I gather that this is a discussion of the famous hybrid circuit for telephones? This is a current loop interface, one that runs in both the balanced and unbalanced domains. It is brilliant, one of the best electronic designs ever made.
It is at least 121; I have in my posession a 1892 Ericsson (well, Electric Bureau, actually, which only later became a part of Ericsson) phone that employs this hybrid circuit.
-- mrr
There he really is right. Oxford (and presumably that other place) award BA and MA to scientists and engineers as well as 'arts' people. BSc is a postgraduate qualification.
Not to anyone who is familiar with Cambridge. You get an MA for (roughly) living a few years after getting a BA, and all undergraduate courses, including science and engineering, lead to a BA.
Alan Braggins wrote
Yes, but you are not exactly "given" an MA. In the immortal words of John Lomax or Lonnie Donegan, first, "you gotta pay the man some money".
You do realise you are looking at the wrong side of the circuit?
The phone line interface is on the left!
You exactly are.
News to me.
Not really (well just possibly indirectly[1]). The argument between "The Natural Philosopher" and the rest of the group has been solely about what constitutes "Balanced Lines" within the context of the use of 2 wire connections in telephone lines over the past 113 years.
I wasn't sure about what your reference to "the famous hybrid circuit for telephones" was but I think you're referring to the ASTIC circuit that was used to reduce the excessive sidetone of the early primitive telephones and improve transmitter coupling efficiency into the line.
Searching for material relating to ASTIC led me to this pdf
which brought back memories of boring afternoons spent reading EIs and EPs as a TTA in the late 60s.
I can understand why you might be impressed by the circuitry of an ASTIC telephone. In common with most strowger vintage circuitry, it was not unusual to see components serving 2 or more circuit functions and the telephone was no exception in this regard.
[1] "The Natural Philosopher" seems to have gone rather quiet on the subject these past 24 hours. I suspect he may have revisited the subject by way of a 'refresher course' to see why his 'off the cuff' remark about PABX lines not being balanced had produced such a 'backlash'.I suppose it's just possible that your reference to "the famous hybrid circuit for telephones" might have some connection to his confusion since, to look at, the typical circuit diagram of a telephone shows no obvious indication of it being a balanced circuit.
In fact, the circuit has all the look and feel of a classic audio circuit one might find connected to a common ground rail with a bias voltage rail thrown in for good measure. It's all too easy to overlook the fact that its only external connections are the two legs of a balanced line connection to the exchange (or PABX) equipment.
For the purposes of balance, no matter how lopsided the telephone circuit might look, it's just effectly a (largely) resistive load terminating the line with no other connection, other than neglibly small stray capacitive coupling, to the local earth to disturb the balance of the line.
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