Miata Mailing List: December 1997, Message #223
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From: 	Chris Chu <cchu@infomatch.com>

Subject: Re: Radio mod.  for the Do-It-Yourselfers

Date: 	Tue, 2 Dec 1997 18:08:28 +0000







On Tue, 2 Dec 1997 MHathaway@IVSinc.com wrote:



> 	Howdy Gang,

> 

> 	As many are aware, there are certain modifications to the stock

> radio (older ones, anyway) which can vastly improve the headrest speaker

> >>> snip



> 	I have taken a look at mine (specifically the amp section, which

> hangs off the back end of the radio), and it appears that there are

> several capacitors which, if changed to caps of smaller size, would

> allow more bass to get to the HR speakers.  These caps are in parallel

> to the speakers, and go to ground on their other ends.  Unfortunately,

> they are not clearly marked as to capacitance, and there seem to be

> identical ones on the speaker lines going to the front speakers.  This

> puzzles me, since I would expect to find smaller ones on the front

> speaker lines, since they take greater bass levels.  I suppose that

> these caps might not be the filter caps, and that this filtering occurs

> further upstream, before the chip that is the last major component

> before the signal leaves the amp.  

> 

> 	Any thoughts?

> 

> 	Mac

> 	'90 MBRG

> 



The stock radio was one of the 1st few items removed from Shirotora.  But

electronics has been my hobby since I was 12 and I designed and built my

own power amplifier at home, so I'd like to contribute my $0.02 worth.



Usually, a power amplifier limits its bass response thru' a capacitor

connected in series with the amp output to the speaker. Judging the front

speaker output from the stock miata radio, I think the output stages are

bridged and so NO output capacitors were used in series with the speakers.

The caps you found that are connected parallel to the speakers are for

limiting high-ends. So for bridged amps, it limits low-end thru' the input

coupling caps, those that connected in series with the input signal to the

amps. These may or may not be traced as easily as the output high-limiting

caps, depending on your experience. To give you some hints, these caps are

usually of values in the range of 1uf (micro-farad) to 4.7uf. In the case

of miata stock radio's HR amp output with that much low-end suppression, I

guess the caps would be a order of magnitude smaller. 



Again, it's my $0.02 worth. I haven't opened up the miata stock radio. The

above was based on my electronics experience. Hope it helps.



--

Chris and Shirotora

'90 Crystal White

whose designed-from-scratch 30Wx2 home power amplifier was praised by its

solid and accurate bass output

http://infomatch.com/~cchu/

--