zombie chorus help needed ...build report

[makkimo]

hi guys!!!
i've just finished my zombie (used two tl062 amps, a 220uf for c6 and a 100uf for c7 as suggested by MH) chorus woow it has a great sound but...
readed posted about it before take the soldering and knew the many problems to make it work well ; finding the ticking for me was a kind of succes it means that my build was done just fine i'll try out the mod  to detick it soon and i'll post the results.... :lol:
....but... playing a little with it  i've founded a more sad problem :cry:  :cry:
it sounds really cool with single and high notes but suffers a lot with low ones, i mean, a little and unpleasant distortion comes out of it expecially hitting hard and low chords i really want it to work well so please help me!!! is this a correct beaviour!! what can i do to fix this problem
i hope
thanks to all

[Mark Hammer]

One of the "tricks" that John Hollis used in the original design was to use one single bias voltage throughout the entire circuit, for the BBD, the LFO, and the audio path.  One of the things this introduces is the *requirement* to have NO capacitors between the op-amp input stage and the BBD, or else the DC bias voltage would be eliminated.

If the audio path from op-amp to BBD *has* to be direct coupled, that means that in order to feed a DC bias voltage through to the BBD, you are obliged to accept ALL low frequency content.

This is a problem, especially since there is no companding or input level adjustment feature like one sees on some chorusses.  It would be nice if one could "shave off" some of the bass to reduce the likelihood of overdriving the BBD (bass content is usually much higher amplitude and at greater risk of distorting).

Looking at RG Keen's layout at GEOFEX,n you can see that the bias voltage formed by the unequal pair of resistors R12/R13 enters U1a via R3.  Whatever voltage U1a is biased to, that is exactly what the BBD (U2) sees on its input (pin 3).

The "ideal" is to do the following:

1) Provide a separate bias voltage for both dual op-amps which is exactly V+/2 (4.5V).

2) Cut the trace between pin 1 of U1a and pin 3 of U2 (making sure to leave the connection between pin 1 and R5 intact).

3) Insert a moderate-value cap after U1a, such as .1uf or something like that, to both"strip" or block the DC voltage, and attenuate all AC (signal) below about 200hz or so.  You can experiment with different cap values to suit your taste, but the idea is to trim bottom end which isn't all that important to a good chorus sound anyways.

4) After the cap, insert a separate bias circuit for the MN3007.  You can see plenty of examples in the various MN3007 pedals that have schems floating around.

I am not promising this will completely eliminate your distortion problems, but there is a great chance they will be reduced.  Note that keeping full bandwidth for the dry signal requires leaving the pin 1 to R5 connection intact.

We too often mindlessly pursue full bandwidth without stopping to think about the costs vs benefits of doing so.  Very often retaining full bandwidth at the low end can cause us problems because of the relative amplitude of lower vs higher frequency signal components.

[aremesnik]

This is what mine does also, but it is only a problem for me if I play chords that are drop tuned to A (like a Seven string guitar), or Play dubble stops on below A on my Bass.

Personnal I like the way this unit sounds

[makkimo]

:cry:  :cry:
thanks for explanation i think thah is a little too difficult for me....probably i just can do that by myself without a good layout od schematics of mods... :?
the tick is still when effect is bypassed this is really a nasty problem....
but tragic is: i've just tried the simple detick mod (the one with 2 10k resistors)suggested by MH and the result is:::::::::::::: :cry:  :cry:  :cry:
the problem is not fixed at all!!! seems only reduce the distortion
i'm so sad i could cry i've made this one with my hands and really i'd like it to live.........
thanks a lot