audio attenuator
Some time ago "keith's electronics blog" featured an attenuator. In the comments on his blog it is pointed out that an attenuator is not as trivial as it seams.
To understand why let's have a look at a typical input filter of an amplifier, in this case a sony gx70 receiver. Notice a series resistor of 1k and a 220p capacitor to ground. When driven by a low source impedance the cutoff frequency will be above 500kHz. You can do the math yourself.
When the source impedance is high then the cutoff frequency will be lower. I am a lazy sod so I spiced the circuit, less calculating more fun.
You must also take the cable capacitance into account, I measured the capacitance of the cable below to be 130p.
japanese solution
I started off with a real life example. Below is the output section of a vintage teac V-2RX cassette machine. Output level is set via a 20k log pot (A is log in japan).
The worst case is when the wiper is in the divide by 2 position.
The value of R1 is more like 100 ohm in reality, with 1k it is actually worse. Roll off starts around 30kHz but you will have to be careful with a long cable.
keith's solution
Keith uses a large resistor in series with the potmeter. Again we consider the worst case scenario. The 1 meg resistor can be ignored since it is 10 times higher than the pot. That leaves us with 100k source resistance. You can see for yourself what happens.
cable attenuator
Recently I made an attenuator for my active speakers. I crammed the resistors in a xlr plug with a short lead to the input of the active speakers.
I only needed roughly 10 dB attenuation and I didn't want the source resistance to be more then 2k, hence the rather low load resistance. One must be careful though when lowering the load resistance, there is also the output capacitor to be taken into account.
To understand why let's have a look at a typical input filter of an amplifier, in this case a sony gx70 receiver. Notice a series resistor of 1k and a 220p capacitor to ground. When driven by a low source impedance the cutoff frequency will be above 500kHz. You can do the math yourself.
When the source impedance is high then the cutoff frequency will be lower. I am a lazy sod so I spiced the circuit, less calculating more fun.
You must also take the cable capacitance into account, I measured the capacitance of the cable below to be 130p.
japanese solution
I started off with a real life example. Below is the output section of a vintage teac V-2RX cassette machine. Output level is set via a 20k log pot (A is log in japan).
The worst case is when the wiper is in the divide by 2 position.
The value of R1 is more like 100 ohm in reality, with 1k it is actually worse. Roll off starts around 30kHz but you will have to be careful with a long cable.
keith's solution
Keith uses a large resistor in series with the potmeter. Again we consider the worst case scenario. The 1 meg resistor can be ignored since it is 10 times higher than the pot. That leaves us with 100k source resistance. You can see for yourself what happens.
cable attenuatorRecently I made an attenuator for my active speakers. I crammed the resistors in a xlr plug with a short lead to the input of the active speakers.
I only needed roughly 10 dB attenuation and I didn't want the source resistance to be more then 2k, hence the rather low load resistance. One must be careful though when lowering the load resistance, there is also the output capacitor to be taken into account.
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