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lonewolf
Diamond Member
Joined: 25 Sep 2003
Posts: 6249
Location: Anywhere, Earth
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 Posted: Saturday Mar 19, 2005 |
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Some models of the ADA MP1 preamp have a balanced input on the rear jack. Not a real useful thing on a 1987 guitar preamp. Anyway, the balanced circuit and jack is on a separate PC board from the mainboard, so I simply removed the circuit components/mainboard cable and wired the rear jack in parallel with the front jack with shielded cable, using the tip shunt switches for front panel priority and unplugged grounding.
Ada used to do this for a small fee and made documents for the wiring harness and where to solder the wires, etc. In their version, they use stereo shielded cable. One of the shielded wires is tip and the other sleeve. The shielding is wired to the mainboard remnants of the balanced circuit which consists of a 200 ohm resistor to ground (sleeve). The shielding is not terminated anywhere else and is open ended at the rear jack.
I have tried both configurations and I'm not very happy with either. Does anybody have any suggestions?
One idea I had was to use ADA's setup, but replace the 200 ohm resistor with a capacitor, like 10uF so it would act like an AC ground, but keep the DC separate.
Another would be to replace the 200 ohm resistor with a 4.7uf to ground and terminate the shielding at the rear jack with another 4.7uf to ground.
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Ron
Site Admin
Joined: 07 Dec 2002
Posts: 2031
Location: State College, PA
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| Posted: Saturday Mar 19, 2005 |
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A cap may make a difference lonewolf, but I doubt it.
Did it have the noise before the mod? What kind of noise, the usual 60Hz?
It could be a tough troubleshoot because of the preamp tube. Do you have schematics?
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Big Jim
Senior Member
Joined: 09 Dec 2002
Posts: 279
Location: Altoona, PA
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| Posted: Saturday Mar 19, 2005 |
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I suggest that you not cook the beans so long.
Sorry Jeff. It's been a long day. |
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