Killion had it all figured out in 1988

Marshall Chasin
November 6, 2012

When it comes to listening to music through hearing aids, it is not a “complex and amorphous” task.  There are well-defined requirements for a music program and there are well-defined hardware technologies that a hearing aid must have.  These have been reviewed in previous blogs and can be found throughout the literature.

The frustrating thing is that Mead Killion had it all figured out in 1988 when he came out with his K-AMP.  For those of you who remember 1988 (and I guess that if you do remember 1988, then you weren’t really there… people used to say that of the 1960s), it was a time of great change in the hearing aid industry. For the first time there were multichannel hearing aids; digitally programmable hearing aids were everywhere; the class D output stage (developed  for Knowles Electronics by Killion and his engineers that year) was replacing the old style class A amplifiers that distorted with high level inputs; and the K-AMP was introduced.  (The ER-15 Musicians Earplug was also introduced in 1988, but that’s a different blog.)    I guess that one could say that 1988 was Mead Killion’s Annus Mirabilis.

I still have musician clients who were initially fitted with the K-AMP and are madly trying to find a new hearing aid that sounds half as good as their old one.  I did a home visit on an elderly jazz musician last week and as soon as I gave him back his old repaired K-AMP hearing aid, his shoulders dropped from being in a tense position to a relaxed one; he heaved a sigh of relief and a smile crossed his face.

I should point out that for me to get his old K-AMP repaired, I needed to bug a friend of mine who works for a hearing aid manufacturer to look through old dusty boxes of IC chips and other long-retired hearing aid components.  He found one K-AMP circuit that he was able to use to get this man’s hearing aid working again.

So what did Mead Killion know in 1988 that we seem to have forgotten?  Well, the one big issue is that the K-AMP was analog.  Before we roll our eyes, it’s not necessarily that the hearing aid was analog- we can make many of the modern day hearing aids sound just like the olden days if we desire- but analog hearing aids do not have an analog-to-digital (A/D) converter .  It is the A/D converter that is the weak point in modern digital hearing aids.

And, oh yes, there are a couple of other things that Killion did in 1988- the K-AMP was single channel (which treated the lower frequency fundamental energy in substantially the same way as the higher frequency harmonic energy); the compression was WDRC, but had a very mild compression ratio (typically 1.5:1 to  2:1), and the output for loud sounds was uncompressed (linear), which Bill Johnson (a well respected engineer in our field) once said was the true genius of the K-AMP design.  Most importantly, it had a “front end” that didn’t distort with the higher levels of music.  In fact, the K-AMP could handle inputs of 115 dB SPL, which was (and still is) the maximum level of hearing aid microphones.  You could stick your head into a piano and pound the keys and perceive no distortion.

In contrast, modern digital hearing aids have had to play “catch up to 1988.”  Technological “tricks” are sometimes used that fool the microphone into being less sensitive for music such that what is applied to the A/D converter is within its operating range.  These include actual microphone attenuators, microphone frequency response alterations, analog compressors prior to the A/D converter (with a digital re-establishment of the input after the A/D converter), and even dynamic range shifters that takes a 16 bit, 96 dB dynamic range and shifts it up to 15 dB SPL to 111 dB SPL – a range that is more optimal for music.

I recall having a hearing aid specially built by a manufacturer that had two processing paths- one was their proprietary digital route and, with a flip of a button, the music route went through a K-AMP IC.  Both routes used the same microphone and same receiver.  It was larger than the client wanted it to be, but the sound quality of the “music route” was outstanding.

If anyone has a secret stash of K-AMP ICs in their back room, please let me know.  I am at a point in my career that I will buy them and have a local manufacturer “custom” make a Killion-designed hearing aid for my musicians. Of course, in the United States, there is one manufacturer that still markets the K-AMP (General Hearing, but in Canada, Europe, and the rest of the world, we need to settle for second best.

If I sound frustrated, I am, and so are my musician clients. There are some really neat innovations for musicians out there (and many of my musician clients are “almost” successfully wearing them) but sometimes a solution needs to be “low tech” even if the market place frowns on this.

  1. Then lets build dual chip hearing aids for all those people who are deprived of the joy of music with their current hearing aids!!

    1. A dual route hearing aid would be a wonderful solution- the K-AMP route along side a manufacturer’s proprietary digital route that can be selected for speech or music. We do however need a supply to K-AMP ICs though.

  2. Marshall you definitely hit the nail on the head. I have yet to obtain a K-Amp hearing aid for my own ears to hear, but anyone who has heard it, sings its praises (sometimes literally).

    So what can be said about the digital hearing aids from Bernafon, and even Unitron that people whisper your name when they talk about the modifications that were made to the aids for improved musicality? Can any manufacturer now using multi core processors build a hearing aid that will be as good at music processing or not?

    1. It is true that I consult with a number of manufacturers- they never pay me though! But to be fair I don’t ask for anything. I just want as many options out there for the hard of hearing consumer who likes (and needs) to listen to music. The Bernafon innovation involves moving the maximum input level for a hearing aid from 96 dB SPL and shifts it up 15 dB to 111 dB SPL- a range that is better for music. Bernafon calls this their Live Music Plus option and it works really quite nicely. The Unitron innovation involves using a hearing aid microphone that is less sensitive to the intense lower frequency components of music so that these don’t overdrive the analog-to-digital converter as easily with music. The lower frequency elements of music still enter through the non-occluded hearing aid (unamplified).

  3. Yes,indeed, sound quality matters. A lot. And the K-amp sounds terrific.

    And since my hearing aid does not provide adequate sound quality for music (either recorded or live), I simply take my hearing aid out and use high quality Etymotic earphones and an iphone rig outfitted with an external cardioid microphone and a hearing app (I use SoundAmpR and FiRe2).

  4. According to Patty Niquette, Etymotic is using a slightly modified K-AMP circuit in their active hearing protection devices: This makes sense, as I designed active earplugs for two normal hearing people with severe hyperacusis using the K-AMP and a smaller Knowles Class D receiver than called for (I forget the part number) with 1kΩ & 2200Ω Knowles dampers.

    Feast your eyes on these pictures:

  5. GN Resound is now touting higher input level limits in their new Verso hearing aids. Marshall, do you know which method they are using to achieve this, and have you heard if it is successful from any musician patients? The more options we have, the better!

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