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Old 14th July 2014, 11:05 PM   #1
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Default input load/mic impedance

Hi,

This may be an obvious question, but I have not been able to find the answer to this. How can you modify the input impedance of a mic preamp circuit? I have searched in as many ways as I could think of, and found no specific answer. As I understand it, the input load should be 10x the impedance of the mic for optimum performance. I would like to build a preamp based on the NE5532, and I am going to experiment with a variety of capsules with very different impedances (680-2200ohm) as well as a piezo film, which ideally requires a 10Mohm load. What part of the circuit determines the load?

Thanks for your patience with my query!
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Old 14th July 2014, 11:25 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Low noise design is not that simple, and no design will
accomodate such a wide range of input impedances.

The NE5534 is lower noise, here is a simple balanced design :

Click the image to open in full size.

Add supply decoupling capacitors. I'd halve R1 to R5.

If you want lower noise you need transformers or to add
a low impedance low noise front end to the op-amp.

Note input impedance is nominally 600 ohm for mic's,
source resistance is often 100 to 150 ohms, and in
the circuit input impedance is mainly R1 and R2.

Circuits can get far more complicated,
with phantom powering and the like.

rgds, sreten.
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Last edited by sreten; 14th July 2014 at 11:31 PM.
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Old 15th July 2014, 02:00 AM   #3
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Thanks so much for the detailed info! I should be more clear that I intend to breadboard a design to see how different capsules perform. The lowest impedance is a Primo, which IIRC is 600 ohm. These are electret capsules. I would ideally be looking to test how each capsule performs with different loads, so what I need to understand is how to modify that load, what are the elements in the circuit that effect the change in load.
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Old 15th July 2014, 04:02 AM   #4
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Sreten, Does this mean that by changing the values of R1 and R2, you change the input impedance? Is the input impedance of this circuit 1Kohm? I apologize if I sound ignorant, but there good cause for that!
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Old 15th July 2014, 04:04 AM   #5
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I notice there is apparently nothing in the power supply connection for the opamp, is that normal?
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Old 15th July 2014, 03:07 PM   #6
AndrewT is offline AndrewT  Scotland
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There may be some mics that require the load impedance to match their own source impedance. The ten times rule would not apply in this case.

Matching of source and load impedances maximises power transfer and at very low signal levels, max power is equivalent to minimising noise.
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Old 15th July 2014, 03:08 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by nycellist View Post
I notice there is apparently nothing in the power supply connection for the opamp, is that normal?
The Power side MUST follow the recommendations of the amp manufacturer unless you have evidence to vary that.
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Old 15th July 2014, 03:10 PM   #8
AndrewT is offline AndrewT  Scotland
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Electret capsules often have a driver circuit inside the microphone casing.
These driver circuits "boost" the mic signal and prepare it for driving a line level input and all the cable normally associated with mics.
You really have to find out more about your mic and how to interface it with the other electronics.
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Old 15th July 2014, 03:11 PM   #9
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.
Another way to go is with a buffer. NE5532s are very commonly used this way.

Buffers have unity gain, and extremely high input impedance. For practical purposes a buffer loads the preceding circuit not at all.

I just happen to have a schematic handy. Circuit notes:

R1 and R4 are overcurrent protection for the NE5532, they may be omitted.

C1 is to shunt radio frequencies to ground, it may be omitted.

R2 provides a path to ground for the input signal, to ensure there will be a voltage at the input side of C2. It's there because you usually don't know what the input signal is going to be, and might not be necessary. When R2 is used, then input impedance is the parallel value of R2 and R3 (in this case 50K).

R2 (if used) and R3 may be changed for whatever input impedance is desired. Remember that high values (megs) can multiply noise problems severely.

C2 and R3 are the actual input circuit. Adjust their values for whatever low frequency rolloff might be desired. If R2 is not used, then R3 sets the input impedance.

The amplifying circuit (U1b) is just there to use the other side of the NE5532. It could be another buffer for the other side of a stereo circuit, or whatever you desire (but putting two buffers in series would be pointless).

In a sidebar, I don't know that a piezo source really needs to feed a 10meg input impedance. On the other hand, I can't prove it doesn't. But 0.5meg to 1meg are values more commonly used, as far as I know. Up at 10meg noise problems can be horrendous, every passing breeze can throw things out of whack. Shield everything!

Credit where it's due, the input side of U1a is a rip from Douglas Self. Hope this might be of some interest.
.
Attached Images
File Type: jpg NE5532_buffer_a.jpg (36.0 KB, 55 views)

Last edited by bentsnake; 15th July 2014 at 03:27 PM. Reason: clarity
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Old 15th July 2014, 04:19 PM   #10
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Thanks for the replies! I have considered a buffer as an option, and I appreciate your explanations, this makes more of this clear to me. I would like the bass roll of to be @30hz, so what mod to R3 would accomplish that?

As I understand the issue with piezo film, the impedance relationship of 10x produces a more linear response, as a match (or ineffectual mismatch) produces a high pass filter effect and other anomalies that give piezos a bad reputation in amplifying stringed instruments (my interest). I am well aware of the shielding requirements, as well. Any clarification in this regard is really welcome!
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