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DIY Stompboxes => Building your own stompbox => Topic started by: Derringer on March 25, 2009, 10:47:25 PM

Title: best way to implement a TL072 as the gain recovery stage of a Big Muff?
Post by: Derringer on March 25, 2009, 10:47:25 PM
I actually have the first half of it as a buffer right before the tone stack and the second half as the recovery stage.

I pretty much copied and pasted these values for the related opamp designs found on Jack's site:
http://www.muzique.com/lab/buffers.htm
and then
http://www.muzique.com/lab/boost.htm

The volume I'm getting is ridiculous loud. I tried a 100K (ouch) 10K and 5Kpots in the feedback loop and even the 5k is loud.


what would you tweak? How can I get the volume more usable?

thanks
Title: Re: best way to implement a TL072 as the gain recovery stage of a Big Muff?
Post by: petemoore on March 25, 2009, 11:31:40 PM
what would you tweak?
  The values Jack shows in the diagrams at the bottom of the booster page linked.
  Perhaps a fixed value input voltage divider [fixed R's mimicing pot setting].
  Then gain control [the '14k'].
  maybe fix gain so it's just a little too loud with the volume control ATW up.
Title: Re: best way to implement a TL072 as the gain recovery stage of a Big Muff?
Post by: brett on March 26, 2009, 05:23:38 AM
Hi
check the value of the 1k resistor.  Maybe you have a short to 4.5V there, or used a 100 ohm ohm or other resistor..
With a 1 k resistor and 5 k pot, the gain should be low at low pot settings (ie barely audible at "9 o'clock", but significant at "3 o'clock".
cheers
Title: Re: best way to implement a TL072 as the gain recovery stage of a Big Muff?
Post by: Ben N on March 26, 2009, 03:18:44 PM
A useful opamp primer (http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm), including the gain formulas. (I know there is something like this on Geofex, but I can never find it.) Go back to your recovery amp and change to taste.
Title: Re: best way to implement a TL072 as the gain recovery stage of a Big Muff?
Post by: Derringer on March 26, 2009, 06:05:37 PM
cool

thanks for the pointers guys

I just went over it again (on a breadboard) and everything "appears" to be wired right .... but it can't be

with the feedback pot turned so that there's zero resistance, I'm still getting a strong signal ... I should be getting no signal if there's no resistance between pins 6 and 7 right?

Title: Re: best way to implement a TL072 as the gain recovery stage of a Big Muff?
Post by: Ben N on March 26, 2009, 06:34:12 PM
No. A closed loop in a non-inverting opamp = unity gain.
Title: Re: best way to implement a TL072 as the gain recovery stage of a Big Muff?
Post by: Derringer on March 26, 2009, 11:18:26 PM
hmm ... well it's still really loud with a closed loop
Title: Re: best way to implement a TL072 as the gain recovery stage of a Big Muff?
Post by: Ben N on March 26, 2009, 11:34:29 PM
Is your tone stack working? If the output of the opamp is really connected to the (-) input, all the gain must be coming from before that point.
Title: Re: best way to implement a TL072 as the gain recovery stage of a Big Muff?
Post by: Derringer on March 27, 2009, 07:24:48 PM
I ran the output directly after the tone stack to see what the gain was like

I stuck a 100K pot in after the last cap there for volume
with the pot dimed I was getting unity ... maybe a touch more

so I stripped it all down and just wired up the muff stock for now to get used to how it operates

I might try using a fet for a pre-tone stack buffer and another for gain recovery


the gain with the opamp was just ridiculous
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