amplifier for "BIG EAR" listening device

[MrDEB]

Have a groupe of cub scouts that I want to build a "BIG EAR" to demonstrate what a parabolic dish is and does as well as have fun.
Have a Dishnetwork dish w/ LNA but can the LNA be used for audio?
PLAN = have an amp (thinking a cheap LM386 perhaps in a push pull setup for increased amplification) and a mic placed at the focal point (where LNA is located.
Using headphones and/or speaker, we can hear a conservation several hundred feet away.
any suggestions or ideas?

 

[BrownOut]

I think the LNA is a RF amp and cannot be used for audio. Remove it and replace it with your home brew amp. You might have to adjust the arm so that the mic is in the focal point, or make the placement of the amp adujstable, so that you can fine tune the final placement. Or you might try to make the amp so that it fits in the LNA enclosure, to make mounting a snap.

 

[Nigel Goodwin]

You need a preamp before the LM386, use a low noise opamp with adjustable gain.

 

[OldTechie]

The satellite dish LNA is for RF, and can't be used for audio. I suggest that you look into the audio amp at BIG EAR AMP KIT AA-1D-MPJA, Inc.. It's specifically made for the purpose you're after. There's a project for an audio parabolic dish at **broken link removed**. Just remove the microphone from the amplifier board and put it into the dish pickup as shown.
Another web site with some good info is at Do-it-Yourself Parabolic Microphone | eHow.com. Probably the most difficult part of the project will be finding a suitable parabolic dish. The more it conforms to a true parabola, the better it will perform. And, of course, the larger, the better.

Good luck with the project - sounds like fun.
Cheers
Dave M

 

[DerStrom8]

Here's a circuit I built a while ago that blew me away (almost literally) with how sensitive it was:



I apologize for the poor picture quality.
I also found that if you add a 100µF capacitor between the positive and negative rails, it increases the sensitivity greatly. To give you an idea of how powerful it is, when I tested it, I could easily hear a very faint whisper from 50 feet away!
Hope this helps!
Der Strom

 

[chemelec]

Check out this article on my website:

**broken link removed**

 

[4pyros]

Yes a sat dish is no good. It has the wrong shape.

 

[JimB]

Yes a sat dish is no good. It has the wrong shape.

???
Would you care to explain that rather wide sweeping statement?

JimB

 

[BrownOut]

There's nothing wrong with your dish. You need a parabolic shape, and that's the shape of the dish.

 

[DerStrom8]

Yes a sat dish is no good. It has the wrong shape.

The satellite dish is shaped so that it can focus the tv signals to the receiver. It should work just fine to focus sound waves to a mic, instead.
Der Strom

 

[chemelec]

A Satellite Dish works OK for Audio.
It is just a Bit more Difficult to AIM IT!
Its focus has a offset of about 30 degrees.

 

[DerStrom8]

I've heard of some people using Chinese cookware (often shaped like a parabola) as a reflector. I, for example, have a lid to a wok that has the perfect shape. Putting a mic in the center, the lid would reflect the sound waves to the mic and greatly improve the performance. Just another idea, and it's a lot easier to AIM IT!
Der Strom

 

[4pyros]

The only problum is the SAT dish is not that deep, you would be better off with something deeper. A full half round is the best. Andy

 

[BrownOut]

The only problum is the SAT dish is not that deep, you would be better off with something deeper. A full half round is the best. Andy

I disagree. I think the dish is better, but you may have some expertise that I don't.

 

[DerStrom8]

I disagree. I think the dish is better, but you may have some expertise that I don't.

I think it really depends on how directional the OP wants it to be. A half round "bowl" would pick up sounds from a more specific spot, whereas a dish would pick up sounds from a wider area. In this case, I think Andy is right, because in order to hear a conversation from 100 feet away, you wouldn't want to pick up lots of different sounds. They would likely drown out the conversation. I would personally go with a bowl shape because it would pick up less ambient noise and capture more of the conversation.
Der Strom

 

[BrownOut]

The great thing about the dish is that it only focuses sounds that inpinge parallel to the axis of the dish, and not those that are off-axis. I really doubt a bowl would attenuate enough ambient noise to make much of a difference, unless if there is a very unusual amount of it. Every listening device I've ever seen was a dish, and not a bowl. Even those used at sporting events, with cheering crowds, are dishes.

 

[chemelec]

A Half Round Dish would be good, ONLY IF it Were a Real Parabolic Shape.
Just being a Bowl shape, Doesn't mean it directs much sound to the mic.

A true parabolic is Quite Useful and will direct all sound to the mic.
And just put a Cup around your mic, to help prevent pickup from the sides.

I have a lot of experience with this and made Many of these devices.

 

[MrDEB]

a bowl is NOT a parabolic shape

There is a definitive math formula for the parabolic curve as used in a sat dish.
I really dout the quality or even validality of using a common bowl.
At summer camp I had my scouts build hot dog cookers with an aiming device. I would like to see a common household bowl cook a hot dog using the sun's energy.
Maybe a sat dish would be hard to aim but thats an easy fix.
I looked at the links posted and the amp with a bandpass filter looks ominious as well as out dated. A preamp using descrete componets?? Wonder how old that schematic is?
Plan is to use a simple LM386 or maybe a 7w chip amp I got from electronic goldmine
(G14042
and add a preamp using an op-amp.
One of the links suggested using a speaker in place of a mic. need to test this out but should work but better??
As for the bandpass filter, not really sure I need but?
going to simulate using TINA as soon as I get MY computer rebuilt motherboard went south)
Hopefully demonstrate to a bunch of young boys (cubscouts) what a parabolic dish / curve is as well as build some hot dog cookers.

 

[chemelec]

There is a definitive math formula for the parabolic curve as used in a sat dish.
I really dout the quality or even validality of using a common bowl.
At summer camp I had my scouts build hot dog cookers with an aiming device. I would like to see a common household bowl cook a hot dog using the sun's energy.
Maybe a sat dish would be hard to aim but thats an easy fix.
I looked at the links posted and the amp with a bandpass filter looks ominious as well as out dated. A preamp using descrete componets?? Wonder how old that schematic is?
Plan is to use a simple LM386 or maybe a 7w chip amp I got from electronic goldmine
(G14042
and add a preamp using an op-amp.
One of the links suggested using a speaker in place of a mic. need to test this out but should work but better??
As for the bandpass filter, not really sure I need but?
going to simulate using TINA as soon as I get MY computer rebuilt motherboard went south)
Hopefully demonstrate to a bunch of young boys (cubscouts) what a parabolic dish / curve is as well as build some hot dog cookers.

Yes that preamp is an Older Design, But it works Very Good.
Older YES, But what is wrong with transistors?
A LM386 is Good for a power amp.

Why would you want 7 Watts of power to HEADPHONE?

And What is wrong with Transistors?
Its a Good LOW Noise Preamp.

Bandpass Filters Help to Tune to Specific Sounds "LIke BIRDS", and helping to Eliminate Unwanted Sounds.

So it really depends on What your wanting to hear.

 

[4pyros]

Ok I could not find anything to backup my claims as to a SAT dish not being that good for a parabolic microphone. It should be fine for expariments. MY Bad Andy