need to reduce audio feedback in amplifier circuit

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k2panman
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Re: need to reduce audio feedback in amplifier circuit

Post by k2panman »

thanks JR. There are some Roland VT-1 units (nearly same functions as VT-3 - older model) if the VT-3s weren't so close to delivery (2 weeks) and the VT1's weren't priced at over 2X the VT-3 price, I'd buy one. I haven't found another voice pitch shift unit - I've checked with Musician's Friend, who carries most everything in pro audio equipment, and there seems to be nothing for voice that has the same features (altered sound "wet" output only, manually adjustable pitch shift control). They tell me I can't plug a mic into the guitar pedals - I'm not sure why not - I didn't look at the specs, but is a guitar input/output that different than a microphone? The Boss PS-6 is a guitar pedal that looks like it would work.

I'll try some more mechanical changes to my microphone setup, and do some other work on the project until then. However, I do need to find a good electronics design engineer for hire. If you do know of anyone you could recommend, I'd appreciate the contact info.
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JR.
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Re: need to reduce audio feedback in amplifier circuit

Post by JR. »

http://www.galls.com/galls-streetthunde ... -megaphone
The Galls StreetThunder Megaphone features feedback-suppressing circuitry for improved sound quality.
Or just buy one of these... :lol:

JR
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k2panman
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Re: need to reduce audio feedback in amplifier circuit

Post by k2panman »

I explain my product as a megaphone as I do not want to reveal it's true function. I've disassembled a few of those to see if I could reverse engineer anything. The ones I took apart didn't have anything special electronically.

I am redesigning the microphone - have it half built. The VT3 will get here just as I'm leaving on a short trip. I'll hook it up when I get back. If that doesn't work, I may try a delay.
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JR.
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Re: need to reduce audio feedback in amplifier circuit

Post by JR. »

This one says they use "feedback suppressing circuitry" , maybe they are lying, it didn't look very expensive.

JR
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k2panman
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Re: need to reduce audio feedback in amplifier circuit

Post by k2panman »

It might be worth the $85 to see what they used. If the pitch change circuit doesn't do well, I'll check this out. The more options the better. I'm not looking for the cheapest way to accomplish feedback reduction, I want the best - that is, most stable, most reliable.
ricardo
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Re: need to reduce audio feedback in amplifier circuit

Post by ricardo »

Use a "Noise Cancelling Microphone". There are cheap electret capsules meant for mobile phone use.

These are cardioids with VERY limited LF response. They get good voice quality by relying on Proximity to boost the LF.

The BBC designed Coles 4104B for sports commentators is the arch-type and is described in a BBC Engineering Memo. It gives good quality speech when the noise level is so loud you can't hear someone next to you shouting. The BBC report describes other useful techniques too.

Always better to reduce the problem before trying to treat it. Is your speaker pointing away from the mike? Inches make a difference in a megaphone.
k2panman
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Re: need to reduce audio feedback in amplifier circuit

Post by k2panman »

Yes, my mic is pointing 180 degrees away from the speaker. The VT did not reduce the feedback at all. I'm using a dynamic microphone, and I haven't found an electret that will work in my application. I've tried quite a few, but I have some particular design constraints that limit the units that will work in my application.
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JR.
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Re: need to reduce audio feedback in amplifier circuit

Post by JR. »

It has been a while since I thought about this.

If the mic, and speaker are permanently fixed or located wrt to each other, the sound leakage path should be constant. So in principle, characterizing that path response, creating a reciprocal or polarity inverted version of that signal and subtracting it from the mic output/amp input could cancel out that feedback signal.

In reality there may be more than one path, and there may be some delay, but if the path is short enough the delay could be inconsequential.

This is surely harder than it sounds, but I would start by feeding a pre recorded signal to the speaker and then listening to what the mic picks up (no not with the same speaker).

The goal is to replicate that same transfer function electronically, but with opposite polarity so it can be subtracted.

JR
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ricardo
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Re: need to reduce audio feedback in amplifier circuit

Post by ricardo »

k2panman wrote:Yes, my mic is pointing 180 degrees away from the speaker. .. I'm using a dynamic microphone, and I haven't found an electret that will work in my application. I've tried quite a few, but I have some particular design constraints that limit the units that will work in my application.
You want something like http://www.transsound.net/electrets/tsb-68d.htm. Note the different frequency responses at 1cm and 50cm. Buy small qtys from JLI

I wouldn't use a dynamic mike for this .. unless it was designed by the BBC and made by virgins from solid Unobtainium. Cheapo dynamics have nowhere near the consistent directivity and their peaky response makes feedback worse.

I'll repeat, "inches matter in your application.
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