On Aug 30, 5:54*pm, "Michael J. Mahon" <mjma...@aol.com> wrote:
> On 8/28/2010 10:34 PM, Osgeld wrote:
>
>
>
>
>
> > On Aug 29, 12:20 am, Polymorph<mike.a.steph...@gmail.com> *wrote:
> >> 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...nterfacing.jpg
>
> >>>> *From analysis of the high resolution scans of the Mockingboard V1on
> >>>> 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
>
> But I expect your diagnosis is correct--the noise is coming from
> the +5v power bus, plus any "ground bounce" on the ground lines.
>
> The Apple +5v supply is quite noisy for any audio application.
> I expect that your experiment with a (quiet) external supply will
> produce results very similar in S/N to your amplified speakers.
>
> The easiest solution for deriving quiet power from the Apple bus
> is to use the +12v line, and regulate it down to, say, +8v for the
> 386's. *Also add an RC decoupling circuit to the +12v line prior to
> the voltage regulator--say, a 22 ohm series resistor and a 100uF
> cap to ground, to reduce high-frequency noise. *The regulator should
> do a good job attenuating the low-frequency supply noise.
>
> For the AppleCrate II, I built an outboard audio processor powered
> by the 'Crate. *I regulated the +12v down to +8 for the 386's, and
> further regulated the +8 down to +5 for the "quiet" +5 supply for
> the 17 1-bit DACs and the opamps.
>
> BTW, if you were unsatisfied with the noise levels with externally
> powered speakers, you'll need to begin by using a large decoupling
> capacitor from +5v to ground--at least 50uF for starters. *It's easy
> to try this out for effect by connecting and disconnecting one end
> of the capacitor while listening to the output with no sound being
> generated.
>
> -michael
>
> NadaNet 3.1 for Apple II parallel computing!
> Home page: *http://home.comcast.net/~mjmahon/
>
> "The wastebasket is our most important design
> tool--and it's seriously underused."
Michael,
As I mentioned in a separate post, I had originally wired the LM386's
to +5V, but then checnged to using the +12V line in the exact same way
as on the Mockingboard V1 (and presumably the original Mockingboards).
But after doing the switch to 12V, I heard little or no difference.
But I plan on rigging up a 9V battery as the power source to see if
that eliminates the noise.
If that fails, I'll probably ditch the suggested schematic from the
AY-3-8910 manual, and just implement what I see on the Mockingboard
V1.
Thanks for the help.
Cheers,
Mike