Input Clipping of Digitech XP-100

[Govmnt_Lacky]

Perhaps the best course of action might be to simply remove the offending pot from the board, mount a 9mm panel mount pot in the existing hole, and then wire it to the board. It will make removal easier (clip lugs, and desolder the remnants after), as well as not having to worry about not being able to find an A taper pot.

+1

This is an excellent temporary (or permanent??) solution. Just use jumper wires to connect to the empty lug holes once you have removed the bad pot.

[Baran Ismen]

Perhaps the best course of action might be to simply remove the offending pot from the board, mount a 9mm panel mount pot in the existing hole, and then wire it to the board. It will make removal easier (clip lugs, and desolder the remnants after), as well as not having to worry about not being able to find an A taper pot.

+1

This is an excellent temporary (or permanent??) solution. Just use jumper wires to connect to the empty lug holes once you have removed the bad pot.

Will do.

I just want to thank you all for assisting me with such a vintage unit and guiding me.

This pedal (as a nu-metal fan) means a lot to me, I've been dreaming of getting a whammy whatsoever for years. The GT-10 I use for live shows is overall good, but its pitch bend sucks. And as there's no official Digitech distributor in Turkey, people are selling Whammy IV,V, or DT ones at least double of their retail price as a 2nd hand. Somehow I found this unit from a very good and helpful guy from South Korea and financially pushed my limits to get it (200$ may not sound much actually, but considering the current currency in Turkey which is 1 $ = 27 TRY, is a lot  )

Upon its first arrival, it failed to work as it needed a 9V AC adapter -which is something that I've never heard of my entire 35 years of lifetime lol-, then I got one last Saturday, and voila it worked, but then I noticed this clipping issue. Seeing that the input level is not responding at all, I used my 101 knowledge of electronics and tried to find the root cause.

Anyway, I'll be writing the results.

I'll also share the pics of the V9 Muff that I've recently finished (you can see on the pics I shared; it was on the same desk where I work lol). I'll be printing a 3d enclosure soon and wrapping everything up.

[Baran Ismen]

Hi again.

Okaaay.. After a very tedious work for desoldering and slowly moving (which is not a habit of mine), I've successfully managed to remove the pot.

Immediately I've measured, and obvious is there. It's dead..

Then I soldered a new 100k just as you suggested, and boom!

It worked! Perfect!

One thing is that I've to use it around %15 percentage, %20 at most maybe, in order to achieve the best clipping/tone loss sweet spot..

As some say though, these pickups are "too hot" for XP-100, that's for sure.

I wonder what happens if I solder a higher K pot, like 220k ? Would it be %50 more or less sensitive?

[GGBB]

Based on the schematic posted earlier, a higher value pot will make the input more sensitive, because the pot resistance controls gain by acting as the gain resistor in the feedback loop of an opamp stage. More resistance = higher gain = more sensitive.

[Baran Ismen]

Based on the schematic posted earlier, a higher value pot will make the input more sensitive, because the pot resistance controls gain by acting as the gain resistor in the feedback loop of an opamp stage. More resistance = higher gain = more sensitive.

I see, so maybe half then, like 50k would be less sensitive and will give me more tweaking chance.

Thanks again, everyone!

[Rob Strand]

Based on the schematic posted earlier, a higher value pot will make the input more sensitive, because the pot resistance controls gain by acting as the gain resistor in the feedback loop of an opamp stage. More resistance = higher gain = more sensitive.

I see, so maybe half then, like 50k would be less sensitive and will give me more tweaking chance.

Thanks again, everyone!

I don't know what taper the existing 100k pot is but an audio taper or log taper (often marked A taper) is much easier to adjust.   Way better than replacing a 100k linear taper (B taper) with a 50k linear.

[Baran Ismen]

Based on the schematic posted earlier, a higher value pot will make the input more sensitive, because the pot resistance controls gain by acting as the gain resistor in the feedback loop of an opamp stage. More resistance = higher gain = more sensitive.

I see, so maybe half then, like 50k would be less sensitive and will give me more tweaking chance.

Thanks again, everyone!

I don't know what taper the existing 100k pot is but an audio taper or log taper (often marked A taper) is much easier to adjust.   Way better than replacing a 100k linear taper (B taper) with a 50k linear.

Current one says W100 on it, but found on an older post of TalkBass that its a A taper.

[Rob Strand]

Current one says W100 on it, but found on an older post of TalkBass that its a A taper.

A taper makes a lot of sense.

Some brands of pot use different letters to the common letters.



You can see here the W tapers are graphic equalizer tapers, more commonly call G-tapers.

Maybe easier to measure it:
- power off
- set pot to maximum.  measure resistance.  let measurement settle.
  check it's 100k.  If not the in-circuit measurement is affecting the measurement.
- set pot to half (position 5, 12 O'clock).  measure the resistance. let measurement settle.
  Linear and G tapers will measure 50k
  log/audio/A tapers will measure around 20k (anything 15k to 25k)


Here's one curve where W means log taper,

https://i0.wp.com/mathscinotes.com/wp-content/uploads/2015/08/Tapers.png?ssl=1

[Baran Ismen]

Current one says W100 on it, but found on an older post of TalkBass that its a A taper.

A taper makes a lot of sense.

Some brands of pot use different letters to the common letters.



You can see here the W tapers are graphic equalizer tapers, more commonly call G-tapers.

