oscilloscope, spectrum analyzer, and signal generator. One the blog, they've just released the source and application for an LCR measuring setup.

Does anyone know anything about this or seen it in action? The specs look good for what I need, but there are some (intriguing) differences between what you tend to see on other DSO. The max sample rate is lower (125MS/s versus more like 1GS/s) but the bit depth of the AD/DA is significantly higher (14bit versus 8bit) so the overall information rate is something like 8 times higher. I'm not sure how that's going to play out in the end. The specs mention a 50MHz bandwidth but that is low for the parts on it, so I;m guessing that it can actually go higher.

Oh yeah, it's a web connected device, so you run the software from a browser on any device you want. It'll work from any PC or tablet, phone, etc.
General Questions / Harbinger LDR specs
October 14, 2014, 02:03:45 AM
I'm getting the parts in line to build a Harbinger and I've got to place an order for some other parts at Allied, so I figured that I'd look at getting the LDR's there. But, they don't have an exact match for the Smallbear part. I looked at the part that is spec'd (but no loner available) in the Neo-Vibe article. That part had a 50k-200k light resistance at 10 Lux and a dark resistance of 20M with a wavelength of 550nm for the peak response. I can find LDR's at Allied for around seventy cents that are close to those specs. The NSL-5542 for example. So, the general is, do I need to look for anything else in the LDR or is that going to be good enough?

A related question is, is the hermetically sealed bit important?
Open Discussion / Rocking before mounting?
October 03, 2014, 11:57:09 AM
... Pots and switches.

I'd like to be able to test a build before soldering in the board mounted pots and switches. I tried doing it with header pins but the connections were just too unreliable. I've seen expensive automated machinery that'll do this, but that's just a tiny bit over the top for DIY (at least for my DIY).

Are there any good methods/tools to do this?
Open Discussion / Not really sure what to think of this
September 05, 2014, 10:10:08 AM
New pedal from Korg.

It sounds like it's based on the Vocaloid synth stuff. My son showed the vocaloid synth to me a few years ago and it's some pretty interesting tech. I'm not really sure about the translation to a stomp box though. Still, it could be fun to play with.
I brought up the idea a bit ago of having a dedicated thread for known good voltages for the projects. It seemed at least some folks thought it might help, but then I got sidetracked and it went to the back of my mind. Anyway, I still think it could be a useful resource and I just happen to have to voltages for my Stage Fright (which by-the-way, I love) sitting at hand, so here goes.

Stage Fright:

Supply voltage in: 9.4

IC1: (TL074)
1   4.67
2   4.67
3   4.25
4   9.33
5   4.64
6   4.65
7   4.65
8   4.67
9   4.64
10   4.55
11   0
12   4.62
13   4.65
14   4.64

IC2: (TL062)
1   Moving (.69 to 8.7)
2   4.65
3   2.8 to 6.5 or so
4   0
5   4.69
6   4.68
7   3 to 6 range
8   9.35

IC3: (NJM13700D)
1   1.16 to 1.27
2   0.291
3   4.57
4   4.57
5   5.72
6   0.31
7   5.6 to 5.78
8   4.58
9   4.59
10   5.64 to 5.7
11   9.16
12   5.63 to 5.73
13   4.58
14   4.58
15   0.38
16   1.13 to 1.27

1   1.12 to 1.3
2   0.286
3   4.61
4   4.61
5   5.72
6   0.3
7   5.61 to 5.78
8   4.58
9   4.56
10   5.66
11   9.15
12   5.58 to 5.69
13   4.58
14   4.58
15   0.317
16   1.14 to 1.28

1   1.11 to 1.3
2   0.29
3   4.63
4   4.61
5   5.64
6   0.4
7   5.68 to 5.75
8   4.58
9   4.60
10   5.75
11   9.23
12   5.60 to 5.76
13   4.62
14   4.61
15   0.363
16   1.14 to 1.28

Q1:  (2N3904)
E   3.90 to 3.98
B   4.56
C   9.14

Q2: (2N5457)
G   0
S   1.184
D   3.90 to 3.98

Q3: (2N3906)
E    4.56
B   3.90 to 3.98
C   1.13 to 1.38
I've been breadboarding with the Fuzz Face circuit to do a positive rail version. This is using PNP Ge transistors. I've read that there are oscillation issues that some people experience when doing this. And, I did an etched PCB for this a bit a go that was a failure because of oscillation at high gain. But, I hadn't been seeing any oscillation with the stuff I had on the breadboard at any gain setting with any set of transistors.

