// This is a netlist description for the sound circuits of Midway's Gun Fight,
// based on Midway's schematic "Gun Fight Sound Generator Detail P.C.
// 597-907E".
//
// (This sound circuitry seems to have evolved from that for an older Midway
// game, an electromechanical rifle game from 1974 called Twin Pirate Gun. The
// schematics for Twin Pirate Gun's sound circuitry (P.C. 569-907-88),
// although not completely identical to Gun Fight's, are similar both in
// general structure and in many details.)
//
// Gun Fight's sound effects are simple "bang" sounds, both for the shooting
// of bullets and for bullets hitting targets. The effects are directed to the
// left or right speaker, depending on whether the left or right player is
// shooting or being hit. (Hits to obstacles get the same sound as hits to the
// right player and thus play from the right speaker.) Each sound effect gets
// a different pitch, with shots being higher pitched than hits. Shot sounds
// also decay faster than hit sounds and have slightly separated initial and
// secondary attacks, resulting in a "ka-pow" effect.
//
// The sounds are generated in stages by different sections of the circuitry.
// A noise generator, based on a zener diode, continuously produces white
// noise that forms the basis for all the sounds, and this noise is fed to
// four separate sound generators. Each generator filters the noise to produce
// the distinct pitch of its sound. Each generator's sound is triggered by a
// switch, activated digitally by the CPU. When this switch is turned on
// momentarily, a storage capacitor is charged up, and that sends power to the
// sound generator's transistor amplifier. This amplifies the filtered noise
// with a sharp attack as the switch is turned on and gradual decay as the
// capacitor drains. The generated sound is filtered further and then
// attenuated by a potentiometer which controls that sound's relative volume.
// Each side's two sound effect signals, after being generated and then
// attenuated by their respective volume pots, are mixed together and then
// further amplified by a second-stage transistor amplifier for that side.
// This mixed and amplified signal is itself filtered and then attenuated by
// the side's master volume potentiometer. Finally it gets sent to the power
// amplifier IC for that side's speaker. (This emulation does not handle the
// final power amplification.)
//
// The different sound effect generators have a common structure which is
// repeated four times. The mixers and second-stage amplifiers likewise have a
// common structure which is repeated twice, and their amplifiers are also
// rather similar to the effect amplifiers.
//
// To make the netlist easier to follow and verify, I've grouped its
// components and connections by function, with the groups ordered roughly
// according to the flow of signals:
//
// * the activating switches,
// * the shared noise generator,
// * the sound-effect noise filters,
// * the sound-effect initial amplifiers, output coupling capacitors and
// filters, and effect-volume potentiometers,
// * the mixing circuits,
// * the second-stage amplifiers, output coupling capacitors and filters, and
// master-volume potentiometers.
//
// Within each group, I've placed the sets of four similar components or
// connections together and listed them in the order: left-shot, right-shot,
// left-hit, right-hit. (For mixing and second-stage amplification components,
// the order is simply left, right.) Individual components are labeled the
// same as on the schematic.
// There are some uncertainties in this emulation about sound levels. The
// average level of the initially generated noise is uncertain, which poses a
// problem because all of the sound effects are based on amplifying that
// noise. The noise is generated by passing a small amount of current through
// a zener diode on the verge of its breakdown voltage, which results in very
// noisy current flow. Even though both the type of zener and the average
// current are known, the average strength of the noise is still uncertain,
// both because zener manufacturers do not specify performance under these
// conditions (they recommended zeners for voltage control under more stable,
// relatively noise-free operating conditions) and because the amount of noise
// may vary greatly from one zener to the next, even within the same
// production batch--let alone from one manufacturer to another. This is an
// inherent problem with zener diodes when used for noise generation.
//
// I have chosen a round figure of 2 mV RMS for the zener's average (RMS)
// noise level. Although this is only a guess, it seems to be in the ballpark
// for similar zeners operated with similar amounts of average current. It
// also keeps the noise component of the sound effects strong enough so as not
// to be overwhelmed by the pulse of the sound effects' initial attacks; this
// pulse is created by switching on their generating amplifiers and is
// independent of noise level. Meanwhile, this noise level is still low enough
// that, with all the volume potentiometers set to their midpoints, the noise
// won't be clipped by any subsequent amplification stage in the netlist, up
// to the power amp inputs. (Some such clipping may occur if a sound effect's
// volume is turned up well beyond its midpoint. That may also be true on real
// hardware.)
