Dr Quack allegedly not suiting the customer!

[bluebunny]

I built a Dr Quack for my tame German guitarist - it's exactly Jack's version using a board from Poodle in the UK.  I tested it with my Ibanez RBM-1 and turned into an instant funk machine (not bad for a cack-handed bass player).  I was dead pleased with it.      However, Herr Gitarrenspieler was reportedly less impressed.   

Apparently, (and I'm still waiting for the video evidence) he would like it to be more "open" when the signal is low.  So instead of going from filter very closed to filter very open, he'd prefer filter half-open to filter fully open.  Where would I need to start looking if I were to implement such a mod?  I'm guessing somewhere in the region of the BJT...

Apparently, also, when the filter closes it's not smooth.  It "stutters" on the way down...  Any ideas on what might cause this?

FYI, he's using a regular SSS Strat.  My axe does humbuckers and/or single coils (and sounded great).

Cyber-sausage on offer to anyone with interesting answers!   

[duck_arse]

the last I saw, I think it was jack said use uber-low gain wahing transistor.

[cortezthekiller]

The "range" trimpot should allow the low point to be set where you want.

The 10uF cap right before it to ground could be increased to 22uF. This effects the decay time of the filter, so increasing it's value should smooth things out.
(Thanks Mark Hammer).

[Mark Hammer]

There's a few ways.

The filter centre frequency is a function of the effective resistance to ground from the junction of the two 4700pf caps.  Normally, that resistance is entirely provided by the collector-emitter path, but it could be easily supplemented by a parallel resistance, which I've seen in other auto-wahs that use a transistor the same way.  Try starting out with 1M, and proceed either upwards or downwards from there.

A second way is to combine the fluctuating envelope voltage with a fixed voltage source, as is frequently doen in many "tunable" auto-wahs.  This would consist of a pot with outside lugs to V+ and Gnd, and the wiper feeding a fixed resistor of suitable value to limit the current.  The resistor feeds the input of the existing tuning trimpot so that the two voltage sources sum.

A third way is to make the tuning trimpot a panel/chassis-mount control so that you can adjust the range of the filter.

And Dave/Cortez is both correct, and very welcome (see you on Sunday, Dave) regarding the ripple/stutter.

There's probably others, but these ones come to mind easily.

[R.G.]

Yep, Mark's right. Go there first.

Here's another, perhaps more subtle point: you need to leave the word "allegedly" out of the subject line.

If the customer says they're not happy, they're not happy. There's no "allegedly" about it. Perhaps a better subject would be something like "how do I adjust a Dr Quack to suit my customer's preferences?"

It's a subtle difference, but it's an enormous issue in business. It's one of the things that separates building hobby pedals and selling a few of them from being in business. Any other approach ultimately puts you out of business.

[bluebunny]

Thanks for the interesting replies.  I shall be trying these out.  Please enjoy the prize, as promised.



@R.G. Yes, you're quite right.  He's a "customer", but he's also a pal.  I can take a few liberties and he takes it all in good humour.  I'd already been poking fun at him with the all-too-ubiquitous "it works for me" claim!  Were I a business, things would indeed be different...  Please enjoy this bonus:

   

[deadastronaut]

as i'm feeling generous and its wednesday ,  i'll donate a bonus prize too.

[cortezthekiller]

Now it should be called the Dr. BBQ