Low noise 9V to 33V DC/DC converter

This DC/DC converter is well suited for guitar effects projects where you have 9V input power but need 33V for your circuit as well (e.g. for tubes or for discrete transistor circuits with a lot of head room). The boost converter IC in use switches at high frequencies. It is very quiet for the guitar (audio) projects I used it for.

The output voltage can be varied by using different values for R1 and R2. 33V is about as much as you can get out of the converter (I actually measured 32.6V with R1 and R2 as shown). See the LT1613 datasheet for the formula to calculate the values for R1 and R2 for lower voltages.

R3 in the schema above is optional. It is not needed for the LT1613 (input voltages of less than 10V). There's a jumper in the diagram below (just above the 39K resistor) in place of R3.

C2 is optional too. It helps reduce output ripple. I simply soldered a small ceramic 2.2nF cap over the resistor.

Layout is the most crucial design aspect for obtaining low noise (and to actually make it work - because of the high switching frequency it won't run as a bread-board prototype). Use big area trace to minimize impedance. The LT1613 GND pin (center left pin in the diagram below) carries high speed, switched current; its path to the circuit's power exit should be direct and highly conductive at all frequencies. R2's return current (bottom left pin), to the extend possible, should not mix with pin 2's large dynamic currents (center left pin). C1 and C3 should be located close to pin 5 (top right) and D1 respectively. Their grounded ends should tie directly to the ground plane. Pin 1 (top left) has a small area, minimizing radiation. (Source: Analog Circuit Design Volume 2: Immersion in the Black Art of Analog Design, Volume 2 and LinearTechnology LT1613 data sheet.)

The following picture shows a possible component layout on a small PCB using both sides for the components. The components drawn in black are visible (front side), the components drawn in gray are located on the back side. The 5-pin component in the center is the LT1613.

Component selection is also important. I used the following components:

• Inductor: Coiltronics DR73-100, 10uH
• Capacitors (C1, C3): Kemet T495D226K035ATE300, Tantalum 22uF 35V
• Schottky-Diode: ST 1N5819RL, 40V

Creating the PCB

This PDF (editable with Adobe Illustrator) shows the mirrored trace layout in actual size. Use the PDF to create your own PCB. I did it as follows:

• Print the layout file on transparent laser printer foil using a laser printer
• Put a flat board on your work desk
• Put a piece of circuit board (one sided 35um copper layer) on the board. Copper up. Make the printout slightly smaller than your circuit board
• Put your printout on top, Toner against the copper.
• Use Scotch Magic tape to tape your printout to the copper around the edges.
• Put a thin piece of linen cloth on top.
• Fire up your household iron. Set it to high (linen or similar)
• Iron the toner onto the copper using a lot of pressure and, in my case, one to two minutes of time. You may have to experiment a little at this point to get perfect results.
• Peel the foil of the circuit board. All toner should stay on the copper.
• Cut your board to size
• Put the board in a bath of natrium persulfate (a little more than hand-warm) for an hour or two (you'll see when it's done). Don't put your hand in the solution. I actually used to baths: a bucket with hot water and a small tupperware container with the persulfate solution - the tupperware container swimming in the bigger bucket.
• Here's a site that provides much more info on this process: https://www.smallbearelec.com/HowTos/DirectPCBoards/DirectPCBoards.htm

And here some pictures for illustration

A laser printer printout ironed onto copper and then peeled off.

The PCB in the natrium persulfate bath

Not pretty but effective: thick short straight connections from the IC to the caps.

I first used a conductor with through-hole leads. I then switched to a different surface mount inductor which required improvised solder pads. 

Ready for a test drive.

The DC/DC converter in my RealTube-One pedal. L2 and C4 are for additional noise filtering.

Without additional noise filtering.

With additional LC noise filter (L2 and C4)

RealTube-One - A 12at7 Tube Overdrive Pedal

In an earlier post I presented the BluesBox - my own design of a valve-based overdrive pedal. It was a great learning experience but didn't quite hit the sweet spot I was looking for with respect to sound and usability.

I'm presenting now the RT-1 pedal. A complete overhaul of the original BluesBox design. This time... yeah! Exactly what I was looking for. A fantastic sounding pedal. My specs to start with:

• very transparent and open
• very dynamic
• a large amount of sweet sounding even order harmonics
• full, rolling tube clipping and distortion
• tone controls which, at their 12 o'clock position, provide a linear (or near linear) frequency response

The Result - RealTube-One

Technical Specs

• two valve (triode) gain stages (a 12AT7 twin triode)
• three discrete transistor gain stages
• maximum gain of 30dB (mileage varies based on component selection)
• controls for level (volume), "crunch" (gain/distortion), treble, bass
• 9 to 12V DC power input at about 170mA
• 33V DC from an internal DC/DC converter (top-right in the picture above) to run the triodes and two of the transistor stages
• 1M Ohm input impedance
• 3.3K Ohm output impedance
• high quality foil capacitors in the signal path

Below I'm providing the schematic, strip-board layout and other helpful resources. 

