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Can it be done?

Mike Alf gmalf19 at mail.idt.net
Thu Jan 30 07:49:54 EST 1997

>>What I am hoping to find is a hearing aid that can boost the 2k+
>>frequencies to help me understand conversation (I don't hear the
>>sibilance). One ITE pair I tried (around $900) did help in this
>>way BUT the compression circuit took away the dynamics (change in
>>voulme) of the music. I asked the audiologist if a 'defeat' switch
>>could be incorporated so I could either use the compression to
>>protect my ears from unwanted loud sounds (squeel of bus brakes)
>>OR turn off that circuit so that the *dynamics* of the music
>>wouldn't be altered.
>There are some two-channel devices where one band is linear while
>the other is compression (i.e. Oticon Multifocus).  Can anyone
>clarify or verify this, and if so, which one is which?

That is the function of a standard TILL circuit.

>>After checking with the manufacture, the audiologist sent the
>>aids back for this operation. What came back was nothing like we'd
>>hoped for. Turning the switch off (eliminating the compression
>>circuit) dropped the volume to almost nothing. And the frequency
>>curve (the high end boost) was nothing like it had been before the
That is how an expansion circuit works.  I call it backwards agc.
Expansion is different from compression (good old agc).  In the
typical agc circuit the threshold pot sets the level at which
attenuation begins.  Defeating that pot gives maximum gain.

In an expansion circuit (typical dual channel circuits) maximum gain
is set by the gain pot (or program setting) for that channel.
Threshold is either fixed or set by another pot or setting.  Defeating
the gain pot results in minimum gain.

This is what the user in question is running into.  From what he wants
it seems to me that standard agc or adaptive compression (Telex) with
a agc defeat switch would come closer to what he is looking for.  

>>I assume my wish for a 'simple' bypass switch isn't possible
>>because all these circuits are inside a chip and you just can't
>>get into those chips to do any rewiring.

It can be done but the proper circuit must be used.

He should be able to find what he wants at the price he wants with a
little shopping but he'll need to look into a standard agc or adaptive
compression circuit.


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