Micro Amp clone issue

[Rick C]

Hello,
I built an MXR Micro Amp clone from GGG. I'm having an issue with the pot. It calls for a 500k reverse log which I couldn't find, so I used a 500k audio taper. I've been told that the tapers are so similar that you can barely tell the difference.
Anyway, the boost doesn't kick in until the pot is turned almost all the way up. ( 7 or 8 ) So there's no way to get any fine adjustments.
I tried a linear taper pot and it did the same thing.
Is there somrthing else I should be looking at or do I actually need to use the reverse log?
Thanks.

[RDV]

If you wire the audio pot backwards, you will have a reverse log pot. You'll at least hear what one will respond like(only backward).

HTH

RDV

[Rick C]

If you wire the audio pot backwards, you will have a reverse log pot. You'll at least hear what one will respond like(only backward).

HTH

RDV

Just tried that, very similar results.

[DryRoasted]

I made a D+ which is essentially the same circuit with a few bits added, and forwent the rev log pot in favour of an ordinary pot and got extactly the same results as you did.  Get a rev log pot from Small bear and you'll really hear the difference.  The tapers make all the difference.

[jxoco]

If you wire the audio pot backwards, you will have a reverse log pot. You'll at least hear what one will respond like(only backward).

HTH

RDV

Just tried that, very similar results.

Try wiring the Audio taper in reverse.

[Rick C]

If you wire the audio pot backwards, you will have a reverse log pot. You'll at least hear what one will respond like(only backward).

HTH

RDV

Just tried that, very similar results.

Try wiring the Audio taper in reverse.

I did, same results.

[Mark Hammer]

Pot taper is fundamentally a question of where the settings of most interest to the user are situated with respect to the full rotation of the pot.  If ALL possible settings are of interest to the user, then pot taper, or rather having a pot with the "right" taper, becomes more critical.

Of course if only a portion of the range of possible settings is of interest to the user, then pot taper may matter much less, especially if the physical pot is being used as a variable resistor, rather than voltage divider.

In the case of the Micro Amp (I'm looking at the Tonepad document http://www.tonepad.com/getFile.asp?id=6 ), there are actually a number of other pot values and gain-adjustment arrangements that you can consider without driving yourself nuts about pot taper.

The Micro-Amp schematic shows the feedback resistor (R9) as 56k, and the two series resistances from the inverting pin (pin 6) to ground as a 2k7 fixed resistor (R8) and 500k pot (R13).  Note that the gain of this op-amp is set by the ratio of [R9+R8+R13]/[R8+R13].  So when the pot is set to max gain (min resistance), this is equal to (56k+2k7+0)/[2k7+0] = 58k7/2k7 = 21.7.  With the pot at max resistance, this is now (56k+502k7)/502k7 = 1.11 .  However, note that if we were to use a linear pot, and set it to the middle (250k), our gain would be (56k+252k7)/ 252k7 = 1.22.  That's not much change, is it?  In fact, the gain doesn't really change all that much until R13 starts to get down to a value of around 50k.  In truth, if a person is interested in gains greater than 2 from this pedal, there really isn't much point to using a pot value greater than 50k, is there? 

Of course, with a 50k pot, the default/min gain would be [56k+2k7+50k]/[2k7+50k] = 108k7/52k7 = 2.06.  The interesting thing is that it doesn't take as much movement of the pot to noticeably influence gain.  Reduce the pot resistance from 50k (max) to 40k, and the gain is now 98k7/42k7 = 2.3.  Drop it to 30k and the gain is now 88k7/32k7 = 2.71.  At 25k (halfway on a linear pot), the gain is now 3.  Beyond this, of course it changes even faster.  With our 500k pot, moving the wiper from full to half resistance only changed the gain by a factor of 0.1.  Here, it changes the gain by a factor of x1.0

Note that the maximum gain is completely unaffected.  What has changed is that we have taken the last 50k of resistance from the original stock 500k pot, and spread it out over the full rotation of a 50k pot, where gains between x2 and x21.7 will be easier to locate.

