Author Topic: Volume boost via larger pot?  (Read 7448 times)

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Volume boost via larger pot?
« on: November 25, 2007, 07:01:39 PM »
Is this possible? I am modding a friend's pedal (I made him an Ampeg Scrambler clone) and he wants to get a bit more volume out of it. I was going  to put a pot between output and ground but if I was to use, say, a 500k pot instead of the usual 100k, would this give any kind of boost to the signal? Would I be better off increasing the output cap value? Sorry if it seems like a noobish question, I'm just confusing myself the more I think about it. Also, any opinions as to whether I should use Log or Lin for this pot?

Re: Volume boost via larger pot?
« Reply #1 on: November 25, 2007, 08:09:25 PM »
15 views and no opinions/suggestions? C'mon guys

slacker

Re: Volume boost via larger pot?
« Reply #2 on: November 25, 2007, 08:22:56 PM »
You can't get a volume boost just by using a pot, that only lets you turn the volume down. You need some sort of amplifier to get a real boost.
Changing an existing volume pot to a larger value might give a slight increase in volume as the larger pot won't cut as much signal.
If you want to make the scrambler louder I think you'll have to add some sort of booster on the end, something like the last stage of a bigmuff would do the job. Then stick a volume pot after that.

Re: Volume boost via larger pot?
« Reply #3 on: November 25, 2007, 08:40:05 PM »
You can't get a volume boost just by using a pot, that only lets you turn the volume down. You need some sort of amplifier to get a real boost.
Changing an existing volume pot to a larger value might give a slight increase in volume as the larger pot won't cut as much signal.
If you want to make the scrambler louder I think you'll have to add some sort of booster on the end, something like the last stage of a bigmuff would do the job. Then stick a volume pot after that.

You have confirmed my suspicions there, thanks. You don't think a bigger output cap would help?

petemoore

Re: Volume boost via larger pot?
« Reply #4 on: November 25, 2007, 09:12:07 PM »
  [caps increase resistance as freqency of their input drops]
  rolloff curve.
  Bigger caps have a lower frequency at which resistance curve starts rising..they pass HF's, attenuate LF's.
  The output cap is blocking DC, and frequencies 'nearer' DC..low frequencies are met with resistance, this is a high pass filter.
  A simple low pass filter [cap from signal path to ground] uses the cap to 'dump' high frequencies, and their higher resistance to low frequecies allows them to pass, they block the LF's from being shunted to ground.
  You don't think a bigger output cap would help?
  8 ball says doubtful, and it would depend on the end result, increased low frequency output, being percieved as helpful.
  Volume pot value increase wins very slight increase in voltage output, at the expense of current output [someone kick my wrongness if so deserved].
  Very slight frequency changes also happen, all the way up a 500k might sound a touch brighter, set pot to 1/2 way [=R both sides] and you've now got *250k series in SP, that can noticably roll highs off, not often desired ['loudness' controls on stereos do opposite, boost hi's or hi's and lows for low volume freq compensation].
  Those lost highs can pass the *250k [in the pot] if a teeny cap is parallel to it.
  Then you have whatever the output impedance ends up at...and very little output increase.
  Scrambler should be loud already ? [dont have one, guess it'd be high output]
   Booster on the end brings whatever you have to...'X' gain increase' [you choose], and low impedance [you choose], can be made to be relatively frequency balanced [compared to input], high voltage output, low impedance output...whether that fits the box or the bill is another matter.
« Last Edit: November 25, 2007, 09:19:57 PM by petemoore »
Convention creates following, following creates convention.

Re: Volume boost via larger pot?
« Reply #5 on: November 25, 2007, 09:25:34 PM »
So... what about doing both? If the higher value output cap effectively increases lower frequency output and the bigger pot increases higher frequency output, wouldn't we end up with a 'bigger' signal at the end? Or did I read that post all wrong?

John Lyons

Re: Volume boost via larger pot?
« Reply #6 on: November 25, 2007, 09:31:06 PM »
If want to add a simple gain stage at the end try a LPB2 or the last stage of the big muff.
I don't have the scrambler circuit in front of me but it there is a resistor ground right before the output before the output cap you can make this larger to get a little more juice out of the circuit.

Try the big cap and bigger volume pot thing, see what you think.

John

Basic Audio Pedals
www.basicaudio.net/

petemoore

Re: Volume boost via larger pot?
« Reply #7 on: November 25, 2007, 09:33:17 PM »
  bigger output cap lets more bass out = louder
  bigger resistance to ground shunts less signal = louder
 1+1= more output.  
Convention creates following, following creates convention.

Re: Volume boost via larger pot?
« Reply #8 on: November 25, 2007, 09:43:27 PM »
If want to add a simple gain stage at the end try a LPB2 or the last stage of the big muff.

I'm trying to avoid this due to limited space in the enclosure I made it in.

