01-10-2016, 09:09 AM
Hmm....
Looking at a random selection of cables:
50 ohm RG58: 82pF/m
75 ohm RG59: 60pF/m
75 ohm CT100: 52pF/m
BBC PSN2/3 (general purpose audio cable): 86pF/m
Canford HST (line or mic level cable): 143pF/m
BBC star quad (high quality mic cable): 160pF/m
RS 749-2554 microphone cable: 145pF/m
I see nothing in those figures that suggests that 75 ohm coax is "useless for audio".
I humbly suggest there was something else happening that night. We don't know what the microphones were (source impedance obviously being an important factor), and it wasn't stated that the fault stayed with the cable and didn't move with the microphone or amplifier input channel. And if the fault did stay with the cable, perhaps the cable was faulty?
To cause the subjective effects reported, you'd need serious HF loss at the 5kHz region. Let's assume the extension was 10m (easy to scale if not), and the cable was comparable to CT100, giving 520pF. To cause 3dB of loss at 5kHz, the microphone's source impedance would need to be 62k. In practice, probably higher, as 3dB down at 5kHz is not a big deal in a noisy pub environment where speech is the programme material. So, perhaps more like 100k? Typical microphones are in the 120 ohms region IME. Of course, the input impedance of the amplifier is a factor - mic inputs are usually 1.2k in the professional world, so if a faulty cable had significant resistance, you'd think there would be significant attenuation long before HF loss, but we don't know how much "gain in hand" the amplifier had. More data needed!
As I say, something was most definitely faulty that night, but it wasn't the choice of cable type.
EDIT: Lawrence beat me to it, but his microphone cable was even "better", at nearly double the capacitance/metre than one I found at RS.
Looking at a random selection of cables:
50 ohm RG58: 82pF/m
75 ohm RG59: 60pF/m
75 ohm CT100: 52pF/m
BBC PSN2/3 (general purpose audio cable): 86pF/m
Canford HST (line or mic level cable): 143pF/m
BBC star quad (high quality mic cable): 160pF/m
RS 749-2554 microphone cable: 145pF/m
I see nothing in those figures that suggests that 75 ohm coax is "useless for audio".
I humbly suggest there was something else happening that night. We don't know what the microphones were (source impedance obviously being an important factor), and it wasn't stated that the fault stayed with the cable and didn't move with the microphone or amplifier input channel. And if the fault did stay with the cable, perhaps the cable was faulty?
To cause the subjective effects reported, you'd need serious HF loss at the 5kHz region. Let's assume the extension was 10m (easy to scale if not), and the cable was comparable to CT100, giving 520pF. To cause 3dB of loss at 5kHz, the microphone's source impedance would need to be 62k. In practice, probably higher, as 3dB down at 5kHz is not a big deal in a noisy pub environment where speech is the programme material. So, perhaps more like 100k? Typical microphones are in the 120 ohms region IME. Of course, the input impedance of the amplifier is a factor - mic inputs are usually 1.2k in the professional world, so if a faulty cable had significant resistance, you'd think there would be significant attenuation long before HF loss, but we don't know how much "gain in hand" the amplifier had. More data needed!
As I say, something was most definitely faulty that night, but it wasn't the choice of cable type.
EDIT: Lawrence beat me to it, but his microphone cable was even "better", at nearly double the capacitance/metre than one I found at RS.