Some ideas for the P90

[Kipper4]

"maybe if you jamb in a pair of opamp mixers, and have one normal and one kipper inverted, send nomal to out one, and inverted to out two."

Yer that was a cool mod. Completely different feel to both.
it's in the treadle phaser thread.

Nothing wrang with the Gnat Duck you need to wire it up right. Ahem.

Nothing wrang with black Ben. Blakie did a great job on his multiFX. Did you spy that?

[Ben Lyman]

Thanks guys, is there a link explaining how to add the opamp mixer?
Will a single dual opamp like a TL072 do the trick?

[Kipper4]

reply #50 switch sw1 A


http://www.diystompboxes.com/smfforum/index.php?topic=115042.40

[Ben Lyman]

Thanks Rich, i will look into that. Right now i have it back on the board with 2n5458's and a trimmer for the bias, it has to be turned more off center than i did with the 5457's so my previous bias network wouldn't have worked without adjustment.

I got a nice sweep pot going here, tried a 2M2 fixed with a B1M pot, then a 2M fixed w/same pot.
Now it is 2M fixed with B500K pot and i think i like it.

[Kipper4]

You'll probabaly get more rotation usage from the bias pot by adjusting the 2x 100k Vbias resistors.
try a 100k + series 22k to ground (R32) . If its worse adjust the other 100k (R31).

offsetting the bias resistors might help.
if 22k aint enough try bigger eg 33k
or smaller depending eg 10k

I'll go listen now.
Good work mate.Did you try swapping the 58's in after listening to the 57s. I get different tones. I find the 58's more Chewy. Technical jargon.

[Ben Lyman]

Yes, chewy 

[Kipper4]

If you like chewy.
Try making R5 bigger. I only got as far as 2M2 and got seasick

[Ben Lyman]

Thanks Rich. It's funny how everything Mark already told me was in the back of my mind, it all made sense, the starting point and the point where it rises before falling again, etc. but I couldn't really make use of it until you said bigger R5=chewy
I think i avoided it before because i didn't want an external bias AND a variable R5 but I really like it. I think I can be happy with this extra chewiness as long as i have the sweep control, it sounds great. I think the sweep control covers even more ground now for some reason, goes super thick and chewy all the way down to very subtle. The vibrato can be quite enhanced with the sweep control now too.

[Kipper4]

I think if I was to offer such a pedal for sale. I'd advertise it as tuneable for your rig.
I.e. The bias is set up for the customers needs. trimmer and inside  the box
Because the phase/vibe set up through a bright fender and a dark Marshall will sound totally different.
Or if the customer where a distance away you could advise on setting it up.

The sweep control is nice especially when in vibe mode.
Btw I'm currently using 1M2-1M5 R5
I might change back to 1M. I'm gonna go look at different versions of the p45.

[Mark Hammer]

Thanks Rich. It's funny how everything Mark already told me was in the back of my mind, it all made sense, the starting point and the point where it rises before falling again, etc. but I couldn't really make use of it until you said bigger R5=chewy
I think i avoided it before because i didn't want an external bias AND a variable R5 but I really like it. I think I can be happy with this extra chewiness as long as i have the sweep control, it sounds great. I think the sweep control covers even more ground now for some reason, goes super thick and chewy all the way down to very subtle. The vibrato can be quite enhanced with the sweep control now too.

The irony here, Ben, is that all of the mods described are normal parts/controls of many standard phasers.  My 30 year-old Pearl PH-44 phaser has variable sweep width, variable feedback, and an offset ("Manual") control, in addition to two speeds and ramp up/down between them.  The MXR units have the misfortune of originating during an era when having two kobs was a really big deal.

[Ben Lyman]

Wow Mark, "gnarly" is the only word i can think of for that thing... maybe "insane" as well... yeah, gnarly and insane, that's what that is 

the other irony is that, for me two knobs is still a big deal!
I'm pretty happy to have something on the bb now that looks like it will be good enough with two knobs and a switch.