//
// The other big uncertainty is the audible effect of the power amp ICs. These
// amplifiers add both power and voltage gain, which may be enough to distort
// or clip the output. They may also have an audible filtering effect. In the
// Gun Fight schematic, and apparently in real machines, they are configured
// for a voltage gain of 15. Furthermore, any input voltages beyond a few
// hundred mV (positive or negative) should produce at least some clipping
// distortion, with the distortion getting worse for stronger signals. With
// all potentiometers set to the midpoint, the power amp signal inputs should
// see extreme pulses of +/- 3 V from the initial attack for sound effects,
// and even past those initial attack pulses, the sound effect noise should
// kick in at levels above +/- 1 V on the power amp inputs. If these levels
// are correct and the power amp ICs work as described, the noise of the sound
// effects should initially be heavily distorted, but since the original
// signal is already random noise, it's not clear whether that distortion
// would be apparent. Anyhow, the power amp ICs are completely unemulated
// here, and any distortion effects they might produce would be quite
// different from the hard clipping produced when limiting output levels
// digitally.
//
// I have compromized by setting the volume multipliers for the netlist stream
// outputs so that output levels of +/- 3 volts will produce the maximum
// allowed stream output magnitudes of +/- 32767. Voltages beyond that will be
// clipped. This at least produces some distortion if the volume
// potentiometers are adjusted above their midpoints.
//
// Further improving accuracy would require testing signal levels on actual
// hardware, with allowances made for variations in components, as well as a
// better understanding of the electrical and sonic behavior of the power
// amplifiers, speakers, and cabinet. It's questionable whether doing so is
// worth the effort.
// As I've said, this netlist does not include the final stage of sound
// generation, the twin audio power amplifier ICs. Although it is the norm for
// MAME analog sound emulation to omit this stage, some discussion of the
// power amplifiers is worthwhile in this case. Each is an SGS-ATES
// TAA-621-A11 single-channel amplifier, driving one 8-ohm speaker of about 4
// or 5 inches.
//
// (On the Gun Fight schematic, the TAA-621-A11 is also labeled "LM354". The
// TAA-621-A11 was introduced in 1970 by the Italian firm SGS. The very
// similar LM354 was introduced in 1972 by European Electronic Products.
// However, this company seems to have been a mere U.S. importer and
// distributor of European components, making the LM354 just a rebadged
// TAA-621-A11. The LM354 occasionally comes up in discussions of Gun Fight
// audio but seems to have been otherwise short-lived. One problem with its
// name is that it can easily be mistaken for a National Semiconductor LMxxx
// linear IC, even though it's unrelated to that company. To further confuse
// matters, National Semiconductor had already introduced an LM354 of their
// own in 1970 which wasn't an audio amplifier at all, but a second-source
// version of Texas Instruments' SN7525 sense amplifier for minicomputer
// magnetic-core memory systems.)
//
// As is normal for TAA-621-A11 amps, those in Gun Fight are configured with
// some negative feedback to control their voltage gain. The gain is
// determined by the ratio of the chip's internal 15-kohm feedback resistor to
// the external resistor connected to the feedback terminal, pin 10. In Gun
// Fight this external resistor is 1 kohm, so the ratio, and thus the voltage
// gain, is 15.
//
// The TAA-621-A11 is rated at 4 watts at 10% total harmonic distortion (THD)
// when supplied with 24-volt power and driving a 16-ohm load. (Although not
// explicitly stated, this appears to be an RMS rating rather than a peak
// rating.) In Gun Fight's case, the speaker load is only 8 ohms, and the
// power supply voltage is a bit lower, about 22 volts (it comes from
// rectified 16.5-volt-RMS AC, buffered by a large smoothing capacitor). Both
// of these factors lower the power rating somewhat. Also, a power rating at
// 10% THD implies clipping is already heavy. The unclipped, clean power
// rating in Gun Fight would be lower, probably no more than 2 watts RMS,
// giving output around 4 volts RMS or about 5.5 volts in extreme amplitude.
// With a power amp voltage gain of 15, this would mean that any input signals
// with extreme amplitudes greater than about a third of a volt would be
// subject to power amp clipping.
