I am looking at Benchmark AHB2.
https://benchmarkmedia.com/products/benchmark-ahb2-power-amplifier
Regarding input impedance, it says:
50 k Ohms, normal mode
1 M Ohm, common mode
If I were to use a single ended passive "preamp" which number should I be looking at?
If I were to use a balanced passive "preamp" which number should I be looking at?
Thanks for reading.
https://benchmarkmedia.com/products/benchmark-ahb2-power-amplifier
Regarding input impedance, it says:
50 k Ohms, normal mode
1 M Ohm, common mode
If I were to use a single ended passive "preamp" which number should I be looking at?
If I were to use a balanced passive "preamp" which number should I be looking at?
Thanks for reading.
Lower impedances are more used with balanced inputs (XLR), so that would be 50kΩ.
The 'common mode' I read as unbalanced (RCA), 1MΩ.
The 'common mode' I read as unbalanced (RCA), 1MΩ.
50 kohm.
That 1 Mohm is when you apply equal signals to XLR pin 2 and pin 3, which is not very useful as it produces no sound.
That 1 Mohm is when you apply equal signals to XLR pin 2 and pin 3, which is not very useful as it produces no sound.
50k is for the wanted signal, basically the impedance seen between pins 2 & 3, 1M is the impedance between 2/3 and chassis (pin 1).
Generally you want the common mode impedance as high as possible as that minimises the influence of unequal source impedances on noise pickup.
I would note that when driven differentially each driver will see a virtual 'ground' behind a 25k resistance as the other end of the 50k load is being driven with the opposite polarity. When driven balanced but NOT differential, you will see a 50k load.
It pays to be clear on both terms 'balanced' refers to the impedance seen by both legs, 'differential' is opposite voltages driving both legs, balanced gets you most of the win at the cost of typically 1 resistor and maybe a cap in the gear doing the driving, differential is an extra amp or a transformer.
I would note that in situations where interference is not a major factor (most hifi), balanced lines are generally inferior as the line receiver can have significant Johnson noise from its resistors and indeed the classic (And not very good) 4 * 10k around an opamp is about 14dB nosier then the unbalanced equivalent! Of course in its native environment (Pro audio) that is still so much better then the interference that would blow straight thru an unbalanced input that it is just not a major issue. While it IS possible to build a balanced line receiver that is as quiet as the simple unbalanced input, it is a fairly heroic thing, Doug Self demonstrates one approach in 'Small Signal Audio Design', that gets really quite silly (But very, very, quiet)!
Generally you want the common mode impedance as high as possible as that minimises the influence of unequal source impedances on noise pickup.
I would note that when driven differentially each driver will see a virtual 'ground' behind a 25k resistance as the other end of the 50k load is being driven with the opposite polarity. When driven balanced but NOT differential, you will see a 50k load.
It pays to be clear on both terms 'balanced' refers to the impedance seen by both legs, 'differential' is opposite voltages driving both legs, balanced gets you most of the win at the cost of typically 1 resistor and maybe a cap in the gear doing the driving, differential is an extra amp or a transformer.
I would note that in situations where interference is not a major factor (most hifi), balanced lines are generally inferior as the line receiver can have significant Johnson noise from its resistors and indeed the classic (And not very good) 4 * 10k around an opamp is about 14dB nosier then the unbalanced equivalent! Of course in its native environment (Pro audio) that is still so much better then the interference that would blow straight thru an unbalanced input that it is just not a major issue. While it IS possible to build a balanced line receiver that is as quiet as the simple unbalanced input, it is a fairly heroic thing, Doug Self demonstrates one approach in 'Small Signal Audio Design', that gets really quite silly (But very, very, quiet)!