But what about if you want no gain?  Simple.  Lift the connection between C5 and ground and the Micro-Amp now becomes a unity gain high-impedance buffer  Indeed, if you wanted to use the pedal as an always-on buffer with switch-selectable gain, you could do the following:
1) wire in a 1meg resistor between R13 and C5
2) connect each end of this added resistor to two adjacent lugs of a stompswitch

When the switch is closed and this added resistor is bypassed by straight wire, you have whatever the gain is set to.  When the switch is open, the 1meg resistor is added to the resistance of R8 and R13, making the pedal effectively a unity gain buffer (well, a x1.056 buffer, but who's quibbling?).  The settings of a 50k gain pot *will* make a measurable difference but not an audible one (gain will be 1.053 at max R13 setting under "bypass conditions").  The nice thing about this is you can use one set of contacts from a cheaper DPDT stompswitch for the "bypass" function, and the other set of contacts for the status LED.  The buffering effect of the pedal remains in place at all times.

Note that if the maximum gain of the pedal is either too much or too little for you, you can always change R9's value.  Hiking it up to 68k will give you a max gain of 26.2, using R8=2k7 and R13=50k.  Dropping it to 47k gives a max gain of 18.4.  The min gain will also change, since you will remember that it is the ratio of resistances that sets the gain.  But the gain will never go below unity.

[jxoco]

have a read of this artice
http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

the stuff for you is in the bottom third of the article, does that make it a reverse log article

[jxoco]

and this article will start to drive you nuts;

http://tangentsoft.net/audio/atten.html

I've sort of been down the same road you are following.

[delbowski]

hi...

i've experienced the same problem with my microamp clone... seems like all the gain is bunched up in the last half to 1/3 of the pot rotation.  i'm using a 500k reverse log as per the schematic, and i've tried mark's suggestion of using a different value pot (i put a 56k resistor across my 500k pot - (500 x 56)/556 to make a 50k pot (50.359)).  this 50k pot still seems to have all of the gain bunched up.  the minimum gain (max resistance) is a little higher than unity as mark said, and the max gain (min resistance) is still the same, but the gain is still not evenly distributed throughout the rotation (almost like a spike in gain after the halfway point).  is there anything else i could do?  as i said i'm using an audio taper pot... any ideas?  thanks

del

[markm]

I had the same problem with my micro-amp clone and finally gave up trying to solve it!
   MM

[Rick C]

I had the same problem with my micro-amp clone and finally gave up trying to solve it!
   MM

I'm thinking the same thing.
Hell, it sounds best cranked anyway!!!

[delbowski]

I had the same problem with my micro-amp clone and finally gave up trying to solve it!
   MM

I'm thinking the same thing.
Hell, it sounds best cranked anyway!!!

yeah i've been living with this for a while too, but there's gotta be some answer!  i can't leave it alone... the perfectionist in me won't allow it!

i'm assuming the original microamp doesn't have this problem, does it?  i've never had one... this has been driving me nuts for months!

del

[anti-idiot]

Pot taper is fundamentally a question of where the settings of most interest to the user are situated with respect to the full rotation of the pot.  If ALL possible settings are of interest to the user, then pot taper, or rather having a pot with the "right" taper, becomes more critical.

Of course if only a portion of the range of possible settings is of interest to the user, then pot taper may matter much less, especially if the physical pot is being used as a variable resistor, rather than voltage divider.

In the case of the Micro Amp (I'm looking at the Tonepad document http://www.tonepad.com/getFile.asp?id=6 ), there are actually a number of other pot values and gain-adjustment arrangements that you can consider without driving yourself nuts about pot taper.