//
// The power amps also have a few other connections, including ripple bypass
// capacitors to suppress ripple from the unregulated power supply, frequency
// compensation capacitors to prevent inducing unstable amplifier oscillations
// at high frequencies, and some filtering on the output to the speakers.
// All of the amplifying transistors in this circuit are NPN transistors of a
// particular type, marked as "A-138" on the Gun Fight schematics. This seems
// to be the Amperex A-138, a low-power silicon NPN transistor made by Amperex
// Electronic Corporation, which was a U.S. division of Philips. This
// transistor, being a vendor-specific type long out of production, seems to
// have fallen into obscurity. The only source I could find with any data on
// it (given in greatly abbreviated form) is _Transistor Discontinued Devices
// D.A.T.A.Book, Edition 16_ (1980), by Derivation and Tabulation Associates,
// which lists the A-138 on p. 76, line 86.
//
// This source shows the A-138 to be an ordinary amplifier transistor of
// fairly high gain, with a maximum small-signal forward current transfer
// ratio (h_fe, AC current gain) of 650. The minimum h_fe is not given, but it
// can be estimated using the fact that such transistors are often graded and
// designated by gain. The A-137, another Amperex transistor listed on line 85
// of the same D.A.T.A.Book page, has the same limits on power, collector
// current, and junction voltages, and thus appears to be in the same series,
// but it has a maximum h_fe of 415. If the A-137 was the next lower grade of
// gain from the A-138, the latter should have a minimum h_fe around 400.
// Values for h_FE, the DC forward current transfer ratio (DC current gain),
// aren't given for the A-138, but the range would be about the same or a bit
// lower, perhaps 350-600, with an average around 450-500.
//
// The high gain of Gun Fight's A-138 transistors causes a "ka-pow" effect in
// its shot sounds; I explain how later. (Andy Wellburn, in a Discord chat on
// 2020-06-05, confirmed this effect can be heard in some actual machines. He
// measured an h_FE of 238 for one A-138 from a Gun Fight sound board, but he
// warned that for old transistors like this, h_FE measurements can vary a
// lot, and I'm not surprised that the transistors' gain might be reduced more
// than 40 years later. But even a gain of 238 turns out to be high enough to
// give the "ka-pow" shot effect.)
//
// Considering the A-138's high gain and other limits, a decent match for it
// here seems to be the BC548C, which is modeled in the netlist library. The
// BC548C is the high-gain version of the BC548, a widely used Pro Electron
// standard type of general-purpose small-signal transistor in a through-hole
// TO-92 package. All A-138 transistors in this netlist are modeled as
// BC548Cs.
//static NETLIST_START(gunfight_schematics)
public static void netlist_gunfight_schematics(netlist.nlparse_t setup)
{
netlist.helper h = new netlist.helper();
h.NETLIST_START(setup);
// **** Sound effect activation switches.
// These switches are triggered by digital logic signals activated by
// the CPU. A high TTL logic level turns the switch on, allowing
// 16-volt power to flow through the switch into the sound effect
// generator's amplifier and storage capacitor, and a low logic level
// turns the switch off, cutting off the power flow. In practice, each
// sound effect is triggered by turning its switch on for about 50 ms
// and then switching it off again.
//
// Each switch is built from two transistors: a "low-side" NPN
// transistor which is switched on when a high TTL output level drives
// the NPN's base high, and a "high-side" PNP transistor which is
// switched on when the now conducting NPN pulls the PNP's base low.
// It is the high-side PNP transistor that actually switches the
// 16-volt power, hence the term "high-side".
#if FAST_SWITCHES
// Use abstracted activation switches instead of a detailed circuit
// model of the dual-transistor switches. This gives faster emulation
// while not making any audible difference in the sound produced.
h.SYS_DSW("SW_LEFT_SHOT", "IN_LS", "I_V16.Q", "R130.1");
h.SYS_DSW("SW_RIGHT_SHOT", "IN_RS", "I_V16.Q", "R230.1");
h.SYS_DSW("SW_LEFT_HIT", "IN_LH", "I_V16.Q", "R117.1");
h.SYS_DSW("SW_RIGHT_HIT", "IN_RH", "I_V16.Q", "R217.1");
// Lower the on-resistance to a more accurate value. The charging
// resistor which follows is only 15 ohms, so the default
// on-resistance of 1 ohm might noticeably affect the result.
h.PARAM("SW_LEFT_SHOT.RON", 0.1);
h.PARAM("SW_RIGHT_SHOT.RON", 0.1);
h.PARAM("SW_LEFT_HIT.RON", 0.1);
h.PARAM("SW_RIGHT_HIT.RON", 0.1);
#else
// Detailed circuit model of the dual-transistor switches.
