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/