Adding a volume recovery stage in Dist. Plus?

[B Tremblay]

Low output volume is mentioned within many of the Distortion Plus threads that I've read.  My Ross Distortion (nearly identical to the Dist. Plus) uses a dual op-amp, so the unused half could be utilized as a volume recovery stage.

I looked through the archives and saw this:

I encourage you to consider using the second op-amp in the 4558 as a simple gain recovery amp with maybe a gain of 2 or 3 just to bring the signal up to a respectable level. This will let you set it for milder distortion and overdrive your amp with a boosted output level.

Has anyone added such a stage, and if so, could they describe it in more detail or provide a link to a schematic ?

[spongebob]

Something like that? Basically it's just another opamp with gain of 3 in place of the original volume pot, and the volume pot is moved towards the output of the 2nd opamp.

Hope this helps!

[B Tremblay]

I tried your suggestion and it works, but I'd like it to be even louder.  Which components do I change to increase the gain?

[petemoore]

Increasing the value of the 20k resistor would increase the gain of the second OA.
 Hopfully someone else will jump on if that needs compensated for elsewhere in the circuit, or if there are other ways to do it.
 A pot [2nd OA's feedback loop] there might be something to try...1meg or smaller...1meg is max Gain for a feedback loop I believe.
 You might like a gain switches between 2 different size feedback resistor values.
 You could try a Larger or smaller volume pot in combination with the 2nd OA's feedback loop.
  Sometimes I'll go say 1/2 again as large or more with a volume pot, then 'tag' resistors across lugs 1 and 3 till the output volume comes down to where I want it.
  A 100k pot with a 100k resistor across lugs 1 and 3 is now a 50k pot.
 Start large and work your way down.

[spongebob]

Gain = 1 + R8 / R9. Simply increase R8 if you need more gain.

[Chris R]

just add a trimpot for R8.

or use a 100K pot.. and measure the resistance when you get the sound you want.

C

[Jay Doyle]

You may want to add a 10uF cap in between R9 and Vref. Without the cap you are applying the same gain to DC as you are to AC.

[gez]

You may want to add a 10uF cap in between R9 and Vref. Without the cap you are applying the same gain to DC as you are to AC.

With both inputs held at Vref (same voltage) any offset is going to be small and I shouldn't think it would cause a problem (not with that low a gain!).

[B Tremblay]

Success!  Many thanks to all who replied.

Spongebob's schematic worked for me initially, but when I increased the feedback resistance, the output slowly faded and sputtered away.  The 10uF cap suggested by Jay seems to have cured that phenomenon.  I'm currently using a 68k resistor for a near-unity output when the Gain is at minimum.

I really like the sound of my Ross at minimum Gain, but it's somewhat quiet since the diodes knock the output down so much.  The unused op-amp half can now solve that annoyance.

Thanks again to everyone.

[WGTP]

I know I'm twisted, but why not build a second distortion, use LED's for the diodes to keep the output high and a 100K output pot.  Sorry, I couldn't resist.

[petemoore]

Dist+
 Dual OA
 Booster
 TL072
 The voltage dividers resistors are getting real hot
 The battery is losing .01v every seven seconds
the DMM says .1ua draw...? I think ...the only reading I could get is with it set at 200u on the ~A setting so far.
  I used two 100k divider resistors, and it would go for a second then conk. I figured something to do with current draw so...
 I added two more, this time 33k bringing the divider resistor values to just less than 33k on each side of Vref. These are the ones that gget hot.
 My Dist+ schematics show no divider, I used the one above, so as to try the Dual with gain recovery.
 The diodes only shut the circuit off in one direction, and nothing noticable in the other...[]
 The only other thing straying from the schematic is the 1meg gain pot is instead a fixed resistor, the second OA's feedback loop R is at 100k for now.
 Pin 8 is er was 8.90v pin one was 4.46v, all the other pins are around
 3.86v, +readings all, Gnd is Gnd where Gnd should be.
 It's actually quite a nice and powerful booster, but it seems to chow battery's with the heater resistors...

[gez]

With both inputs held at Vref (same voltage) any offset is going to be small and I shouldn't think it would cause a problem (not with that low a gain!).

gez was talking utter bollocks...apologies to Jay!

[Mark Hammer]

Stick a 68k fixed resistor in the feedback loop of the second op-amp and a 330pf cap in parallel with it.  With the 10k resistor and 10uf cap to Vref, that will give you a gain of 8 in the recovery stage and a rolloff around 7khz.  A gain of 8 should be enough to make up for whatever you lose from germanium diodes, and a rolloff around 7khz will tame the fizz without cutting out too much bite.  If you want as much fizz and sizzle as you can get,  drop the cap value to 220pf.

The suggestion of sticking in some clipping diodes in the second stage as well makes some sense but you should note the following:

a) If you use Ge diodes for the first stage (i.e., the stock diodes) the input signal into IC2 will require considerably more boost to make that second set of diodes do anything useful.  I did this with the "Chaos" thingie I threw together a year ago and it sounds nice but the level emerging from the first stage is much hotter than what you'd get from a pair of Ge's.

b) Why invent the wheeel again?  The BMP does the cascaded clipping thing nicely and I doubt whether this one would be any sort of improvement.  The real improvement here is taking the sound of the stock Dist+ and giving it enough power to push an amp's front end harder.  The secondary improvement comes in the form of tone shaping via the second stage.  Indeed, you may be able to stick in a standard BMP tone control between IC2 and the volume control and make it even more flexible.  If the passive tone control eats up too much signal, simply up the feedback resistor of IC2.

[B Tremblay]

Here's a schematic of all our contributions:


Thanks again to all!

[WGTP]

Looks nice.  

WGTP Mods to the gain recovery stage, may be switchable.

Change 68k to 680K 330pf to 100pf and add 1 LED for more output and asymmetry or 2 LED's for less output and more symetrical distortion.

Just dreaming