The Micro-Amp schematic shows the feedback resistor (R9) as 56k, and the two series resistances from the inverting pin (pin 6) to ground as a 2k7 fixed resistor (R8) and 500k pot (R13).  Note that the gain of this op-amp is set by the ratio of [R9+R8+R13]/[R8+R13].  So when the pot is set to max gain (min resistance), this is equal to (56k+2k7+0)/[2k7+0] = 58k7/2k7 = 21.7.  With the pot at max resistance, this is now (56k+502k7)/502k7 = 1.11 .  However, note that if we were to use a linear pot, and set it to the middle (250k), our gain would be (56k+252k7)/ 252k7 = 1.22.  That's not much change, is it?  In fact, the gain doesn't really change all that much until R13 starts to get down to a value of around 50k.  In truth, if a person is interested in gains greater than 2 from this pedal, there really isn't much point to using a pot value greater than 50k, is there? 

Of course, with a 50k pot, the default/min gain would be [56k+2k7+50k]/[2k7+50k] = 108k7/52k7 = 2.06.  The interesting thing is that it doesn't take as much movement of the pot to noticeably influence gain.  Reduce the pot resistance from 50k (max) to 40k, and the gain is now 98k7/42k7 = 2.3.  Drop it to 30k and the gain is now 88k7/32k7 = 2.71.  At 25k (halfway on a linear pot), the gain is now 3.  Beyond this, of course it changes even faster.  With our 500k pot, moving the wiper from full to half resistance only changed the gain by a factor of 0.1.  Here, it changes the gain by a factor of x1.0

Note that the maximum gain is completely unaffected.  What has changed is that we have taken the last 50k of resistance from the original stock 500k pot, and spread it out over the full rotation of a 50k pot, where gains between x2 and x21.7 will be easier to locate.

But what about if you want no gain?  Simple.  Lift the connection between C5 and ground and the Micro-Amp now becomes a unity gain high-impedance buffer  Indeed, if you wanted to use the pedal as an always-on buffer with switch-selectable gain, you could do the following:
1) wire in a 1meg resistor between R13 and C5
2) connect each end of this added resistor to two adjacent lugs of a stompswitch

When the switch is closed and this added resistor is bypassed by straight wire, you have whatever the gain is set to.  When the switch is open, the 1meg resistor is added to the resistance of R8 and R13, making the pedal effectively a unity gain buffer (well, a x1.056 buffer, but who's quibbling?).  The settings of a 50k gain pot *will* make a measurable difference but not an audible one (gain will be 1.053 at max R13 setting under "bypass conditions").  The nice thing about this is you can use one set of contacts from a cheaper DPDT stompswitch for the "bypass" function, and the other set of contacts for the status LED.  The buffering effect of the pedal remains in place at all times.

Note that if the maximum gain of the pedal is either too much or too little for you, you can always change R9's value.  Hiking it up to 68k will give you a max gain of 26.2, using R8=2k7 and R13=50k.  Dropping it to 47k gives a max gain of 18.4.  The min gain will also change, since you will remember that it is the ratio of resistances that sets the gain.  But the gain will never go below unity.

What if, instead of a 50k Rev Log, i want to put a Linear one? what value would u recom'nd?

[Mark Hammer]

50k will probably cover the range of most interest to most people here.  All that log/antilog stuff is about where you're going to find that last 50k when using a 500k pot.  It it's all squisshed into a tiny bit of knob rotation, then it will be hard to nail a setting easily nad make it stay there.  If that 50k is spread out over the entire rotation of the pot, then the taper starts to matter much less.

[Fret Wire]

Hello,
I built an MXR Micro Amp clone from GGG. I'm having an issue with the pot. It calls for a 500k reverse log which I couldn't find, so I used a 500k audio taper. Is there somrthing else I should be looking at or do I actually need to use the reverse log?
Thanks.

A 500k rev. log will progress the most naturally to your ears. Assuming you're in the US, and shipping won't be a problem, you can get them here for $2.75:
http://www.smallbearelec.com/Detail.bok?no=116

Next time you do an order, grab a few. The Micro Amp, Dist.+, DOD 250, phase 45 & 90 all ramp up the most naturally with their original spec'd rev. log pots.