Author Topic: Input impedance selector - explanation needed  (Read 1068 times)

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Offline TomerTopic starter

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Input impedance selector - explanation needed
« on: March 03, 2019, 10:23:30 pm »
Hello All, and thank you for your help.

In Pro Audio pre-amps, there is usually an input impedance selector. A good example can be a classic pre-amp and compressor from UA model 6176.
https://www.sweetwater.com/store/detail/6176--universal-audio-6176

The 6176 offers an impedance range of 50Ohm to 2.2MOhm, and apply the first 3 options to balanced inputs and the last 2 option to an unbalanced inputs.

Appreciate if you can explained how the impedance selector is working and what circuitry is behind such selector.
Any referral to reading materials will be great.

Thank you for your help, helping me to understand the topic.
Tom

 

Offline ataradov

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Re: Input impedance selector - explanation needed
« Reply #1 on: March 03, 2019, 10:52:32 pm »
On a basic level it simply connects a resistor with a specified value parallel to the input. Assuming  that input amplifier itself has an infinite impedance. If not, then the resistor is calculated so that total impedance matches what's on the label.

In real life it may be a bit more complex that this, there may be some frequency compensation circuitry as well.
Alex
 

Offline Audioguru

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Re: Input impedance selector - explanation needed
« Reply #2 on: March 04, 2019, 01:25:08 am »
The very old vacuum tubes circuit claims to produce "vintage coloration" instead of high fidelity. The variable input impedance is part of that.
An electric guitar can sound muffled or bright by changing the input impedance of its preamp. An old moving magnet phono cartridge is the same.
 
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Offline TomerTopic starter

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Re: Input impedance selector - explanation needed
« Reply #3 on: March 04, 2019, 09:47:38 pm »
Thank you ataradov.

Can you explain why the connection needs to be in parallel for this case ?

In addition, doesn't the input device see the load as a complete unit (the impedance sum,  I am lacking a better explanation here) and not only the first resistor ?

Thank you,
Tom
 

Offline ataradov

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Re: Input impedance selector - explanation needed
« Reply #4 on: March 04, 2019, 10:30:09 pm »
Because that's what impedance is if we only consider the real (resistance) part of it. 

As I said, a resistor of that value is only valid if impedance of the amplifier following the input is large (close to infinite). This way your total input impedance of the device is mostly represented by the input resistor, which is adjusted by the knob.
Alex
 


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