// "Low-side" NPN transistor switches, driven by TTL logic inputs.
RES(R134, RES_R(470))
RES(R234, RES_R(470))
RES(R121, RES_R(470))
RES(R221, RES_R(470))
NET_C(IN_LS, R134 .1)
NET_C(IN_RS, R234 .1)
NET_C(IN_LH, R121 .1)
NET_C(IN_RH, R221 .1)
RES(R133, RES_K(5.1))
RES(R233, RES_K(5.1))
RES(R120, RES_K(5.1))
RES(R220, RES_K(5.1))
QBJT_SW(Q108, "BC548C")
QBJT_SW(Q208, "BC548C")
QBJT_SW(Q105, "BC548C")
QBJT_SW(Q205, "BC548C")
// These all go to TTL ground at pin 7 of 7404 IC H6, rather than the
// ground used for the other sound circuits.
NET_C(GND,
R133 .2, R233 .2, R120 .2, R220 .2,
Q108.E, Q208.E, Q105.E, Q205.E)
NET_C(R134 .2, R133 .1, Q108.B)
NET_C(R234 .2, R233 .1, Q208.B)
NET_C(R121 .2, R120 .1, Q105.B)
NET_C(R221 .2, R220 .1, Q205.B)
RES(R131, RES_K(1))
RES(R231, RES_K(1))
RES(R118, RES_K(1))
RES(R218, RES_K(1))
NET_C(Q108.C, R131 .1)
NET_C(Q208.C, R231 .1)
NET_C(Q105.C, R118 .1)
NET_C(Q205.C, R218 .1)
// "High-side" PNP transistor switches, driven by "low-side" NPN
// switch outputs. The PNP switch outputs charge the storage
// capacitors that supply power to the sound-effect amplifiers; in
// addition, while they are on they power the amplifiers directly.
RES(R132, RES_K(5.1))
RES(R232, RES_K(5.1))
RES(R119, RES_K(5.1))
RES(R219, RES_K(5.1))
// The actual transistors used here are 2N4125s:
QBJT_SW(Q107, "PNP")
QBJT_SW(Q207, "PNP")
QBJT_SW(Q104, "PNP")
QBJT_SW(Q204, "PNP")
// All connected to 16-volt power.
NET_C(I_V16.Q,
R132 .1, R232 .1, R119 .1, R219 .1,
Q107.E, Q207.E, Q104.E, Q204.E)
NET_C(R131 .2, R132 .2, Q107.B)
NET_C(R231 .2, R232 .2, Q207.B)
NET_C(R118 .2, R119 .2, Q104.B)
NET_C(R218 .2, R219 .2, Q204.B)
NET_C(Q107.C, R130 .1)
NET_C(Q207.C, R230 .1)
NET_C(Q104.C, R117 .1)
NET_C(Q204.C, R217 .1)
// End of switch description.
#endif
// **** Current supply and storage capacitors for sound-effect
// **** amplifiers.
h.RES("R130", RES_R(15));
h.RES("R230", RES_R(15));
h.RES("R117", RES_R(15));
h.RES("R217", RES_R(15));
h.CAP("C122", CAP_U(10));
h.CAP("C222", CAP_U(10));
h.CAP("C116", CAP_U(20));
h.CAP("C216", CAP_U(20));
h.NET_C("GND", "C122.2", "C222.2", "C116.2", "C216.2");
h.NET_C("R130.2", "C122.1", "R126.1", "R124.1");
h.NET_C("R230.2", "C222.1", "R226.1", "R224.1");
h.NET_C("R117.2", "C116.1", "R113.1", "R111.1");
h.NET_C("R217.2", "C216.1", "R213.1", "R211.1");
// **** Shared white-noise generator circuit. This is the basic noise
// **** source which is filtered and amplified by the sound-effect
// **** circuits.
