Jam Box

This is my latest project. I call it "Double Trouble" - it is a two guitar input cross-switch with optional buffering and a switchable FX loop for each channel.

The purpose is to be able to connect two guitars to two amps and to cross-switch the guitars between the amps by pushing one button.

The top-right switch engages the buffer. There are two independent single JFET buffer circuits inside, one for each channel. The buffers are true by-pass using relais. This means that if the power goes out, the pedal switches itself to true by-pass and keeps operating (un-buffered).

The bottom-right switch switches the channels.

The bottom-left switch engages FX loop 1,

the top-left switch engages FX loop 2.

The following picture shows the circuit board with the components:

This is the schematic:

The following pictures show the strip-board layout, the templates for the cuts and jumpers, followed by illustrations of the work in progress and a couple of files for download.

DIY layout (http://bancika.github.io/diy-layout-creator/)

Position of strip cuts (copper side)

Jumper layout (component side)

Cutting the strip cuts

Running the jumpers

Resources:

• The LTSpice layout file: schematic
• The DIY routing and component layout file: layout

My messy pedal board

This is what it looks like. The bottom right pedal and the RT-1 in the top row are my own designs. The bottom right one is a dual input buffered cross-switch with FX loops. The RT-1 is a transparent overdrive / boost with a 12AT7 tube inside. The black bricks top-right are a TC Electronics tuner, a 12V lead battery for effects power and a 9V Sanyo Eneloop battery also for effects power.

To get this more organized, I'm planning to build a board for home use like this: it will be a more narrow design with space for three to four pedals across and three sloped boards on top of each other.

My favorite guitar

This is the guitar I've been playing since about a year:

• Fender Custom Shop 1965 built to spec Stratocaster with relic/closet-classic finish (Nov. 2012)
• Serial #R53483, Part # 11180111-49
• Black, alder body, rosewood fingerboard, white pick-guard
• Mid 60's style oval "C" neck with custom 9.5" TO 12" compound radius
• Mother of pearl inlays
• Abby hand wound 65 pick-ups
• Vintage 2 7/32" string spacing at bridge
• Vintage tone pot wiring (no pot on bridge p/u)

I found this guitar (or it found me) in January 2015. I walked into a guitar store with the aim to get a new guitar (my second one). I didn't have a specific model in mind but I always liked strats. It was a quiet afternoon in the store and I just sat down and took my time trying out about 10 different guitars. Of the ones I tried, this one was just different - the feel, the sound, the look! I didn't know it was a custom shop model until after I had played it. The price was about twice my target budget but I had to have it. Checking the specs against the Fender Custom Shop options and price list after I had bought it, I realized that I actually got a good deal from the shop (a 35% discount).  

Fatter sound? Turn your pick!

I play with my pick turned 90 degrees, the pointy side pointing to the neck. I started doing this because for me, the pick stays in place with less pressure coming from my thumb and index finger.

Over time I noticed a positive change in tone compared to the normal way of holding the pick (warmer, fuller tone). The effect is subtle but consistent across different types of picks.

To analyze it, I recorded a picked A string into my laptop and ran it through Audacity's frequency analysis (Hanning window).

The result is shown below:

• With the turned pick, I get a few decibels more of the 3rd and 4th order harmonic, which explains the fatter sound.
• There also seems to be more overall distortion/noise in the -70dB range with the turned pick. However, I never noticed that through the amp.
• The main frequency is at 220Hz (A3). The harmonics depicted in the diagram are: 440Hz (A4), 660Hz (E5 - harmonically the fifth with respect to A4) and 880Hz (A5).
• I ran several samples through the frequency analysis. The increase on the 3rd and 4th order harmonic was consistent in all the tests.
  

On the side: I recently read in the German magazine "Guitar" (issue 9/2015), that Stevie Ray Vaughan allegedly played like this.

DigiTech TRIO Band Creator Sound

I recently got a DigiTech Trio. A really cool device. I use it to add some background to my blues practicing.

Out of the box, plugged into my guitar amp, there was a problem though. I didn't like the sound of it. It actually hurt my ears. I kept going back to the DigiTech web site to check for firmware updates hoping that the scratchy sound of the drums and snares was something that might be corrected with a software update.

There were no updates and I started thinking about other ways to improve the sound. The solution is actually simple: don't run the bass and drums output from the Trio through your guitar amp. Use a dedicated, linear loudspeaker.

Check out the following video for a sound sample:

Electric Guitar Output Voltage Levels

I was recently interested in how much output voltage you get from guitar pickups. To find out, I connected a guitar to an oscilloscope and did some measurements. Here are the numbers:

• The values are peak voltages in millivolts (double the values for peak-to-peak)
• The 'A' values represent the maximum transient peak voltage I observed (just after the string leaves the pick)
• The 'B' values are measured after about two seconds into the tone.
• I picked (strummed) hard. That's obviously a very subjective statement. Your mileage will vary.
• The values are averages over three to five repetitive measurements.
• The scope used has an input impedance of 1MOhm and an input capacity of 18pF. Probe attenuation was at 1X.

The guitar I used has a volume control and a tone control. For the measurements, volume was on maximum output and the tone control was on minimum impact.

I used the following pickups:

• Single coil (neck and middle): GFS Pro-Tube lipstick
• Humbucker (bridge): Artec Vintage Humbucker LPC210N

The measured DC series resistance of the pickups are as follows:

• Neck: 4.8K
• Middle: 6.2K
• Neck & middle (parallel): 2.7K
• Neck & middle (series): 10.9K
• Humbucker: 8.3K

The difference between soft, medium-hard and hard picking was in my case about a factor of 2 to 3. Meaning: picking the open A string softly I got about 10mV, medium-hard 20mV and hard 30mV (which you'll find under 'A' in the table, above).

And here the screenshots referred to in the table:

a.bmp - neck pickup, open A string.

b.bmp - neck pickup, open E chord. This is one of the lower samples. Most other measurements came in higher.

c.bmp - bridge humbucker, open A string.

d.bmp - bridge humbucker, open E chord.

April 22, 2025: Thanks for all the comments so far. If I wont reply it's probably because I'm busy working on my new thing: InvMon, a professional investment monitoring and portfolio management application.

Scratchy Pots?

Your pot is...

• either too old (resistive layer worn off; wiper doesn't connect well anymore...),
• is dirty (has dust particles under the wiper),
• has DC on the wiper,
• or... (let me know if you've experienced other sources)

For a lot of people obvious, for me something I learned recently: If you have DC voltage at the pot in your audio circuit, your pot will sound scratchy as you adjust the level. Once adjusted, no problem, but as you move the dial you hear an annoying scratchy noise.

A typical cause for the DC are defective caps that leak DC into the circuit. In my case, however, it came from an error in the schematic.

Here's the problem area in my tube overdrive circuit (see the earlier RT-1 post):

And here's the fix to the circuit:

To fix it, I added an additional cap after the pot wiper (C17, you may have to scroll to the right to see it). This reduces the effective coupling capacitance by 50%. To remedy this, I roughly doubled the input impedance of the following buffer circuit (R26 & R27).

By the way, see this very useful page about pots in general: http://www.geofex.com/article_folders/potsecrets/potscret.htm