Specifications
Specifications for the original first-generation MX-5050 ("Mini Pro"), with sources noted — this page prefers gaps over guesses. Not sure this is your machine? Start with Is it a Mini Pro?.
Where the factory and the reviewers disagree, both numbers are given. They usually aren't really disagreeing — they're measuring different things.
The machine
| Parameter | Value |
|---|---|
| Tape width | ¼″ |
| Reel capacity | 12.7–26.7 cm (5–10.5″), NAB or EIA hub |
| Tape speeds | 7.5 / 15 in/s on the 2SHT; the line also offered a 3.75 / 7.5 pairing |
| Track formats | 2-channel, 4-head — erase, ½-track record, ½-track playback, ¼-track playback behind a changeover switch. That is the 2SHT: one of eight configurations the line offered on quarter-inch tape — full-track, ½- and ¼-track monaural, ½- and ¼-track stereo, and ¼-track four-channel (Electronics Australia, June 1976). Otari's suffix digit counts tracks, not channels: the US catalog's 4SH is a ¼-track two-channel deck, not a four-channel one. The half-inch MX-5050-8 and the one-inch MX7308 are different tape widths and sit outside that eight — the eight-track has a history of its own |
| Capstan motor | AC hysteresis-synchronous, driving the flywheel by flat belt (2 + 0.5 µF run cap). A DC-servo capstan was a factory option from the launch — Otari's first ad offers an "optional DC capstan servo system" (db, September 1974) — but the named DC variants come later: the ±10 % varispeed 2SH-2 in the 1976 catalog, and the 2SD/4SD of the factory maintenance manual. The DC machines are true direct drive: in the maintenance manual's parts lists, the AC machine's capstan and flywheel are a single part (TC-423-001) turned by a belt, while the DC list has no flywheel and no belt at all — the servo motor's shaft is the capstan |
| Reel motors | Two 6-pole AC induction motors (5.5 µF run cap each) — mine are TOWA, 250 WV.AC, dated 1977 |
| Dimensions & weight | 21.5 in W × 21 in H × 17 in D with 10.5″ reels (Audio); from 24 kg in the standard cabinet to 34 kg for the heaviest portable (Electronics Australia); Otari's own S-111 sheet: 21⅝ in H × 21⅜ in W including reel overhang, 7⅝ in D bare (the portable case's covers bring depth to 14½ in), 50 lbs in the cabinet, 62 lbs in the portable case |
Performance: the factory's own figures
These come from Otari's maintenance manual for the 2S/4S/2SD/4SD — the tolerances a technician is expected to hold the machine to, transcribed in full on this site at Performance Tests. They are the most authoritative numbers here, and they are stated per model, because a ½-track machine and a ¼-track machine are not held to the same standard.