// Gun Fight's noise generator circuit is based on a reverse-biased
// 9.1-volt zener diode (D304, a 1N5239) whose noise current is then
// amplified by an A-138 NPN transistor, producing white noise at
// audio frequencies.
//
// (Strictly speaking, this is not a *pure* white noise generator,
// because the generator's bypass capacitor and biasing resistor, in
// combination with negative feedback from the amplified signal, act
// as a high-pass RC filter, filtering out the lowest noise
// frequencies.)
//
// Figuring out how strong the noise signal should be for this circuit
// is difficult. The noise generator's biasing resistors and
// transistor gain limit the average current through its zener to
// around 2 microamps, but the 1N5239 is normally expected to be used
// with much larger currents, in the range of 250 microamps to 50
// milliamps.
//
// Zeners are most often used for smoothly controlling and limiting
// voltage with minimal fluctuation, like in a power regulator. But a
// zener can only do this smoothly if it passes a large enough
// current. This is especially true for "zeners" of voltages of 9.1
// volts or more, which properly speaking are "avalanche diodes" that
// don't user the actual Zener effect but rather a different effect,
// avalanche breakdown, which is capable of generating far more noise.
// Standard zener specifications include "knee" figures which in
// effect give the minimum expected current, along with the maximum
// "dynamic impedance" at that current: how much the voltage may vary
// if the current changes, or vice versa. For the 1N5239, the knee is
// at 250 microamps, and the maximum dynamic impedance at the knee is
// 600 ohms, so a 1 microamp increase in current at that point would
// raise the voltage drop by up to 0.6 millivolts. Noise values for
// zeners are often not given at all, or if they are (as with
// Motorola's zeners), they are given at the knee current. In short,
// the manufacturer expects the current to be kept above the knee
// value, and if your current is lower, you're on your own.
//
// At very low currents, still within the breakdown region but well
// below the knee current, the zener's dynamic impedance is much
// greater, and more importantly, so is its noise. The conduction of
// current is no longer smooth and regular, but pulsing and episodic,
// as small conducting "microplasmas" within the diode's junction are
// repeatedly triggered and extinguished, like microscopic versions of
// lightning bolts in a thunderstorm.
//
// The netlist library includes a Zener diode model, but this model
// does not simulate the Zener's noise behavior. Instead I generate
// the noise from a noise voltage source in series with the Zener.
// Simple model of a 1N5239 9.1-volt Zener diode. The 1N5239 is
// specified to conduct 20 mA of current at its nominal breakdown
// voltage of 9.1 V. The model produces an exponential I-V curve,
// passing through this point, which has the same general shape as
// that of a normal forward-biased diode. NBV is an exponent scale
// factor; its value here of 1 gives the curve a steep rise and a
// relatively sharp knee. Actual breakdown I-V curves have an even
// steeper rise and sharper knee, too steep and sharp to be
// represented by an exponential, but this model is good enough for
// this emulation, since the diode operates very close to a single
// point on the curve.
h.ZDIODE("D304", "D(BV=9.1 IBV=0.020 NBV=1)");
// 24 kHz noise clock for the noise source, chosen to retain noise
// frequencies as high as possible for 48 kHz sample rate.
h.CLOCK("NCLK", 24000);
h.NET_C("I_V5.Q", "NCLK.VCC");
h.NET_C("GND", "NCLK.GND");
// Normally-distributed noise of 2 millivolts RMS voltage.
// This level was chosen to have a strong amplified noise signal that
// won't be clipped by any subsequent stages of amplification before
// the power amps, if the volume potentiometers are not raised beyond
// their approximate midpoints.
h.SYS_NOISE_MT_N("NOISE", 0.002);
h.NET_C("NCLK.Q", "NOISE.I");
h.RES("R302", RES_K(6.8));
h.RES("R303", RES_K(6.8));
h.CAP("C307", CAP_U(10));
h.NET_C("C307.2", "GND");
h.QBJT_EB("Q302", "BC548C");
h.NET_C("Q302.E", "GND");
h.NET_C("I_V16.Q", "R302.1");
h.NET_C("Q302.B", "NOISE.1");
h.NET_C("D304.A", "NOISE.2");
h.NET_C("R303.2", "C307.1", "D304.K");
// Coupling capacitors from noise generator to sound effect frequency
// filters. (These coupling capacitors, together with the resistances
// beyond them, act as high-pass filters with very low cutoff
// frequencies.)
h.CAP("C303", CAP_U(0.1));
h.CAP("C306", CAP_U(0.1));
h.CAP("C304", CAP_U(0.1));
h.CAP("C305", CAP_U(0.1));
h.NET_C("R302.2", "Q302.C", "R303.1", "C303.1", "C306.1", "C304.1", "C305.1");
// **** Sound effect frequency filters.