Quote
I don't have the scrambler circuit in front of me but it there is a resistor ground right before the output before the output cap you can make this larger to get a little more juice out of the circuit.

Try the big cap and bigger volume pot thing, see what you think.

John

Here's the schem - http://www.generalguitargadgets.com/diagrams/scrambler_sc.gif

Re: Volume boost via larger pot?
« Reply #9 on: November 25, 2007, 09:44:09 PM »
  bigger output cap lets more bass out = louder
  bigger resistance to ground shunts less signal = louder
 1+1= more output. 

*High fives*

John Lyons

Re: Volume boost via larger pot?
« Reply #10 on: November 25, 2007, 10:08:46 PM »
Ok, now I see...
Making the 47K resistor to ground at the circuit's end 100K should make the volume increase.
The blend will be affected as well so may as well make both 47Ks to ground at either side of the blend pot 100K to keep the same balance.

JOhn

Basic Audio Pedals
www.basicaudio.net/

Re: Volume boost via larger pot?
« Reply #11 on: November 25, 2007, 10:29:40 PM »
Aha! So R6 and R16 in that schem?

Just for the record, increasing the output cap and adding a 1M pot have made pretty much no difference at all to the volume. Sound has changed midly but I think I prefer its original sound

Re: Volume boost via larger pot?
« Reply #12 on: November 25, 2007, 11:08:12 PM »
GRRR! Trust me to have run out of 100k resistors. Closest thing I have is 220k... might be worth a try methinks  :icon_twisted:

petemoore

Re: Volume boost via larger pot?
« Reply #13 on: November 26, 2007, 09:01:32 AM »
  I see a 'Blend' pot, different there than a 'volume' pot wiring, there is no connection to ground.
  Calculating impedance...beyond me other than to say I see...well if the 47k which sets the output impedance gets large enough so it won't matter so much [like 120k+?]...the output impedance looks as though it'd largely be set as going through the blend pot to 47k emitter resistor [a ground feed to 50k blend pot from front end, or about 100k], not much mileage to be wraught from +ing^ the 47k value.
  The blendpot, made as 1meg. larger resistances in series with output = lower output. [now that i know what we weren't talking about], and would sound mud I guess, especially anywhere toward a blend position.
  If anything were to be done to the blend pot value, it'd be at sacrifice some of the blends ability to go one side or the other [say use 20k], again, for a very slight volume increase, greater inability to separate inputs.
  So...20k blend w/100k where the 47k was?, lifts output a touch not much, changes impedances [ :icon_evil:?], loses blend separation..etc. [I'd probably just leave the blend section alone]
  Looking through the rest of the cirucit, all the emitters are bypassed large, one twisted circuit, R15, a 4k7 emitter R on a transistor gain stage w collector resistor at 1k...?..
  Someone else explain or try reducing R15, or a 22uf emitter bypass cap across it, increase R14 value, this last gain stage is wierd [k..seems wierd], I'm used to seeing small E resistor w/ larger C resistor...
  At any rate, the signal path above the last transistor is a mere 1k away from the PS, a larger resistor there will reduce the shunting effect *and...worth a try?...looks like where I'd start.
  R15 should be easy decrease with testclips?...you might not have to lift from PCB to test smaller values there.
« Last Edit: November 26, 2007, 09:07:19 AM by petemoore »
Convention creates following, following creates convention.

MartyMart

Re: Volume boost via larger pot?
« Reply #14 on: November 26, 2007, 10:12:43 AM »
GRRR! Trust me to have run out of 100k resistors. Closest thing I have is 220k... might be worth a try methinks  :icon_twisted:

Parallel 2 x 220k's for 110k .... close enough !
MM.
"Success is the ability to go from one failure to another with no loss of enthusiasm"
My Website www.martinlister.com

John Lyons

Re: Volume boost via larger pot?
« Reply #15 on: November 26, 2007, 10:48:39 AM »
Try using the parallel 220Ks to make 110K, marty is right...close enough!
I don't think changing the impedance is going to be an issue.

Using a volume pot will do nothing as the circuit has no volume pot in the first place.
Making the 47Ks 100k (er.. 110K)  is esentiall the same as making a 47K volume pot a 100K pot set at max gain... which should get you some more volume. I can't see how it would change the blend any since the resistor swaps would be symetrical.

John
Basic Audio Pedals
www.basicaudio.net/

Mark Hammer

Re: Volume boost via larger pot?
« Reply #16 on: November 26, 2007, 11:57:33 AM »
Whenever tacking a volume pot on the end of a circuit, or changing the value of the one already there, you need to examine the pot value in the context of all those other resistiors that might be in immediate direct contact with it.

