[Fogbrain Freefairy]

Thanks for the info. I had no previous knowledge of the principles you used.
@Ted Fletcher wrote:

The TBU2 had auto correction for both level and phase, and I believe this was where I started to use a partial ducking circuit.

Was the ducking in addition to ‘balancing’ (as in a pair of hybrid transformers or the electronic equivalent)?

It worked reasonably well, and was a competitor of the Studer unit.

IMHO a tin can on a string would have been a good competitor to the Studer! (No insult to the Alice TBU intended!). By co-incidence I have recently opined on the Studer in a thread on a broadcast engineering forum – naff balancing, no on/off hook switching, XLR for phone line connection, ect. I have only worked with Studer TBUs at one station, which in another co-incidence, was also the only station at which I have worked with an Alice TBU.

In the early 80s I used a more sophisticated version of the TBU2 with very fast acting balance, in communications systems for money brokers. These multi-line ‘broadcast’ systems were very successful until everything went digital!

Ironically, fixed balance TBUs really came into their own once the whole inland PSTN had gone digital. The SLICs in the exchanges are very good at balancing and thereby preventing echoes getting back into the network. The TBU only has to deal with the local line and the reactance of this doesn’t vary so once the R and C have been correctly set they work very well. International calls are obviously a bit hit and miss, and for calls to/from mobiles even an ISDN codec on G.711 can’t touch a new digital TBU. Phone lines have become so good that they can be used with simple TBUs, but the TBUs have become so complex that they can be used with poor phones lines – the technologies rushed towards each other, but missed and continued hurtling in opposite directions!

I’m afraid I don’t have any pictures of these old units.

I have little doubt I had the A4 glossies. Moved house several times since, but they may still come to light! (Virtually certain I have one for a 12.48).

Thanks again for the reply.

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