AC bias circuit for a tape recorder

Question

I was wondering if I can "upgrade" a DC biased tape recorder to AC bias with a simple circuit.

I found some schematics for AC bias circuits, but they were all very different, and I'm not sure which one is suitable for tape recorders. I was thinking maybe there's an IC for that available, but again, I'm not sure on that one.

I'd like the most simple solution, which could be easily put on a daughter board and hidden in the tape recorder. I was thinking using a 70kHz oscillator, and adding it to the signal somehow (before the DC bias, as in, removing the DC bias parts).

This is just a little project to play around, nothing serious. I'm not planning to make "professional" recordings on the tape recorder that will emerge at the end of the project. I just want a proof of concept type of thing.

OK, this is how far I've got:

Consider this circuit of the LA4160: This is a screenshot of the datasheet, page 4.

Now as you can see in the lower left corner, there is a device "Bias OSC". Given the rest of the Datasheet, I might be able to locate the point where this oscillator has to go.

But how exactly do I build this thing? Or is it an IC that eludes me?

Would be nice if someone could take a look at the datasheet I've linked, and maybe point out a thing or two, if I missed something.

Now, the oscillator needs to be a sine wave, but that's pretty much all I know. I don't know which peak-to-peak level, and I'm not entirely sure what frequency. I think somewhere around 100kHz makes the most sense.

Answer 1

1. Check the type of your recorder erasing head. It's likely just a permanent magnet that saturates the magnetic tape in one direction. It's OK to record to such tape with DC bias, but I don't know how it is suitable for AC-bias recording. Anyway, it's better to replace it with a ferrite erasing head because it demagnetizes the tape and prevents it from producing low-frequency saturation noise.

2. The bias current is several times higher than the recording current and also its frequency is higher. The head's resistance is primarily inductive at such frequencies, so it makes the AC bias voltage many times higher than the recording voltage or even the amplifier's supply voltage. It means you need a separate bias generator anyway.

3. The simpliest bias generator circuits contain one transistor, a transformer, and several capacitors and resistors. An erasing head is commonly attached to the generator directly, and a secondary winding of the transformer is capacitively coupled to the recording amplifier output.

4. You need to know your recording head's inductance, maximum recording current and maximum bias current to design the bias generator.

Answer 2

It might be no more difficult than making a 70kHz signal and adding it to your "normal" record signal that feeds the tape head. It might be as simple as mixing those two signals using resistors.

On the other hand, it may not work because the "simple" resistor circuit above may cause the amplifier producing the regular record signal (the audio) to clip and distort. You may have to use a transformer to couple the two signals together and this still may cause the regular amplifier some issues.

It's a bit like when someone asks if you can fix their TV - I always reply "get me the circuit diagrams and I'll have a look (sometime)".

The bottom line is try it and see and if it doesn't work then no harm done.