Sound Skulptor MP573
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I am proud to announce the release of our first ‘500’ design.
Schematic: http://www.soundskulptor.com/pdf/mp573-schematic.pdf
Circuit description:
Input:
The MP573 is based on the preamp part of the Neve 1073.
The signals from XLR and Di inputs are sent to a relay that makes the selection. The relay is automatically switched when a jack is inserted into the front panel Di. The signal then goes to the input transformer (Carnhill VTB9045). The ratio is set to 1:2 but can be modified to 1:4 by a jumper.
Gain switch:
The gain switch is 4 circuits, 6 positions. The first 2 circuits (SW2a & SW2b) are used to insert various pads after the transformer. The third circuit (SW2c) is used to switch the gain of the first stage (BA283NV) and the fourth circuit (SW2d) allows bypassing it.
The input impedance of the MP573 is set by the reflected impedance at the transformer secondary which must be 4800 ohms, in order to get 4800/22 = 1200 ohms at the primary. The input impedance of the BA283NV varies considerably with its gain: from 12K at 18dB down to 4K6 at 35dB. This is the reason why the impedances set by resistors R4 to R12 seem all over the place. They actually compensate the impedance variations of the BA283NV.
The fourth circuit of the gain switch allows bypassing the first stage in the 10 and 20dB positions. Here, the transformer drives directly the potentiometer P1a and the line output stage BA283AM. This configuration allows running high line levels on the input without resorting to a huge -40dB attenuation pad like in the 1063, which is not great for noise.
First stage:
The first stage is a typical BA283NV. The gain is set to 18, 25 and 35 dB by gain switch SW2d.
Output stage:
The output stage is similar to a BA283AM where the gain is made variable from 14 to 25 dB by the action of P1b. This, combined with the attenuator action of P1a allows a smooth and precise +/-10dB trim with linear pots.
The power transistor is a 2N6488 which is remarkably close to the 2N3055 but in a smaller size, confirmed by listening tests.
The phase switch is placed after the output transformer.
The transformer is continuously loaded by R22, a 820 ohms resistor. This resistor paralleled with the actual load of the preamp will be approaching the typical 600 ohms. This ensures that the transformer will work without any ringing. If the preamp should be connected to a real 600 ohms load, it would be preferable to omit this resistor.
The total max gain of the MP573 settles as follow: 6dB (input TX) + 35dB (stage 1) + 25dB (stage 2) + 4dB (output TX) = 70dB
Peak detector:
U1a and U1d form 2 rectifiers and peak detectors that monitor the signal level at the output of both stages. The voltage dividers R41/R42 and R43/R44 are set to fire the LED red 3dB before clipping and green 40dB below.
Power supply:
The ground level of the preamp is shifted down to -9V by the regulator U2 between the input and output transformers. This brings the available +15V (16V minus one diode voltage drop) to +24V above the local ground. This setup allows avoiding the use of a switchmode power supply.
Two thermistors PTC1 and PTC2, acting as resetable fuses, protect the power lines from a local failure: In case of a breakdown of the unit, the other modules in the rack will keep on working.
Di circuit:
This is a simple one FET affair, high input impedance (>1M) low enough output impedance (200 ohms) to drive the input TX. No gain here but a 20dB attenuation.
http://www.soundskulptor.com/uk/proddetail.php?prod=MP573
[br]
[br]
I am proud to announce the release of our first ‘500’ design.
Schematic: http://www.soundskulptor.com/pdf/mp573-schematic.pdf
Circuit description:
Input:
The MP573 is based on the preamp part of the Neve 1073.
The signals from XLR and Di inputs are sent to a relay that makes the selection. The relay is automatically switched when a jack is inserted into the front panel Di. The signal then goes to the input transformer (Carnhill VTB9045). The ratio is set to 1:2 but can be modified to 1:4 by a jumper.
Gain switch:
The gain switch is 4 circuits, 6 positions. The first 2 circuits (SW2a & SW2b) are used to insert various pads after the transformer. The third circuit (SW2c) is used to switch the gain of the first stage (BA283NV) and the fourth circuit (SW2d) allows bypassing it.
The input impedance of the MP573 is set by the reflected impedance at the transformer secondary which must be 4800 ohms, in order to get 4800/22 = 1200 ohms at the primary. The input impedance of the BA283NV varies considerably with its gain: from 12K at 18dB down to 4K6 at 35dB. This is the reason why the impedances set by resistors R4 to R12 seem all over the place. They actually compensate the impedance variations of the BA283NV.
The fourth circuit of the gain switch allows bypassing the first stage in the 10 and 20dB positions. Here, the transformer drives directly the potentiometer P1a and the line output stage BA283AM. This configuration allows running high line levels on the input without resorting to a huge -40dB attenuation pad like in the 1063, which is not great for noise.
First stage:
The first stage is a typical BA283NV. The gain is set to 18, 25 and 35 dB by gain switch SW2d.
Output stage:
The output stage is similar to a BA283AM where the gain is made variable from 14 to 25 dB by the action of P1b. This, combined with the attenuator action of P1a allows a smooth and precise +/-10dB trim with linear pots.
The power transistor is a 2N6488 which is remarkably close to the 2N3055 but in a smaller size, confirmed by listening tests.
The phase switch is placed after the output transformer.
The transformer is continuously loaded by R22, a 820 ohms resistor. This resistor paralleled with the actual load of the preamp will be approaching the typical 600 ohms. This ensures that the transformer will work without any ringing. If the preamp should be connected to a real 600 ohms load, it would be preferable to omit this resistor.
The total max gain of the MP573 settles as follow: 6dB (input TX) + 35dB (stage 1) + 25dB (stage 2) + 4dB (output TX) = 70dB
Peak detector:
U1a and U1d form 2 rectifiers and peak detectors that monitor the signal level at the output of both stages. The voltage dividers R41/R42 and R43/R44 are set to fire the LED red 3dB before clipping and green 40dB below.
Power supply:
The ground level of the preamp is shifted down to -9V by the regulator U2 between the input and output transformers. This brings the available +15V (16V minus one diode voltage drop) to +24V above the local ground. This setup allows avoiding the use of a switchmode power supply.
Two thermistors PTC1 and PTC2, acting as resetable fuses, protect the power lines from a local failure: In case of a breakdown of the unit, the other modules in the rack will keep on working.
Di circuit:
This is a simple one FET affair, high input impedance (>1M) low enough output impedance (200 ohms) to drive the input TX. No gain here but a 20dB attenuation.
http://www.soundskulptor.com/uk/proddetail.php?prod=MP573
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