The Shure SM7b has a tiny transformer inside the capsule

At your risk. Older posts stating that SM7 does not have a transformer are most likely wrong, Theodoris who had the chance to open up a defective capsule said he had found a tiny transformer inside it. So it at the secondary of this transformer that we have 150 ohms output impedance.

Testing Electro-Voice 664 variable D moving coil microphone with Pleiades 2 k117 preamplifier old version

At your risk. The older 2 k117 version has a lower load resistor on first stage than 100K, no potentiometr between the 2 stages, a lower load resistor on second stage, so gain is lower. The mic tested is new old stock bought from Yiannis who supplied these in the 60's to the military television. At that time the goverrment in Greece was miliotary so was the TV station. From the mics tested they liked most the 664. The particular mic not sold to them is s/n 164995. After a new flexible Eurocable of blue color was soldered to the Amphenol connector at the low impedance terminals, the mic sings, amazing sound and detail. Tested at singing male voice 24in from nose, about 30 degrees towards head from above.

Increasing the gain of Pleiades 2 K117 microphone preamplifier part 2

At your risk. A tiny output transformer had been used on part 1. A much larger Belclere output transformer was tried, somehow more clear sound. Then a 220K potentiometer was connected between stage 1 and 2. It works great, very low noise, gain of preamplifier can now be adjusted. So it is certainly possible with just one input transformer, 2 JFets, one output transformer, no negative feedback, to go from dynamic microphone level to line level, feeding Sennheiser HD580 headphones for example. Battery supply is an ordinary 9V battery which should last long time.

Listening to Guilty - Barabara streisand, Barry Gibb CD on the Pleiades 2n3053 single transistor per channel power amplifier in class A with no feedback

At your risk. Today Eleni from the excellent Record shop Record House handed me the anticipated CD. The amplifier is in more detail described on older posts. The output transformers are the hand made ones wound on small Siemens ferrite E inverted E cores with air gap added. Frequency respone which number of turns described on previous posts is flat 40hz to 40KHz with no negative feedback of cource. The sound quality is amazing. Breathtaking, promising. deep but not boomy bass, smooth mid and amazing extended treble whith no harshness, with sweetness and cymbals appearing to be real ones inside the room. Barbara and Barry in breath taking peformance. Signal path, cD - Sony CD Walkan line out - Pleiades 2N3053 2 channels - Celestion Ditton 150 at 2 m in a small bedroo with 3 differenct size panel low frequency absorbers. The big panel is behing the speakers away from the wall, its upper side only touching it and its lower side touching the wooden floor. panel in landscape orientation. Panels made of drawing frames with canvas with betumin roofing felt panel fed at the back side of them. Linoleum must be more suitable for enviromntal and cleaner air of room condition.

Increqsing the gain of the Pleiades 2 K117 microphone preamplifier

At your risk.It is a 2 JFET microphone preamplifier, mic feeding the input transformer and 2nd JFet feeding the output transformer which feeds for example the Sennheiser Hd580 headphones. Vb has been changed to a 9 Volt battery. Load resistor of first stage has been changed to 100Kohm, and of 2nd stage to 22Kohm. Now gain is very high and the 2nd stage clips. There is again FM radio reception, floating wire picking up RF etc. Maby a potentiometer should be connected between the 2 stages to attenuate what the 2nd JFET gets from the 1st. Also to check if the whole thing does not oscillate.

Shure SM7 internal switchable filters

By looking at the datasheet of the SM7 it makes sense that when the single 240mH inductor is connected, there is low cut. And when the other filter of 11mH in series with 220nF is connected the high frequency response is flat. When no filter is connected the capsule, similaer to SM57, or SM58 manifests its presence peak that the inventor Ernie Seeler did not like. Would an SM58 sound very similar to SM7 if similar band arrenuate filter is used, and similar high pass filter is used?. Attention should be made to the fact that the impedance of the SM7 is 150 ohms whereas of the SM58 is 300ohms or perhaps more. Also one must take into account that rea; inductors have series resistance which is a good thing for example making the slope of the hi pass gentler, and the Q of the resonant filter lower increasing the bandwidth of the band reject filter and arranging the maximum reduction of the peak to be such that a flat frequency responce is obtained. Would it be possible to use variable series resistors for that purpose? The SM7 seems to be a very good microphone if Michael Jackson, Whitney Houston were cutting vocals in the studio with it.

There are mistakes on the constant k, T, PI filters which are now corrected on the previous posts

At your risk. Please also note that these filters have by definition the useful property that ZL times ZC equals R times R for any frequency. (the impedance of an inductance L is 2 times PI time L, the impedance of a capacitance C is 1 over (2 times Pi times C). It would be interesting to use as image impedance the output impedance of a Shure SM58, eg R+320 ohms and calculate suitable imductors, capacitors for various high pass cut of frequencies, or even a band reject filter of say 5Khz to convert the peak to flat as is done of the SM7 microphone?