Connecting directly a microphone to an RS61 will not reveal the rich sound of the RS61 unless that mic is a good mic and has a pre preamplifier inside or outside it. An obvious example is a Neumann U47 with its VF14 world class quality prepreamp.
It was decided to drive the RS61 with a very similar and even simpler circuit. A top quality one electron tube prepreamp, the Pleiades V0, so that the RS61 can be properly hotly driven and its magic revealed.
The microphone chosen was the Beyer M55 as it gives a almost flat frequency responce to listener's brain without the need of EQing.
The recorder used is the M-Audio Microtrack. It has variable sensitivity so it allows comparison of the (Beyer M55 to Pleiades V0) sound with or without the EMI RS61.
The RS106A at flat position was used as a variable pad to make shure the signal is low enough to avoid any internal overload of the Microtrack.
A final comparison was made with an M55 HN driving a Pleiades K117 JFET prepreamp directly connected the Microtrack.
The result is that the combination of the Pleiades V0 to EMI RS61 sounded fantastic as if the sound
was made by the best reel tape recorder running at an as much high speed as one can think of. Very loud sound, with amazing treble and edge. As if one is listening to a finished product. An edge similar to what we hear on Beatles records for example.
By comparison the Pleiades V0 on its own sounded very clean, lifeless, not loud, not very interesting.
On a previous experiment of just connecting an M55 to RS61 the RS61 sounded under driven, with a lot of bass to listener's brain. The RS61 and an input inductance of 10H so by directly connecting a mic the frequency responce is flat to a reproducing loudspeaker but not flat to the listener's brain. Even an M55 which has bass reduction for compensating for Fletcher-Munson and voice effort curves sounded bass heavy.
So the best combination by far was the RS61 (hotly?) driven by the V0.
Bass compensation (reduction) is done by the input and output transformers of the Pleiades V0 which have low inductance. The input inductance is 400mH. And the inductance of the primary of the output transformer connected to the ECC82 anode is 29H.
There may be systematic errors such as the V0 military output transformer be directly terminated by the M-Audio Microtrack as opposed to be terminated by the input transformer of the RS61 but the difference was so great that it makes the poin
A Beyer M55 HN driving a Pleiades K117 driving in turn the Microtrack sounded bass heavy too as it is a direct connection with no bass compensation for Fletcher-Munson and voice effort curves again. To repeat, on this setup the sound is made correct to listener's brain by the relative low inductance of the input and output transformers of the Pleiades V0, please see other euroelectron posts.
The schematic of the Pleiades V0 is very similar to V6 but it uses an external input transformer, the electron tube is the ECC82 biased by a 10ΜΩ resistor from plate or anode to grid and the power supply is 12V is series with a 4R7 wire wound power resistor supplying both heaters and anode circuit. More details on another euroelectron post describing the first connection of the V0 with the RS61.
On a previous listening test a sinewave from an Advance tube signal generator was after attenuation applied to RS61. The output of the RS61 was then constant impedance attenuated by the RS106A in order to listen with headphones on how the RS61 overloads. How gently, what harmonic components are produced and how quickly as the amplifier is further driven.
The result was not very impressive as when reaching the overload point sounded quite abrupt with many harmonics at the same time.
Nevertheless the listening test with human voice revealed that the RS61 makes even a whispering human voice sound loud, edgy, natural. There may be various EF37a stages adding character or ...
Asian audiophiles know very well what they doing when the use paper in oil capacitors. The RS61 is full of them and this may be a part of the sound too. The oil is toxic so they have to handled with case.
The RS61 of course sounds amazing connected directly to 300Ω precision headphones.
An important point is that although the HF edge is as edgy as can be, it is not harsh, and there are no sibilant problems. This is a very important criterion of quality of a preamplifier or in fact any audio component. By connecting the Pleiades V0 to the ordinary used Realistic descrete transistor disco mixer driving the same headphones although the sound is very nice, sibilant problems are apparent.
It seems the EMI RS61 "distorts" or adds, in the way you want and does not distort in the way you do not want.
Allan Blumlein and all the team at EMI Central Research Laboratories designed possibly the best preamplifier on the planet with no compromise.
Setup, signal path:
Male singing voice at 1in - Beyer M55 - Canford 400mH input transformer - Pleiades V0 - EMI RS61 - EMI RS 106A - M-Audio Microtrack - Sennheiser HD580 precision
References:
EMI RS61 schematic - euroelectron
Neumann U47 schematic
Pleiades V6 schematic - euroelectron
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