// Each sound effect has a pair of passive low-pass RC filters with
// cutoff frequencies determined by their component values. The
// different capacitor values produce each sound effect's distinct
// pitch.
h.RES("R129", RES_K(20));
h.RES("R229", RES_K(20));
h.RES("R116", RES_K(20));
h.RES("R216", RES_K(20));
h.NET_C("C303.2", "R129.1");
h.NET_C("C306.2", "R229.1");
h.NET_C("C304.2", "R116.1");
h.NET_C("C305.2", "R216.1");
h.CAP("C121", CAP_U(0.015));
h.CAP("C221", CAP_U(0.015));
h.CAP("C115", CAP_U(0.033));
h.CAP("C215", CAP_U(0.033));
h.NET_C("C121.2", "C221.2", "C115.2", "C215.2", "GND");
h.RES("R128", RES_K(30));
h.RES("R228", RES_K(30));
h.RES("R115", RES_K(30));
h.RES("R215", RES_K(30));
h.NET_C("R129.2", "C121.1", "R128.1");
h.NET_C("R229.2", "C221.1", "R228.1");
h.NET_C("R116.2", "C115.1", "R115.1");
h.NET_C("R216.2", "C215.1", "R215.1");
h.CAP("C120", CAP_U(0.01));
h.CAP("C220", CAP_U(0.015));
h.CAP("C114", CAP_U(0.1));
h.CAP("C214", CAP_U(0.047));
h.NET_C("C120.2", "C220.2", "C114.2", "C214.2", "GND");
// **** Sound effect amplifier circuits.
// Each sound-effect amplifier is a single NPN transistor wired as a
// common-emitter amplifier. The amplifiers for "hit" sounds also have
// a bypass capacitor at the emitter, while those for "shot" sounds
// have no bypass capacitor and a much lower emitter resistance. The
// attack and decay of the sound effects is handled by controlling the
// current supply to each amplifier, which is done by the switching
// circuits and supply capacitors described above.
// More explanation is needed for the "shot" sounds. Apart from their
// higher frequency and faster decay, the "ka-pow" effect in their
// initial attack further distinguishes them from the "hit" sounds.
// This effect comes from the high current gain (around 450-500) of
// the amplifier's A-138 transistor together with the low emitter
// resistance. When the current supply for the sound is switched on,
// the collector voltage at first spikes upward as the supply
// capacitor is charged. But the transistor's base voltage and base
// current also rise, which "turns on" the transistor, and as its
// collector current increases through its biasing resistor, the
// collector voltage plummets. For the "shot" sound transistors,
// because of their high current gain and low emitter resistance, the
// collector current grows so much that the collector voltage is
// pulled below the base voltage, pushing the transistor into
// saturation. This persists for as long as the current supply switch
// remains on; the collector voltage stays low with little variation.
// In this state the amplifier's input noise signal is being clipped
// rather than amplified.
// The result is that the sound effect's initial voltage spike is
// followed by a relatively prolonged low with almost no noise. As
// this signal passes through the output coupling capacitor, the
// intervening filter and potentiometer, and then the second
// amplification stage, it becomes a series of strong oscillations,
// followed by a momentary silence which lasts as long as the sound's
// switch is held on: around 50 milliseconds.
// Finally the sound switch is turned off, and the amplifier's supply
// voltage and current begin to drop as the supply capacitor
// discharges. The base current and collector current drop also, and
// the collector voltage begins to rise, eventually rising above the
// base voltage again. The transistor leaves saturation and returns to
// forward active mode. Now the noise signal is not being clipped but
// gets properly amplified on the collector output. So the momentary
// silence is followed by a sudden burst of noise, which then dies
// away as the supply capacitor is drained.
// The result is the shot's distinct "ka-pow" sound: an initial
// punctuating crack, a very brief silence, and a sudden noise burst
// that quickly fades.