| Parameter | 2S / 2SD | 4S / 4SD |
|---|---|---|
| Signal-to-noise, 15 or 7½ in/s (ASA weighted) | 68 dB | 65 dB |
| Signal-to-noise, 3¾ in/s (ASA weighted) | 67 dB | 64 dB |
| Crosstalk, adjacent channels | 60 dB | 55 dB |
| Parameter | Specification |
|---|---|
| Flutter and wow, 15 in/s | 0.05 % (NAB weighted) |
| Flutter and wow, 7.5 in/s | 0.06 % (NAB weighted) |
| Flutter and wow, 3¾ in/s | 0.15 % (NAB weighted) |
| Speed accuracy | 3,000 Hz ± 0.2 % (a 2,994–3,006 Hz reading on the 3 kHz test tone) |
| Erasure | better than 65 dB |
| Second-harmonic distortion | not to exceed 0.5 % |
| Equalization | NAB (50 µs + 3180 µs at 7.5 and 15 in/s; 90 µs + 3180 µs at 3¾), with IEC alignment tapes listed as the alternative (35 µs at 15 in/s, 70 µs at 7.5) |
What Audio measured on the bench
Audio, April 1976, put a US-market 2SH on the bench (transcribed here). These are measured results, not specifications:
| Parameter | Audio measured |
|---|---|
| Signal-to-noise | −66 dB, ANSI "A" weighted, referred to 0 VU (−75 dB at the 3 % distortion reference some makers used) |
| Wow and flutter | 0.07 % at 7.5 in/s, 0.06 % at 15 in/s — DIN weighted |
| Crosstalk | −58 dB |
| Erase efficiency | better than −60 dB |
| Frequency response, record–playback | −3 dB at about 18.5 kHz at 7.5 in/s; at 15 in/s the −3 dB point moves up to 28 kHz |
| Headroom | 3 % distortion not reached until nearly +9 VU |
What the sales sheet promised
Otari's own sales literature is now on this site: Specification Sheet S-111, effective August 1976, covering the FL, 2S and 4S. It fills the gap the maintenance manual leaves — the marketing figures. These are promises to a buyer, not test tolerances:
| Parameter | S-111 says |
|---|---|
| Line input | −15 dBm minimum, unbalanced 50 kΩ; balanced 600 Ω with optional transformer |
| Microphone input | −70 dB minimum, unbalanced, nominal 50 kΩ; balanced 150/250 Ω with optional transformer |
| Line output | variable, or fixed +4 / −10 dBm on a rear-panel switch; headroom +19 dBm before clipping |
| Frequency response | 15 in/s at 0 VU: 50–22,000 Hz ± 2 dB; 7½ in/s (at 0 VU −10 dB): 50–18,000 Hz ± 2 dB; 3¾: 50–12,000 Hz ± 2 dB |
| Signal-to-noise, two-track | 68 dB weighted / 65 dB unweighted at 15 and 7½ in/s — and the sheet states its reference: 520 nWb/m record level against biased Ampex 456 / 3M 250 tape |
| Crosstalk | greater than 60 dB, adjacent tracks at 1 kHz |
| Distortion | less than 1 % at 1 kHz at 200 nWb/m |
The weighted signal-to-noise figure matches the factory tolerance above — the sales department and the service department, for once, agreeing — and the sheet is unusually honest for sales paper: it names its reference fluxivity and even its reference tape.
S-111 is now the middle of three sales sheets on this site — the September 1975 tri-fold came before it, the April 1978 re-set after — and the printed numbers moved between them:
| Parameter | 1975 (B-103A) | 1976 (S-111) | 1978 (S-117A) |
|---|---|---|---|
| Signal-to-noise, two-track 15 ips | 68 dB weighted / 65 dB unweighted | 68 / 65 dB | 65 / 63 dB |
| Signal-to-noise reference | "1000 Hz reference" on 3M 250 / Ampex 456, unbalanced output | 520 nWb/m, Ampex 456 / 3M 250 | 520 nWb/m, 3M 206 / Ampex 456 / 3M 250 |
| Crosstalk, two-track | greater than 60 dB | greater than 60 dB | greater than 55 dB |
| Frequency response, 15 in/s ±2 dB | 35–22,000 Hz (no level stated) | 50–22,000 Hz at 0 VU | 50–22,000 Hz at 0 VU |
| Flutter & wow, 3¾ in/s | 0.15 % | 0.15 % | 0.12 % |
| Distortion reference | 200 nWb/m | 200 nWb/m | 185 nWb/m |
| Varispeed (dc-servo option) | ±10 % | — (not stated) | ±7 % |
| Cabinet weight, two-channel | 50 lbs (22.7 kg) | 50 lbs | 53 lbs (24 kg) |
Resist the obvious reading that the machine got worse in 1978. The 1978 sheet is the only one that names its measurement basis — Ampex NAB alignment tape and 3M 206 — and it moved the distortion reference from 200 to 185 nWb/m; the 1975 sheet doesn't even state a reference level for its response figures. These are the same lesson as the weighting-filter trap below: before comparing two of these numbers, ask what each one was measured against. What did change in print, plainly: the four-channel QX has its own S/N columns in 1975, is a name-drop by 1976, and is gone from the 1978 sheet — whose head list carries only the FL and the 2S.
