Tonepad DS-1 : Setting Min Distortion

[niggez]

I breadboarded the Tonepad DS-1 . I want to be able to dial in less distortion - i tried a lot of components if not all of them, but i just cant lower it...i got everything else to sound the way i want to, but i am not able to lower the minimum distortion further... any idea which component could do this? Help would be very appreciated!
http://www.tonepad.com/getFile.asp?id=78

[chizkop]

Try lowering the value of R7. I've heard this helps.

[chizkop]

That is if you've already tried increasing r13. You will need to compensate with c8. i.e if you double r13, halve c8. this is to maintain the freq. response.

[5thumbs]

I'm not sure the Tonepad DS-1 board uses the same component markings as the BOSS board, so I'll describe it differently.

Like chizcop suggested, by increasing the value of the resistor between the DIST pot and ground, you can reduce the level of gain increased by the op amp.  If you increase that resistor from 4K7 to 10K -and- decrease the capacitor between that resistor and ground from 0.47uF to 0.22uF, you will keep the same corner frequency of that R-C filter.  This will also reduce the maximum-available volume boost out of your DS-1, which you may or may not want.

Another option would be to leave the op amp gain alone and increase the proximity to clip on the clipping diodes.  Stock, the DS-1 uses 1N4148/1N914-type diodes, which clip somewhere around 550-600mV.  If you want to reduce the distortion at minimum DIST settings, replace the clipping diodes with something that has a higher proximity to clip.

Some options for clipping diode replacements:

• 
  LEDs                                                                                         ~1000-1200mV (and higher for some of the super-bright varieties)
  
• 
  1N34A Germanium diodes                                                        ~300-350mV
  
• 
  1N4002 silicon power diodes                                                   ~450-500mV
  
• 
  1N914/1N4148 silicon diodes                                                   ~550-600mV
  
• 
  NPN silicon transistors (Base->Emitter –or– Base->Collector) ~650-700mV.
  

You can also wire combinations of these in series to get higher proximity to clip.  In series, add up the proximity to clip voltages and you'll get the proximity to clip voltage for that series combination.

For a quick test, replace the clipping diodes with two standard-brightness LEDs.  This will about double the proximity to clip.  If the volume increase is too much for your liking, then replace the resistor/capacitor combination between DIST pot and ground as mentioned previously to drop the op amp gain.  If the LEDs are still too distorted, add more diodes in series on each clipping diode position until you get the proximity to clip that you want.

[chizkop]

Speaking of clipping diodes!
I swapped one clipping diode (d5) for a led and didn't notice to much difference in sound.

I later went back and measured the led and got a reading of 749 mV. This is way below the value red leds should have!
I then measured the stock diodes and got a reading of around 740 mv. This is way above the 550 - 600mv you're supposed to get. It also explained why the distortion character didn't change much (I was looking for more assymetrical clipping).

I then measured the 1N001's I had lying around and got readings from 690 to 715?

Is this all within spec?

[flo]

@chizkop: Did you measure this with a scope or with a multimeter?
What you want to measure are the maximum and minimum signal peak voltages of your signal. With a scope you can do this and should yield signal peak values like stated by 5thumbs. If you measure with a multimeter you are probably measuring DC voltages which will not give you the correct signal peak voltages.
You can connect the clipping diodes with a DC voltage that you vary manually, with for instance a pot. Then you can measure the clipping voltages with a multimeter set to DC voltage.
Hope that helped...

[chizkop]

It was a multimetre.  The instruction booklet refers to it as "forward voltage drop in mV".

Thanks for pointing that out..

[5thumbs]

Speaking of clipping diodes!
I swapped one clipping diode (d5) for a led and didn't notice to much difference in sound.

I later went back and measured the led and got a reading of 749 mV. This is way below the value red leds should have!
I then measured the stock diodes and got a reading of around 740 mv. This is way above the 550 - 600mv you're supposed to get. It also explained why the distortion character didn't change much (I was looking for more assymetrical clipping).

I then measured the 1N001's I had lying around and got readings from 690 to 715?

Is this all within spec?

Specs can vary widely even between parts from the same manufacturer, so it is often difficult to say what is "within spec" without reading that manufacturer's spec.  Like R.G. has said in other posts, component manufacturers will often state only a MIN or MAX value...or even only a TYPICAL value when they want to be really slippery about what their component will really do.  Components can and often do vary a lot, so I can't say what youv'e tested are within spec for that manufacturer's published specifications.

I will tell you that I came up with the values in that table after measuring all the components of that type I had in stock with a DMM.  My DMM was set to "diode forward voltage test" and I verified the first DMM value with my "cheap" DMM I keep in my runabout backpack.  Both DMMs agreed on the voltages, so I averaged out the results and put together that table.  The LEDs were standard-brightness LEDs of various colors.  The 1N34As were green-band NTEs (so the black-band folks might see a different value.  Heck, even some in the lot of green-banded NTE 1N34As varied below 300mV and a couple were above 400mV.)  NPN Silicon transistors were 2N3904, 2N5089, 2N3859A, 2SC1815, 2SK30A.  The only Si NPN transistors I had that didn't clip as documented was the metal-can 2N3440s, which clipped about the same as the 1N4148/1N914 diodes.

It was intended to be a general guide, not the be-all-and-end-all authoritative table on proximity-to-clip values for various diodes.

Getting back to your task of increasing the proximity to clip (PTC), just add more diodes in series to get the PTC value you want.  Si diodes will have a harder clipping shoulder than Ge diodes, so bear that in mind as you pick your series combinations.  If you don't know what that means, just try out an equivalent PTC set of Si versus Ge and let your ears tell you which you like better.

Good luck!

[flo]

Ok right, measuring a diode (not connected to a circuit) with your multimeter with that "forward voltage drop in mV" function is fine. I was assuming that you were measuring the "live" clipped guitar signal peaks on the circuit board across the diodes. Sorry... 

[5thumbs]

Ok right, measuring a diode (not connected to a circuit) with your multimeter with that "forward voltage drop in mV" function is fine. I was assuming that you were measuring the "live" clipped guitar signal peaks on the circuit board across the diodes. Sorry... 

No need to apologize.   I just wanted to share how I came up with those numbers so folks would understand as they measure their own parts.

I don't usually measure standard-brightness LEDs, but I do measure all my other clippers for a given circuit so I can have an idea of how they'll actually clip.  Even if I'm using one the same type of diode in each clipping direction, I usually pick one with the highest forward-voltage reading and put one with the lowest forward-voltage reading on the other side.  Symmetrical clipping has its uses, but I usually like some level of asymmetry in my clipping arrangements.

[5thumbs]

Just to correct my post above, I didn't test 2SK30A.  Those are FETs, not NPN.  My fingers were going faster on my brain on that one.   The rest of the list stands as listed.