Re: [Harp-L] Feedback

[Greg Heumann <greg@xxxxxxxxxxx>]

Hi, Vern

I think Rick's answer is pretty accurate but I will take exception to a few items. First of all, gain is not the same as volume. You cannot change a mic's gain - you can only change its volume. Think of gain this way: Imagine you have a 400hp sports car - with a throttle pedal that only moves 1/2 inch. It is very hard to drive smoothly. Now the same car, with a throttle pedal that moves 5 inches, would be easier to control. A typical Fender circuit 3 12AX7 amp, for harp, is like that first example. The single most effective thing you can do to help with feedback is to reduce the gain by substituting some lower gain tubes into the preamp section. For the above circuit which you'll find in MANY amps, replacing V2 and V3 with 12AU7 tubes is usually the right choice. Foot to the floor, you will still have full throttle, because there is STILL enough gain to drive the power tubes to their saturation point. The original setup has WAAAY more gain than is necessary to fully drive the power tubes. That is done because guitar strings and pickups do not couple nearly as efficiently to sound waves as does a microphone. Guitar players like very gainy amps so they can get Hendrix-style feedback - but for harp it just makes the amp edgy and twitchy and hard to drive. 

So - to your first question - the gain of the mic is fixed and is a function of the design of its element. Lowering its volume doesn't change the gain (much) - it just lowers the input signal to the amp, including the mximum input the amp will see. For that reason, volume controls are indeed very handy because they allow you to kill feedback instantly if it does occur. But they don't change the fundamental sensitivity of the amp.

Phase is not so important - if you picture a sound wave from the speaker in relation to your mic, you only need to move 1/2 a wavelength to be in or out of phase with that signal. Once you have sound at multiple frequencies, you just can't win. 

As for the directional properties of mics, all bets are off when you hand hold them. Directionality of vocal mics like an SM57 or 58 is accomplished by allowing some stage sound in behind the mic's diaphragm - which does provide some phase cancellation. But this assumes the mic is in free air, mounted on a stand. Once you put your hands around the mic, you're creating a resonant chamber AND a "funnel" - sort of like a satellite dish - that totally changes the mic's directional properties. You CAN help to control feedback by leveraging this, but it has nothing to do with the directional properties designed into the mic. 

And yes "If the responses of two mics are different at different frequencies" you could correct that with an equalizer. 

But when all is said and done, the single most valuable change you can make to make an amp more harp friendly is to reduce the gain of the amp through tube substitution. 

/Greg

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> On Fri, Apr 29, 2011 at 9:28 PM, Vern <jevern@xxxxxxx> wrote:
> 
>> Every time I see a report on the feedback tendencies of a mic, I have the
>> following questions.
>> 
>> Q. Why is the gain of the mic more important than the gain of the entire
>> system including the mic and the amp?
>> 
>> It seems to me that the following things are important concerning feedback.
>> - The amount of sound (at each frequency) from the speaker that enters the
>> mic.
>> - The phase of that sound with respect to the source and the speaker.
>> 
>> Q. How can that involve the mic unless it is allowed to affect the overall
>> system gain which would affect speaker loudness?
>> 
>> Q. If the loudness at the speaker and the positions of the speaker and mic
>> don't change, how can a different mic affect feedback?
>> 
>> Q. If the responses of two mics are different at different frequencies,
>> could not this be corrected by equalizer controls?
>> 
>> Vern
>>