Carver M-500
A power amplifier that shipped with a service bulletin attached to its own turn-on circuit, and a six-pound, four-ounce magnetic field coil doing the work of a forty-pound toroid.
Overview
Lift the lid on an M-500 and the power supply tells you what era of Carver engineering you're dealing with. It isn't a transformer in the conventional sense. It's a magnetic field coil, a six-pound, four-ounce assembly. Carver's own advertising claimed it delivered instantaneous high peak and long-term power that could react to musical transients as brief as one millisecond. Audio (February 1986, p.87) described it as a patented Magnetic Field Coil with an output current of plus or minus 100 amps at 10 percent regulation. A conventional amplifier stores energy in a heavy toroid. The M-500 modulates a magnetic field instead, so the power supply weighs a fraction of what the rated output would suggest.
The manual lists dynamic headroom at 240 watts into 4 ohms and 230 watts into 8 ohms in stereo. In mono it rises to 520 watts into 4 ohms and 620 watts into 8 ohms. Rated output is more than 200 watts per channel, with total harmonic distortion of 0.05 percent across 20 Hz to 20 kHz with both channels driven. HiFi-Stereo (January 1982, p.58) pushed a test unit hard enough to measure 0.0018 percent distortion at 1 watt and 0.0066 percent at 50 watts, with 0.4 percent at the clipping point of 85 watts. Those numbers suggest the 0.05 percent rating was written with a generous pen. The same review noted the amplifier was not intended to drive 2-ohm loads, yet under that severe condition it still held together.
Signal-to-noise is where the M-500 pulls ahead of most of what shared a shelf with it. The manual rates S/N at more than 100 dB IHF-A with the main input shorted. Audio (February 1986, p.87) went further, reporting that the circuitry had the best signal-to-noise ratio of any production amplifier, better than -120 dB. The review also flagged a practical benefit: the M-500 does not require an external fan, so it stays quiet inside and out.
Frequency response is specified at 20 Hz to 20 kHz, with a 1 kHz reference at 20 watts held to plus or minus 0.5 dB. Input impedance is listed at 15 kilohms. Crosstalk between channels measures better than 80 dB at 1 kHz and better than 60 dB at 20 kHz, both taken at 20 watts output with the main input shorted. Damping factor, residual noise, and IM distortion figures appear in the manual's specification block, but the OCR is too degraded to read reliably. Work from a clean scan rather than a forum transcription if you need those numbers.
Physically, the M-500 is a 440 by 110 by 348 millimeter chassis weighing 10.5 kilograms, with a maximum power consumption of 1100 watts. Speaker output is specified for 8 ohms. The manual carries a note that relay speaker switching, the 42 to 82 rail adjust switch, and a front-panel on/off switch are not available on the M-500. This model sat below its more feature-laden siblings in the lineup. The parts list confirms a speaker selector switch and a power switch, along with two relays, two lamps, and a meter panel, so the front panel is not bare. What it lacks is the convenience switching found higher up the range.
The turn-on circuit is the M-500's main maintenance issue, and Carver knew it. Service Bulletin M-500-2, dated February 1991, addresses amplifiers that fail to turn on reliably or at all. It blames the Silicon Bidirectional Switch SBS501, a BS08A part located on the GT-377 sub board. The original BS08A was no longer available from Carver by the time the bulletin was written, so the fix substitutes a Motorola MBS4991 and changes R527 from a 56k to a 39k quarter-watt resistor. The bulletin also walks technicians through adjusting SVR503, which sets the high rail voltage. If that rail is set too low, the amplifier simply will not start. Carver specified that the amplifier should start with a minimum of 90 VAC line voltage. The procedure also warns about bypassing the triac: a Variac must be used and line voltage held to no more than 50 VAC, because the magnetic field coil cannot accept 120 VAC without triac control.
The bulletin carries a second warning. When removing and re-installing the GT-377 sub board, do not overheat the connecting pins, because the solder connections can become intermittent. Notes like that usually get written after enough units come back.
Bias adjustment is straightforward for anyone with a meter. The procedure calls for preheating more than five minutes, then setting the speaker impedance selector to 8 ohms and adjusting SVR101 and SVR102 for 6 mV and 11 mV respectively, measured at test points TP1/TP2 on the left channel and TP3/TP4 on the right. The power supply alignment starts by rotating SVR503 and SVR504 fully counter-clockwise, then setting SVR503 for 74 volts at test point R178. Meter calibration is done by feeding 1 kHz into the input jacks and adjusting SVR601 and SVR602 until the meters read 100 watts at the speaker output.
The M-500's reputation rests on that power supply. A conventional amplifier with 230 watts per channel into 8 ohms and 620 watts in mono into 8 ohms would be a much heavier box, and Carver's whole argument was that the weight was unnecessary. The tradeoff is a circuit that depends on a triac and a bidirectional switch to bring the rails up in the right order. That is why the service bulletin exists, and why a non-starting M-500 is more often a control-circuit problem than a blown output stage. Check SBS501 and R527 on any unit that has not been serviced, and verify that SVR503 has not drifted.
The naming is confusing. The magazine coverage of the period repeatedly refers to the M-500t, and the M-500t appears in the same review pages as the M-500. The available documents do not establish whether the "t" denotes a variant, a later revision, or simply a typographic habit of the era's reviewers. The service bulletin and manual both address the M-500 without the suffix. When comparing listings, read the model designation on the rear panel rather than trusting the headline.
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