And now it sounds pretty lush with the recent resistor changes, I will try to make a new vid soon

[Kipper4]

Quick question.
What is the purpose of R8 ?
Is the op amp not biased by R9 ?

[Ben Lyman]

Quick question.
What is the purpose of R8 ?
Is the op amp not biased by R9 ?

I really don't know, maybe someone else can answer that
I just copied it from GGG(20k) and JCM(22k) schematics

[duck_arse]

R8 - I'll have a swing. being as it is wired as a non-inverting amp, we set the gain (mostly) with the feedback resistor and the (-)in resistor, which has to go to a low impedance point, in this case V/2. and as we still need to centre bias the output, the (+)in gets a resistor to V/2, also setting the input impedance. it's just drawn different, and so perhaps confusing.

ben - are you keeping the external bias pot? and if you give me the day, I'll draw the mixers circuit I was thinking of.

[Kipper4]

Whats confusing to me is when i take R8 out. It sounds brighter than with.
Maybe some trickery going on.
If the - input is at V/2 wont the +input try to maintain that and also the output?


edit
defo some trickery. It's not brighter.

[Ben Lyman]

ben - are you keeping the external bias pot?

Not for now Duck, I messed around with it last night for an hour and wasn't too happy with the results. I figured out the working range, again it was about 50K but this time with a 160K to ground and a 39K to VR. It made some good sounds but I think I will leave it on an internal trimmer this time.

I changed R5 to 2M and R25 to 3M with the B500K sweep pot, I like it a lot. When the bias trimmer is set just right I can get a wide range of speeds and depths.

One problem remains: When I pull the dry path and go for vibrato only, BIG volume jump. I think I have to leave the vibrato switch off this one too, I can't see myself ever using it if I have to adjust my amp volume every time. I haven't tried the mix pot yet, I will test it out next but I expect a volume drop in the middle of the pot with a boost at each side.
Maybe a B10K mix pot in between a couple of 5K resistors?

Whats confusing to me is when i take R8 out. It sounds brighter than with.

I've noticed that sometimes when I change something I have to pay a quick visit to the bias pot

[Kipper4]

This is where I'm at.

Extrenal pot for Bias

[Mark Hammer]

One problem remains: When I pull the dry path and go for vibrato only, BIG volume jump. I think I have to leave the vibrato switch off this one too, I can't see myself ever using it if I have to adjust my amp volume every time. I haven't tried the mix pot yet, I will test it out next but I expect a volume drop in the middle of the pot with a boost at each side.
Maybe a B10K mix pot in between a couple of 5K resistors?

Now that's weird.  I tend to have the opposite problem.    On the one hand, killing the clean/dry signal for vibrato means the overall sum of the two signals is reduced.  On the other hand, in phasing/Uni-vibe mode, a big wide dip is created (Vibes create dips, rather than notches), which, in principle, should reduce the overall signal amplitude.

[Ben Lyman]

Looks great Rich, mine is very close. I like how you have the external bias pot and a simple switch for the sweep. I also like my sweep pot so i'm still undecided at the moment.

Mark, when I made my P90 I did not have the volume prob; could it be somehow related to the P90 "output mixer" stage? Could it be you haven't had this problem because you included the mixer?

[Mark Hammer]

I'm more inclined to think it has to do with whether dips or notches are being created.  Vibes create wide shallow dips that are much less focussed than phasers.  The "dreaminess" they produce is partly a function of how that aspect almost yields a tremolo effect at the same time as it is doing other things.  That's also why Vibes sound better ahead of clipping pedals.  The broad shallow dip moves whole regions away from and closer to the clipping threshold.

So, my gut sense - which can always be corrected by empirical evidence to the contrary - is that the broad shallow dip reduces overall signal amplitude moreso than a few narrow steep notches do.  So, remove the dry signal, such that all you have is the subtle pitch-shifting of the whole signal, and the volume level will appear to increase.

The drawn schematic shows a bit of boost in the input stage.  The RC network around each JFET serves to reduce increase headroom, so it is feasible that a vibrato/phaser switch could simultaneously cancel the dry signal AND adjust the gain of the input stage so as to maintain level balance.