Tape recorder bias frequencies

The discussion of what bias frequencies were used over time keeps recurring. Special thanks to Jay McKnight of Magnetic Reference Lab, Tom Fine, and Brian Roth  for input to this list. I posted this to the ARSC list, but wanted to include it here as well. This knowlege is useful for those who wish to archive the bias along with the audio for future application of time-base-error correction tools such as the Plangent Processes.

Bias frequences started low, apparently with 60 kHz for early consumer recorders, but Ampex started with 100 kHz. Other later machines used different bias and erase frequencies. We can see with a few exceptions, the top bias frequencies were commonly limited to 250 kHz for audio, with the Sony APR series and the Ampex ATR series in the 400 kHz region. For cassettes, a practical maximum appears to be about 150 kHz. Much higher frequencies (up to at least 8 MHz) were used in instrumentation recorders. An enumeration of several machines follows.

Early Wire and Reel Audio Tape Recorders

In the early days, apparently, wire recorders used bias as low as 30-40 kc, but Jay McKnight recalled in the pre-Ampex days, 60 kHz was common.

Ampex
The Ampex 200A used a bias frequency of 60 kHz [Jay McKnight 2012-03-25 post to Studer List] He indicated that this was probably the lowest for any professional recorder.

The Ampex Standard was 100 kc up to the MR-70.

With the MR-70, Ampex switched to 150 kHz bias frequency (and adopted the Hz) [Larry Miller, ex Ampex]

Ampex AG-440 (A) stayed with 150 kHz [manual]

Ampex ATR-100: 144 kHz erase, 432 kHz bias (1:3) [manual]

MCI-Sony
MCI JH-24 Multitrack: 210 kHz bias, 105 kHz erase [manual via Brian Roth]
Sony APR-5000, APR-24: 100 kHz erase, 400 kHz bias (1:4) [manual]

Otari
Otari MTR-10/12 II: Bias 250 kHz (erase not spec’d) [manual]
Otari MTR-90 (original): 246 kHz bias, 123 kHz erase [manual via Brian Roth]
Otari MX-80 (c. 1986) 192 kHz for bias and erase [manual via Fred Kaeppel]

Scully
Scully 280 (original, Bridgeport, CT): 60 kHz erase, 180 kHz bias [datasheet – added 2021-03-03]

Studer
Studer A80VU (All versions): 80 kHz erase, 240 kHz bias (1:3) [manual]
Studer A80 RC: 150 kHz [manual]
Studer A810, A807, A820 2CH, A820 MCH, A827: 153.6 kHz [manual]
Studer B67; ReVox A700, B77, PR99: 150 kHz [manual]
ReVox A77: 120 kHz [manual]

Tascam
Tascam 32/44-OB: 150kHz [manual via Tom Fine]

Technics
Technics 1500/1506/1520: 120kHz [manual via Tom Fine]

Cassette Audio Tape Recorders
Here is a quick sampling of published bias frequencies for two top-of-the line cassette recorders, a better-than-average portable, and an early compact portable.

Nakamichi Dragon (Along with the Nakamichi CR-7A, perhaps  the finest machines ever made for overall audio quality): 105kHz [Service manual dated 1985 (scan) 1990 (copy)]
Nakamichi MR-1: 105kHz [manual via Tom Fine]

Studer A710 (a high-end cassette recorder, without the auto-azimuth that makes the Dragon superior): 150kHz [no date, scan on Studer ftp site]

Sony TC-D5M (a workhorse, good quality stereo portable): 85 kHz [Svc Manual dated 1980]

Sony TC-55 (an early compact — jacket pocket — mono portable): 41kHz (as low as I’ve ever seen) [Svc Manual dated 1972]


Reel Instrumentation Recorders
Honeywell 101 Medium Band: 4 MHz, Wide Band: 8 MHz [1977 manual] (note that the direct recording bandwidth on Medium Band was 600 kHz and 2 MHz on Wide Band at 120 in/s)

Racal Store / DS (Dual Standard) series: 1.2 MHz [1982 manual] (note that the direct recording bandwidth was 300 kHz at at 60 in/s, the same wavelength limit as the Medium Band version of the Honeywell 101)