The Schematic

Download LTSpice (.asc) file here (Note: I used LTSpice just for drawing. The schematic cannot be run as a simulation). LTSpice is from Linear Technology.


The Strip-Board Layout

Download the layout file as PDF, or as a complete DIY project file. The DIY file opens with the DIY Layout Creator.


The Jumpers

Download the jumpers file as PDF.


Trace Cuts and Solder Bridges

Download the file as PDF. This file is a mirror image, looking at the board's trace side. The blue jumpers are provided as a reference. They help locate the correct positions of the cuts.

Notes on the DC/DC converter and a PCB layout for it will follow in a separate post.

I used the Type BB enclosure from Musikding. It's easy to work with (like drilling the holes) and has pleasing dimensions. Fitting everything into the box, however, was a bit of a challenge.

One of the coming weekends I'm planning to spray-paint the box and put some nice white lettering onto it (using the DecalPro system).


Possible Modifications

• The two bias trim pots are not too important. They can be replaced with 100 Ohm resistors.
• The first transistor stage in the design is a JFET based input buffer / booster. With a 1.2K drain resistor (or 1.3K as shown in the schematic), it provides very little gain. By incrementing the drain resistor you can get a little more gain out of the input buffer stage. At the same time, you will also start to get distortion from the transistor. The level of this distortion depends a lot on the output level of your pickups. By tickling the MPF102 just a little, you get very nice first and second order harmonic distortion. By pushing it too far, you get some pretty bad sounding distortion. However, this can be used in an interesting way: driving the signal further, through the next transistor stages and finally through the tube stages, you will get some pretty cool fuzz-like sounds out of the box. The modification I have in mind is a three way switch allowing to switch between three sizes of the drain resistor: 1.2K, 1.6K and 2K (clean, harmonic and fuzz).
• The tone controls allow to adjust for bass boost, bass cut, treble boost and treble cut quite nicely. You can also get a nice mid scooped kind of frequency response. Mid boost is more difficult to get. I'm considering modifying the tone stack to also be able to shape a mid-boost kind of frequency shape.

Disclaimer: The schematic and all other resources of this post are provided "as is". I put a lot of effort into reviewing the schematic and layout for errors. However, I do not provide any guarantee as to their correctness. Do not use the pictures as a reference (for example, the transistor orientation is wrong on some of the pictures). Always refer the provided PDFs and DIY file.

Should you decide to build this for yourself: it is not an easy project! If you can't read the schematic at all you will not be able to turn it into a functioning device. I will not be able to provide any support. General questions and comments, however, are welcome.

Copyright notice: All resources provided are my copyright. I'm granting you the right to use these resources for your own use with no limitations, except when used as a basis for commercial production of sound effect products. 

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.

BluesBox - Tweaks and Samples

The source of a lot of noise and the chirping sound turned out to be the tone stack driver. The noise could be eliminated by bypassing the tone stack. So the noise was obviously added to the signal path. However, the chirping sound remained even after bypassing tone stack. Therefore, the tone stack driver circuit introduced the chirping sound on the signal ground. Nasty!

Since I had enough of overall gain to work with, I decided to go for a simple solution. I disconnected the tone stack driver circuit (the recovery amplyfier part of it) and put the passive tone stack after the tube gain stage. To be able to model the sound going into the distortion stage a little bit, I added a sinple passive variable bass cut between the pre-gain stage and the tube stage.

The following movie shows the final result. The second movie shows the BluesBox in action.

BluesBox: First Tests

Plugging in the guitar and turning on the BluesBox for the first time was a pretty cool experience. It almost worked as expected! Testing it for about an hour with various settings and levels revealed following problems:

• Turned all the way up it produced a lot of gain but was too noisy. The source of the noise turned out to be the DC/DC converter. That's something I expected - I also had an idea what I could do about it.
• The tone stack driver stage produced a lot of noise on its own. The source of that wasn't clear to me. By using the tone stack bypass switch, this noise could be eliminated (loosing the tone controls doing so).
• There was a low but audible high frequency chirping/beeping sound. The source of it I couldn't easily identify at first.
• It looked like I needed to modify the pots for the tone stack. In the 12pm position of both the bass and treeble controls the sound didn't seem to have a flat frequency response.

Regardless of the above issues, it sounded really cool. With the separate bass, treeble and the two gain controls I was able to mimic the sounds of a DigiTech Bad Monkey in an A/B switch setup very well. My box was noisier, but the sound hat more dynamics and overall sounded very promising.

The following frequency diagrams show some of the characteristics of the BluesBox. The diagrams were created with Audacity on a laptop. I generated a sine sweep from 20Hz to 20000Hz, fed that into the BluesBox and then recorded the BluesBox output.

Frequency response of the 12at7 tube. The two cascaded triodes of the tube produce a maximum gain of about 25dB in my circuit. (The output level with the tube stage on bypass lies at -60dB. With the tube stage on, the level is at about -35dB.)

Frequency repsonse of the tone stack at positions 12pm and 12pm. It should be flat!

Frequency response of the tone stack at positions 8:30am and 8:30am. Expected would be a mid-boost around 700Hz.

