The Signal Transfer Low-Noise Balanced Input Card
The conventional balanced input stage built with 10K resistors and a 5532 op-amp has a far worse noise performance than a simple unbalanced input, and is also much noisier than any of our power amplifier designs. Its output noise is approximately -104 dBu.
The Signal Transfer low-noise balanced input card solves this problem by the use of a multiple amplifier array that causes the noise from each amplifier to partially cancel, and in a similar way improves the common-mode rejection ratio; this array is driven by a multiple-buffer structure that allows the input impedance to be much higher than usual, preventing loading of external equipment and also further improving the CMRR. The elegant design does not require selected or exotic components. The noise output is less than -115 dBu.
The principle of operation is fully described in the Fifth edition of Douglas Self's book "The Audio Power Amplifier Design Handbook".
This card handles a single channel, so that the two required for stereo can be spaced apart to minimise interchannel crosstalk.
- PCB 008: £19:80 each
- Kit 008: £38:00 each
- Built and tested: £58:00 each
F E A T U R E S
- Very low noise. Less than -115 dBu at the output.
- Very low distortion. THD better than 0.0007% 10 Hz- 20 kHz at 5 Vrms. This is well above the normal operating level, so in real use the distortion introduced is vanishingly small.
- High input impedance to maximise common-mode rejection.
- RF filters on inputs.
- Three-pin output connector to allow a local balanced connection between the input card and the power amplifier.
- Non-polarised input capacitors to allow for offset voltages of either polarity on the outputs of extenal equipment.
- Easily obtainable parts. The only op-amps used is the 5532 which is easy to find world-wide. Their performance is exceptional here simply because of the way they are used.
- High-quality double-sided plated-through-hole PCB with solder mask both sides and full screen print.
All solder pads gold-plated.
- Fully ROHS compliant in both PCB and all components.
S P E C I F I C A T I O N S
- Gain is unity (0.0dB)
- Noise out: Less than -115 dBu ((Rs=50 Ohms, 22-22kHz bandwidth, rms sensing)
- Input impedance: 47 kOhm (customisable)
- Maximum output level: 11.5 Vrms +/-18V supply (at 1% clipping THD)
- Output impedance: 70 Ohms
- Minimum recommended load on output: 500 Ohms
- Distortion : Less than 0.0005% THD 10 Hz - 10 kHz, at 10 Vrms output.
Less than 0.0015% at 20 kHz
Level in actual use is approx 1 V rms so distortion will be much lower.
- Common-Mode Rejection Ratio: Better than 50 dB 10 Hz - 50 kHz
- Frequency response: -1.0 dB at 90 kHz, -0.1 dB at 20 kHz (customisable)
-0.1 dB at 10 Hz
- Power supply: +/-15V to +/-18V; an +/-18V supply is recommended to maximise headroom.
- Power consumption: 50mA from +/-18V supply.
- PCB size is 62 by 76 mm. Incorporates four M3 fixing holes.
C O N N E C T O R S
- Input 3-pin
- Output 3-pin
Graph of THD at 5 Vrms and 10 Vrms. Testgear AP SYS-2702.