Saturday, 21 October 2017

EHX Big Muff - Basic build

This must one of the most studied, rebuilt, cloned, copied circuits ever.
The famous Mike Matthews Electro-Harmonix Big Muff guitar pedal. Pink Floyd used it. So did the Smashing Pumpkins. Jimmy Hedndrix gave it immortality. The list of artists is endless.

There are countless variations too. You can buy kits on the net with PCB & parts supplied to save you the hassle of understanding the circuit. But I like to know how things work. I'll build this on perf board.
So here is the basic circuit.
So how does it work?
The signal comes in through R2 and C1 before hitting the first stage, a basic transistor signal booster (Q4). Once the signal is boosted, it goes through C4 and enters R24 (the sustain potentiometer). This controls the gain. 

The signal goes through C5 and R19 before hitting the next stage, which is doing 2 things. Boosting  and clipping the signal. The clipping is achieved via the diodes (D1 and D2) & C6/C12.

The signal then goes through C13 and R12 before hitting the next stage : tone control.
It's identical to the previous diode clipping stage apart from the tone pot (R25) which pans between a high-pass and a low-pass. This explains why when you turn the tone control down the sound is very bassy, while turning it up cuts the bass, and it gets very bright.

After leaving the tone pot, the signal goes through C3 and enters into the final signal boosting stage before exiting through C2 and going out of the volume control into the pedal’s output.


Many thanks to Kit Rae for a wonderful website.
If you haven't visited it yet here is the link:
This is the site to go to for everything you could ever need on this topic.

The first pedals came out in the late 1960s and over the last nearly 60 years the designs have barely changed. It says a lot about how good the basic design is. Of course there has been lots of tweaking over the decades. At one point the four transistor design was changed to op amps. And for another one of those decades Electro-harmonics transferred production to Russia.

Today the company is strong and still building Big Muffs. I highly recommend the modern versions (I own a few and love them) but I think it is fun and educational to explore the older variations. These go for astronomical prices on Ebay & Reverb today. Vintage "Triangle" and "Foxey Lady" pedals regularly change hands for over $1500.
If you can afford the real vintage thing go for it as I think nothing really beats vintage components. They have a persona which only the years can give. But probably the only chance I will have to experience something like the vintage sound will be to build my own.  So below is a brief comparison between the different variations. It's a great way to explore the evolution.

Starting with the early "Triangle" pedal

Triangle V1

Circa 1971 Circa 1971/2 Circa 1972-73

R2 33k 33k 33k
R3 100K 100K 100k
R4 2.7K 2.7K 2.7k
R5 33k 33k 39k
R6 10K 12K 12k
R7 390K 390K 390k
R8 33k 33k 39k
R9 470k 470k 390k
R10 150R 100R 82R
R11 15K 15K (or 12K) 22k
R12 8.2k 8.2k 8.2k
R13 15K 10K (or 15K) 22k
R14 100K 100K 82k
R15 470k 470k 390k
R16 100K 100K 82k
R17 470k 470k 390k
R18 10K 10K 12k
R19 8.2k 8.2k 8.2k
R20 100K 100K 82k
R21 100R 100R 150R
R22 100R 100R 82R
R23 1K 820R 820R
R24 100K 100K 100k
R25 100K 100K 100k
R26 100K 100K 100k

C1 0.12uF 0.1uF 0.1uF
C2 1uF (or .1uF) 0.1uF 0.1uF
C3 1uF 0.1uF 0.1uF
C4 1uF 0.1uF 0.1uF
C5 0.12uF 0.1uF 0.1uF
C6 0.12uF 0.1uF 0.05uF
C7 1uF 0.1uF 0.05uF
C8 0.01uF 0.01uF 0.01uF
C9 0.004uF 0.004uF 0.004uF
C10 500pF 500pF 500pF
C11 500pF 500pF 560pF
C12 500pF 500pF 560pF
C13 0.12uF 0.1uF 0.1uF

Q1 2N5133 FS36999 FS37000
Q2 2N5133 FS36999 FS37000
Q3 2N5133 FS36999 FS37000
Q4 2N5133 FS36999 FS37000

D1 1N914 1N914 1N914
D2 1N914 1N914 1N914
D3 1N914 1N914 1N914
D4 1N914 1N914 1N914

The values seem to have changed slightly over those 3 years.
There is a lot of scope to do some tweaking to find that sound that's right for your needs.
Maybe play around with carbon vs metal resistors? In the 1970s those carbon resistors would have been 10% tolerance or more ???

R24, 25 & R26 are the pots.  
Vintage USA Big Muffs (V1, V2, V3) used single-gang, linear taper, 24mm,100k potentiomers.
A possible future mod could be to use B100K for R25 & R24 & A100k for the Tone (R25).
Log pots (A-100K type) have a longer usuable mid position and less at the ends. ... better fine tuning.
Could be useful for the tone section.

 Leakage current for the 1N4148 as 5 µA .... I think the same as the old 1N914

What transistors to use???
They need to be NPN bipolar
BC549C, BC550, BC239, SE4010, and 2N5210 work according to Kit Rae.
2N5088, 2N5089, MPSA18, 2N3904, 2N4401 are possible candidates.
It might be best to use sockets to allow experimentation.
Were the transistors matched in the early pedals?

Kit Rae's valuable diagnosis identifies three sets of components which alter the BMPs the most.
1. The clipping blocking caps. (C6, C7)
2. The feedback / filter caps. (C10, C12, C11)
3. The Tone stack filter. (C9, C8, R8, R5).

The clipping blocking caps
" C6 & C7 determine the bandwith to be clipped by the diodes". They are usually identical in value and have a great effect on the sound. The smaller the value, the more bass. From the photos I've seem it looks like these are ceramic.

Feedback Filter caps;
C10,C12,C11 usually have identical values. They determine how buzzy/fuzzy the pedal sounds. "They filter the amount of high frequencies. Larger values roll off more highs and smaller values give more crunch and buzz to the sound. The better sounding vintage Big Muffs tend to have the larger values and sound smoother and warmer. Modern Big Muffs tend to have the lower values and sound a bit more buzzy/fuzzy." (Kit Rae).

The pics of vintage units indicate ceramics.

The ToneStack filter.
These 4 components surround the tone pot (R25).
C8, C9, R8 & R5. This section influence the mid frequencies. The more mids that are removed, the deeper & darker will be the sounds and the more the muff will probably stand out in a mix. The two resistors determine the amt of mids removed. Looks like 33K was the most common value for R8 & R5.
"Higher values have less mids scooped out and lower values have more mids scooped. This resistor also works in conjunction with the high pass tone capacitor (.0004uF in the example above) to affect the treble at low tone settings. Reducing this cap to .003uF alters the range and scoops slightly more mids. Increasing to .004uF reduces the scoop. Combined, a larger high pass cap and smaller resistor decreases the treble at low tone settings." (Kit Rae)

The pics of vintage V1 boards suggest C8 is a polyester  & C9 be ceramic.

Muffs Guts (Kit Rae)
Electro-Harmonic's Time Line

To be continued......

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