G8MNY  > TECH     20.05.21 09:15z 123 Lines 6105 Bytes #175 (0) @ WW
BID : 34616_GB7CIP
Subj: Driving Inductive Loads
Sent: 210520/0901Z @:GB7CIP.#32.GBR.EURO #:34616 [Caterham Surrey GBR]

By G8MNY                                    (Updated Aug 13)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

If you have ever tried to get a tape recorder head current flat, magnetic
cutting pickup, LF scope scan coils, hearing aid loop etc, you will be aware it
is not very efficient & impossible to match over a wide frequency range.

                   Z                        RMS Volts
                100k                     .' 100
                 30k                  .'     30
                 10k               .'        10
  =====           3k            .'           3
  (((()           1k         .'              1
               300      .'                 .3
  Z=R+Xl         100___.'                    .1  
                    1   10   100  1k   10k  100k Hz

Theory says the impedance Z starts at the DC resistance & then the coil
inductance reactance Xl takes over as you increase frequency. As the impedance
gets higher at higher frequency the self & lead capacitance will resonate coil
making the total Z go very high.

So trying to design a driving stage to give constant magnetic field (or force)
is really impossible. What is normally done in the Valve era was to use a high
series resistor from a high power stage, so the constant current was fed in as
the head Z was always much lower than the series R.

                                                       Bias Osc
    )||( PLayback LS                  Input>Ĵ\.        ====
    )||( Winding          HF Record           ,>Ĵ((()Ŀ
    )||             Ĵ<  Bias          Ĵ/               )||Low Z
   Ĵ100kĴ                     Rfb           )||Record
 /~~\  0.2u         )||High Z            R  ===C               )||Read
                   )||Head              The R & C give 6dB/Octave lift
   Old Valve Circuit                  Typical Semiconductor Circuit

For tape record a large HF bias voltage is applied too. For transistorised
equipment especially battery powered using such a wasteful approach is not
used. Instead pre-emphisis is used to predict the head current & drive the head
from a lower impedance. This gives a flat response OK, but it has the problem
that higher drive voltage is not available as needed for LOUD treble, so most
simple designs suffer from treble clipping (SHUSHY treble) in the driving amp &
this is often mistaken for poor tape etc.

Many years ago I made a tape recorder from scratch using an old Valve type tape
deck & I knew there would be this problem with a semiconductor design. I came
up with this approach still using the AF amp as a drive source, but a step up
transformer that gradually came in at HF to boost the voltage available at the
treble end to give the full unclipped recording level.

                   10kĴ<HF Bias
            Rec     )||        
\  1000u    _____  )||48mH     )||HIGH Z
 >Ĵ\      _)||         )||HEAD
/          Play   )||1:10     )||
AF                          __  
AMP         LS    === C
           __    __

At bass to mid the constant current came from the series 10k, from mid to
treble the step up came in as the cap C & the tapping ratio were chosen to give
the drive required step up to maintain the full head current for tape

I have a 12" one of these, I use it for displaying a spectrum analyser adaptor.
It uses constant current power output drivers with current NFB. The whole thing
runs terribly hot, as any display offset (e.g. trace at the bottom of screen)
needs huge standing current from a low voltage supply, & at only a few 10s of
kHz the supply is far to low to give much deflection current.
             +10V @8A             Drive ._    Supply
Display Amp                          _   _            DISPLAY
   _   Input Ĵ\                    '    '              ____
         >Ŀ                  _10v            
            Ĵ/  220  1)|| Y Yoke    Clipped               
                       )|| Coil     Transiant                  
            )Ĵ                          ____      ____
                       0.1R                     Slopping sides on large
             -10V @8A   __                    verticals due to lack of volts

Putting the yoke coil in the NFB loop gives the required voltage to frequency
uplift of 6dB/Octave, but due to the coil there is a time delay that causes
instability, so the amp gain has to be reduced at HF to make this circuit work.

Unlike with audio, the signal phase is important on scopes. so no bodges really
work well. :-(

Here is another inductance load! The main problem here is not to overload the
driving amp etc. & still give plenty of treble.

 TV           Ŀ                  Equivalent Loop Circuit
 >Rs  LARGE                         L
/            ROOM                    ___()))))_______R cable____
     Ĵ    LOOP                                
        Cs                               R radiation
  _____________________                  (very little)
                                             (at audio)

Rs makes up the total DC resistance to not less than 8 ohms, & it has to handle
most of the LS amp power (1-5W). Cs (e.g. 1-10uF) can then boost some of the
treble if needed, as the loop Z goes up due to it's inductance. The lower the
loop DC resistance the bigger Rs needs to be, & the more Cs may have an effect.
If the loop is made more sensitive with several turns, it may be too inductive
(<1mH for 8 system) & treble will be reduced!

Why don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP

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