Audio Subsystem Block Diagram
Audio Source for Audio Over MCA
Common Sound Circuits?
On the 95 and 90, look next to the operator panel header or the power switch
header and you will notice the small LM386
Audio Op Amp.
Driving Linear Amp Input Node
The audio subsystem is a speaker driven by a linear amplifier.
The linear amplifier input node can be driven from the following sources:
Tone Generator (timer 2) output
Enabled using bit 1 of I/O port 0061h set to 1. (For information
about timer 2 see Programmable Timers.)
GATE 2 is controlled by bit 0 of System
Control Port B.
CLK IN 2 is driven by a 1.193-MHz signal.
CLK OUT 2 has two connections. One is
to input port hex 0061, bit 5. CLK OUT 2 is also logically ANDed with port
hex 0051, bit 1 (speaker data enable). The output of the AND gate drives
the "Audio Sum Node" signal.
System Channel, using the 'audio sum node' signal.
Audio Subsystem Block Diagram
Each audio driver must have a 1200-ohm source impedance, and a 7.5-kilohm or
greater impedance is required for each audio receiver. Volume control is
provided by the driver. Output level is a function of the number of drivers and
receivers that share the AUDIO line.
The logic ground is connected to AUDIO GND at the amplifier. Placing
components between pins 1 and 8 on the LM386 sets the gain.
Single channel analog audio signal. Synthesized voice or music generated on
a Micro Channel adapter can be routed as a complex analog waveform directly to
the amplifier and speaker inside the computer. The Micro Channel allows
expansion cards on the channel to exchange and independently process audio
A more detailed schematic of the LM386 stage can be found
Audio Signal Group
Consists of an analog voltage sum node and an audio ground.
Audio Sum Node (Signal pin B02)
Used for communication of audio signals between devices on the bus or to the
Frequency range 50Hz to 10 kHz, +/- 3db. Maximum analog noise level is 50 mV.
Analog signal amplitude is 2.5V peak-to-peak with a DC offset of 0 +/- 50 mV.
Audio Gnd (Signal Pin B01)
Used as the audio ground return signal.
Audio Source for Audio Over MCA (from Peter; edited)
On MCA systems the audio signals supplied by certain cards (M-ACPA namely)
is passed through the MCA connector (auxiliary audio channel) and fed to the
planar audio amplifier. Therefore the ACPA indeed can play audio via system
speaker with no external speakers connected. At least OS/2 supports this
The main problem with the faint beeps on later machines is the tiny SMD-version
of the LM386M (or compatible) audio amplifier they used - along with a SMD
47 µF (or similarly small) output capacitor and pretty high input
resistors. That adds to a mere whisper rather than a real good beep at all. The
audio amplifier is good for 0.5W output - enough to beat the crap out of these
8 Ohms "Taiwan" speakers - but the signal is dampened too much on the input
side... and the tiny output capacitor cannot transfer the required energy to
For the always curious: on the 95A board the audio amp sits bottom / left at
position U5, close to panel and FDD connectors. I *think* the 220 µF cap
C26 is the output capacitor, but I haven't verified that. So the output stage
should have reasonable good energy transfer. I'm going to analyze the rest of
the audio amp later.
Mod. 56s have the LM386 towards the rear of the planar. The output cap is
at the underside - a 22 µF at the underside of the board if I have tracked
it correctly. Bah. Way too small. The sound is not throttled... it is
*strangled* by the circuit design.
Speaker Power (from Don Hills)
> But on some machines the beep is rather faint.
This is for a good reason...
Back in the days of the PC-AT, I was intrigued by a driver that played WAV
files (or their precursors) over the internal speaker. I found some technical
details and wrote my own, learning a lot about the PC timer chip in the
process. PC-AT systems had a good speaker and plenty of drive to it, as did the
first series of PS/2 systems (50/60/70/80). The M30s didn't have a speaker as
such, just a tiny (1 American cent) size "squeaker". Later models had
progressively more wimpy sound systems.
I think the reason the beep started out loud and got fainter in succeeding
models was that in the first machines, there was no affordable alternative for
producing sounds. Games, in particular, benefited from a good speaker system,
even for the limited range of effects that could be achieved. Trouble was,
customer feedback said that too loud a "beep" wasn't welcome in corporate
applications. So as sound cards became available for those who really wanted
sound, the volume of the internal speaker dropped to a suitable level for
"corporate" beeps. It was cheaper to do this than put in a volume control.
I still have all the code (for real mode DOS) for driving internal speakers,
and the sound card option for the original 10 MHz 286 PS/1 systems, and the
Disney Sound Source, and the basic "resistor network on the parallel port"
sound adapter. I'll make a distribution of it if anyone's interested.
Ed. Most PS/55 machines
have a handy potentiometer on the front of the unit that can be used to adjust
the volume of the internal speaker (or to silence it completely).