The three signal-to-noise numbers
This is the figure most often quoted wrongly, including — until recently — on this page. Three different numbers circulate for the same machine, and all three are honest. They differ because of what filter sits in front of the meter:
| Number | What it actually is |
|---|---|
| 62 dB, unweighted | Otari's own published specification — this is the figure printed in the manufacturer's-specifications table of Audio's review, and it is what most catalogues repeat |
| 68 dB, ASA weighted | Otari's factory tolerance for the 2S/2SD in the maintenance manual (Table 3-1); 65 dB for the 4S/4SD |
| −66 dB, ANSI "A" weighted | What Audio actually measured on a real deck |
A weighting curve is not a rounding error. An unweighted figure and an A-weighted one describe the same noise floor through different filters, and a few decibels of apparent difference is the filter talking, not the machine. Always ask which weighting a number refers to before comparing this deck to anything else — and be especially wary of any comparison that doesn't say.
The same trap sits under the flutter figures, and it is why this page will not tell you whether Audio's deck met the factory tolerance. Otari's Table 3-3 is measured NAB weighted — the manual's own procedure says to set the flutter meter to the NAB position — while Audio reported DIN weighting. Those are different curves. Putting 0.06 % NAB and 0.06 % DIN side by side and declaring a verdict would be exactly the error this section warns about.
What a fifty-year-old deck actually does is, in any case, a question about that deck rather than about the model — which is what the performance tests are for.
Sources
- Otari's maintenance manual for the 2S/4S/2SD/4SD — the factory tolerances above, from Tables 3-1 (signal-to-noise), 3-2 (crosstalk) and 3-3 (flutter and wow), plus the erasure, speed and distortion steps. Transcribed in full: the digital edition.
- Audio, April 1976 — the bench measurements, on a US-market 2SH.
- Electronics Australia, June 1976 — the review of the 2SHT, the exact designation on my deck's plate, and the source of the weights.
- The 1976 Stereo Directory — the catalog listing and prices.
- Specification Sheet S-111 — Otari's August 1976 sales sheet for the one- and two-channel machines: the marketing figures and suggested list prices, transcribed in full.
- The 1975 sales brochure (B-103A) and the April 1978 re-set (S-117A) — the sales sheets either side of S-111; the figures that moved between the three printings are tabled above.
Every one of those period sources is mirrored in full on the Period Press page.
The document wish-list this page carried is now empty: the sales spec sheet arrived in August 2026 (its marketing figures are above), and the operation manual (Otari's OIM-1005E) surfaced on paper the same month — an original, scanned and now transcribed in full. Its own specifications table, Table I-6, mostly corroborates rather than adds: the same 68/65/67/64 dB weighted signal-to-noise figures as the maintenance manual's Table 3-1, the same 0.05/0.06/0.15 % NAB-weighted flutter as Table 3-3, and the same −15 dBm line input and +19 dB headroom already printed from S-111 above. On the one row where the sales sheets disagree with each other, it takes a side: its frequency response prints the 1975 sheet's 35 Hz low bound at 15 in/s (and 30 Hz at the lower speeds), not the 50 Hz of the 1976 and 1978 sheets — and it settles, from the factory's own operator-level book rather than a magazine table, that the 62 dB unweighted signal-to-noise figure in "The three signal-to-noise numbers" is genuinely Otari's own published number, not just Audio's secondhand copy of it. What it actually adds: a headphone-jack spec printed nowhere else on this site (−24 dBm, nominal 8 Ω unbalanced) and the bias-and-erase oscillator frequency (133 kHz) — both small enough to note here rather than build new rows for; read them in full in Table I-6 itself.