Dr. Quack 6 String Bass with Active Pickups

[wingfijm]

I am going to be building a Dr. Quack for a friend of a friend who lives out of state. He plays a 6 String Bass with Active Pickups. I have read the bass mods for the Dr. Quack, but what should I do to account for his active pickups?

The AMZ schematic notes that the 470K resistor did not provide enough gain for some guitars, so it was  increased to 2.2M. Since this effect will be used with active pickups which produce a higher output, should I go with the original 470k?
Reference: http://www.muzique.com/schem/quack.gif

Also, I don't have any instrument with active pickups. What can I do to test it out? Can I try running an extra hot signal into it? Perhaps from a preamp directly into the Dr. Quack?

Thanks in advance!

[solderman]

Hi
I suggest you build the Nurse quacky instead from ROG

This one does the same. It is a devbeloped and a bit mor versetile circuit. When I tested it I thought that the high pitch was to pronaunced and a bit distorted so I added a on-off-on switch to change the filtering range. The left position addes a 10nF cap in paralell to each of the 4,7nF filter caps. The right position adds 22nF caps and the middle position is stock. This way I can get a bassier sound without the real high pitch. http://www.home-wrecker.com/nurse-quacky.html

//Solderman

[Mark Hammer]

The Dr. Quack is a better circuit, but it's "better" because it provides a high input impedance so as to avoid the loading that normally occurs at the junction of the Sensitivity pot and 47k resistor to ground on the filter side.  Your friend's active bass presumably provides a low-ish impedance out so the FET front end that Jack added is not strictly necessary.

The range set by the 4n7 caps is for guitar, so shifting that down 1-2 octaves is wise for bass, depending on what those extra 2 strings are.  I am assuming they are higher, rather than lower strings, so one octave should be sufficient.  Replace the 4n7 caps with 10n, and you're good to go.  If that is still not low enough for your friend's needs, then make them 15, 18, or 22n, depending on taste.

At one level, yes, basses put out a heftier signal, so the gain of the rectifier stage can afford to be more modest.  At the same time, guitar players will slam 6-string chords, and I don't see too many bassists doing that, so the 1-2 string output of a bass may well be comparable to the 6-string output of a guitar.  Again, this will depend on how much gain the bass circuitry provides.  It is entirely reasonable to replace the 2M2 resistor shown with a 1M5 and 1M resistor in series.  Use a SPST toggle to bridge/shunt one of those resistors so that you get a high/low sensitivity range function.

Finally, if one is strumming chords, then a long decay, while the filter is hanging around up in the higher parts of the spectrum, can be a pleasing thing.  For bass, it's plain wrong.  Bass filter sweeps need to get the hell out of the way fast, because you're using the filter to enhance the percussive feel of the bass.  A slow decay eradicates any hope of a percussive feel.  In which case, do NOT add the attack mod.  Leave a 47-68R resistor in place for fast attack time, and run a pot+resistor arrangement in parallel with the 22uf cap to provide adjustable decay.  A 33k resistor and 500k-1M pot, wired as variable resistance, is appropriate.

While Jack Orman adds a silicon diode between that electrolytic cap to ground and the trimpot, the Nurse Quacky does not.  If you plan on using the suggested variable decay mod, I recommend reinstating that diode, simply to isolate the decay pot from the trimpot.  Note that you can probably improve feel a bit by using a germanium diode, rather than the 1N914 shown.

[wingfijm]

Thanks for the excellent advise!

[wingfijm]

A couple questions:

1) you said: "Leave a 47-68R resistor in place for fast attack time, and run a pot+resistor arrangement in parallel with the 22uf cap to provide adjustable decay.  A 33k resistor and 500k-1M pot, wired as variable resistance, is appropriate."

Are refering to the Dr. Quack or Nurse Quacky? The Nurse is the only one with the 51 Resistor or 22uf cap. The Dr. has a 100 Resistor and a 10uf cap. Should I change those Dr. Quack values or change to the Nurse's values?

2)You said: "you can probably improve feel a bit by using a germanium diode, rather than the 1N914 shown."

Would a 1n34A, 1N100, or a 1N60 be prefereable?

[Mark Hammer]

1) you said: "Leave a 47-68R resistor in place for fast attack time, and run a pot+resistor arrangement in parallel with the 22uf cap to provide adjustable decay.  A 33k resistor and 500k-1M pot, wired as variable resistance, is appropriate."

Are refering to the Dr. Quack or Nurse Quacky? The Nurse is the only one with the 51 Resistor or 22uf cap. The Dr. has a 100 Resistor and a 10uf cap. Should I change those Dr. Quack values or change to the Nurse's values?

A fast attack will depend on the value of both the resistor and cap.  The attack time is a function of how quickly the cap charges up, which in turn is a function of how much there is to charge up (cap value) and how much current you let in to accomplish that task (resistor value).  If one uses a larger 22uf cap like in the Quacky, you need to drop the resistor value to achieve the same charge-up/attack time.  So, roughly speaking, 100R+10uf = 47R+22uf.  You're welcome to use either combo.  Note, however, that the decay time is also a function of how much stored charge there is to dump.  A larger cap value gives longer decay.  Once you have the suggested 500k pot down to zero (i.e., parallel resistance is 33k), you wouldn't notice the difference between 10uf and 22uf, but at longer decay times you will.

2)You said: "you can probably improve feel a bit by using a germanium diode, rather than the 1N914 shown."
Would a 1n34A, 1N100, or a 1N60 be prefereable?

Any of those is perfectly acceptable.  You're not using it for its sonic qualities.  If you don't have one, just go through your pile of silicon diodes and pick the one with the lowest forward voltage.

Make sense?

[wingfijm]

That makes perfect sense. Thanks a million!

[Vitrolin]

First of all sorry about diggin up this old thread.
I was thinking a bout make a envelope filter for bass, but also suiteble for guitar, and i like the idea of adjustable attack and decay, so from the rog nurse quacky, i should maintain the 51R, 1kpot and the 22uF, the 22uF should then be paralleled with a 33k+500kpot.
Now i wonder, wouldn't these two controls become interactive?
of course the two 4n7 cap should be replaced or maybe swicthable with a larger value