Re: Help With Mockingboard Amplification

[Polymorph <mike.a.stephens@gmail.com>]

On Aug 29, 3:34 pm, Osgeld <osg...@cheesefactory.us> wrote:
> On Aug 29, 12:20 am, Polymorph <mike.a.steph...@gmail.com> wrote:
>
>
>
>
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> > On Aug 29, 1:47 pm, Osgeld <osg...@cheesefactory.us> wrote:
>
> > > 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?
>
> > Thanks, I'll give that a try tonight and see what happens.
>
> > One thing that has occurred to me just now is that the resistors I've
> > used are all 0.5 Watt. The addition of the amplifier would be causing
> > the output to go greater than 0.5 Watts now would it not? Could this
> > be the cause of the noise? Should I replace all resistors with 1 Watt
> > (or higher) resistors, or are the 0.5 Watt ones I'm using sufficient?
>
> > Cheers,
> > Mike
>
> half watt resistors are really overkill (but not doing any harm) the
> wattage rating of a resistor is not how much you can pass though it,
> but how much energy it can dissipate before breaking, with the current
> and voltages involved your more than ok
>
> the issue is there is noisy power, your amplifying it (just like the
> hum of a guitar amp) so you are looking to remove it or filter it
> out...
>
> I dont know how you have this constructed, if its soldered together,
> trial and error could be a big PITA, that is why I suggested running
> it off of a separated power supply first, you could also increase the
> value of the 300pf capacitor or omit it and the 500 ohm resistor, as
> that is not usually seen on basic applications of the amp
>
> it also could be coming from the passive mixer since its taking 3
> inputs and running them through a voltage divider to ground
>
> dunno I am not an analog electronics guru

Yeah, its soldered together. Details are on my website as to how I
constructed it:
http://apple2.sytes.net/Mockingboard/

It has been modified since those photo's to accommodate the
amplifiers. I took a couple of quick shots of the card as it is now.
The image links are:
http://apple2.sytes.net/Mockingboard/images/Amplified/Mockingboard1.jpg
http://apple2.sytes.net/Mockingboard/images/Amplified/Mockingboard2.jpg

One thing I forgot to mention is that I had originally wired the
LM386's using the +5V line (as all the other chips are), however, I
noticed that the Mockingboard V1 (and original Mockingboards) used the
+12V line, so I changed to using that; with the 12V wired to a 220uF
cap going to ground (as per the Mockingboard V1). However, after doing
this, the results sound exactly the same.  :-(

I'll try rigging up an external power source and see what happens.

Thanks for your help!

Cheers,
Mike