Author Topic: Why would you lower the input impedance to a pre-amp?  (Read 607 times)

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Offline KarlTopic starter

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Why would you lower the input impedance to a pre-amp?
« on: October 08, 2021, 03:46:55 am »
Hi,

I've been working on an old Roland mixer/PA combo and I've just noticed something that has me scratching my head.

The mixer has a total of eight channels - each channel has an input impedance of 10kohms. Someone in the past has added a 2.2kohm resistor across each of the inputs which, in parallel with the 10k input impedance reduces the input impedance to about 1.8kohms.

My question is why would someone do this? For general source connectivity to a preamplifier we require a low source impedance while the input impedance to the preamplifier needs to be high for voltage sources. In this case the sources would likely be dynamic microphones or outputs from electric guitars - these have an output voltage of 20 - 100 millivolts or thereabouts (peak to peak). The preamplifier specification for these inputs is -50dBm to -10dBm.

The only reason I can think why this was done was to use these inputs as line in inputs with a higher input voltage. Line out signals are generally about 1.0 volts, but for this to work, if my understanding is correct, then the source impedance would also be quite high (say 600 ohms) in order to see an effective reduction of voltage due to the combination of the source and input impedances.

If the source impedance is low (say less than 100 ohms) then there would be little reduction of the source voltage and the lower input impedance wouldn't have much effect.

I'm not sure if my thinking on this is correct so any advice would be appreciated.

Regards,

Karl

 

Offline bdunham7

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Re: Why would you lower the input impedance to a pre-amp?
« Reply #1 on: October 08, 2021, 03:58:13 am »
Your thinking appears sound and it may well have been an ad-hoc modification to interface with an older system of some type that did have 600 ohm outputs, possibly transformers. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline bob91343

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Re: Why would you lower the input impedance to a pre-amp?
« Reply #2 on: October 08, 2021, 04:03:27 am »
Perhaps to control noise.  Hum pickup from a high impedance source for instance.  Or even rf.
 


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