Re: Help With Mockingboard Amplification

[Osgeld <osgeld@cheesefactory.us>]

On Aug 28, 8:35 pm, Polymorph <mike.a.steph...@gmail.com> wrote:
> Hi Guys,
>
> I decided to add the LM386 amplifiers to my Mockingboard that I posted
> about a few weeks ago. Prior to adding the amplification, the board
> was working fine with powered speakers. This might be another case of
> "if it aint broke, don't fix it", but I wanted to hear the difference
> the amplifiers would make, and I had already bought them as well, so
> the temptation was too strong.  ;-)
>
> I completed the amplifier add-on and have got it working as such, but
> with *heaps* of bus noise being output. I can play music, and I know
> the amplifiers are obviously working as the sound is loud even on un-
> powered speakers. The trouble is, as soon as I turn on the Apple II, I
> can hear fairly loud bus noise.
>
> Now I've used the circuit as described in the AY-3-8910 data manual
> which can be seen here:http://apple2.sytes.net/Mockingboard/images/AY-3-8910_Interfacing.jpg
>
> From analysis of the high resolution scans of the Mockingboard V1 on
> the ReactiveMicro site, this circuit appears to be very similar to
> what is on the Mockingboard V1. Is there anything obviously wrong
> with the circuit above?
>
> Another thing I've noticed is that the Mockingboard V1 has some
> additional 0.1uF capacitors on the +5V line going into the AY-3-8910
> and 6522 chips. Could the absence of these capacitors be responsible
> for the noise I'm hearing?
>
> I've gone over the additional amplifier circuitry that I added, and
> everything appears to be wired correctly. Remember, the rest of the
> circuit was working fine (albeit without amplification).
>
> Is anything else I should check to eliminate the bus noise?
>
> Any help would be greatly appreciated!
>
> Cheers,
> Mike

nothing looks offensive, although the 386 is an especially noisy chip
in the first place, the 0.1uf caps going to the digital chips are
decoupling caps, they keep bouncy power from effecting the chips
(false triggers and whatnot) and they should not have any effect on a
analog output

what happens if the amp is powered externally?