Pleiades V5 headroom

This is a two stage electron tube (EF183 triode connected) prepreamplifier with anodes operated at less than 7 volts.


The output is resistive potential divider attenuated to get back to mic level.


The objective is to get extremely low hiss noise and as much the dynamic signal conditioning of electron tubes for use in music production.


When a signal is conditioned it sounds loud but the objective dynamic range is reduced so that it can safely be sent to op amps or an analog to digital converter and still sound loud.


If we send a live music signal directly to an op amp or an analog to digital converter in order to avoid the very high peaks to be abruptly and unmusically clipped, it's magnitude has to be reduced and so does its volume.


Singal path, setup:


Male soft singing voice - Electro-Voice RE15 with low cut on - Pleiades V5 (47mH inductance of input transformer) - 560Ω termination - Belco VT-250 VTVM


At soft singing voice the average level was -60dBm.


At progressively singing at a very loud voice no more than -35dBm could be got out of the prepreamp.


Does this mean that the headroom is 25dB?


The question is if it has already started conditioning the signal when the singing was soft. For example instantaneously peak limiting the extreme peaks.


It was also connected as mic - V5 - Altec 1566A - 20dbB attenuator - Sennheiser HD580


The sound quality was amazing with no hiss, a very clear and full sound and no hint of s problems.


There were interfacing buzzing problems as the V5 output is unbalanced. Perhaps a ground problem. When the output connector of the V5 was touched the problem almost stopped. When connected to the realistic disco mixer which is unbalanced there are no buzzing problems.


S problems started when the signal inside the Altec was getting hotter by increasing its volume control above about 7/10. See also EMI and Altec comparison being driven by Pleiades V0, the RS61 exhibiting no s problems at all.


It seems the Pleiades V5 is conditioning the signal as it sounds more full than one stage.


Since it operates at very low anode voltage it seems that the sound is in a way  already there, processed. It is felt that it could be driven a bit more.


For a louder singing voice it may be even better.


It could be that 3 stages may be avoided especially if a higher ratio Pleiades input transformer is used. Or for example 4 mics are connected in series parallel combination at the input to get back to the same mic impedance and a signal of double voltage ie 4x the power or 6dB more. Another trick to further increase gain is the use of a small ratio step up interstage transformer, this would also present a nice low noise load for the first stage. If still more gain is needed, one more stage could be added, a potentiometer after the first stage making the prepreamp suitable for the faintest to the louder signal while conditioning the signal too.


References:


Tubes vs Transistors, is there an audible difference? - Russel O. Hamm - JAES