TFPRO P10 dual/stereo compressor and mic preamp
	THE FRONT END My experience says that there’s nothing quite like a transformer for an input buffer, it’s the perfect isolator, and it eliminates the need for electrolytic capacitors at the audio input.. The P10 uses a substantial UK made transformer, but because of the novel design of the current mode input stage, the transformer operates with predominantly current. With almost zero voltage there is no possibility of overload or distortion at the input, so the input stage can handle outrageous levels with no distortion, even at very low frequencies. P10 front panel WIDEBAND Another great advantage of the current mode transformer is the frequency response. With zero voltage there is less actual ‘work’ being done in the transformer, and so the phase response and therefore the frequency response remains true across the windings. This allows me to let the frequency response remain ‘flat’ from about 8Hz right up to 50KHz, with a natural and very smooth roll-off above that. (the response remains significant at 100KHz). The amplification after the transformer is shared across 4 gain stages so that no single amplifier is ‘stretched’, this minimises impulsive distortion; the effect that makes so many so called ‘good’ preamplifiers sound over clean and brittle. The design of the input gain control ensures that gain is evenly proportioned across the 4 stages. All the selectable inputs go through the transformer, even the instrument in! The normal mic input has a huge gain range, from ‘off’ to 75dB gain (with another 10dB in hand at the output). When the input selector switches to ‘Capacitor mic’, the gain is reduced by 15dB, and 48 volt phantom power is applied to the XLR mic input socket. THE COMPRESSOR/LIMITER THE INSERT From the first experiments with optical compressors in the 1950s, the designs by Urei, Fairchild, and myself; and the derivatives copying those originals, the problem has always been the difficulty of achieving stable high ratio compression. I took another look at the problem and designed a new circuit where the compressor forms one leg of a precision ‘bridge’. This design makes it possible to achieve compression ratios up to 100:1, a true limiter! (Previous circuits have been restricted to ratios of about 6:1 maximum).The compressor make-up amplifier, which is normally a low noise op-amp, in the P10 it is a classic design class A discrete transistor amplifier, this adds to the cleanness and ‘weight’ of the sound With further departures from my older designs in the cell type, the way the ratio is determined, the compression metering and the sidechain filtering and rectification, I’m looking forward to the P10 providing a new set of standards. Output from the vari-phase is balanced and appears on a TRS jack socket at near zero level. Inset return is balanced and is via another TRS jack socket. THE FILTER An optimised Sallen and Key filter provides the high pass filter. It gives 12dB per octave attenuation of frequencies below 75Hz for situations where the ‘flat to 8Hz’ may be an embarrassment. THE VARI-PHASE From the filter amplifier the signal goes through a pair of ‘all-pass’ filters; the first is switched to give a perfect click-free phase reverse, the second has a variable control and gives phase shift from 0 to 180 degrees in one sweep.This facility is important for matching multi mic systems on drums or other instruments. It is an immensely powerful studio tool and probably a first for a channel module. STEREO The compressors work independently in the two channels, but a link switch takes the operation of the compression controls from channel 2 and makes them work from channel 1. The stereo linking is accurate enough for precision mastering of stereo tracks. THE EQUALISER As there is already a classic mastering equaliser in the TFPRO range, I decided to use a simple but effective semi-parametric type of equaliser rather than attempt to duplicate the (very expensive and delicate) passive coils of the P9. The EQ circuits are based around detuned gyrators subtracting or adding at specific selectable tuned frequencies. The ‘Q’ values are all around 1.5 giving detailed control of overlapping frequency bands that are particularly effective for drum recording. TECHNICAL SPECIFICATIONS MIC IN to LINE OUT. At 60dB gain Amplitude frequency response +0 -1dB 8Hz to 46KHz (5dB down at 100KHz) IInput overload margin 30dB Harmonic distortion 1KHz 0.002% 3 rd harmonic 0.015% 2 nd harmonic for +10dB output Noise 125dB below input referenced against 200ohm resistor. (The current mode input amplifier measures 130dB below input for open circuits.) Maximum input; mic in, +8dBu Line in, +28dBu Insert in, +22dBu Maximum output, +23dBu For the technically minded, these are actual measured figures from the first prototype. METERING VU meters show the audio level immediately before the output gain control circuit. When the ‘READ GR’ button is pressed, the VU meter shows the gain reduction of the compressor in dB. In addition to the large VU meters there are LED indicators showing ‘signal present’ (when there is a signal greater than -26dB in the system) and a channel overload LED that flashes on brightly when the signal hits a level within 6dB of a real overload. GENERAL SIGNAL PATHS All inputs and outputs of the P10 are balanced and operate at professional levels.