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Ashdown ABM500 preamp Schematic here

Discussion in 'Amps and Cabs [BG]' started by kringle77, Jul 25, 2007.


  1. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    I love my ABM500 but wihs it were brighter. I have a feeling that the FET Stage located between the input jack and first opamp labled "input" is some sort of filter that make the tone warmer or less bright. Some would call this "tubey". Not me. What would happen if I jumpered around this section directley to the "input" stage? I would leave in the high impedance part of course.
     
  2. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    This file is too big. If you would like to help my out and take a look at it, I can e-mail the schematic. Thanks guys.
     
  3. WillPlay4Food

    WillPlay4Food Now With More Metal! Supporting Member

    Apr 9, 2002
    Orbiting HQ
    PM me and I'll host it.
     
  4. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    Schematic sent, thanks.
     
  5. WillPlay4Food

    WillPlay4Food Now With More Metal! Supporting Member

    Apr 9, 2002
    Orbiting HQ
  6. Jeff Scott

    Jeff Scott Rickenbacker guru.......... Supporting Member

    Apr 11, 2006
    As best as I can tell from the schematics there is nothing before the first input stage FET. Something I read a lot about these amps is that they are too warm; IIRC, the out position of the preshape button make this circuit active and when the button is pushed in, then the amp's FR is flat. Could this be the source of your disappointment in the sound of the ABM?
     
  7. Jeff Scott

    Jeff Scott Rickenbacker guru.......... Supporting Member

    Apr 11, 2006
    BTW, It looks like this is just the schematics for the preamp, I can't seem to find the output stage there.
     
  8. sconix

    sconix Supporting Member

    Feb 23, 2004
    High Point, NC
    The Fet is a buffer stage and the opamp is a straight gain stage albeit an inverted current feedback thing. Each is basically flat with compensation for RF considerations. I use the first version of the ABM EV500 and get mild to wild stuff out of the eq section. Would think you could suck out some thick thing or boost something, anything ;)
     
  9. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    No! You guys have it all wrong. I love this amp but wish it was brighter. It seems like it needs a bright switch like on the ampeg gear. I really have to boost the highs and high mids through some cabs to get the sound right. If you set the ashdown flat and compare it to and eden or other amps, it will sound darker in comparison. I believe in the manual it says that the fet section at the input is there to add "warmth" Since it is a buffer, then maybe this it true which is why I wondered if I was to bypass that and go directly to the first opamp gain stage, then the sound would brighten up. I have a print for the power amp section but wanted some of you fellows to analyze the preamp section for me. I know alittle about this stuff, having an electrical background but I didn't pay enough attention in some of my eletronics classes! ;)
     
  10. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    Oh yeah, I think that I had the op-amp gain stage figured out. The high value cap in there should be to filter out noise. I did't really understand the grouping of capacitors around the FET buffer. It looked like some sort of filter and I thought this could be causing some sort of high end roll off.

    1)Anyone have a clue what would happen if I was to jumper from the high impedance resistor at the input to the variable potentiometer in front of the first op-amp gain stage?
    2)What is the value of the fet buffer stage when there is already a high impedance input and an op-amp gain stage anyway?
    3)Any other way to brighten this circuit up aside from outboard gear.

    This would be my perfect amp 100% if it were alittle brighter. I will continue to gig it for a long time to come one way or another. It's pretty much the best thing out there for rock IMO.
     
  11. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    A big thanks to "WILLPLAY4FOOD" for hosting the schematic for me! :) :hyper:
     
  12. JAUQO III-X

    JAUQO III-X Banned

    Jan 4, 2002
    CHICAGO,IL.
    Endorsing artist:see profile.
    If you want brighter than the ABM 500 check out the Ashdown MK500.
     
  13. sconix

    sconix Supporting Member

    Feb 23, 2004
    High Point, NC
    Don't do number 1, bad idea, you really don't understand ...The resistors and caps you refer to are power supply decoupling.
    2 .. the opamp really has a low input z hence the need for a buffer.
    3 .. you could do a cap bypass on the volume control, just like bright switches used in fender or ampeg and likely get the effect you want. 1nf or 1000pf across input and wiper.
    I design and manufacture this kind of stuff for a living in another industry.
     
  14. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    Thanks sconix. I have an electrical degree but mainly work with power utility relaying and protection. I have just enough electronics knowledge in my memory to get into trouble and have fixed or modified some of my gear in the past. I had the fender/Alembic preamp circuit in the back of my mind. So your saying that I could put a capacitor from point 3 to 2 on the output pot? (RV3 , 50K ohms). It seems like the high frequencies would have been drained off by then but, you really do more about this than I do! I just read the manual and it did say that the fet stage was for "warmth". Would there be any truth to this?

    I really appreciate your input.
     
  15. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    Oh yeah, I understood your point about #2. I just was in hurry and didn't explain myself very well.
     
  16. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    It looks like a 120 pf cap was used in the Alembic preamp design in the same spot that you suggested. This is very encouraging.
     
  17. sconix

    sconix Supporting Member

    Feb 23, 2004
    High Point, NC
    The cap between pin 3 and 2 on the pot isl how bright switches or bright channels are done in most all amps. Simple and easy to tag one on and see what happens. It may or may not be what you're looking for, but worth a shot. The fet buffer just gives a nice almost 4 meg load for passive input and Being single ended it's likely to have even order distortions, kinda, sorta, maybe almost like a tube would. Certainly prettier than an opamp buffer in that position. Really this thing would have no roll off in the audio band at all post the opamp amp stage. Try the cap/pot bypass with a mylar, mica or anything other than a ceramic. You can use the fender/alembic bright scheme as a guide, just scale the cap for the pot's value. Bigger pf brassier treble, smaller pf daintier treble. You other kids... don't try this at home.
     
  18. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    Sconix, I will try this in the very near future and I can't thank you enough for the advice. And yes.....don't try this at home kids. Especially, right before a gig if you don't have another amp!!! :smug:
     
  19. seanm

    seanm I'd kill for a Nobel Peace Prize!

    Feb 19, 2004
    Ottawa, Canada
    OT, but for those who wonder what the difference is between a preamp in the bass and a preamp in head, notice the +/- 15V on the input FET. 30V of headroom.

    I like issue 3, "Buffer added to tuner out". Oops!
     
  20. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    I used a 5nf cap that I had laying around the shop and put across pins 2 and 3 of the output pot on my Ashdown electric blue combo. It really brightened the amp up. It has no horn so this value or maybe alittle lower is nice. When I get some different value caps of better quality, I will try it with my ABM500 through some cabs w/horns. This may be the answer to my questions. I'll keep you guys posted.
     

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