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Old 11th February 2009, 02:17 PM   #1
diyAudio Member
 
Join Date: Jun 2005
Location: Fort Collins Colorado
Default Dean markley DR-80 Pre Need help

Greetings all.
I need a little theory help here understanding how this preamp works. It's a Dean Markley DR-80 preamp board raw out of a board trash pile from an auction.
I fixed most of it's issues and it now appears to be working but now I'd like to tweak the overdrive chanel a little as it sounds like a Marshall type whimp for a Blues sound.
Of interesting note... It appears to me that the Marshall Valvestate is actually based on this thing. The first stage is identical. The DR80 has an active tonestack while the Marshall's is passive. I was originally planning on tweaking the valvestate I have (also from a junk pile) for a more fendery tone on the clean channel but I believe that is impossible.

My question is regarding the two different style preamp stages in the unit. Here is a pic of the area in question.


I understand the first stage at IC1a.. Identical to the Valvestate by the way. Hmmmmmm. .

1) On the OD channel there are what appear to be two clipping diodes shown. There is only one and no space for the other.
Removing this diode has no effect on the signal. Swapping in a dual led array has no effect.

2) With the tube removed, when the FET switch is engagued the OD channel does not come through at all. I would have expected a little.

3) I don't understand the orientation of the feedback loop on IC 1b and 2a. Specifically the different reasons why they are configured that way.
Am I correct that the cap across the gain pot on the OD channel is a HF limit and should be moved to eliminate the cap changing my tone as I raise the gain. Should I just copy the clean channel?

4) My desire is to smooth the OD channel and lower the gain for
Blues lead playing while restoring some of the low end.

5) Is that .01 uf cap at the output of the OD ch a lf cutoff? Seems a little small for a decoupling cap.

I have plenty of Marshall, Fender, Mesa, and Engl tones but I am in dire need of a tone driven blues tone with whatever I have available.
Actively used gear: Tech 21 tri-ac, Modded Warp pedal (ENGL tone), Atrec analog delay, Tech 21 Double Drive, Rocktron Metal planet. All feeding a TC M300 fx unit and a Stewart pa=1400 poweramp. Cabinet is a Peavey sheffield 15 and Stamford acoustics 2" horn in a monitor type cabinet. (It's an analog modeling rig, Meyer esque). I just got a Behringer GX212 preamp section which is like a dual sansamp with FX, I don't expect it to have a good tube tone when it comes. I want to add the DR80pre as either a parallel preamp chain for blues only, or as a pedal.


Thanks
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Old 11th February 2009, 08:16 PM   #2
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Join Date: Jun 2005
Location: Fort Collins Colorado
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I've had a chance to play around with it today. The clean channel is quite nice but doesn't break up well. That's O.K.

I really dont like the OD gain affecting the tone via that cap across it. I tried different caps, got some nice tones. Unfortunately
the gain screws it up.

So I'm guessing I'd like to alter the od stage like the clean channel
but with some nicer distortion.
I do see why some people love these preamps.
suggestions?
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