Hearing Aid Transducer Question

Hi guys, I follow a crystal radio forum and thread about hearing aid transducers came up. One fellow found a transducer with higher impedance and is looking at making crystal radio earphones with them.

Yes, there are matching transformers involved.

But I'm lost on some specs.

Here's the datasheet,

formatting link

It says 0.33ma RMS AC and 0.47ma DC.

I don't know why a hearing aid amp would put DC on the transducer.

It reads to me like the hearing aid companies don't use a capacitor on the output of their amps. I assume that is to simple, but what other explanation is there for having DC on the output of your amp?

Also why on a battery operated device with a tiny battery would you waste 0.47ma for 15hrs a day? (DC current)

What is the advantage of eliminating an output capacitor. (other than saving six cents on a $1,000 item)

Can I assume the transducer diaphragm is centered to make equal maximum excursions in both directions? And, that any DC bias will lower the maximum excursion in one direction? Thus lowering the maximum level the transducer can be driven before distortion? Here's a bulletin about testing.

formatting link

What do you think? Thanks, Mikek

--
This email is free from viruses and malware because avast! Antivirus protection is active. 
http://www.avast.com
Reply to
amdx
Loading thread data ...

To save a few pennies, but mostly to reduce component count and save space inside the hearing aid. Most schematics show DC on the EP:

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
 Click to see the full signature
Reply to
Jeff Liebermann

I agree that the only reason for the DC is to simplify the drive circuit but this is a very old fashioned approach.

I don't think that there is any need for the DC for the BK series parts to work - Knowles are just telling you the maximum it will stand. (If you put to much DC current through the armature will lock up on one pole or the other..)

Michael Kellett

Reply to
MK

"amdx"

** FFS read your own links.

The transducers shown use DC bias and modulate it with AC signals.

The *peak value* of 0.33mA rms IS 0.47 mA !!!

So that is the level where the bias runs out.

.... Phil

Reply to
Phil Allison

Yeah, sorta. Todays hearing aids use BlueGoof instead of wires and DSP filters. BlueGoof is also used to connect to phones and computers, usually from a central pendant hung around the neck. The high level of sophistication and price in hearing aids is astounding. Yet, despite all these advances, the end result is driving some kind of audio transducer in order to produce the sound in the ear (unless it's an implant). Whether the hearing aid is driven by a simple analog audio amp or a complexicated DSP based system, the output will still come out of an audio transducer, which requires a drive amplifier, and which needs to deal with the DC "problem". It may be old fashioned, but the "in the ear" audio transducer is hear to stay.

That's true. There's probably a tiny coil inside with about 6K ohms of resistance. I have no idea what gauge wire is used, but I'm sure it's very fine. Therefore, the max current rating. Discharging the coupling cap through the earphone might also be a rather bad idea, which might be another reason for its unpopularity.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
 Click to see the full signature
Reply to
Jeff Liebermann

When Phil says "the bias runs out", what does that mean? Bias of what? Is he talking about a prebias on the diaphragm. Does the diaphragm need the DC bias to be centered?

OR, does 0.47ma dc provide maximum excursion of the diaphragm? In both directions?

Have at me Phil, but when you're done can you provide the info I'm seeking. Mikek

Mikek

--
This email is free from viruses and malware because avast! Antivirus protection is active. 
http://www.avast.com
Reply to
amdx

Hmm I know nothing, but I'd read that as

0.33mA (rms) to do the AC impedeance test (6k at 1kHz, 2.3k at 500Hz) and 0.47mA to measure the DC resistance. (895 ohm)

But if you care ask Knowles. George H.

Reply to
George Herold

** The bias current in the drive transistor, your second link has all the details.

Your question is really about why do it that way - ie direct coupled, class A.

To which the short answer is that it's the most efficient method, given that the component count has to be very low to fit in the available space.

The transducer is not described, so it may or may nor need DC bias to work.

.... Phil

Reply to
Phil Allison

After much google searching for this mysterious bluegoof (wire?) that must me very miniaturized, I figured out... ahh well you know what I figured out.

