Denon PMA-850 (1977–1979)
A pre-main amplifier with an odd output stage and a cartridge-matching computer behind a slab of extruded aluminum. People have argued about its dates for four decades.
Overview
The Denon PMA-850 is an amplifier collectors argue about before they've plugged it in. The first argument is the date. Denon's own German-language sales literature for the TU-850/PMA-850 combination is dated 6/78. The Japanese documentation points to a release in 1977 - thevintageknob.org pins it to November 1977, audio-database.com says 1977, and a European catalogue lists the pair under "Denon Hifi-Program 1978/79." The 1984-1987 window that has circulated in English-language research is almost certainly wrong. The period documents don't support it, and the 1978 brochure date sits squarely inside the 1977-79 span. Treat the PMA-850 as a late-1977 introduction with a run into 1979.
The second argument is the power rating, and the manufacturer's own brochure is the authority. Denon's German literature rates the PMA-850 at 2×95 W into 8 ohms and 2×110 W into 4 ohms at 1000 Hz. audio-database.com lists 85W + 85W into 8 ohms across 20 Hz - 20 kHz at 0.01% THD, and 110W + 110W into 4 ohms at 1 kHz at 0.05% THD. The "110W per channel into 8 ohms" figure in some English write-ups appears to be a 4-ohm number that landed in the 8-ohm column. Assume roughly 85-95 watts per channel into 8 ohms, not 110.
Nobody argues about the topology. Instead of the flat "plate" amplifier layout common in solid-state designs of the era, the PMA-850 used a single-ended push-pull (SEPP) output stage. That approach has more in common with tube amplifier thinking than with typical transistor amps of the period. Denon's documentation describes an all-stage pure complementary push-pull design, with both NPN and PNP transistors at every stage and a differential first stage of high-gain NPN/PNP devices feeding a push-pull output. The symmetry is there for distortion cancellation: even-order harmonics generated in one half cancel against those in the other, and the total drops accordingly.
The rest of the chassis backs that up. Dual mono construction gives each channel its own power transformer winding. Denon's literature specifies a 420 VA toroidal transformer with independent left and right windings in a single package, chosen partly because toroids leak less flux and so protect the signal-to-noise ratio. Each channel gets its own rectifier circuit and its own large block capacitor (20000 µF per channel per Denon; some English sources describe twin 10,000 µF per channel, which amounts to the same reservoir). The front panel is machined from a high-grade aluminum extrusion. The whole thing weighs 17 kg per Denon's figures. The 12.8 kg figure in circulation doesn't survive contact with the manufacturer's documentation.
Key Features
SEPP Output Stage with Emitter-Ballast Transistors
Denon's literature credits the output devices as newly developed E.B.T. (emitter ballast transistor) power transistors, said to switch at around 100 MHz, roughly ten times the speed of conventional devices of the period. That is where the high-frequency distortion performance comes from. The amplifier is built as ICL and OCL DC-coupled stages, chosen for transient response and phase behavior. This is the part of the PMA-850 that owners talk about: it doesn't sound like a mid-70s transistor integrated, and the reason is in the output stage, not the spec sheet.
Cartridge-Impedance-Sensing MC Head Amp
This is the PMA-850's most unusual feature, and it separates the amp from almost everything else in its price class. Denon built a moving-coil head amplifier into the phono section, using newly developed ultra-low-noise transistors in the first stage. Its gain is inversely proportional to the impedance of whatever cartridge is connected. The head amp output is automatically equalized as a result, so the owner doesn't have to re-trim the volume control when swapping a low-output MC cartridge for a higher-impedance one. For a listener running a DL-103 or similar, that means no outboard step-up transformer, no separate pre-preamp, no fiddling.
The phono section itself is discrete-FET based with both MM and MC inputs. Denon's specs give the MM input at 2.5 mV into 50 kΩ with an RIAA deviation of ±0.2 dB or less from 20 Hz to 20 kHz, and the MC input at 0.37 mV at a cartridge impedance of 40 Ω. Signal-to-noise is quoted at 89 dB or better on MM and 74 dB or better on MC (IHF, A-weighted, input shorted). There are three phono inputs total - Phono 1 and 2 for MM, Phono 3 for MC - and the Phono 3 terminal is gold-plated.