Maybe easier to measure it:
- power off
- set pot to maximum.  measure resistance.  let measurement settle.
  check it's 100k.  If not the in-circuit measurement is affecting the measurement.
- set pot to half (position 5, 12 O'clock).  measure the resistance. let measurement settle.
  Linear and G tapers will measure 50k
  log/audio/A tapers will measure around 20k (anything 15k to 25k)


Here's one curve where W means log taper,

https://i0.wp.com/mathscinotes.com/wp-content/uploads/2015/08/Tapers.png?ssl=1

Hi Rob,

Here's the link the message I mentioned. Its on harmonycentral btw, not talkbass lol

https://www.harmonycentral.com/forums/topic/120949-digitech-xp100-whammywah-repair-question/

As an update, I just received a response from Digitech about the W100k part:


W just referes to the pole piece. It is a 100K audio taper pot.


Regards,


William Clayton

Harman Music Group

Technical Customer Service

[Rob Strand]

Hi Rob,

Here's the link the message I mentioned. Its on harmonycentral btw, not talkbass lol

https://www.harmonycentral.com/forums/topic/120949-digitech-xp100-whammywah-repair-question/

As an update, I just received a response from Digitech about the W100k part:


W just referes to the pole piece. It is a 100K audio taper pot.


Regards,


William Clayton

Harman Music Group

Technical Customer Service

In that case it doesn't make sense that is hard to adjust.

[ElectricDruid]

In that case it doesn't make sense that is hard to adjust.

It does if the pot is busted, which is pretty likely on a front-panel input level control.

Slammer, Glad that Harmon/Digitech got back to you about it. So many "customer service" departments seem to do precious little to actually serve their customers.

[Govmnt_Lacky]

Although Digitech responded to you by saying that the pot was an "audio taper," I will say that I have tried an "A" taper and a "B" taper pot in that position and both of them did not quite work like the "W" taper pot that is installed at factory.

Bottom line is that if you want it to work as advertised, I would go with the W-taper pot. IF you are in a pinch and can live with it, then use the A-taper 

[Rob Strand]

It does if the pot is busted, which is pretty likely on a front-panel input level control.

That will put a big spanner in the works.

Although Digitech responded to you by saying that the pot was an "audio taper," I will say that I have tried an "A" taper and a "B" taper pot in that position and both of them did not quite work like the "W" taper pot that is installed at factory.

Good info - although I'm not surprised  there are some manufacturers who produce funky tapered pots even though generically they are 'log' or whatever.  The percentage resistance at 50% rotation is something which does vary with subtle taper differences (perhaps 10% to 30%, which will have a large impact on the gain).

You could try to chase-up the exact brand of pot and then find the data and taper curves - assuming the data is actully given by the manufacture, and represents the product.   Measuring the taper is much more convincing but setting pot positions accurately needs some care.

[Matthew Sanford]

funky tapered pots

The Funky Tapered Pots sounds like a great band name to me! I have a feeling most would believe it a drug reference though...

[Rob Strand]

If you do a bit of digging here's some of the variations you might see, and this is a small cross-section.

K vs A vs D:


Reverse T vs W:


In the old days the generic log pots were 10% log and labelled D and the audio taper pots were 15% to 20% log labelled A.  As Japanese made products began to dominate around the late 70's or so, log pots generically became A tapers.   IIRC, the A pots in Ibanez products are around 20%.    As general figure across A taper pots I've measured 13% to 23% or so.

Not only does the resistance at 50% change, so does the actual profile.   For example, there are substantial differences around 50% to 70% between the ALPs profiles.

On top of that are 20% to 25% resistance tolerance for pots in general - which need to be factored in if you are trying to measure the profile.

[Baran Ismen]

Now thats a lot of taper going on therr

I seriously have doubts that i could find a A tapered pot in Turkey in such small and spesific type, let alone finding the others. There are some as standard type A pots like alpha in 100k though.

Maybe someone with a good heart from here can find one and send me with international post? 

[Govmnt_Lacky]

Maybe someone with a good heart from here can find one and send me with international post? 

Have you checked your PM messages?? 

[Baran Ismen]

A rather late update that, I've finally soldered A100K pot to its place and it helped up to a point.
It was starting to clip around %20 of the pot, now its safe until %50-55 of the pot position.

I'm now sure that my pick-ups are hot for this unit.

Is it better to increase the R1 to 1M or do the modification mentioned here? Either case, I've never played with SMD parts before, I think I'd ask some technician to do this for me 

https://postimg.cc/Xpn9XTmm

[Govmnt_Lacky]

A rather late update that, I've finally soldered A100K pot to its place and it helped up to a point.
It was starting to clip around %20 of the pot, now its safe until %50-55 of the pot position.

I'm now sure that my pick-ups are hot for this unit.

It sounds like the Input Gain is doing exactly what it is supposed to. The purpose of that control is to adjust the amount of input that goes to the circuit. If you read the User Manual, it specifically says this. It is there to allow the user to adjust the input level for guitars with different pickups and gains.

[Baran Ismen]

A rather late update that, I've finally soldered A100K pot to its place and it helped up to a point.
It was starting to clip around %20 of the pot, now its safe until %50-55 of the pot position.

I'm now sure that my pick-ups are hot for this unit.

It sounds like the Input Gain is doing exactly what it is supposed to. The purpose of that control is to adjust the amount of input that goes to the circuit. If you read the User Manual, it specifically says this. It is there to allow the user to adjust the input level for guitars with different pickups and gains.

I know yes, but there's a real sweet spot right now, just a bit more clips, and just a bit less sucks the signal.

I do suspect about the R2, the 470k input impedance resistor still. I'd like to know if making it 1m would help whatsoever.

Meanwhile, as I couldn't remove the R10 (I think its glued or something), I soldered a parallel 1k and 200ohm resistor to it, tried both, and 200ohm seemed to help a bit with the clipping, but the signal tended to decay more quickly.

If I have a chance (or make one), I'll put a compressor in front of this unit and try that way, maybe it'd help with it.