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-   -   DABAR mystery circuit (https://www.vintage-radio.net/forum/showthread.php?t=140452)

suebutcher 11th Oct 2017 1:50 pm

DABAR mystery circuit
 
3 Attachment(s)
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?

Herald1360 11th Oct 2017 5:55 pm

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.....

Herald1360 11th Oct 2017 9:15 pm

Re: DABAR mystery circuit
 
1 Attachment(s)
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).

suebutcher 11th Oct 2017 11:59 pm

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.

suebutcher 12th Oct 2017 6:28 am

Re: DABAR mystery circuit
 
1 Attachment(s)
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.)

suebutcher 12th Oct 2017 7:15 am

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!

Radio Wrangler 12th Oct 2017 7:34 am

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.

David

trsomian 12th Oct 2017 8:06 am

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

Herald1360 12th Oct 2017 12:05 pm

Re: DABAR mystery circuit
 
I'll modify the ingredients and cook it again later.....

suebutcher 12th Oct 2017 1:02 pm

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.

Herald1360 12th Oct 2017 5:32 pm

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.

suebutcher 13th Oct 2017 12:57 am

Re: DABAR mystery circuit
 
I imagine a high value of collector resistor would save on 9V batteries.

Radio Wrangler 13th Oct 2017 7:21 am

Re: DABAR mystery circuit
 
Distortion could be seen as a plus point. Characterful, even.

David

AC/HL 13th Oct 2017 12:07 pm

Re: DABAR mystery circuit
 
Preferably if first used by a "Legend" cobbling a future myth together.

ms660 13th Oct 2017 12:10 pm

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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