// The "hit" sounds don't have this effect, despite using the same
// high-gain transistors, because their transistor amplifiers have a
// bypass capacitor and a much larger emitter resistance, 1 Kohm
// versus 100 ohms. That higher resistance keeps the collector current
// low enough that the collector voltage never drops below the base
// voltage, so the transistor never saturates, while the bypass
// capacitor allows the amplifier's AC gain to remain very high.
h.RES("R126", RES_K(330));
h.RES("R226", RES_K(330));
h.RES("R113", RES_K(330));
h.RES("R213", RES_K(330));
h.RES("R127", RES_K(30));
h.RES("R227", RES_K(30));
h.RES("R114", RES_K(30));
h.RES("R214", RES_K(30));
h.NET_C("R127.2", "R227.2", "R114.2", "R214.2", "GND");
h.RES("R124", RES_K(5.1));
h.RES("R224", RES_K(5.1));
h.RES("R111", RES_K(5.1));
h.RES("R211", RES_K(5.1));
h.RES("R125", RES_R(100));
h.RES("R225", RES_R(100));
h.RES("R112", RES_K(1));
h.RES("R212", RES_K(1));
h.NET_C("R125.2", "R225.2", "R112.2", "R212.2", "GND");
h.CAP("C113", CAP_U(50));
h.CAP("C213", CAP_U(50));
h.NET_C("C113.2", "C213.2", "GND");
h.QBJT_EB("Q106", "BC548C");
h.QBJT_EB("Q206", "BC548C");
h.QBJT_EB("Q103", "BC548C");
h.QBJT_EB("Q203", "BC548C");
h.NET_C("R128.2", "C120.1", "R126.2", "R127.1", "Q106.B");
h.NET_C("R228.2", "C220.1", "R226.2", "R227.1", "Q206.B");
h.NET_C("R115.2", "C114.1", "R113.2", "R114.1", "Q103.B");
h.NET_C("R215.2", "C214.1", "R213.2", "R214.1", "Q203.B");
h.NET_C("R125.1", "Q106.E");
h.NET_C("R225.1", "Q206.E");
h.NET_C("R112.1", "C113.1", "Q103.E");
h.NET_C("R212.1", "C213.1", "Q203.E");
// **** Coupling capacitors, high-pass (pulse-differentiator) filters,
// **** and volume potentiometers for sound effect amplifier outputs.
// These circuits act as high-pass filters on the sound effect
// generator outputs, with very low cutoff frequencies. Because the
// cutoff frequency is so low, one of the main effects of the filter
// is to remove any flat areas from the initial turn-on pulse of the
// sound effect generator amplifier. The filter effectively
// differentiates the pulse, producing output voltage proportional to
// the steepness of its slope. This replaces the single wide pulse of
// the initial attack with a sequence of sharp spike pulses.
h.CAP("C119", CAP_U(0.047));
h.CAP("C219", CAP_U(0.047));
h.CAP("C112", CAP_U(0.1));
h.CAP("C212", CAP_U(0.1));
h.NET_C("R124.2", "Q106.C", "C119.1");
h.NET_C("R224.2", "Q206.C", "C219.1");
h.NET_C("R111.2", "Q103.C", "C112.1");
h.NET_C("R211.2", "Q203.C", "C212.1");
h.CAP("C118", CAP_U(0.022));
h.CAP("C218", CAP_U(0.022));
h.CAP("C111", CAP_U(0.033));
h.CAP("C211", CAP_U(0.033));
h.NET_C("C118.2", "C218.2", "C111.2", "C211.2", "GND");
// There are four sound-effect volume pots, for shot and hit sounds on
// left and right.
h.POT("R123", RES_K(50));
h.POT("R223", RES_K(50));
h.POT("R110", RES_K(50));
h.POT("R210", RES_K(50));
h.NET_C("R123.3", "R223.3", "R110.3", "R210.3", "GND");
// Reverse the sense of pot adjustments so that larger values result
// in greater volume.
h.PARAM("R123.REVERSE", 1);
h.PARAM("R223.REVERSE", 1);
h.PARAM("R110.REVERSE", 1);
h.PARAM("R210.REVERSE", 1);
h.NET_C("C119.2", "C118.1", "R123.1");
h.NET_C("C219.2", "C218.1", "R223.1");
h.NET_C("C112.2", "C111.1", "R110.1");
h.NET_C("C212.2", "C211.1", "R210.1");
// **** Mixing of shot and hit sounds for each side.