Then I tried to add a sag control. Oscillation everywhere with every transistor at a gain above about 2/3 max. Interesting. I pulled the sag circuit back out and tested using the variable voltage on my bench supply -- no oscillation at any voltage setting. Weird. So, I put a 100Ω resistor on the power input before the filter cap. Oscillation again. I tried it with a 10Ω resistor and it still oscillates, not with all transistor pairs but with at least a few.

So far, it looks to me like I can do this with a positive rail with no risk of oscillation as long as I don't have any restive filtering on the power input. This feels like it makes sense because, to this circuit, it's like putting a resistor on the ground input and I can see that causing some issues.

Does this make sense? Is it consistent with what you've seen or heard? Or, really to the point, has anyone experienced oscillations on a positive rail fuzz face circuit that didn't have any restive filtering on the positive power rail?

Also, it seems workable to add a sag control as long as it's based on an active variable voltage circuit that can sink and source current. The one I'm using that works is built around an LM317. But, simply buffering a pot based voltage divider followed by an emitter follower did have oscillation problems.
Open Discussion / Sticky Thread for Voltages?
July 21, 2014, 04:27:41 PM
Maybe this has already been suggested (and rejected) or it's here and I missed it, but a sticky thread in the Tech Help section with the voltages from known good builds could be really helpful. I know that there are lots of good voltages floating around, but finding them by searching the forums can be pretty hard because the good levels mainly get masked by the requests for help with not quite right (or really wrong) levels. If such a thread were created, I'd be happy to get out the meter and post some levels from the builds of mine that I know work.
Open Discussion / Luxo magnifier?
July 15, 2014, 06:03:10 PM
My not so trusty lighted magnifier died (well, it's got floppy anyway, so it's near death). In looking for a replacement, I've come across the Luxo KFM LED magnifiers and they do look really nice. But, they are really pricey and I can't find any reviews of them, so I was wondering if anyone here has tried them and if they are actually decent and will last a for a good long time.

Here's a link to the product page,
Open Discussion / Under saddle acoustic pickup
July 07, 2014, 02:30:59 PM
I'm looking for recommendations for under saddle acoustic pickups. All of the nice looking pickups I've seen come with preamps and I don't want/can't use a standard acoustic preamp setup on this guitar. The guitar originally had an under saddle pickup in it that really sucked, so I replaced it with one of the Schatten soundboard transducers. I'm actually really happy with the sound of the transducer as it is now, but I'd like to experiment with adding in an under saddle that didn't suck and making a preamp to mix the two.

I see tons of them on Amazon and Ebay, but I have no idea if any of them are any good or not, so I'm hoping someone here has had some direct experience, good or bad, with some of them.

I'm working on a couple of layouts that I want to use 9mm pots and board mount everything. They also have some switches involved. It looks like there could be some height issues between the pots and standard toggles. Does anyone have any experience with mixing these two on a board?

In case it does turn out to be a problem, I've been looking for other switch possibilities and there are two on commercial pedals that look like they might work. The first is the SPST and SPDT rotary toggle that are like those in the Danelectro Cool Cat series. The second would be the push button type like those in the MXR pedals. Problem is, I can't figure out who makes them. I've got a couple of the Cool Cat pedals, and the switches are completely unmarked. I don't own any of the new MXR pedals, so I can't check if they are marked. Does anyone know what brand these switches are or what type. My searches on Mouser have been fruitless. Any other suggestions about ways to do this would be much appreciated too.