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Old 11th Oct 2017, 12:50 pm   #1
suebutcher
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Default DABAR mystery circuit

I found this in my junk box. I think the person who gave it to me said it was part of a wah-wah pedal. Can someone confirm this?
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Old 11th Oct 2017, 4:55 pm   #2
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Default Re: DABAR mystery circuit

Varying the resistance between the two points O------?------O will do something odd to the frequency response of the circuit so Wah-Wah might well be the effect. Time to SPICE things up, methinks.....
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Old 11th Oct 2017, 8:15 pm   #3
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Default Re: DABAR mystery circuit

OK, with 0-50-100K between the points we get the results attached.

I guess it would give a sort of wah-wah sound if waggled.

FWIW the emitter resistor had to be set at max to get the operating point even vaguely usable (about 1.5V on the collector).
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Old 11th Oct 2017, 10:59 pm   #4
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Default Re: DABAR mystery circuit

Thanks. Perhaps it had a log or antilog pot attached to the pedal, because the middle curve's not much different from the top curve.
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Old 12th Oct 2017, 5:28 am   #5
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Default Re: DABAR mystery circuit

I've found a very similar one transistor wah-wah circuit online in which the equivalents of the two 3300pF caps are connected, with the connection going via the pedal pot to earth. This creates something like a bridge circuit. It could be that the DABAR is meant to be connected this way. (My apologies if I've trodden on someone's copyright here, but circuits are really difficult to explain in words.)
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Old 12th Oct 2017, 6:15 am   #6
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Default Re: DABAR mystery circuit

I've re-checked the DABAR by holding it to the light. Yep, the two 3300pF have a common terminal, and the missing component, undoubtedly the pot, goes between this terminal and the earth. Sorry!
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Old 12th Oct 2017, 6:34 am   #7
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Default Re: DABAR mystery circuit

It should act as a filter giving a peak whose frequency can be varied with a foot pedal on the resistor in the T network. The resistor in the emitter controls the height and sharpness of the peak by setting the transistor's gain.

A bridge-T network acts as a notch filter. this bridge-T is in a feedback circuit around a transistor amplifier. Outside of the notch, feedback reduces the amplifier gain. inside the notch the gain is not reduced. so an active circuit transforms a bandstop function into a bandpass.

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Old 12th Oct 2017, 7:06 am   #8
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Default Re: DABAR mystery circuit

Are you sure the collector load resistor is 56K, could it be 5K6, because 56K seems very big for a collector load on a 9V circuit. 5K6 would also mean the driving impedance to that feedback circuit would be smaller than the input impedance of it.

I have certainly seen resistors where the colour bands are ambiguous as to whether they are red or orange, and had to take a meter to them to check
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Old 12th Oct 2017, 11:05 am   #9
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Default Re: DABAR mystery circuit

I'll modify the ingredients and cook it again later.....
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Old 12th Oct 2017, 12:02 pm   #10
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Default Re: DABAR mystery circuit

The collector resistor is 56k according to my meter. Perhaps it's an assembly mistake. And the output buffer resistor is 10k, I forgot to mark it.
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Old 12th Oct 2017, 4:32 pm   #11
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Default Re: DABAR mystery circuit

Well the circuit does work with 56K, just not much headroom on the collector swing.


I suspect a few of these types of devices were probably just cooked up rather than designed and if they worked, they went into production.
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Old 12th Oct 2017, 11:57 pm   #12
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Default Re: DABAR mystery circuit

I imagine a high value of collector resistor would save on 9V batteries.
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Old 13th Oct 2017, 6:21 am   #13
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Default Re: DABAR mystery circuit

Distortion could be seen as a plus point. Characterful, even.

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Old 13th Oct 2017, 11:07 am   #14
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Default Re: DABAR mystery circuit

Preferably if first used by a "Legend" cobbling a future myth together.
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Old 13th Oct 2017, 11:10 am   #15
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Default Re: DABAR mystery circuit

It's no mystery it's a Mess of The Dabarvilles.... I'll get m'coat.

Lawrence.

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