Tape Head2Tape Head Saturation Box Help...

[abechap024]

on my workbench is a project that I'm fooling around with. I put 2 tape heads together to try to get a box that can color the sound. So far its working except there is no bass. Its high passed around 700hz and I've checked all the circuitry and located the problem to the two tape heads, all the other circuitry is passing 20hz to 20 khz (I've disabled all the pre-emphasis circuitry for the tape). So my question is... I know Ii'm doing a perverse thing by putting the heads together but why aren't they passing all frequency? Thanks in advance!
AC

 

[abbey road d enfer]

The transfer between "record" head and "playback" head is done via induction B. If you feed an inductance (that's what a tape head is) with a constant voltage level, the impedance at low frequencies is very small and the current increases, as the induction, at a rate of -6dB/octave. At the playback head, if the induction was constant, the voltage would increase at 6dB/octave. But since the current increases at low frequencies, the compensation is perfect.
Except that nothing is perfect.
The driver stage may not be capable of delivering the current at low frequencies. Or very simply, in order to deliver enough current, the source impedance of the driver must be significantly lower than the head impedance at LF. And the magnetic material of the head would saturate.
In fact, in tape recorders, the record head is current driven, with one of the following methods:
- True current generator (Studer)
- High impedance driver stage (very often with a simple resistor in series)
- Low impedance driver stage with 6dB/octave pre-emphasis
As a result, the playback head shall deliver a reasonably flat response.
Still, there are HF losses that need compensating (some are due to magnetic tape, they should not be of concern to you).

 

[abechap024]

Thanks!

 

[Kingston]

Also know that you're not going to get soft saturation from the tape heads alone, more like hard clipping and ring modulation-like nasty artifacts. Tape saturation comes entirely from the media itself and the tape heads alone have more headroom above the actual media saturation point. But the head clip might still make a cool distortion effect.

 

[abechap024]

I've abandoned the project. While it did have some very cool distortion/clipping - IT seemed to almost "launch" the transients out of the speakers. But still just a distortion box. and I couldn't get it to pass L/F. The heads were from a machine used to record speech (8 track quarter inch!!!) I don't know if they were designed to be able to handle Low F. it was becoming a PITA. Thanks