To: Michael J. Mahon
Michael J. Mahon wrote:
>
> The original Mockingboard was also pretty noisy, so don't think that
> will cure the problem!
>
> It's quite possible that your card has always had a fair amount of
> background digital noise, and that the 386's are just amplifying it.
> (Did you have the treble control turned down on your powered speakers?)
>
> To get reasonably quiet audio out of an Apple II will require good
> filtering on all supplies--including the +5v which powers the source
> of the audio signal.
>
> Extra bypassing may help, but the ultimate solution is to double
> regulate down from the +12v to get quiet +5v for the 8910's.
>
> You may also want to increase the value of the capacitors shunting
> the input to the 386's. That will roll off the high frequencies,
> attenuating the most annoying noise and making the sound richer
> and less shrill (a big issue with squarewave sound generation).
>
>> Thanks for the help.
>
> You're welcome--I hope you get it working to your satisfaction!
>
OK, so I bit the bullet today and un-did all of the amplification
circuit and put things back to how they were before (as described on my
web site).
I found the noise from the LM386's unacceptable. I tested the LM386's in
isolation (with no input signal) with only power and ground wired up as
per the Mockingboard V1 (i.e. with the 12V line attached to a 220uF
filter capacitor) and the noise was the same as noted previously.
I notice that I have LM386N-1 chips as opposed to the Mockingboard V1's
LM386N-3 chips, but I don't think this would make any difference from
reading the specs (slightly different voltage tolerance and output
IIRC). So I have no idea how they've got the Mockingboard V1 to not
produce the noise I was getting. The only thing I can think of is that I
noticed that the power and ground rails on the Mockingboard V1 are
thicker. Could using thicker wire for power and ground have reduced the
noise I was getting? I'm not going to try it, but would be interested to
hear thoughts on this.
Without the LM386's in place, I can crank the volume/treble/base up to
maximum on my powered speakers and I can only then just faintly hear
noise when no music is playing.
Based on my findings, anyone looking to make a Mockingboard, my
recommendation is to forget the LM386's and look to another chip, or do
what I did and just forget amplification altogether and run it with
amplified speakers.
Cheers,
Mike
--
Mike Stephens
Central Coast, Australia
Find my Apple II projects online at:
http://apple2.sytes.net/