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.

The BOSS DM-2 analog delay uses the discontinued MN3005.

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.

 

BBD "MN3205" with different specifications is produced.

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

Tried to make my own based on 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)

DM-3 has a double noise reduction circuit, which gives very good S / N characteristics!

The DM-3 has a double noise reduction circuit. First, it has pre-emphasis and de-emphasis circuits that use operational amplifiers for input and output. In order to improve the S / N of the BBD delay part, the treble range is raised at the input, and after passing through the delay circuit, the treble range is lowered at the output. This can reduce the high-frequency noise generated in the BBD circuit. Similar technology is used in FM radio broadcasting to improve S / N.
The DM-3 consists of a high shelving filter and a low shelving filter that use operational amplifiers. It raises 1kHz or more by a maximum of 14dB (amplitude 5 times), passes through a delay circuit, and then lowers it by 14dB.

Moreover, about the processing of a delay sound, the compressor and the expander are constructed by using half each of V571 before and after the BBD circuit. Thereby, BBD noise is reduced. This is a circuit similar to the noise reduction of DBX and Dolby used in analog tape recorders. This expander raises the volume only when there is a delay sound, and turns down the volume when there is no delay sound, so BBD noise when there is no sound is almost inaudible. The DM-3 is a delay effect pedal with low noise and very good S / N.

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: This adjustment determines the maximum delay time. Set the RATE pot in advance so that the delay time is the longest. Turning the CLOCK trimmer all the way to the right maximizes the delay time, but increases the aliasing distortion (the delay sound sounds jerky and muddy). Gradually move it back to the left and set it where you can't hear the aliasing distortion (where you can tolerate it) while listening to the delay sound. By design, the maximum delay time is about 300ms.

BIAS adjustment: This is a distortion adjustment. Input a 200Hz sine wave and monitor the output pin 10 of the compander V751. Gradually raise the input level and stop where either the top or bottom of the sine wave clips. Turn the BIAS trimmer to adjust to the point where the sine wave distortion is the smallest (where the sine wave shape is clean).
If you don't have an oscilloscope, set the effect level pot to MAX, connect it to an amp, listen to the guitar, and turn BIAS to find the place where the delay distortion is minimized.

I don't have a signal generator, so in order to generate a 200Hz sine wave, I use the Windows free software "WaveGene" to generate the signal. The signal from the home recording audio interface "Steinberg UR22mkII" was used as the input for this delay.

CANCEL adjustment: Shorts the input to GND. CANCEL Monitor the semi-fixed pin 2. Turn the semi-fixed to adjust to minimize clock leakage.
If you don't have an oscilloscope, set the effect level pot to MAX, connect it to an amplifier, listen to the delay sound, and turn CANCEL to find a place where the noise on the delay sound (It sounds jerky) is minimized. If the input is silent, the V571 expander is closed and there is no delay sound itself. You need to add some input.

With this adjustment, the repeated delay sound 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 pedals 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

[Additional MOD] Added delay time indicator

I added an LED indicator to show the delay repeat interval. This analog delay doesn't have a TAP function, and the delay time is as short as 300ms in the first place, so it may not be useful as a repeat indicator. This is a self-satisfying mod.

The method is to divide the clock for driving the BBD (V3205) generated by the clock generator (V3012). Since V3205 is a 4096-stage BBD, the clock is divided by 2048 (a half of the clock) to create a clock with a repeat interval of the delay sound.

Since 2048 is 2 to the 11th power, the clock of V3205 was divided using the binary counter "TC4040" with 12 stages. The schematic is below. (Click here to see the entire schematic.)

Since the TC4040 is a CMOS-IC, the current that drives the LED directly cannot be obtained, so it is driven via a transistor (2SC1815). Furthermore, in order to prevent noise at the edge of the blinking clock of the LED, a time constant is added by CR (R2 and C1) so that the LED lights are turned on gently.

I made it with a universal board, put it in a shield box made of aluminum foil tape and vinyl sheet, and stored it in the battery space.

It flashes as below. It seems to be a guide for the tempo (about quarter notes).


(This is a GIF animation created as an image. It is not an actual video.)

[Additional MOD Part 2] Add BBD and extend the delay time to 600ms

I added another BBD (V3205) and modified it to extend the delay time. As a method, the clock generator (V3101) can drive two BBDs even if they are connected in parallel, so simply connect another BBD in series to double the delay time.

The connection from BBD1 to BBD2 has a DC offset on the BBD output, so I cut the DC with a capacitor (C116) and gave it the same bias as BBD1.

BBD output requires a BBD odd-numbered and even-numbered mixing pot. The circuit diagram of the BBD part is as follows.

BBD addition modification for DM-3

The BBD to be added is taped directly above the original BBD. The pins that connect in parallel to the original BBD are soldered directly, the other pins are flipped up and the wires are soldered to the pins. Additional parts such as semi-fixed capacitors, resistors, etc. are made on a universal board cut into small pieces and attached to the empty part of the printed circuit board with double-sided tape.

Sound sample with extended delay time

These are sound samples with a delay time set to about 560ms. With the original 300ms, the time is too short and it often interferes with the rhythm, but if you can set it up to nearly 600ms, the usage will expand.

Also, if two BBDs are used, the number of stages (the number of BBD buckets) will be doubled. Therefore, if you want to get the same delay time, you can double the clock frequency feeding to BBD. As a result, the folding noise (digital distortion caused by sampling and discretizing) can be reduced and the delay sound becomes clearer.

Sample #1: Short chord(MP3)

Sample #2: Solo phrase(MP3)