Re: tuner capacitor question

In article <ifCTf.4051$J43.2587@edtnps90>, "Dave" <dspear99ca@xxxxxxxxxxxxxxxx> wrote:
In a previous post ("tuner repair problems") I outlined some steps that I
had taken to repair a Sansui T-80 AM/FM tuner. In particular I replaced all
of the electrolytic capacitors.

This seems to have fixed the unwanted behaviors. However, quite by accident
(rec'd the wrong part from DigiKey) I replaced the series output cap with a
much smaller value cap. The cap in question was spec'ed at 0.15uF. There
was actually a 0.22uF in there when I started, and the cap I inadvertently
put in was 0.0015uF. It sounded bad. Tinny. Not much volume. I
understand that this cap in conjunction with my pre-amp's input impedence
makes up a high-pass filter. Frequencies below 3.5KHz would have been
attenuated with this tiny cap, hence my tinny sound.

Well, I put the 0.22uF caps back in while I waited for the correct 0.15uF
caps. The sound improved dramatically. I measured the input imepdence of
my amp at 30.2K. As an aside, I also calculated it from the schematic and
it should have been about 44K. Anyways, using the measured impedance, I
should be using a 0.26uF filter cap if I want to roll off at 20Hz. Hmm,
wonder what would happen if I stuck in a 1uF cap (the next higher-size cap I
happened to have in my inventory). Wow. What a difference! The bass is
tight and defined INDEPENDENT of volume. Not only do I not need to use the
loudness button on my preamp at low volume, I can't use it without it
generating more bass than I want! This has made such a huge difference to
the sound of this tuner that whereas before I was thinking of replacing it,
now I definitely will keep it.

Is there any downside (potential harm) to what I've done? I have a subsonic
filter (15Hz) on my amp which I have switched on to avoid any subsonic
damage to my speakers (shouldn't matter if sound below human hearing is
filtered out, right?). The 0.15uF cap in the design spec would start my
bass roll-off at 35Hz. Wouldn't it make sense to size the filter cap in
each component to jive with the pre-amp's impedence? You'd almost think
manufacturers would put in some jumpers and a few ten-cent caps so you could
adjust it to suit.

It might seem a bit strange to be working on an inexpensive tuner to
such a degree. Most mods to tuners involve getting more sensitivity
and more selectivity. Perhaps there are things to be learned.
It seemed to me a long time ago, that FM had rolloffs, or
perhaps it was TV sound. 50 Hz rings a bell. Today, there
is plenty or too much bass. Many brodcasters are using 3 band compression,
and many songs have way too much low bass compared to listening
to it direct from disk.

greg
.

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