I'm looking to build a passive preamp/volume control that will allow me to switch between 2 separate amps as well as 2 or 3 inputs, Can someone point me to a schematic that shows this? Basically, I want to power 2 sets of speakers (Suoer Pensils and Forte II's) from 2 separate amps (SET for the Pensils and a t-amp for the Fortes) while sharing sources (1 or 2 phono preamps/turntable and a DAC/CDP). This will allow me to do all switching from the passive without having to switch interconnects or speaker wires constantly. I'd also like to wire in dual mono Alps pots instead of one stereo volume pot, for better control and channel balance.
I'm a new builder, so while I can envision what I want, I'm not sure how to design and build it. Any help or direction is appreciated!
Tom
I'm a new builder, so while I can envision what I want, I'm not sure how to design and build it. Any help or direction is appreciated!
Tom
Hi,
using Potis for a passive volume control can present problems if the resistance value of the Poti is high, the cabling to the following device is long and if the input inpedance of that device is low.
A poti has a constant input impedance of its specced value, but its output impedance varies between 0R for volume level=0 and 1/4th the specced value for the volume level=max.
The output resistance forms a lowpass filter with the capacitive load of the cable and the following device´s input.
The bandwidth of the whole system becomes level dependent and could even fall below 20kHz.
If the input impedance of the following device(s) is too low this may also generate problems with the full power bandwidth as the high resistance poti output can´t supply for enough current to drive that load.
For short cable lengths and defined impedance values a poti may well suffice though.
Its prefferable to work with a attenuator that features a lowish and constant output impedance.
You can find such devices working with switched resistor networks.
The imho best implementation use bit-wise switched networks, so that You switch 63 volume steps with a 6bit and 127 steps with a 7bit network.
Here You find only 2-3 resistors within the direct signal path at typical listening levels.
Such a network is best controlled by a microcontroller, which can offer a lot of comfort in handling and input channel switching at the same.
Have a look at this "Audio Volume Relay Attenuator with IR Control"
See als o the RelayXed on the same website.
There´s also a calculator if You want to design eg. a lower impedance network.
I think, that besides the great sonic qualities possible with this kind of attenuator, the suggested device comes a close as possible to Your demands.
jauu
Calvin
using Potis for a passive volume control can present problems if the resistance value of the Poti is high, the cabling to the following device is long and if the input inpedance of that device is low.
A poti has a constant input impedance of its specced value, but its output impedance varies between 0R for volume level=0 and 1/4th the specced value for the volume level=max.
The output resistance forms a lowpass filter with the capacitive load of the cable and the following device´s input.
The bandwidth of the whole system becomes level dependent and could even fall below 20kHz.
If the input impedance of the following device(s) is too low this may also generate problems with the full power bandwidth as the high resistance poti output can´t supply for enough current to drive that load.
For short cable lengths and defined impedance values a poti may well suffice though.
Its prefferable to work with a attenuator that features a lowish and constant output impedance.
You can find such devices working with switched resistor networks.
The imho best implementation use bit-wise switched networks, so that You switch 63 volume steps with a 6bit and 127 steps with a 7bit network.
Here You find only 2-3 resistors within the direct signal path at typical listening levels.
Such a network is best controlled by a microcontroller, which can offer a lot of comfort in handling and input channel switching at the same.
Have a look at this "Audio Volume Relay Attenuator with IR Control"
See als o the RelayXed on the same website.
There´s also a calculator if You want to design eg. a lower impedance network.
I think, that besides the great sonic qualities possible with this kind of attenuator, the suggested device comes a close as possible to Your demands.
jauu
Calvin
A vol pot with a load does not have a constant input impedance.
A high receiver to source impedance ratio helps to reduce the effect of variable vol pot impedance but does not eliminate it.
Two examples follow. One using 1:5 and the the other using 1:20 ratio of impedances.
Vol pot = 10k, Receiver Rin = 50k
Vol pot input impedance = 10k||50k when set to -0dB i.e. 8k3
But when set to -infinity the Rin = 10k
Vol pot = 10k, Receiver Rin = 200k (Pass B1)
Vol pot input impedance = 10k||200k when set to -0dB i.e. 9k5
But when set to -infinity the Rin = 10k
Both the input impedance and the output impedance of standard vol pot attenuators vary for different settings of the volume.
There are special attenuators that can maintain close to zero change in input and output impedances, but we tend to see these in measuring instruments, not audio gear.
A high receiver to source impedance ratio helps to reduce the effect of variable vol pot impedance but does not eliminate it.
Two examples follow. One using 1:5 and the the other using 1:20 ratio of impedances.
Vol pot = 10k, Receiver Rin = 50k
Vol pot input impedance = 10k||50k when set to -0dB i.e. 8k3
But when set to -infinity the Rin = 10k
Vol pot = 10k, Receiver Rin = 200k (Pass B1)
Vol pot input impedance = 10k||200k when set to -0dB i.e. 9k5
But when set to -infinity the Rin = 10k
Both the input impedance and the output impedance of standard vol pot attenuators vary for different settings of the volume.
There are special attenuators that can maintain close to zero change in input and output impedances, but we tend to see these in measuring instruments, not audio gear.
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Something like these perhaps?
Modular Preamplifier Switching Center - RED - Page149
Passive Line Stage Schematic
Or you could use some of the boards from eBay if you're not into building from scratch like these. I put up a post recently asking about these boards and it seems these will work as both 3 in and 1 out or 1 in and 3 outs.
In all these cases you'd have to wire in your mono pots as well but that's the easier part.
Modular Preamplifier Switching Center - RED - Page149
Passive Line Stage Schematic
Or you could use some of the boards from eBay if you're not into building from scratch like these. I put up a post recently asking about these boards and it seems these will work as both 3 in and 1 out or 1 in and 3 outs.
In all these cases you'd have to wire in your mono pots as well but that's the easier part.
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