With all the headphones worn now, the stigma of wearing a set of earphones connected to a box is gone. Just put a Sony sticker on it, people will think you aren't listening or just antisocial, which is better than not hearing what they say. Mikek

Reply to
amdx

"MK"

** The ear transducer has a resistance of 895ohms and an impedance of 6000 ohms at 1kHz ( see page 2 of the data sheet):

formatting link

So is basically an inductor of about 0.9H at the frequencies of interest.

Using it as the collector load of a single transistor makes a class A output stage, with the drive voltage able to swing above the DC (ie 1.5V) rail - plus you get current drive which helps the EQ the transducer.

I guess that with a suitable mic transducer, a very simple, one or two transistor hearing aid is possible.

.... Phil

Reply to
Phil Allison

As in the Emperor's New Clothes fable, BlueGoof(tm) virtual wires and all things RF are invisible to the unworthy. If you fail to see the connections, don't worry. It takes a few months of blown deadlines, overwork, and sleep deprivation before the missing wires manifest themselves as hallucinations. The only problem is that you'll then be seeing wires everywhere.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
 Click to see the full signature
Reply to
Jeff Liebermann

I sent an email to Knowles asking several questions and got back a minimum amount, but useful information.

"Mike: The DC centers the reed as you guessed. If you do not have DC, better to choose a zero bias for max output capability.

BK will require a seal to the ear canal ? otherwise the bass with not be there." Mikek

--
This email is free from viruses and malware because avast! Antivirus protection is active. 
http://www.avast.com
Reply to
amdx

This may be of interest, part of a write-up on low voltage/low current designs I did for ICE more than 40 years ago...

On the S.E.D/Schematics page of my website...

HearingAidSlidingClassA.pdf

IIRC I did that chip design for Beltone. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
 Click to see the full signature
Reply to
Jim Thompson

Nice writeup Jim, I followed it though to the end, but I didn't see a link for samples of the IC on the last page. Thanks, Mikek :-)

--
This email is free from viruses and malware because avast! Antivirus protection is active. 
http://www.avast.com
Reply to
amdx

I'm sure it isn't made anymore... it was bipolar, thus a current hog by today's standard's... I change out my 312A's (zinc air batteries) every Saturday morning, though they will often go 9 days before giving me a two-hour warning beep before the aids shut down. And these dudes talk to each other (and the Rx adjustment computer) via Bluetooth ;-) ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
 Click to see the full signature
Reply to
Jim Thompson

I have read the thread though I post up here. A couple of thoughts on the m atter.

I am sure that even though the thing runs on one IC, it is not just a simpl e audio amplifier. It has EQ in it. Limited, but there are not going to be many caps and coils in there, nor is there room for all that many component s. I do not know for sure, but there might even be an I2C interface because they custom set the EQ for different wearers.

Taking the DC off the transducer would require basically twice as many comp onents in the output circuit, which is bad. It would also require quite a b it of bias, because the last thing a hard of hearing person needs is crosso ver distortion. And of course the coupling cap.

The coupling cap could be eliminated by using a bridge amp, which brings in more components, then there's two of them to bias. This thing is not runni ng off a car battery.

So if you figuire this all out, I suspect that the removal of DC from the t ransducer simply doesn't do enough in any respect to be worth it. Just use a single ended class A output and be done with it. What's more, the DC bias can be varied with output level easily enough which would be alot cheaper way to get to a decent efficiency.

It has also been stated in audiophile circles that even order distortion is alot more tolerable then odd order, and this fits well.

It simply wasn't worth it in my opinion, and I would bet, theirs. For good reason.

In a magnetic dynamic transducer, the magnetic field is never absolutely co nstant. This means that when the DC lowers due to lower output level requir ements, some automatic compression could be built in - quite advertently. T o avoid it would also mean more cost.

Even though ever mA/Hr is of the essence, the gain is better than the loss.

I believe that is why they did it that way.

Reply to
jurb6006

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.