AKG D190 E to Pleiades V(-1) low cut implemented after electron tube or to V1 or to Pleiades (130Ω,40mH) filter

At your risk. Take all safety precautions. A fuse should be in series to any battery for fire hazard safety.


V(-1) is one of the first Pleiades mic booster pre preamps.


Made before 2000.


With some mods it now consists of a neat gray painted diecast aluminium rectangular box. One electron tube 12AU7 for each channel.


Long tailed pair with short tail as cathodes of each pair are connected to Vb minus.


Heaters are in parallel so with the 12V battery power supply if one channel is to be used, only one electron tube can be pluged in for less power consumption.


The V(-1) internal signs, path or schematic signal flow is:
Mic XLR connector
Sowter 1:7 input transformer
Secondary feeding grids at space potential
Anodes feeding primary of Sowter 1:1 10K transformer, center tap to Vb=12V
Output XLR connector


Grids are floating as there is no centre tap return.
So amp is sensitive to light falling on electron tubes as possibly the photoelectric effect (photons colliding with matter, electrons etc) changes bias.
The amp might be used as a photo side chain compressor.


This amp has no resistors, no capacitors. Like one stage of a Langenvin amplifier.


So this is what happened by chance:


Since the output impedance is high it was decided to connect a say 10:1 German in line transformer to bring impedance down so that it can then be connected to Sony TC-D5 Pro low impedance XLR transformer input.


There was almost no signal level until some light fell on electron tubes after sunrise. Sound was low and bass heavy.


But fortunately the German in line transformer has a switch making it 1:1. It is clearly "wrong" to use a 1:1 setting but it sounded great, with loud sound and great treble. (Bass heaviness gone).


So this is another case where low cut or high pass is implemented after the first electron tube.


Signal path:


male voice singing Πως θα Ηταν in F minor - AKG D190E at say 3in - Pleiades V(-1) - German 1:1 transformer - Sony TC-D5 Pro or TC-D5M - Sennheiser HD580


Monitoring with the TC-D5M and recording was done to one channel (L) as there is no bridging mono mode.


In the case of TC-D5 Pro there are 3 transformers one cascaded to another, Sowter 10K:10K - German 1:1 - Sony mic input transformer


A recording was also done with Pleiades V1 (CV2269 electrometer electron tube operating with a 1.3V battery for both haters and anode). Anode feeding directly TC-D5M trough coupling capacitor, RL C coupling.


And AKG D190E was also connected directly to TC-D5M through a Pleiades (130Ω,40mH) gentle slope high pass filter.


All sounded nice. The cassette recorded must be listened too again to decide which may be best.


On preliminary real time listening tests on TC-D5 Pro before recording, low cut done on V(-1) by the interaction of anode internal resistance and transformer inductance sounded more natural bright and interesting than the Pleides filter implemented just at mic's output.


It is as if some kind of frequency or even non linear distortion makes the mic more sensitive, more revealing, perhaps more natural.


The Geraman transformer is GSM1 e+p and has at the 1:1 setting secondary inductance 600mH at LF (97Hz), 270mH at MF (around 1.028KHz) and primary inductance 470mH at LF, and if memory is correct 220mH at MF.