h.RES("R122", RES_K(30));
h.RES("R222", RES_K(30));
h.RES("R109", RES_K(30));
h.RES("R209", RES_K(30));
h.NET_C("R123.2", "R122.2");
h.NET_C("R223.2", "R222.2");
h.NET_C("R110.2", "R109.2");
h.NET_C("R210.2", "R209.2");
h.CAP("C117", CAP_U(0.047));
h.CAP("C217", CAP_U(0.047));
h.CAP("C110", CAP_U(0.1));
h.CAP("C210", CAP_U(0.1));
h.NET_C("R122.1", "C117.2");
h.NET_C("R222.1", "C217.2");
h.NET_C("R109.1", "C110.2");
h.NET_C("R209.1", "C210.2");
// **** Second-stage amplifier circuits, which amplify each side's
// **** mixed shot and hit sounds.
// These amplifiers are similar to those for the "hit" sound effects,
// each being a single A-138 NPN transistor wired in common-emitter
// configuration, with a 1-Kohm resistance and a bypass capacitor at
// the emitter. They have no need for an attack-decay envelope,
// however, and so get their current directly from the 16-volt power
// supply.
h.RES("R107", RES_K(150));
h.RES("R207", RES_K(150));
h.RES("R105", RES_K(30));
h.RES("R205", RES_K(30));
h.NET_C("R105.2", "R205.2", "GND");
h.RES("R108", RES_K(5.1));
h.RES("R208", RES_K(5.1));
h.RES("R106", RES_K(1));
h.RES("R206", RES_K(1));
h.NET_C("R106.2", "R206.2", "GND");
h.CAP("C109", CAP_U(50));
h.CAP("C209", CAP_U(50));
h.NET_C("C109.2", "C209.2", "GND");
h.NET_C("R107.1", "R207.1", "R108.1", "R208.1", "I_V16.Q");
h.QBJT_EB("Q102", "BC548C");
h.QBJT_EB("Q202", "BC548C");
h.NET_C("C110.1", "C117.1", "R107.2", "R105.1", "Q102.B");
h.NET_C("C210.1", "C217.1", "R207.2", "R205.1", "Q202.B");
h.NET_C("R106.1", "C109.1", "Q102.E");
h.NET_C("R206.1", "C209.1", "Q202.E");
// **** Coupling capacitors, bandpass filters, and volume
// **** potentiometers for second-stage amplifier outputs.
h.CAP("C108", CAP_U(0.047));
h.CAP("C208", CAP_U(0.047));
h.NET_C("R108.2", "Q102.C", "C108.1");
h.NET_C("R208.2", "Q202.C", "C208.1");
h.RES("R104", RES_K(30));
h.RES("R204", RES_K(30));
h.NET_C("C108.2", "R104.1");
h.NET_C("C208.2", "R204.1");
h.CAP("C107", CAP_U(0.001));
h.CAP("C207", CAP_U(0.001));
h.NET_C("C107.2", "C207.2", "GND");
// There are two master volume pots, for left and right.
h.POT("R103", RES_K(47));
h.POT("R203", RES_K(47));
h.NET_C("R103.3", "R203.3", "GND");
// Reverse the sense of pot adjustments so that larger values result
// in greater volume.
h.PARAM("R103.REVERSE", 1);
h.PARAM("R203.REVERSE", 1);
h.NET_C("R104.2", "C107.1", "R103.1");
h.NET_C("R204.2", "C207.1", "R203.1");
// The potentiometer outputs are used here as the left and right audio
// outputs. In the real circuit they drive the signal inputs of the
// audio power amplifier ICs for the left and right speakers.
h.ALIAS("OUT_L", "R103.2");
h.ALIAS("OUT_R", "R203.2");
// The real outputs are somewhat constrained in that they drive the
// bases of the input transistors within the power amplifiers. If they
// go too low in voltage, there seems to be a peculiar effect on the
// speaker output waveforms, although I'm not sure whether this is a
// real effect or an artifact of the LTspice simulation I constructed.
// Nor am I sure whether it matters in practice. In any case, it's not
// modeled here.
h.NETLIST_END();
}