Direct Coupling Switch
A "Direct Couple" switch on the front panel completely disconnects the control amplifier section, including the tone controls, and feeds the equalizer amplifier's output straight into the power amp. Denon's brochure makes the pitch explicitly: this is for the hi-fi purist who wants an exclusive preamplifier without filters or controls, so the signal path takes no phase-turning detours. Bass and treble are ±8 dB at 100 Hz and 10 kHz respectively when in circuit. There's a loudness contour of +7 dB at 100 Hz and +6 dB at 10 kHz, plus a subsonic filter at 20 Hz (12 dB/octave) and a high-cut filter at 9 kHz (12 dB/octave). Muting is -20 dB.
PCC — Denon's Crosstalk Cancelling Computer
The PMA-850 carried Denon's PCC (PCC) circuitry, a computing circuit that attacked the crosstalk generated inside the cartridge itself. The crosstalk components are measured separately for each channel and subtracted from the wanted signal, and the result is an audible improvement in channel separation. The catch is that the circuit has to be aligned to the specific cartridge in use. Four trim potentiometers and a test record are required to set it up per cartridge. Owners who never bothered with the alignment are running the feature uncalibrated, so ask about it before buying a used example.
Dual Mono Power Supply and Symmetrical Layout
Every assembly in the PMA-850 is laid out symmetrically to keep the two channels as identical as possible and to minimize interaction between them. The mains transformer carries separate windings for left and right, each channel has its own rectifier, and the power supply ground uses a dual bus line. Denon's service documentation also notes that the toroidal transformer is fitted with multiple silicon-steel shields inside its case. They keep leakage flux, which degrades S/N, away from the audio circuits.
Inputs, Outputs and Switching
Denon's literature lists Tuner, Aux, Tape 1 and Tape 2 at 320 mV into 40 kΩ, with pre-out at 1 V rated (20 V maximum into a 50 kΩ load) and record-out at 320 mV rated. Maximum allowable input at 1 kHz is 200 mV on MM and 30 mV on MC. Speaker terminals accept 4-16 Ω on A or B, and 8-16 Ω with A+B engaged. There are switched and unswitched AC outlets on the back - one switched at 100 W, two unswitched totalling 250 W. Power consumption is 215 W. HiFi-Stereo (1980, p.75) included the PMA-850 in an integrated amplifier roundup and noted its tone control defeat, dual tape monitor loops with dubbing, and phono 1/phono 2/tuner/aux input selection. That is the same feature set Denon's own brochure describes.
Historical Context
Denon's documentation frames the PMA-850 as a product developed by integrating the company's accumulated technology for the fundamentals of audio playback: transmission characteristics, dynamic range, distortion, and crosstalk. It sat above the PMA-730 and below the PMA-890DG in the lineup, and it was released earlier than both the PMA-830 and PMA-630. A European owner's comparison notes that the 850 should be "faster" than its switchable sibling, and expresses puzzlement that it cost 2500 DM against 1950 DM for the 830. The documentation doesn't explain the price gap.
The model is easy to confuse with the PMA-850II, which is a different amplifier. The original PMA-850 has no record-out selector and no Class A mode. The PMA-850II adds both, deletes the muting switch in favor of a Class A/B function, and is considerably rarer. A European variant, the PMA-850V, exists as a separate model with undocumented differences. Buyers searching for "PMA-850" on auction sites will regularly turn up 850II listings and vice versa.
Collectibility & Value
Reception at the time was mixed on looks and warm on sound. Thevintageknob.org notes that the series' appearance turned a few customers away - "way too serious looking" for the period - but that once the amplifier warms up, the reaction changes entirely. The site's verdict is that these are healthy units built to make music for decades, "pretty rare nowadays." Original pricing ran ¥109,000 in Japan at release, 2500 DM in Germany, and $540 in the US market.
Current market data is thin. One eBay listing for a PMA-850 has been seen at $550, and a European example described as "in good working order, 85+85 WRMS Vintage 1978" has appeared for sale without a captured price. There is no documented pattern of common failures, no established repair procedure, and no spare-parts availability information. A 17-page service manual circulates as a PDF, and it's worth having before attempting anything inside the chassis. The PMA-850II is the one collectors chase hardest. The original 850 is the more attainable way into the same circuit family.
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