Re: [Harp-L] Astatic (Crystal Balls) problem

[Greg Heumann <greg@xxxxxxxxxxx>]

Richard I think you're barking up the wrong tree here. Hum happens because your amplifier is receiving some 60 Hz RF - which is present to varying degrees all over the place. The higher impedance the input to your amp, the more sensitive it is. I'm willing to bet if you remove the disconnect the mic from the cable but don't unplug the cable, it gets a little worse. If you unplug the cable it goes away - because when you do this the amp's input jack shorts the signal directly to ground. A mic's element effectively places a resistor across the input of the amp. A dynamic mic has considerably lower resistance than a crystal mic and therefore more effectively damps out the 60Hz RF, for which your cable is an antenna.

For all intent no current flows from the amp when you plug in. Mics are "generators" and create the electrical current that the amp then amplifies. As your amp warms up, its input impedance may change or it's gain may increase and it is therefore going to make the hum louder.

My guess is that the problem is your element is getting old, output has dropped, and you've been slowly raising the volume on your amp to compensate. Your cable may have deteriorated too and the shielding quality ain't what it used to be. The solution is a new cable and a stronger element. 

As for modifying the JT30 to take 1/4" - it is a real hack job to do it internally. A more common approach is to install a screw-on connector (which earlier JT30's had). I prefer a screw-on cable with that, but the Switchcraft 332A adapter will screw on and provide an external 1/4" input. I can do this mod for you if you wish.


/Greg

http://blowsmeaway.com
http://facebook.com/blowsmeawayproductions
http://bluestateband.net


> From: Richard Trafford-Owen <richardtraffordowen@xxxxxxxxx>
> Date: October 27, 2011 2:04:54 PM PDT
> To: harp-l@xxxxxxxxxx
> Subject: Re: [Harp-L] Astatic (Crystal Balls) problem
> Reply-To: Richard Trafford-Owen <richard@xxxxxxxxxxxxxxxx>
> 
> 
> Thanks Wendell. This sounds plausible because the hum reminds me of ground loop 
> problems I've had in the past.  Not sure how ground loops get prevented though, 
> since there is no independent ground connection in the cable and amp. I guess 
> I'll just try cleaning off the connection in bullet shell. Having said that, I 
> don't know why it wouldn't happen when I turn everything on. I guess even with a 
> passive mic, plugging it in starts current flowing that changes the crystal in 
> some way. 
> 
> Thanks Buck and Dave for suggestions too.
>  
> Has anyone modified an Astatic to take a 1/4" connection or to be directly 
> wired? Alternatively any customizers out there who would do it?