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What is the RPM of a reel to reel recorder?

  • I just realized that reel speeds change but the capstan which is located between the two reels that does the traction and pulling the tape at certain IPS governs all the tape mechanism while the reels just adapt to it...the reason I was confused about this rpm speed was that I thought the capstan was the pivoting mechanism of the collecting reel...thank you all for all the input and guiding me to the right direction. – texture Aug 27 '15 at 10:38
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Audio tape speeds are measured in ips (inches per second) or in cm/s (centimeters per second), not RPM (i.e. there is no typical/common RPM).

The actual RPM of the reels changes with the amount of tape. The speed of the tape is not controlled by the reel motors them selves, but rather the capstan motor, which then ensures a constant speed (and typically the reels simply follow).

On the capstan a roller of varying diameter from model to model is mounted - typically around 1 - 2 inches, which means somewhere between 4-5 inches of tape is transported per round (1.5 * 3.14159 = 4.7). If the speed is e.g. 30 inches per second, then that gives 30 / 4.7 * 60 ~= 383 RPM for the capstan. The reels are harder because many different reel types exist, with varying starting inner diameter.

Some common speeds are

  • 120 ips (304.8 cm/s): Loop bin duplication, analog instrumentation
  • 60 ips (152.4 cm/s): Loop bin duplication, analog instrumentation
  • 45 ips (114.3 cm/s): Digital audio recording
  • 30 ips (76.2 cm/s): The highest standard professional speed.
  • 15 ips (38.10 cm/s): Most common speed multi-track pro studio recording and otherwise professional speed for reel-to-reel including multitrack recorders.
  • 7-1/2 ips (19.05 cm/s): Most common pre-recorded playback speed, highest domestic speed
  • 3-3/4 ips (9.53 cm/s): Second most common pre-recorded playback speed
  • 1-7/8 ips (4.76 cm/s): Lowest common reel-to-reel speed

A more comprehensive list of different tape speeds and their main application can be found here. Additional info on the reel-to-reel tape recorders, tape speed and fidelity can be found here.

  • So I guess we are looking at the average angular speed of the sum of two reels constant changing RPMs, even though they change their ratio one to another stays the same to keep a constant IPS on the reader head...since one gets faster because its circumference getting bigger, while the second feeding reel gets smaller and slower in RPMs to compensate, the two reels keep an average constant RPM so the IPS always stays the same at the magnetic player head...my question is what would be that average RPM? – texture Aug 27 '15 at 7:25
  • Depends on how much tape is on the reel & how large the reel centres are. You're actually asking a question which is the equivalent of "how long is a piece of string?". It's something no-one has ever considered before; so long as the reels keep sufficient back-tension to prevent tape-slack, no-one has ever cared before either. – Tetsujin Aug 27 '15 at 8:24
  • Alright - you should probably update your question to reflect this. I added some additional information about the capstan role and speed. – Michael Hansen Buur Aug 27 '15 at 8:30
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7.5, 15, 30 are all common. Perhaps there are 3 3/4 ones also. There is a wide range of reel to reel type recorders. If your asking about a specific one you would need to provide some more info.

  • are you sure you are not talking about IPS inches per second? if so how does it relate to RPM...one is inches per second,and other is revolutions per minute, I guess RPM is angular speed, just like hertz...I know we can divide the capstan circumference by the ips to find how many seconds it takes for the reel to complete one revolution or period then take the reciprocal of the period and multiply it by 60 seconds for RPM... – texture Aug 26 '15 at 21:29
  • I am, sorry I read your question incorrectly. – coaxmw Aug 26 '15 at 21:30
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As the tape speed in front of the heads has to be constant (in ips as mentioned), there's no constant RPM for the reels as it is continuously changing as the reel empties or fills.

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