[joefly]

HI guys, hope you can help someone with very basic understanding of circuits. My goal is to make a portable hearing aid for my elderly father.

I found this DYI Kit which is very basic but works well. Goal is to take mic input for line-in output made for the stereos. But, the output is too low for direct headphone.

So my question is if there is a simple (maybe naive) way to increase the volume output so that I can used headphones on it.

is there a way to simple change the resistors or caps to increase the output to the mic linein?

I seen other designs but this is the simplest one which works well for my purposes, only if I can increase the output?

Thanks to the group for your help.

 

[amb]

Try increasing the value of the "470K" resistor.

 

[Fred_fred2004]

I've been down the same path with my father's hearing, eventually I gave up on these type of circuits and settled on a simple amp circuit using a TDA2822 which I found here http://www.crukx.com/education/bac/e...ring%20Aid.pdf

its very simple to make

cheers
FRED

 

[joefly]

hi Amb, thanks, Any idea approximately what value I should be trying. If I use a higher value, would that not decrease the gain?

HI Fred, Thanks, Sounds like you have certainly been done this path as I have searched and tried various kits. I will certainly try yours as it seems simple enough. For the sake of sharing, I have also tried this Velleman Inc. which also works great, but I thought the above was simpler and smaller for portability. Also trying to learn in the process why the vellema can not be simplified? For others the link I provide works and also easy. There is a link within the page for the diagram.

 

[joefly]

*

 

[amb]

That resistor provides negative feedback, so increasing the resistance decreases the amount of feedback and increases the gain. As for how much more resistance, I can't tell you because I don't know how much gain you need. Experiment by going up in increments, such as 680K, 820K, 1M...

 

[joefly]

thanks amb for taking the time to explain the value change, unfortunately, I tried various values upto 2M and no change, the output did not change or at least not noticiable on the headphone.

Any other suggestions? Thanks

 

[nikongod]

How are you driving the headphones? That circuit looks like it is designed to drive a high impedance load, so at the minimum you need a buffer after it. As it sits anything but VERY high impedance headphones (which it probably dosnt have enough voltage swing to drive loud anyways) will load the output terribly which has the effect of reducing gain by a LOT. The caps are also tooooo small (4.7mf=4.7uf) for any reasonable headphone which will cause bass roll-off. Dunno if that matters in this app, but maybe someone else will find it useful someday.

If you dont have an output buffer yet, try that. It should be very easy to put a volume control between that circuit and the buffer.

If you used an op-amp in unity gain mode as the buffer just increase the gain around the opamp.

added:
If you have not yet, you should think about stopping acoustic feedback loops (where the microphone "hears" the speakers and causes the squeal... you know what I mean) and perhaps maximum level protection.

 

[diditmyself]

Are you connecting the phones to this little amp? In that case I don't think it's low gain that's your problem. What headphones do you use? What's the impedance? To drive headphones directly there has to be more parts, a buffer stage. Another problem is the low voltage supply.

I think it would be easier for you to build an opamp based amp. You don't need more parts and it wont be more expensive.

 

[Fred_fred2004]

The resistor you are changing is used to bias the transistor into its operating range and will have very little effect on its gain, transistors are an analogue device and don't use negative feedback, have a look at this its quite a good explanation
The basic transistor amplifier
As others have said you need an ouput stage to provide the grunt you need

 

[amb]

The circuit that Fred+fred2004 linked to is similar to this one, with one key difference -- the resistor in question in this circuit is connected to the output whereas the one shown in the link is connected to the positive rail.

The resistor serves to bias the transistor in both circuits, but in this one also provides negative feedback.

However, the actual gain of both circuits depend greatly on the load as seen at the collector of the transistor. The 1K resistor in this circuit is effectively in parallel with the load as far as AC is concerned. Depending on the load you're trying to drive, it might dominate over the 1K resistor (i.e., if you're trying to drive 32 ohm headphones). In such an event the only solution is to buffer the output with an additional emitter follower stage, or to use an opamp-based design. However, if the load is high-Z, you may get an increase in gain by increasing the value of the 1K resistor.

 

[joefly]

hi Guys, Thanks for are your help and apologize for the lack of response. But I realized that your level of expertise is beyond me as the questions and terminology being thrown around was hard to keep up. .. so I quietly went into my cave and did my own hacking/learning. I am sharing what I came up with if anyone is interested to build a "good" super wireless hearing aid..

I used the original post for the preamp for the mic input. Then I bought a cheapo $10 USB powered mini speaker for portable MP3 players (which probably out of naiveness worked) I disconnected the speaker and replaced it with a audio out socket. Now I have a working Mic input and a regular working Line-in output. I then connect this out to a wireless headset transmitter http://www.e-sense.com.tw/big/produc...ail.asp?id=541 (sorry, I am in Taiwan).

I stripped the packaging on the bought items and then connected their boards one after the other: pre-amp, amp, wireless headset. And stuffed it all into a package about the size of a cigarette box without batteries. Nice thing is that all this works on the same power source of 4.5v. So now my father has a portable wireless headset.

The advantage is numerous 1) since the wireless headsets are cheap about $30US, i bought multiple of them spread at different tvs, so he now just walks around with one headset and he can listens to what he likes by choosing which wireless base to turn on. If he wants to hear a dinner conversation, he can now just turn on the portable mic.

2) Since the mic is wireless from the headset, the mic can now be placed anywhere, in a noisy restaurant, he can place the mic next to the person he is speaking to, or middle of the table to hear all, or in his pocket if he is walking around.

3) the mic socket accept most cheap mics found int he electronic stores, so he can chose how he likes his mic, clipped on, long wire, short etc.

4) It is more powerful and cheaper than the fit in the ear hearing aids...he used to break or lose them at about $1000 per pop, and also complained about the lack of power or power sound.

5) He moved to the wired ear piece. But also complained about the wires tangling up everywhere.

This project probably cost me less than $50 in parts but I think I spent about $200 due to experimentations and burnt circuits.

I am going to tinker with this to reduce packing size and also looking to build my own amp (can not fine a simple one that can work off the same power source)

This is not pretty or elegant.. But I hope this atleast is an idea for DIYers out there.

 

[joefly]

Quote:

Originally Posted by Fred_fred2004 /img/forum/go_quote.gif I've been down the same path with my father's hearing, eventually I gave up on these type of circuits and settled on a simple amp circuit using a TDA2822 which I found here http://www.crukx.com/education/bac/e...ring%20Aid.pdf its very simple to make cheers FRED

Fred_fred, I want to thank you for your suggestion. I was not able to follow your advice as I could not find the component chip locally. So I did not try it at first and did many other experiments. I was finally able to find the required parts for your suggested board, it turned out to be a good hearing aid and worked better than anything I have done, simple and good result. thanks for your advice, nothing loss as I learned a lot along the way.

 

[Fred_fred2004]

I'm glad it worked out, I ended up getting the chip from ebay it was the path of least resistance

cheers
FRED