Self-made analog delay pedal using BBD (BOSS DM-3 MOD) Self-made analog delay pedal using BBD (BOSS DM-3 MOD)

Self-made analog delay pedal using BBD (BOSS DM-3 MOD)

Make my own full analog delay pedal using BBD

BOSS Analog DelayAmong the effect pedals for guitar, analog delay using BBD is very popular. BOSS continues to sell the analog delay model as one of the "WAZA" series.

Regarding the available self-made delay effect pedal, a production article using the easy-to-produce one-chip IC for delay "PT2399" has been published by TONEPAD as "REBOTE Delay". However, I wanted to try making my own delay using a 4096-stage BBD chip because I am a self-making nerd.

MN3005 is used for BOSS DM-2 analog delay pedal

MN3005 BBD ChipThe schematic for the BOSS DM-2 analog delay pedal can be found in the repair service manual. However, the DM-2 uses the Matsushita BBD chip "MN3005". This chip is nolonger manufactured and hard to obtain. The chips on the market are very expensive or suspicious used chips taken from some abandoned gear.


Changed BBD chip to another specification "MN3205"

Currently, the BBD chip has been changed to another specification chip "MN3205". This chip can be purchased, but it is a little expensive. So I decided to use the cheaper Coolaudio "V3205" (the second source product of Matsushita's "MN3205").

You can buy V3205 for around $4. The clock generator V3102 used together is less than $1.

Difference between BBD chips "MN3005" and "MN3205 (V3205)"

Both of these are functionally the same as the BBD with a built-in 4096-stage bucket element, but the new MN3205 (V3205) is designed to operate with a negative power supply. Simply to say, the power line +9V and GND are the opposite of the MN3005. In addition, the maximum power supply voltage is +9V, which is lower than the + 15V of the MN3005. The recommended voltage of MN3205 (V3205) is +7.5V. Therefore, it seems that the circuit of BOSS DM-2 using MN3005 cannot be used as it is.

Self-made using the circuit of BOSS DM-3

BOSS has a model called DM-3 that uses "MN3205"

BOSS's analog delay DM-2 (released in 1982) is considered to be a famous pedal. On the other hand, there is a model called DM-3, which was sold only for a short period of time after the DM-2. The function is the same as DM-2, but the BBD chip is changed to "MN3205" and it supports stereo output. So I searched for the DM-3 schematic. A service manual was found which contains the schematic using MN3205.

The BOSS effect pedals have an ON / OFF foot switch that is not a mechanical switch but FET electronic switches. I want to make my own delay pedal the True-Bypass specification, so I removed all the FET switches from the schematic, and stereo output as well.

Circuit diagram (DM-3 modify, PDF)

Design a 2-layer PCB and order from "Fusion PCB"

This time, I ordered a printed circuit board that I designed from "Fusion PCB" in Shenzhen, China, which produces 10 PCBs for $25. All board design processes were performed using the freeware "KiCAD". I input the circuit diagram, designed the double-sided circuit board, output the standardized board design data called "Gerber data", and sent it to “Fusion PCB”.

The manufacturing fee is only $5. However, DHL shipping costs $20, so the total is $25. Still, it's cheap enough to make your own board for $25, which used to cost hundreds of dollars a decade ago. It will arrive in about a week.

10 original printed circuit boards that arrived from Fusion PCB
10 original printed circuit boards that arrived from Fusion PCB

About board design

I designed the PCB using the freeware "KiCAD". First of all, I input the schematic with the schematic editor, assign the part shape data called 'footprint' to the used parts, and I can design the pattern with the board editor. I wanted to put this analog delay in an MXR size enclosure, so I made it with a size of 60mm x 100mm. The actual area is smaller because the foot switch area is hollowed out. The pattern design is fairly easy since the double-sided board can draw patterns on the front and back. I connected all the lines except GND, and I filled all the remaining areas with solid ground.

Board design requires some experience (I worked for an electronics design company about 25 years ago). There are many books on CAD and board design methods, so you can refer to them and make your own.

Board design free software "KiCAD"
Board design free software "KiCAD"

Component mounting on the board

I put the purchased parts on the printed circuit board that I ordered for design. For me as a self-making nerd, soldering is the most enjoyable task. Since there are many parts, it is challenging to make, but the fun parts mounting is completed in about 3 hours.

Analog Delay Completed mounting of components on the board
Completed mounting of components on the board

About the main parts

For BBD chips, I used Coodaudio's second source products "V3205, V3102". The DM-3 uses a noise reduction chip (NEC uPC1571C) called a compander. This chip was replaced with Coodaudio's second source product "V571".

As for the transistor for small signal, 2SC1685 and 2SC732 are used for the original DM-3. Since these were difficult to obtain, I used the alternatives "2SC945, 2SC1815". The FET "2SK30A" is also difficult to obtain, but I found it by chance and was able to purchase it. If you cannot get it, "2SK2880" can be used instead.

