1970s Design Indulgence

Best connected to the rec out and 3rd head input on block diagram.

Many years ago, part of my day job was to look after several Tandberg "mono" reel-reel machines. I say "mono" but the heads were two channel, but one half wasn't used, and instead its output was taken to a socket simply named "tape head output". The replay preamp (for the half of the head in use) was the "Dinsdale" two transistor stage and very similar to the circuit being developed here, except for the extra two (output buffer) transistors. The reason I know this, is because I took one apart to find out.

Now, the faults with the two transistor circuit are many. For a start it doesn't do a really good transient response, but that's at really high frequencies, but tape doesn't do really high frequencies because of the head gap and also it's inductance, so it mattered not.

But most folk my age listened using the two transistor stages without knowing, and most I talk to were happy enough with the sound, and in some ways far more happy than with modern circuitry. The other great thing is it can't be "chip rolled" (hooray!)

And who cares that it can't do 120dB S/N? Except perhaps the specification nerds, but what would they know about music, and emotion? Obviously nothing! Reminds me of the Blanks on World's End... 

Using a 1k2 dropper and 470uF decoupler, the supply voltage using the PSU1 (24V regulated output) is 22.6V, which is close enough to the circuit diagram value of 22.5V.

The DC output should be around 13V, but started at 16V and now is 14.4V, indicating initial capacitor leakage current, which will, given time, decay to a minimum, and will hopefully reach around 13V.

13V allows an r.m.s. output swing of 6.3V, which should be adequate for signal headroom.