A great sound from the Unidyne III (a passive amplifier)

This was done by connecting it as follows:


Shure Unidyne III - Pleiades (130Ω, 40mH) brain perception EQ - Altec 4722 input transformer - Pleiades 10:1 output transformer - Sony TC-D5 Pro - Sennheiser HD580


The output transformer is the one used on Pleiades V6. See relevant post.


It was by this experiment that it was found it sounded so much better. By bypassing the electron tube inverting stage of the Pleiades V6 (removing batteries and short circuiting the Pleiades resistor) it sounded focused and brilliant.


The above setup is a passive amplifier as the Altec 4722 has a higher turn ratio than 1:10.


By connecting the Unidyne III directly to Sony mic in, the sound output is less, sensitivity is reduced, the Sony gain has to be increased and hiss goes up.


The inductances of course of the cascaded transformers may play a proximity compensation role too. In fact we have 2 cascaded transformers including the Unidyne III internal transformer.


Of course the passive amplification can be done with just a small ratio (1:2?) transformer increasing the mic' output impedance from 200Ω to 800Ω. But it is fun with 2 transformers. And the sound is very nice proving again how miraculous devices transformers are.


By gently cutting perhaps bass and treble the sound becomes immidiately more focused and closer to flat frequency response from producer's brain to listener's brain.


On a microphone system 3 effects are at least into play. Proximity, Fletcher Munson and Voice effort curves. Also the pressure doubling effect at HF on uncompensated omnis.


See the classic D. P. Lowe, K. F. Morgan paper on JSMPE


See also the previous euroelectron post on operating the Pleiades V6 one electron tube head amp without an electron tube.


See also on active active options the Pleiades V6 and Pleiades V1.





http://euroelectron.blogspot.com/2018/07/a-one-tube-microphone-headamp-operating.html


Next day addition: Some impedance measurements at 1028Hz made with Escort LCR ELC-120 digital meter:


Real part of Impedance of Unidyne III = 236Ω


Real part of Impedance of Unidyne III carcaded with Pleiades (130Ω,40mH) = 145Ω


Real part of impedance of Uni III cascaded with Pleiades (130Ω,40mH) after the cascaded transformers = 524Ω


Same as above without Pleiades EQ (130Ω,40mH) = 830Ω


After removing transformer bridge link across Rag and power the Nuvistor 7586 electron tube with anode voltage 4V to 1.3V the sound becomes many times as big. The phase had to be reversed With an XLR adaptor as an anode follower phase reverses. See tomorrow's post.