Impedances and crosstalk: a case history
Capacitive crosstalk between two opamp outputs can be surprisingly troublesome. The usual isolating resistor on an opamp output is 47 Ω, and you might think that this impedance is so low that the capacitive crosstalk between two of these outputs would be completely negligible, but . . . you would be wrong.
A stereo power amplifier had balanced input amplifiers with 47 Ω output isolating resistors included to prevent any possibility of instability although the opamps were driving only a few cm of PCB track rather than screened cables with their significant capacitance. Just downstream of these opamps was a switch to enable bi-amping by driving both left and right outputs with the left input. This switch and its associated tracking brought the left and right signals into close proximity, and the capacity between them was not negligible.
Crosstalk at low frequencies (below 1 kHz) was pleasingly low, being better than 129 dB up to 70Hz, which was the difference between the noise floor and the maximum signal level (the measured noise floor was unusually low at 114 dBu because each input amplifier was a quadruple noise cancelling type, and that figure includes the noise from an AP System 1). At higher frequencies things were rather less gratifying, being 96 dB at 10 kHz, as shown by the ‘47R’ trace below. In many applications this would be more than acceptable, but in this case the highest performance possible was being sought.
I therefore decided to reduce the output isolating resistors to 10 Ω, so the inter-channel capacitance would have less effect (checks were done at the time and all through the prototyping and pre-production process to make sure that this would be enough resistance to ensure opamp stability – it was). This handily reduced the crosstalk to 109 dB at 10 kHz, an improvement of 13 dB at zero cost. This is the ratio between the two resistor values.
The third trace marked ‘DIS’ shows the result of removing the isolating resistor from the speaking channel, so no signal reached the bi-amping switch. As usual, this reveals a further crosstalk mechanism, at about 117 dB, for reducing crosstalk is proverbially like peeling onions. There is layer after layer, and even strong men are reduced to tears.
Excerpt from Small Signal Audio Design by Douglas Self © 2014 Taylor & Francis Group. All Rights Reserved.
About the Author
Douglas Self studied engineering at Cambridge University, then psychoacoustics at Sussex University. He has spent many years working at the top level of design in both the professional audio and hifi industries, and has taken out a number of patents in the field of audio technology. He currently acts as a consultant engineer in the field of audio design.