MRC01
Major Contributor
With all this discussion a summary is worthwhile. That is, what are the limitations, or guidelines for when a passive switch-box with a stepped attenuator should out-perform an active preamp. Put differently, what can go wrong with a passive attenuator:
1. Don't add attenuation unless you need it. If a switch-box does the job, your design will be simpler and cheaper, and your impedance matching is simplified: just make sure downstream device input impedances are at least 10x higher than upstream device output impedances. Note: most typical solid state devices will have a ratio of more than 100:1, like a CD player with a 100 ohm output impedance to a headphone amp with a 30k input impedance = 300:1.
2. If you have attenuation, use a ladder / voltage divider with metal film resistors having the lowest impedance you can: 10k is typical but if all your upstream sources are < 100 ohms then 5k will be better.
3. Upstream source devices have low output impedance, ideally < 500 ohms or at least 10x lower than the attenuator input impedance.
4. Downstream devices (amps) have high input impedance, ideally > 25 kOhm or at least 10x higher than the attenuator output impedance.
5. Use short low-capacitance cables for all connections -- especially to downstream devices (amps).
6. Ensure your design individually grounds de-selected upstream devices, to prevent de-selected devices from driving each other or leaking into the selected source.
Anything else?
1. Don't add attenuation unless you need it. If a switch-box does the job, your design will be simpler and cheaper, and your impedance matching is simplified: just make sure downstream device input impedances are at least 10x higher than upstream device output impedances. Note: most typical solid state devices will have a ratio of more than 100:1, like a CD player with a 100 ohm output impedance to a headphone amp with a 30k input impedance = 300:1.
2. If you have attenuation, use a ladder / voltage divider with metal film resistors having the lowest impedance you can: 10k is typical but if all your upstream sources are < 100 ohms then 5k will be better.
3. Upstream source devices have low output impedance, ideally < 500 ohms or at least 10x lower than the attenuator input impedance.
4. Downstream devices (amps) have high input impedance, ideally > 25 kOhm or at least 10x higher than the attenuator output impedance.
5. Use short low-capacitance cables for all connections -- especially to downstream devices (amps).
6. Ensure your design individually grounds de-selected upstream devices, to prevent de-selected devices from driving each other or leaking into the selected source.
Anything else?