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Old 10th June 2004, 09:47 AM   #1
raf is offline raf  United Kingdom
diyAudio Member
Join Date: Jun 2004
Location: South East
Question New - need advice on OPA2604

Hi there,

I am new to this board. I met a helpful guy in Maplin yesterday who told me about this website. AS i had no idea of electronics I wanted to mod my Super Overdrive Pedal (Boss SD-1) for guitar. I read what I had to do onthe web and it looked striaght forward.
I had done some soldering in the past.

So far I have managed to change 3 capacitors on the PCB board.

What I need advice on is.

The original OP_amp is a JRC4558DD I hav ebeen told to change it to a Burr Brown OPA2604.
Would this be a good move?

Also the original diode on the board is a 1N4148 Silicon.
I have been advised to change this to a 1N34A.

Does anyone know where to buy this parts in the UK, as maplin does not stock these, and where the shipping/handling fee is not rediculos ?(digi-design is expensive shipping if you buy less than £75 or something) .

I have manged to change some capacitors by looking at the picture on the web and comparing it to my PCB, which is identical.
(Not a wise move I supose).

However how can I read a schematic in relation to the PCB, as there is so many traks on the PCB.Any help on this will be great.

I can provide a schematic and a picture of the PCB board, if anyone would like to circle the components I need to change.

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Old 10th June 2004, 10:11 AM   #2
diyAudio Member
Join Date: Jul 2003
Location: Nottingham, England
What are you trying to achieve by modifying the pedal?
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Old 10th June 2004, 10:16 AM   #3
raf is offline raf  United Kingdom
diyAudio Member
Join Date: Jun 2004
Location: South East
I am trying the make the pedal more lively, ie increase bass response and increase fidelity.

So far I have changed some capcitors that were rated at .047uf to 0.1uf metal film capacitor. I have changed 3 so far.
The improvement is quite noticable.

I would also like to increase the fidelity by changing the IC and also increase the amount of distortion possible. I suppose this is where the 1N34A germanium diode comes into it.
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Old 10th June 2004, 03:59 PM   #4
raf is offline raf  United Kingdom
diyAudio Member
Join Date: Jun 2004
Location: South East
Any help on this guys as the message has become active as I am a new member
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Old 10th June 2004, 04:58 PM   #5
joensd is offline joensd  Germany
diyAudio Member
Join Date: Dec 2001
Location: Germany
Google is one of our best friends isn´t it?

Schematic doesn´t load completely at the moment but try it.
Some modifications are given as well.

Try them and compare.
Distortion characteristic will heavily depend on D4-D6.
Change them and see if you like the sound.

And don´t forget that your goal may not be to hunt for lower distortion.

If you like metal, try to have a listen to a Boss Metal Zone (forgot the number MT-2??).
More into tube pedals at the moment, although the best are quite expensive.
You´ll find tons of schematics for pedals.
Can send you some if you´re interested.

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Old 10th June 2004, 05:10 PM   #6
raf is offline raf  United Kingdom
diyAudio Member
Join Date: Jun 2004
Location: South East
cheers joensd

I am infact using the sd-1 schematic and making the changes.
However my Sd-1 does not have C and R numberings like on the schematic but has Ds and Q numberings.

I have changed C2 C3 and C8 by looking at the picture and checking what value the original caps where.

but the only thing I cant figure out is R2 R6 and R30.

Any help on this would be great. I have attached the link and the schematic. However I cant seem to translate the schematic to the PCB as I dont know how.

PS Any mods for MT-2 ? I would be interested in seeing them.
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Old 1st July 2004, 11:13 PM   #7
davada is offline davada  Canada
diyAudio Member
Join Date: Jul 2004
Location: Fort St John, BC Canada
Default OPA2604 opamp

Raf the opa2604 is an ultra low distortion, ultra low noise, high
slew rate opertional amplifier. For a fuzz or distortion pedal it seems rather redundant to select an opamp with these specs. The opa2604 will reproduce a square wave and just about all it's harmonic quit well. I think this would add to the harshness of your distortion/fuzz unit. If thats what you want then the opa2604 is a good choise. However, if you,re wanting a smoother distortion/fuzz sound then I would suggest you use an opamp having the lowest slew rate possible. This would be better for rounding the clipped waveform created by the diodes in your unit, which may produce a more pleasing sound. OPamps are very inexpensive these day so it's reasonable to try an assrotment of them. You could remove the exiting 4558 opamp and replace it with an 8 pin DIP socket. This would make it easy to sub-in different opamps until you find the sound your looking for. You may find that there isn't much audable difference between opamps in your application.

Good luck

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Old 2nd July 2004, 09:01 AM   #8
joensd is offline joensd  Germany
diyAudio Member
Join Date: Dec 2001
Location: Germany
finding your resistors on the PCB is just a matter of tracing where the part can be.
R30 for example sits between the zener diode and the white LED and so on...
Double check before you exchange the part and do one substitution at a time and compare.

Back to the opamp I wouldn´t change it.
As DB mentions as well you are not looking for low distortion.
You actually wanna add it in a nice way.

D4,D5,D6 will change the character of the distortion and if you don´t find some that will suit your needs I´d rather try to build a pedal from scratch.
There are many interesting schematics that are much simpler than this, and those will probably give you much better results.
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