Designing that OP-Amp OR15-soundalike

Stompboxes circuits published in magazines, books or on DIY electronics websites.

Designing that OP-Amp OR15-soundalike

Postby doombox92 » 31 Oct 2017, 17:03

Dear Readers,
I'm parallel-posting this both here and on the SS-guitar forum, since the latter does not show all that much activity.

Firstly I'll list my build goals. Secondly I'll list the constraints I'll subject myself to. Thirdly, I'll propose a method of approach.

1. The goal is to design, prototype and build a solid-state preamp that sounds like the Orange OR15.

-I'd like to limit myself to the use of op-amp gain stages. Although -- and ROG keeps proving this point-- JFETs have proven to work well in solid-state adaptations of valve topologies, J201's are hard to come by in my region. The project will hence serve as an attempted proof-of-concept that op-amps can be used functionally to mimic valve topologies.

-The preamp should preferably work off a single 9V supply, although if nescessary, a dual-rail supply can be constructed and implemented

3. I've spent some time analyzing the OR15-schematic. (I'll appendicize it) and used a calculator to calculate the gain of the first 4 preamp tubes. Negative feedback of the op-amp stages will be set accordingly. I presumed the filters between the stages could be copied one-on-one. To incorporate soft-clipping characteristics , antiparallel diode pairs are placed in the feedback loops. To adjust the clipping threshold, a resistor has been placed in series with the diode pair. (These will require tweaking. Asymmetric clipping in one or more stages might be desirable)

Now I'd like to ask some questions and place some notes:
-In the feedback-loops are radio-frequency pF-value caps. How would I choose appropriate values?
-There are DC-blocking capacitors from the inverting input to ground. How would I choose these values? (They should not have a filtering function, since filtering is done through coupling capacitors and roll-off capacitors)

Of course, feel free to vent your thoughts


I'll add my proposed schematic. If I remember correctly I shouldn't post the OR-15 schematic here, due to forum rules.
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Re: Designing that OP-Amp OR15-soundalike

Postby roseblood11 » 02 Nov 2017, 19:29

Maybe you should add a buffer / makeup gain stage after the tone stack.
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