Built tonepad Phase 90 and...it distorts.

[sircletus]

Hello, all.  I'm relatively new to the wild world of modulation effects, and I thought I'd start out by building a Phase 90 using the tonepad schematic.  Upon completion, I've noticed a nasty distortion when I pick hard ('93 strat with lace sensors).  I did a little test and first disconnected the wet signal from the output and found that the clean signal is not distorting, so it's not the input stage clipping.  Only the wet signal is distorting.  To further test things out, I rebuilt the entire circuit on my powered proto board and it's still distorting.  So there's something about the circuit itself that's causing distortion, as opposed to my screwing up the build.  Any idea how to eliminate this problem?

Thanks all,
SirC

[mdh]

Could you tell us which JFETs you used, whether you matched them, and what the gate voltage is on one of the JFETs (they're connected, so they should all be the same)?

[aab0mb]

A lot of people have encountered this problem w/ the P90.  Do a search here in the forum and in Google and you'll more that likely find what you want to know. 

Start here:
http://www.erikhansen.net/?page_id=31

The link is NOT talking about the tonepad version but it'll help.

Good luck and let us know how it turns out.

Jam,
A

[guitarman89]

I've built it some weeks ago and i noticed that it distorts a very very bit or however crank up the dry signal which is so filtered (i've mod it with depth mod, 1 M pot instead of the 1 M resistor after zener diode and i use tl082)...
But it sounds great! It is one of my preferred stompbox!!  :icon_evil:P90 Rulezzzzzzzzzz

I was thinking.... you can try to change the ICs with others! Try and listen...It can be a good solution!

[sircletus]

I'm using 2N5952s matched to the 1000th of a Volt at the gate (using the GEOFEX JFET matcher).

Since I posted, I've done some poking around and noticed that there are several discussions about R28 and R31.  It seems that R28 is the feedback resistor.  That's removed easily enough.  But since the tonepad schematic doesn't number the components, I haven't any idea which resistor is R31.  Anyone know of a schematic with component labels?  I'd also like to investigate C11 and C12.

Thanks,
SirC

[mac]

I had the same problem with low Vgsoff fets, like 2sk117 and bf245a.
2sk246 and mpf102 sounded without distorting but they were not as matched as the others.
I noticed that using a lower value bias trimmer, 100k, solved the problem with the bf245a.

mac

[Auke Haarsma]

I cannot really explain why, but my Tonepad Phase90 distorted also. Once I removed the zener-diode the distortion was gone and it phases nicely!

[iaresee]

Can I resurrect this thread?

I've got an MXR Phase 90 that, even after clipping R28, continues to distort horribly when I use it with hotter output pickups. I can't tell from the description on Erik's page whether removing C11 and C12 will help. What will removing C11 and C12 do for the sound of the pedal? Erik's description makes me think I'd better off lowering the value of R7 than clipping C11 or C12.

Thanks!

[mac]

I also used 8k2 as feedback resistor which reduced the gain of all stages, distortion was gone.

mac

[iaresee]

Thanks Mac. I'll play with the feedback resistor value then. Much obliged!

- Ian

[iaresee]

Hmm...can't seem to delete a post. I moved my comment to it's own thread. My stupidity should stand on it's own. 

[Mugshot]

I cannot really explain why, but my Tonepad Phase90 distorted also. Once I removed the zener-diode the distortion was gone and it phases nicely!

can someone explain what will happen if the zener diode is gone? mine distorts too.

[vondran]

I cannot really explain why, but my Tonepad Phase90 distorted also. Once I removed the zener-diode the distortion was gone and it phases nicely!

can someone explain what will happen if the zener diode is gone? mine distorts too.

The zener seems to be used as a voltage regulator to set the bias voltage.  I measured the voltage on my MXR Phase 90 at the zener and found that it holds the voltage at 4.05V independent of the input voltages I used (tried batteries that outputted 8.6V, 8.2V, and 7.5V).  My guess is the distortion is from the output transistor, which looking at the circuit can clip when the base goes above the emitter.  By removing the zener it looks like the bias voltage will be raised and likely increase the headroom between the base and emitter on the output transistor.

[Mugshot]

I cannot really explain why, but my Tonepad Phase90 distorted also. Once I removed the zener-diode the distortion was gone and it phases nicely!

can someone explain what will happen if the zener diode is gone? mine distorts too.

The zener seems to be used as a voltage regulator to set the bias voltage.  I measured the voltage on my MXR Phase 90 at the zener and found that it holds the voltage at 4.05V independent of the input voltages I used (tried batteries that outputted 8.6V, 8.2V, and 7.5V).  My guess is the distortion is from the output transistor, which looking at the circuit can clip when the base goes above the emitter.  By removing the zener it looks like the bias voltage will be raised and likely increase the headroom between the base and emitter on the output transistor.

fufu  i DO have to learn more about electronics and stuff. big words like emitter and collector as used in electronics makes my nose bleed for a more concrete explanation.

maybe in layman's guitar-speak terms, what does it mean?