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copper
Bronze Member
USA
73 Posts |
 Posted - 02/07/2009 : 00:24:28
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A friend of mine asked me to do a mod to a PW-2 of his. He mostly just wanted the pedal to be better suited for metal. I told him I'd give it a shot. From looking at Lauries schematic (link at bottom of page), it looks as if the distortion is based off of a chip. I'm thinking if I lowered R17, that would reduce resistince resulting in more signal would would equal more distortion. Right?
Is there anything else that would help maybe make it more suitable for metal? Like make the Muscle (mids) knob more versitile and maybe more treble because I find the Fat (lows) is almost overpowering. Thanks!!
Here's the shematic:
http://members.shaw.ca/lauriepedals/pw2.pdf |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 02/07/2009 : 01:28:42
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Hi Copper!
The distortion is all producted by the Q5/Q6 pair. You could try changing C9 to 0.01 and C10 to 1n. That would move the "muscle" way up in frequency. Or try C9 = 0.047 and C10 = 4n7 to move it up in frequency just a bit.
Hmmm... looking at it some more, if you drop C6 to 1uF that might let a lot more highs and upper mids through? |
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copper
Bronze Member
USA
73 Posts |
Posted - 02/07/2009 : 01:53:08
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Thanks for the help! I have a few questions:
1. How would I raise the distortion level?
2. When swapping C9 and C10, do I have to change them both, because I only have values of .01 and .047 laying around?
3. Does C6 have to be polarized, or can I use a metal film cap?
Thanks for all the help! |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 02/07/2009 : 02:46:54
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Hi Laurie,
Is the value for R47=47k correct ? The PW2 schematic on
http://www.freeinfosociety.com/electronics/schemview.php?id=165
list this value as 4.7k, which sounds more realistic. R47=4.7k would give this stage a gain of ~ 3
You could increase distortion/gain at this stage by increasing the value of R48 to something like 47k, to get a gain of 10. |
Edited by - cctsim on 02/07/2009 12:55:49 |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 02/07/2009 : 03:06:13
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Another quick way to add more distortion is to add back-to-back diodes in the feedback loop of the discrete opamp formed by Q5, Q6 and Q7.
More specifically, between the gate of Q6 and the collector of Q7.
Have a look at the schematic of OD-2's turbo channel to see the details.
You could use either symmetric or asymmetric arrangements.
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copper
Bronze Member
USA
73 Posts |
Posted - 02/07/2009 : 03:16:41
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Explain....
Tell if I'm wrong, but are you saying two diodes, like 4148's (each facing the opposite way), connected straight between the Gate of Q6 and collector or Q7? |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 02/07/2009 : 03:36:37
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Yes, correctly. See the schematic below shown for asymmetric clipping:
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copper
Bronze Member
USA
73 Posts |
Posted - 02/07/2009 : 03:54:17
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| Do I need the cap? |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 02/07/2009 : 04:14:34
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The tubescreamer and most of the mods around use a similar cap to smooth the diode clipping.
It's a subtle effect.
Keeley even suggests to use an expensive silver mica capacitor to keep noise at mimimum. |
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copper
Bronze Member
USA
73 Posts |
Posted - 02/07/2009 : 04:16:47
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One last question and then I will leave you be. This is a newb question:
Which is the gate (middle?) and collect (middle again?) legs?
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Edited by - copper on 02/07/2009 04:22:11 |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 02/07/2009 : 05:18:24
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Yes, for the 2SK184 jfet the Gate electrode is in the middle.
I checked the 2SA1048 pnp bjt, and its pin layout is the same as the one for the 2SC2458 found in Laurie's schematic, e.g. the Collector electrode is in the middle.
Good luck with your mod. |
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copper
Bronze Member
USA
73 Posts |
Posted - 02/07/2009 : 05:29:02
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Ok did the mod.
I upped the gain with the 47k at R48. Added lots of gain.
Added a symmetrical diodes between the yadda yadda. Didn't care for it that much, so I yanked them out.
And I put a 1uf metal film cap at C6.
Everything sounds great. Except there is lots of noise. I do realize that when you turn up the gain you get more noise, but is there anyway to lower the noise? |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 02/07/2009 : 05:56:03
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You can reduce the noise by replacing the opamp chip with something that is more Hi-Fi/lower noise, OPA275 or NE5532.
Looking at the circuit again the discrete op-amp Q5-Q6-Q7 doesn't seem to have enough gain to saturate the diodes in the feedback loop. If you had a 250k potentiometer instead of the 50k that will give you better saturation.
Alternatively, you can try to lower R16 to 47 or 100 Ohm.
Another option you could try is back-to-back LEDs at the output of the opamp ala SD-2. |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 02/07/2009 : 08:00:30
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quote:
Originally posted by cctsim
Hi Laurie,
Is the value for R47=47k is correct ? The PW2 schematic on
http://www.freeinfosociety.com/electronics/schemview.php?id=165
list this value as 4.7k, which sounds more realistic. R47=4.7k would give this stage a gain of ~ 3
There were numerous erros on the freeeinfo society one... I'll double check, but I recall this one being wrong. |
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cctsim
Silver Member
United Kingdom
418 Posts |
Posted - 02/07/2009 : 12:59:53
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If you are correct then the gain of this stage is ~ 0.3 which means it is used to reduce the volume.
I never had a chance to listen to a PW2 over the years. It looks like a cool pedal.
What other boss pedal is closer to soundwise ? |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 02/07/2009 : 16:27:43
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quote:
Originally posted by cctsim
What other boss pedal is closer to soundwise ?
Nuthin'  |
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Topic |
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PW-2 mod
