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what's a T-wah?

8/27/1999 12:24 AM
what's a T-wah?
Hey guys, maybe someone can help me out here.  
A friend of mine had an envelope filter called a 'T-Wah', which got stolen. He's asked me to make him a replacement. Problem is, I've never heard of a T-wah. Does anyone have any info on this, and even a schematic? Otherwise, what is a general feeling on what's a good env filter to make?  
On another matter, I just made myself a phase 90 - type phaser, and the LFO is quite audible in the signal. I used a dual opamp for the LFO and first audio section, so is this creating interference, or would it be something else like pcb layout?  
Oh yeah, one more thing, I made a dyna-comp from the schematic on R.G.'s site, but I don't know what the controls are supposed to do. the volume is obvious, but what's the 2k pot for? I guess the 500k is compression.  
Thanks for the input,  
8/27/1999 2:03 AM
Mark Hammer
The T-Wah is an older BOSS unit that predates the Dynamic Filter. Same deal, though. Up/down sweep, bandpass/lowpass, sensitivity, and resonance controls, but no footpedal control.  
AMPAGE readers know of my obsession with envelope-controlled filters. For my money, asking what's a "good" ECF to build is a bit like asking "what's a good effects chain to buy?". Most ECF's have a pretty unique personality, since there are many parameters to filtering sound, many parameters to deriving an amplitude envelope and control-voltage, many combinations of each of the above, and lotsa room for component tolerances to add a lovely unpredictable spin to the whole shebang. Most of the time, what's "good" depends on what you like to play.  
That being said, many people find there is more than enough personality for them in the Mutron III, schematics for which abound at the various sites like Aron Nelson's, RG's, Justin Philpott's, and an ever-increasing array (not to mention the clone at Mike Burgundy's site). Most parts are easy to obtain, although the photocell/optoisolator can be problematic for some folks.  
Another contender is the EH Bassballs, whose schematic can be found at Mike Burgundy's site (scroll down on your left to almost bottom and you'll find a link to it). This is a pretty flexible unit that uses garden variety components, and has ample room for easy-to-do mods. It works exactly like the famous Dr. Q, which is another decent pedal (albeit a little one dimensional).  
My fave is the MXR Envelope Filter, because of the variable attack time (which I think allows for better matching of the effect to the song, in addition to some interesting special effects). I keep promising to verify the hand-drawn schematic I have of this one and send it out for posting, but I'm about 4 years behind on promises. As much as I like it, I wouldn't recommend novices attempting it because of the parts count, because it uses CMOS chips (static sensitive), and because one has to be particular about which version of the CMOS chips; in short, because too many things can go wrong. On the other hand, I've made two without much problem so they can't be that hard.  
I don't have the schematic in front of me so I may be wrong, but I think the 2k pot on the Dynacomp is a trimpot serving as a "distortion trim" control, something which is normally found on 3080-based devices, and compensates for the quirks and constraints of that chip, rather than any kind of spiffy control to inject more personality into one's sound. This is set for cleanest sound and left alone. It is a board-mounted component, not a pot on the chassis.
8/27/1999 4:17 AM

thanks, mark  
I've been looking to make an env filt for myself, but I haven't tried many, so I don't really know the difference between tham. Do you know if there's any schem's around for the t-wah? Do you think it'd be better to make my friend an MXR version, perhaps? I'm not scared of tricky chips, but things have been known to go wrong. :-)  
thanks for the info on the dyna-comp.  
do you perhaps know of a mail-order place where i could get some CLM6000 optoisolators? I live in Australia, and I can't find any here.  
8/27/1999 11:32 AM

The 2K is a trimpot, just set it for the maximum sustain without distortion. If you've got a sine generator and scope feed a low-level 100Hz sine to the input, put compression at max. and adjust it so output clips symmetrically.
8/27/1999 2:31 PM
paul perry

Rob, I'm in Australia & I've got some spare CLM6000.  
email or ring Melbourne 9347 5853
8/27/1999 2:39 PM

Absolutely correct on the 2K Mark. It's there to balance any oddities between the two inputs. It's a trimmer. The new PCB's for a Dynacomp clone from GEO have the trimmer printed on the board.  
The GEO Neutron board implements the same circuit as the Mutron III, and shows four ways to make the LED/opto circuit. C.J. Landry also has some direct replacements for the LED/opto like the original.  
The MXR Envelope Filter is a digital filter. It uses CMOS switches to vary the duty cycle on its capacitor, effectively varying their value. The digital portion of the circuit is pretty straightforward, but keeping the digital signals out of the analog signal path probably needs some good grounding and layout work - at least for me. I haven't made one of these yet.
9/6/1999 4:30 AM
MXR Envelope Filter
I have one of these that doesn't work- sound comes through it but no filter change (come to think of it I also have a T-wah that does(doesn't?) the same thing). Any ideas of where I should start looking to fix either or both? I have working versions of both as well if it would help to get voltage readings etc.  
Also, I just built Paia's envelope follower. It works pretty good and it's cheap, so if anyone is looking for one more flavour you might want to consider this.
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