Coolaudio's second source ICs
Coolaudio's second source ICs

Operation check

"Oops, I made a mistake!"

I checked the operation with the board on the desk. I connected a guitar to the input and connected the output to an amplifier, but I heard a oscillating sound and no guitar sound. I connected a small digital oscilloscope that I recently bought for $ 50 and followed the signal from the input. The signal is coming up to the input operational amplifier (NJM4558DD), but there is no signal coming out from the output pin 7.

I looked closely at the schematic and found a mistake that pins 5 and 6 of the op amp were reversed. I reluctantly cut the pattern and made the wiring to reconnect the pin numbers in reverse.

Oops, I made a mistake!
I cut the pattern and reconnected it to fix the pin number of the operational amplifier that I made a mistake.

A delay sound came out!

The board has three semi-fixed potentiometers. I adjusted them as follows.

CLOCK adjustment: The MN3102 is making the clock to the BBD chip, and this duty ratio should be 1:1. I checked it with a digital oscilloscope and adjusted it.

At this point, I plugged it to a guitar amp and heard a delay sound. Next, I moved on to adjustments to improve performance.

BIAS adjustment: This is a distortion adjustment. Input a 200Hz sine wave and watch the output pin 10 of the compandor V751. Gradually raise the input level and stop where either the top or bottom of the sine wave clips. Turn the BIAS pot and adjust it to the state where the distortion of the sine wave becomes the smallest (the shape of the sine wave becomes clean).

I don't have a signal generator gear that produces a 200Hz sine wave, so I used Windows free software to generate a 200Hz signal and fed the signal from the audio interface "UR22mkII" into this delay input.

CANCEL adjustment: Shorts the input to GND. Monitor pin 2 of the CANCEL semi-fixed pot. Turn the semi-fixed pot and adjust to minimize clock leakage.

With this adjustment, the attenuation of the sound during repeat can be heard clearly.

V3120 output clock adjustment
V3120 output clock adjustment, duty ratio is being confirmed

Built into the enclosure

Overall structure

I designed the component placement so that the phone jack and foot switch do not interfere with the PCB. The small Switchcraft box-type phone jack (# 112BX) can be used. In order to take a 20mm space between the PCB and the other parts, I taped 2mm thick paper spacers inside the enclosure. Unfortunately there is no space to insert a battery because the PCB length is too long.

I made a 3mm hole and pushed the LED from the inside and fixed it with super glue. I don't use the uncool LED holder. If you want to make it in the same way, please refer to the figure below.

Enclosure processing

I housed this analog delay in a blue-painted enclosure of HAMONND's "1590N1". The enclosure was processed using a hand drill. After positioning, the holes were expanded in the order of 1.5mm, 3mm, and 6mm, and then expanded to the hole diameter of each part using a taper reamer.

My own analog delay enclosure processing

My own analog delay enclosure completed

Housing PCB into the enclosure

The board width is made according to this enclosure, so it fits perfectly. The board is inserted with the component side facing outward so that the board looks beautiful. The input / output jack and DC jack are located near the top of the enclosure so that the PCB can be stored. When I open the back cover, it looks beautiful and looks like a ready-made product, and I am very satisfied!

My own analog delay PCB mounting

My own analog delay beautiful looking

Sound sample

The maximum delay time is 300ms. When the FEEDBACK knob is raised above 2 o'clock, it will start oscillating. If I turn the delay time knob while oscillating, I can create an oscillating sound effect peculiar to an analog delay box. The EFFECT knob works exactly as I imagined and is perfect.

Below is a sample delay sound when the delay time is set to MAX 300ms. You can hear the thick and warm sound that is unique to analog delay and does not interfere with the original sound. The feedback repeat sound is gradually distorted and blurred. It has a nice analog feel.

Sample #1: Short chord phrase (MP3)

Sample #2: Short melody (MP3)

My own analog delay completed perfectly!

What I felt after making an analog delay

Music making has changed from a tape recorder to a personal computer Pro Tools. I think that digital technology is used for guitar effectors and amplifiers without being aware of it. However, analog delay is probably an effect pedal that goes against the times.

In the first place, electric guitars are still connected with old-fashioned thick phone plugs, and they are instruments that make a lot of noise from primitive pickups. Therefore, it may not match digital devices such as multi-effects.

In the sound of this analog delay, the high range is cut, and the feedback repeat sound is gradually distorted. This is it! I feel that this is the sound of an electric guitar that started in the 1950s, shortly before I was born. So is spring reverb.

I'm almost 60 years old and have been playing guitar for 45 years. I'm not in a band anymore, but I still find it most fun to play along with the songs of my favorite artists.

For those who want to make their own effect pedal from now on, I made a page explaining how to select a tool. Please refer to it.

My current efferctor board