Shure Unidyne III with Pleiades V-1 with Altec 1566A

The green input transformer with octal base of the Altec 1566A preamplifier has a primary winding inductance of about 300mH.


This together with the mic output impedance create a low cut filter which compensates some of the proximity effect (Pleiades filter). Still at 4-6 inches the sound was bass heavy but at 9in - 12in it was very natural. The volume control of the Altec had to be at max. There was some buzzing noise (due to lack of mic grounding in the cable used?, it disappeared when the mic was touched, has to be looked at). And some hiss.


When connecting the Pleiades V-1 prepreamp the sound was a few times bigger. Optimum mic distance was at 9in. There was much less buzzing problem even when the mic was not touched and less hiss. More background ambient noise could be heard as on condenser mics but not too much as the mic discriminates greatly off axis. The volume control of the Altec was at about 6-7 and the male voice sounded a few times as loud and big without being bass heavy.


The Pleiades V-1 is an electron tube (a single triode section of 7025) prepreamp operating with one 9V battery for the anode (plate) and a 9V battery regulated to 5V? for the heaters. The Plate battery is fully inserted with a Bulgin battery holder drawer all the time as there is no power consumption until the heater is on. The amplifier starts operating some seconds after the 9V heater battery is fully inserted inside the other Bulgin drawer. Sound starts to gradually build up as on condencer electron tube mics, noise going down while signal goes up until the hiss almost disappears and ambient acoustic noise can be revealed. There is no few megohms resistor from anode to grid as on later Pleiades prepreamps. There is a cathode resistor of low value optimized by starting life as a variable multi turn wire wound resistor.


The Pleiades V-1 must had been built at around about 2000. It is open source, public domain and more details and component values can be found on other euroelectron posts.


The input transformer is a high inductance (1:7?) Sowter and the output transformer is a lower inductance military transformer doing much of the processing for flat frequency responce from singer's brain to listener's brain. It compensates for Fletcher-Munson, voice effort, proximity effects.


Setup, signal path:


Living room with some broadband panel absorbers - male voice singing the end phrase of Words - Bee Gees - Shure Unidyne III at 9in - (Pleiades V-1 on the second experiment only) - Altec 1566A set to input accepting 200Ω mics and 600Ω output impedance - Sennheiser HD-580 (there is some output mismatch since each Sennheiser is 300Ω and they are bridged to mono ie 150Ω but they sound great, at some stage the 150Ω Altec output with be configured for comparison)


Reference:
Flat frequency responce from singer's vocal chords to listener's brain, Sound Picture Recording and Reproducing Characteristics - Loye, Morgan - Journal of the Society of Motion Picture Engineers