[Harp-L] Re: Feedback

[Greg Heumann <greg@xxxxxxxxxxx>]

Pierre wrote:

> This subject keeps comming up, not surprising since setting many amps 
> at more than 3 generates feedback. The problem with this is that the 
> amp may not overdrive below 5 or 6.
>
> So why do miced amps tend to feedback in the first place?
>
> Is it because a mic's output simply has higher voltage levels than a 
> guitar (perhaps due to loudness of the harmonica). If so why not just 
> use volume control on the mic to bring it back to the expected range 
> of a guitar and then jack up the volume and/or drive on the amp?

Actually the reason we get feedback is that our amps are set up for 
guitars, not microphones. Feedback happens when the input device begins 
to resonate with the sound coming from the amp's speaker. A microphone 
is simply much more sensitive to moving air (a.k.a. "sound waves") than 
a guitar string. Amps for guitars are set up with much more gain. The 
solution for harp players is to reduce the gain. 3 12AX7's is a recipe 
for disaster. A 12AX7 has "100" gain. But the 12AY, 12AT and 12AU have 
less, from 70 all the way down to 20. I used to have a Fender Blues Jr. 
It came with 3 12AX7's. I changed to AU, AT, AX - and it went from 
feedback city (volume control at 2) to very tolerant (volume control at 
7-8).

I make microphone volume controls. (See 
http://harmonicamasterclass.com/n-line_volume_control.htm) Regardless 
of the value of the pot you use, a volume control does place some load 
on the mic, and can change its tone. Although this isn't necessarily a 
bad thing,   it is not a substitute for having your amp set up right. 
Some of the distortion we look for in a "Chicago Sound" comes from the 
mic itself, and the mic won't distort the same way when the load it 
"sees" changes considerably. I like to set my rig up so that at full 
volume on the control (where the resistance it puts in the signal path 
is 0 ohms), I'm at the hairy edge of feedback, but can still control it 
by how I hold the mic and where I am on stage. Then I only need to turn 
it down a little for volume management, or all the way when I'm not 
playing.  I wrote a FAQ about this with more information - you can find 
it here:  http://www.heumann.com/volumecontrol/faq.html

/GH

>