Multitap input transformer for the Pleiades V6

For example there can be 4 inputs on the Pleiades V6 but only one input transformer used.


As an example let as suppose that the octagon aluminum die cast box has on some of the sides 3 female XLR female sockets and one 1/4in input socket.


The core has 3 primary windings and each winding is connected to each balanced input.


One of the balanced inputs can be of 140mH by being connected to the 140mH input transformer winding. In fact the wire coming out of the core can be twisted and directly connected to pins live, return ie 2,3. This is useful for connecting 25Ω mics such as Grampian DP4/L or STC 4021 apple and biscuit etc. Also for connecting a Sennheiser 200Ω MD21. Or for directional microphones of about 200ohms such as AKG D112, D1000, Shure SM57 or Unidyne III, Sennheiser MD441, Electro-Voice RE15 etc when these are used for singing at a distance of a few only inch thereby compensating for the proximity and other psychoacoustic effects. See next euroelectron post on the importance of input inductance rather than impedance. Such a transformer has been tested on the V6 and these mics expect the 4021 with very good results. It is wound on the Magnetec 070 ring Nanoperm core, has a ratio of 1:10 and input inductance of 140mH. See other euroelectron posts.


The second input can be of more primary turns having for example an input inductance of 300mH. This winding would having more turns and since the secondary on this example is constant the turn ratio would be less. This winding would give a more bass extension making it suitable for example for... A Sennheiser MD211N for vocal singing. An MD21 for recording drums. 200Ω directional moving coil microphones for vocal singing at 1-2feet for example. It is important to understand that more turns gives a higher input inductance. So the impedance at low frequencies is not reduced. Therefore there is no voltage drop at the mic's terminals at low frequencies. Other possible uses are: Distant mic capture with a Gramoian GR1/L or GR2/L 25 Ω ribbon microphones for full range sound capture to listener's brain. The fact that the turn ratio of this transformer may be less than 1:7 or even less than 1:5 should not matter at all for noise considerations as the equivalent input noise resistance of the Pleiades V6 is of the order of only 2500Ω. In addition microphone loading is minimal. For example with a turn ratio of 1:5 the input impedance of the grid which is 100KΩ is transformed to 100KΩ:5:5 or 5KΩ. So a mic of 200Ω sees a 5KΩ input at the frequencies of interest. An Electro-Voice 635a can be connected for more bass than the previous terminal.


Another winding of very few turns may be connected to the 3rd XLR input. The very low inductance can be of 14mH for example. This roughly means 1000 less than the secondary which is 16H. So a turn ratio of aboit 1:33. This may be used with a low impedance ribbon microphone such as the 25Ω Grampian GR1/L or GR2//L for very close singing voice of correct frequency response to listener's brain. A full spectrum sound with great high end detail. The best value of input inductance is best decided by the application of interest and the desired result.


The other input is an unbalanced one connected to the 22nF input grid capacitor. A switch may allow the secondary to be connected in parallel. The switchable low cut may be useful for example for compensating for exagerated bass frequency perception to listener's brain when an electric bass is directly injected to the control grid circuit.


Reference:


Pleiades V6 Schematic