Setup, signal path:
Male singing voice close to the mic - Pleiades V5 or V6 - Sony TC-D5 Pro - Sennheiser HD580
The V5 is a 2 stage RC coupled pre preamp with battery operated EF183s and the output potential divider arrenuated so that it can feed a mic input. As would a condenser internal preamp driver a mic input. The V5 has no output transformer. And the inducatnce of the 1:10 input transformer is just 47mH. 47mH was chosen so that the low cut introduced would compensate for the proximity effect of mics such as Unidyne III, MD441 U3, RE-15, D112 etc.
The V6 which is one stage battery powered EF183 with transformer input (140mH) and output transformer. Both transformers are 1:10. The inductances of the output transformer are greater, 800mH:80H.
The V6 sounded much brighter which was very nice.
The V5 sounded fuller apparently because the 2nd tube is driven into some instanteneous peak limited.
A reason for the V6 brightness may be the output transformer introducing a gentle 5dB per octave bass roll off below about 100Hz.
Don't let the 100Hz scare you, the V6 sounds as full on bass as can be with the Grampian DP4/L and the Sennheiser MD21. In fact the 140mH input introduces a much higher cutoff with the 200ohm mic impedance, otherwise the MD21 sounds bass heavy for close and not very loudly sung vocals. This cutoff is at about 200Hz. With the Grampian this cutoff is at 8? times less i.e about 25Hz as it is a 25ohm. 8 is found by dividing 200ohms by 25ohms.
References:
V5, V6 schematic
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