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Old 5th December 2012, 06:36 AM   #11
Mooly is offline Mooly  United Kingdom
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Originally Posted by engatslo View Post
I found it, but it dosent look so simple
105kHz is for erase head and 60-200 kHz is for recording. I think you can get better performance with a discret op amp.
Your not understanding how the record process works

The AC bias is derived from an oscillator and "mixed" into the signal fed to the record head. Without bias of some sort (AC or DC) the record process is non linear. All you would hear on playback is distorted peaks of the signal.

Remember the AC bias isn't processed at all by the audio circuitry. It is purely an HF current added to the head to linearise the record process.

The same bias frequency is used for erase too. Its a fixed frequency
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Old 5th December 2012, 08:05 AM   #12
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The recording head should be fed from a current source (or through a series resistor), and the frequency response of recording amplifier is flat. In some cases there is some boost at 15 kHz or so for compensating the head gap HF loss. Bias is applied directly at the recording head, after the aforementioned series resistor.
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Old 5th December 2012, 03:36 PM   #13
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I know how it works.
Click the image to open in full size.

I heard that you can also use a permanent magnet for erase. I read some service manuals and they often have a 3 way switch for amplitude of the bias.

normal 13V
low 12V
high 15V
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Old 5th December 2012, 04:16 PM   #14
Mooly is offline Mooly  United Kingdom
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Low quality portable audio (radio cassette players) often used a permanent magnet type erase head. They work but the erase is "noisy" meaning if you play an erased tape in a high quality unit the background noise is obvious. Play a tape erased with AC erase and its much quieter.

The bias for the record head is a current rather than a voltage as oshifis explained. So yes, its quite possible to have a switched bias scheme for normal/chrome/metal tapes as these all need different bias. Switching the voltage to the bias oscillator is another way of altering the amplitude if using AC bias. Like permanent magnet erase, DC bias is noisy compared to AC bias.
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Old 5th December 2012, 04:19 PM   #15
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I believe that switch is a small frequency change to prevent beat from AM recordings. I fact it adds capacitors to the bias oscillators.
Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001.
Argentine Ham Radio LW1DSE since 1987.
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Old 5th December 2012, 04:54 PM   #16
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Click the image to open in full size.

found something
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Old 5th December 2012, 08:55 PM   #17
x-pro is offline x-pro  United Kingdom
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My advice: find a good quality 3-head cassette deck (can be bought very cheaply) and use the deck's electronics together with externally placed heads and tape transport. Service manuals for most decks can be downloaded for free and that would help too.


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