A recording of the noise and chirping sounds.

Building the BluesBox

The schematic for the first complete build is depicted in the following illustration:

Prototyping the pre-gain and tube (distortion) stages:

Building it:

Top-right is the 9V to 34V DC/DC converter that generates the plate (anode) voltage for the tube. It is based on a Linear Technology  LT1615-1 DC/DC step-up converter. Top left is the tone stack driver, bottom left is the tube driver (pre-gain) stage.

The following picture shows a gut-shot of the completed pedal. There was no space left for a proper tube socket. I therefore decided to solder the wires straight to the tube's pins and hold it in place with a piece of folded sheet metal.

Links and Resources

In my last post I wrote about the conceptual design of the "BluesBox". The main components (or stages) are a simple EQ (tone stack), a booster stage and a clipping (distortion) stage. Various example circuits for these stages can easily be found on the web. The purpose of this post is to list the most valuable resources I came across during my research.

Helpful links about tube amplifier design:

• FUN WITH TUBES
• The Basic Triode Gain Stage
• Valve Technology - A Practical Guide
• Designing Common-Cathode Triode Amplifiers
• The Cool Sound of Tubes
• Building Valve Pedals

For my design I decided to use a 12AT7 tube (a dual triode). Both triodes operate in a Common Cathode circuit, the first feeding the second one. The output volume pot of the preceding booster (a bipolar transistor stage) adjusts the clipping of the first triode and a second volume pot after the first triode stage adjusts the clipping of the second triode. This results in two gain pots allowing to independently control the clipping of the positive and negative semi-cycles of the guitar signal.

Input-buffer and booster circuits

• A Discrete FET Guitar Preamp
• FET Preamplifier
• Design Guidelines for JFET Audio Preamplifier Circuits
• Design Guidelines for Bipolar Transistor Audio Preamplifier Circuits
• Bipolar Transistor Amplifiers
• A Comparison of Various Bipolar Transistor Biasing Circuits
• An Introduction to the Amplifier Tutorial

For the BluesBox I picked a JFet pre-amplifier / input buffer circuit (the second link in the list above) followed by a bipolar (NPN) transistor "booster" circuit.

Tone stacks

• The Tone Stack Explained in English for Humans
• Adam's Amplifiers: Tone Stacks
• Practical Tone Control Circuits
• Passive Baxandall

I decided to incorporate a Baxandall tone stack. It's a two-knob tone control allowing to independently adjust the lows and highs of the signal. Combining the Baxandall tone stack with a booster stage (and booster output volume pot) you get three pots in total allowing you to quite nicely adjust the lows, highs and, with the volume pot, mid-boost or mid-scoop of the signal.

Buffer circuits

• Basic Buffers
• Guitar Buffer Circuits

The output signal of the tube stage I decided to run through an output buffer circuit yielding a low output impedance of the pedal.

General info and calculators

• All About Capacitors
• How to choose an emitter bypass cap value
• Coupling Capacitor Calculator
• Hochpass- und Tiefpass- Berechnung (hi-pass & low-pass calculator; in German)
• Sengpielaudio dB Calculator (in German)

...and some random pages I found particularly cool or enlightening

• GM Arts
• Selected Stompbox Schematics
• Hilltree Productions Projects Page
• The Beast with a Thousand JFETs

DIY Tube Overdrive

One post that really intrigued me and got me thinking about designing my own overdrive effects pedal was Dave Mac's version of the Matsumin Valve Caster. The Valve Caster is a simple circuit built around a 12AU7 tube running at 9 volts. The 9 volts feed the tube's heater and also provide the tube's plate (anode) voltage.

At 9 volts, the tube operates way below its intended operating voltage. Most designs use plate voltages of between 100 and 300 volts. I was skeptical that at 9 volts, the tube would sound and perform the way I had in mind. I decided to use a higher plate voltage. I also decided to use a higher mu (amplification factor) tube.

I finally picked the 12AT7 tube for my design. In terms of mu it lies between the 12AU7 and the 12AX7. The 12AX7 is a very popular tube used in the pre-amp stages of many great guitar amps. The main reason I chose the 12AT7 over the 12AX7 was its higher idle current - something I considered an advantage in a low voltage design.

So the heart of my overdrive circuit would be a 12AT7 tube running at more than 9 volts. However, I wanted my pedal to be able to run off the standard 9 volts of power supply most pedals use. As a result, I needed an internal DC/DC voltage converter in my design. I also decided to run a pre-amp stage in front of the tube with adjustable output in order to vary the level of overdrive the tube would generate. A simple EQ would be nice too I thought, to independently shape the lows and highs going into the tube stage.

The following diagram shows the main stages of what I had in mind for my circuit.

Overall block diagram - the guitar tone going into a simple EQ, then into a booster (with adjustable output) and finally into a tube amplification stage producing (hopefully) a nice over-driven guitar tone.

The challenge now was to turn this conceptual design into a working electronic circuit! Quite a challenge considering that my knowledge of electronics and amplifier design circuits was (and still is) very limited. I needed to do some serious research!