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archimedes
Silver Member
United Kingdom
191 Posts |
 Posted - 06/13/2009 : 19:10:28
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I have an OD-3 which was modded a while back and I have to say that I was never entirely happy with, certainly it was cleaned up and less muddy but there was a harshness to it. It has had a chip adapter fitted in place of the stock chip for a Burr Brown which I didn't like so I ended up trying an NE5532P which was an improvement but still not there.
I decided that I needed to learn about how these things work for myself and what had been done to get it to where I wanted it so I started studying the schematic. Apart from the op-amp change the other changes seem to be some caps for tone (less bass and more clarity), the removal of D12 and a switch to short C22 which adds lots more mid range (Tube Screamer ish I guess but I wasn't keen on that either - some might like it though).
D12 seemed to be the key as I found mention on the net that this clipping stage helps prevent nasty unmusical clipping in the discrete op-amp after it. I put a 1N914 diode back in and hey presto the harshness I didn't like was gone and the pedal was sounding great - the clarity of the tone mods and op amp from the original mod plus the grainy complexity of the OD-3 three stage clipping.
So then I started to wonder what it would sound like if I added another diode to make this first clipping stage asymetric as that's what OD-1s and SD-1s use and lo and behold I think it sounds even better, a tad thicker, warmer, more valve like and three dimensional. I have utlised the C22 switch to short the second diode at D12 so I can switch between sym and asym although I have to say that I think it will be staying in the asym position.
So there's my first mod to a mod... (it's still running the NE5532P)
I have to say I am now a bit suspicious about mods that involve diode removal now, such as the Keeley suggestions to take out D6 and D7 which garners mixed reviews, as they are bound to present a very different signal to the op-amp that follows. Then again what do I know!
Best wishes
David
(very happy with his overdrive sound and playing a Strat and PRS Custom 24 through a Tiny Terror into a Celestion G12 something, Marshall JTM having rest)
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 06/13/2009 : 22:56:23
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D9 and D12 seem to be input protection diodes for the discrete transistor opamp. Up to that point, there are only 2 JFET buffers and a frequency shaping stage.
The main signal clipping happens at D10 and D11. If you like asymmetric distortion similar to SD-1 I suggest you add a diode in series with D11. It can be a 1N4148 or 1N4001. |
Edited by - cctsim on 06/13/2009 22:58:41 |
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archimedes
Silver Member
United Kingdom
191 Posts |
Posted - 06/13/2009 : 23:59:21
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quote:
Originally posted by cctsim
D9 and D12 seem to be input protection diodes for the discrete transistor opamp. Up to that point, there are only 2 JFET buffers and a frequency shaping stage.
The main signal clipping happens at D10 and D11. If you like asymmetric distortion similar to SD-1 I suggest you add a diode in series with D11. It can be a 1N4148 or 1N4001.
Thanks, kind of confirms my suspicion that the removal of D12 was the cause of the harsh unmusical overtones I was hearing and not a great idea. My extra diode at this stage does seem to thicken the sound a little though, more by luck than judgement.
David
(off to see Zappa Plays Zappa in London tomorrow - ya hozna!) |
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hot tubes
Silver Member
Canada
162 Posts |
Posted - 06/19/2009 : 23:47:43
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| also try LEDS, and 1n34a's , they have worked for me in the past. |
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archimedes
Silver Member
United Kingdom
191 Posts |
Posted - 07/18/2009 : 19:01:20
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Just to report back on this one in response to the advice for which many thanks. I have ended up with a 1N4148 in series with the 1N914 at D11. I tried the other suggestions and came back to this one which to my ears makes a subtle improvement to the sound, a a bit more natural, smooth and valve like. I have it on the switch so that I can go from sym to asym but I think asym will be the one I end up using. D12 is back to normal by the way. I think it says something that two of the most popular of the older Boss ODs, the OD-1 and SD-1 use asymmetrical clipping.
David |
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OD-3 Asymetric mod