The textbook case is the MXR Distortion+ and its cousins (DOD 250, etc).  Some of these (certainly the one I borrowed in 1978), had a 10k output volume pot.  The trouble is thet there is a 10k resistor in series with the input to that pot.  True, there are diodes and a cap on the input lug of the volume pot as well, but we will confine ourselves to the resistor.  From the pot's perspective, that resistor adds to the overall resistance of the pot.  Essentially what you have is a 20k voltage divider (10k resistor + 10k pot) that is never allowed to go up past halfway. Even with the wiper at max volume, you'll have 10k on one side of the wiper, and 10k on the other, cutting the signal by half.  If the pot is increased to 50k or even 100k, you end up with the equivalent of a 60k or 110k "pot" that is still never allowed to reach maximum, but comes closer to it from a proportional perspective.  As a result of providing less attenuation, the output volume of those pedals where the pot value was changed is greater.  Less attenuation means more signal level preserved/conserved.

So, changing volume pot value CAN, in some instances, change the potential volume/level of the pedal.

Is the Scrambler one of these?  In a word, no.  Let's take a look at the circuit and see why.
http://www.generalguitargadgets.com/diagrams/scrambler_sc.gif   Imagine the 50k Blend control is set midway so that there is 25k of resistance on either side of the wiper.  In tandem with the 4M7 resistor on the output, each half of the Blend pot serves as a sort of volume control for the signal.  At the exact mid-point, there is a sort of voltage divider formed by R18 and each half of the Blend pot, with a total resistance value of 4.725k (4M7+25k).  It is "turned up" pretty much most of the way since there is far more resistance in R18 than in what is on the other side of the R17 wiper.

So what would happen if you put a volume pot in there?  Let's replace R18 with a 1M log pot (selected because it is a fairly common value).  When the volume pot is set to maximum (wiper at the end where it is essentially in contact with the R17 wiper), you would have a voltage divider (which we'll call R19) with 1M on one side of the wiper (i.e., the full resistance of R17), and somewhere between 0k and 50k (depending on Blend setting) on the other side of the wiper.  If the Blend pot is set to the midpoint, you have 25k on one side of the wiper and 1M on the other; essentially a 1.025M pot turned down a bit but not that much.  If the pot value was dropped to 100k, our R19 would behave like a 125k pot turned down about a 1/4 of the way, which would present a certain amount of volume loss.

So, in this illustration, opting for a higher value volume pot on the output provides for more signal conservation, as a result of its interaction with other adjacent resistors.  Note that simply adding a 1M pot and leaving the 4M7 resistor in place will NOT improve matters.

Re: Volume boost via larger pot?
« Reply #17 on: November 26, 2007, 04:02:45 PM »
Cheers for all the suggestions guys. I ended up using a 1M pot and the 220k resistors either side of the blend pot. Overall there wasn't a huge volume increase, but it was definitely there. Hopefully it's enough for the guy. He said it only needed a little bit more, so cross-fingers eh? If this isn't adequate, I guess I'll just have to squeeze the LPB2 in there too

jakenold

  • Guest
Re: Volume boost via larger pot?
« Reply #18 on: November 26, 2007, 07:46:16 PM »
The danger of making the volume higher on both channels is, that the effect will act as a volume boost when engaged, regardless of which blend-setting is chosen. The booster will indeed be the optimal solution, but fingers crossed that your solutions satisfies your buddy!

mfg jake

Mark Hammer

Re: Volume boost via larger pot?
« Reply #19 on: November 27, 2007, 07:58:24 AM »
I recently made myself a combo pedal with a Dist+/DOD250 derivative as the front end**, going into a Scrambler.  (If the guy I loaned it to would return it, I could post some samples ::) ).

This combination addresses two of the enduring shortcomings of the stock Scrambler design: 1) there is no way to adjust output level, and 2) there is no way to adjust sensitivity.  The output volume pot on the Dist+ front end accomplishes both those goals.

I find that by using this combination, I get a few other advantages as well.  One of them is that by predistorting the signal a bit, and adjusting the level just right, you can get some very nice octave-up sounds, with very robust (and more predictable) octaving that starts to verge on what you get with a Foxx Tone Machine (though not quite).  Frankly, I've never been able to do that with a Scrambler on its own.  The second advantage is that whatever one likes about the sickest sounds a Scrambler is capable of, can be more than satisfied when the input signal is boosted and predistorted.  You can get choking sonic implosions....sweet!  Finally, by simply rotating the Blend control over to its' "clean" side, you can get a Dist+ signal with additional boost. 

Four pots and one whole heckuva lot of tonal flexibility (though including a treble cut control for the front end might make it even more interesting).  Highly recommended.  Just be careful to put it in a well-shielded setting (box, leads, etc) because there is a lot of cumulative gain and opportunity for noise and hum.

**Changes include substitution of 3k3/.22u network for 4k7/.047u network on ground leg of op-amp, 100k gain pot, 47pf feedback cap for op-amp, and 1N914 diodes for Ge.