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Signal Transfer Company Phono Preamplifier, Balanced Output Version

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klem

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Oct 12, 2005
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The Signal Transfer Company Phono Preamplifier, Balanced Output Version

Phono preamp from the man who literally wrote the book on small signal audio design, Doug Self.  This is the type of phono pre-amp you’d use when you don’t want added color, flavor, harmonics, dynamic processing, anything other than absolutely TRANSPARENT wire with gain. 

Specs are listed below. What’s probably the most important follow-up to reading the data is that it sounds absolutely transparent - there’s nothing ‘rigid’ or glaring about this design, a fear I used to have with some op-amps. I’ve shot this out with other preamp designs for both objective and subjective evaluation. This was frankly more fun to listen to, because it simply didn’t add anything; no smearing of transients, the noise floor was non-existent, and front to back imaging was robust. 

It served great for a few restoration projects, directly into my Crane Song HEDD converter.  I'm no longer doing this work, hence selling.

Free shipping to US48, will ship most anywhere else as well provided you contact me beforehand and agree to pay all shipping and handling costs. For the latter, please only contact if serious inquiry!

The $2000 SPL Phonos RIAA Phono Preamplifier has a very similar circuit, but only offers unbalanced outputs. The STC has the added benefit of having balanced outputs and a remote power supply.  I'm selling it for less than I paid for the parts alone.



F E A T U R E S.

Moving magnet and moving coil inputs. Separate input connectors are provided for the two inputs so two sources can remain permanently connected. Selection between the two is by a switch on the PCB.
The moving coil input has two gain options to match a wide range of cartridges.
Selection between the two is by a switch on the PCB.
Fully balanced outputs with high-quality male XLR connectors. Low output impedance to maximise common-mode rejection of interconnect.
Integral 3rd-order Butterworth subsonic filter: -3 dB at 20 Hz
Power indicator LED
Exceptional RIAA equalisation accuracy of +/-0.05 dB. Surprisingly, this does not require specialised and expensive components. This accuracy is obtained done by design rather than just spending money.
Very low noise.
Extremely low interchannel crosstalk. Better than -100 dB in moving-magnet mode. This performance is built into the PCB layout.
Very low distortion. THD better than 0.002% at 6 Vrms. This is about forty times the normal operating level, so in real use the distortion introduced is vanishingly small. It is certainly too low to measure.
PCB positions are provided on both moving magnet and moving coil inputs to allow both capacitance and resistance to be added across the input so cartridge loading can be altered as desired
Absolute phase is preserved from all inputs to output.
Easily obtainable parts. The only op-amps used are the 5534, the 5532 and the TL072, both of which are easy to find world-wide. Their performance is exceptional here because of the way they are used.
Provision for standard-footprint gold-plated phono sockets.
High-quality double-sided plated-through-hole PCB with solder mask both sides and full screen print.
All solder pads gold-plated. PCB size is 220 by 98 mm.
Fully ROHS compliant in both PCB and all components.


S P E C I F I C A T I O N S.

Input sensitivity: (moving-magnet) 5 mV rms for 150 mV rms out at 1 kHz
Gain= +29.5 dB (30 times) at 1 kHz
Input sensitivity: (moving-coil) 0.1 or 0.5 mV rms for 150 mV rms out at 1 kHz (switchable)
Gain= +49.5 dB or +54.5 dB (300 or 1500 times) at 1 kHz

Maximum input level: (moving-magnet) greater than 350 mV rms at 1% THD
Maximum input level: (moving-coil, gain 10x) greater than 53 mV rms at 1% THD
Maximum input level: (moving-coil, gain 50x) greater than 11 mV rms at 1% THD
Input impedance: (moving-magnet) 47K in parallel with 100pF (customisable)
Input impedance: (moving-coil) 100 Ohms (customisable)
Maximum output level: (unbalanced) greater than 10 V rms at 1% clipping THD
Maximum output level: (balanced) greater than 20 V rms at 1% clipping THD
Output impedance: 100 Ohms
Minimum load on output: 600 Ohms

Distortion (moving-magnet input): Less than 0.002% THD 50Hz - 7kHz, at 6 Vrms output.
Level in actual use is approx 150 mV rms so real-life distortion will be much lower.
Distortion (moving-coil input at 10x gain): Less than 0.006% THD 1kHz, at 1.6 Vrms output.
Level in actual use is 150 mV rms so real-life distortion will be much lower.

RIAA equalisation accuracy +/-0.05 dB 100Hz - 50kHz

Noise out (moving-magnet, Rs=50R) Less than -95 dBu (20-22kHz bandwidth, rms sensing)
Noise out (moving-magnet, cartridge load 610R + 450mH) Less than -88.5 dBu (22-22kHz, rms
Noise out (moving-coil, gain 10x, Rs=10R) Less than -90 dBu (22-22kHz bandwidth, rms)
Noise out (moving-coil, gain 50x, Rs=10R) Less than -75 dBu (22-22kHz bandwidth, rms)

L - R crosstalk (moving magnet input) better than -100 dB at 1 kHz
L - R crosstalk (moving coil input at 10x gain) better than -75 dB at 1 kHz

Power supply: +/-15V to +/-18V; an +/-18V supply is recommended to maximise headroom.
Power consumption: 45mA from +/-18V supply.

This graph shows the typical RIAA equalisation accuracy. The error is less than 0.1 dB from 100 Hz to 40 kHz.
Please note that the scale is enormously expanded and covers only 1.5 dB.
The response rolls off very slowly below 100 Hz due to action of the subsonic filter, but is still accurate to -0.5 dB at 50 Hz.
Measurement system Audio Precision SYS-2702.
 
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