Mic Preamp with a 5534

Other Hardware General Electronics
1

Hello everyone!

I have a question about a mic preamp I'm going to attempt to build.

I'm planning on using the 5534, because I've been told they will work well, and I have a few of them already!

The circuit I found online is attached, but in my project, I already have a dual +/- 15V supply, so I'm wondering if I could use that for the circuit.

My other question is, if I'm able to use the full swing of the dual supplies, what would change in the circuit? Would everything remained connected to ground like it is, except for pin 4, which would then be connected to the -15V supply with a bypass cap?

Would it make sense to use two of these in series, at a lower gain each, to get a better signal? Or is it better to just use one at a full gain?

Thanks!

(Credit Andy Collinson for the circuit diagram off the net)

tl071_mic.png

2

The circuit would be radically different.

For a start, the voltage divider (2x47k) would not be needed. The current 0V would become -15V, and most things that connect to it would instead connect to the new real ground.

3

Like this circuit?

Except 15V instead of 12...

Screen Shot 2012-09-12 at 11.15.42 AM.png

4

Yup. That's your basic inverting amplifier.

What are you using as a microphone?

5

Likely going to use a Sennheiser HMD25, or a Remote Audio product that's a combo of a pair of Sony 7506 headphones and a Shure headset gooseneck.

The 5534 will be able to use the +/- 15V correct?

6

If the microphone is a little electret mic (which it probably is) you'll need to provide it with power. Usually a resistor of (around) 3.3K between the mic side of the input capacitor and V+

As for can the 5534 use +/- 15V - what does the data sheet say?

7

Yes the SA/NE 5534 will work from +/- 3 to 20 V (data sheet attached). Sure seems like overkill for an Arduino, not necessarily for the microphone you mentioned but a good quality very low noise professional grade amplifier. An LMC622 would work as well slightly noisier but Much easier to implement especially with a single ended supply... and it will go Rail to Rail both input and output. However you use what you have and know...

Doc

NE_SA_SE5534_A_2.pdf (109 KB)

8

This is for another part of a project that I'm trying to fit the Arduino into. I have a little pre-built mic preamp that I might have to put in this enclosure, but I'm trying to create one instead, so I can save some space.

I tried a really quick build with the 5534 before I had to leave for work, and I'm sure I'm doing something wrong somewhere, maybe too much gain?? I set the resistors at 1k for the Input resistor and 100k for the feedback resistor to try to get some more voltage.

All I got was just a bunch of noise, no signal. I didn't have the first cap on hand, so I didn't use it... Could that have caused my problem?

Today I was using an SM58.

I'll have to investigate to figure out what I did wrong. Any thoughts?

Thanks!

9

Just as a side comment, the 5534 was a very popular op-amp choice for the Hi-Fi stereo folks to use in audio circuits. Very good noise specs for the price.

10

If you are doing a low-noise amplifier you can't just stick 2k2 resistors in the input signal path, the thermal noise from such a large resistance is likely to completely limit the noise performance. IIRC the NE5534A has 4nV/root-Hz so its pretty good - keep input resistors below a few hundred ohms or so to avoid compromising that performance. 4nV/root-Hz is equivalent to 1k resistor noise, adding another 2k2 in circuit will up total noise to about 7nV/root-Hz (3 times the noise power).

11

I tried a really quick build with the 5534 before I had to leave for work, and I'm sure I'm doing something wrong somewhere, maybe too much gain?? I set the resistors at 1k for the Input resistor and 100k for the feedback resistor to try to get some more voltage.

A gain of 100 seems about right, but with acoustic audio you always need a gain control or a volume control. From what I've read, an SM58 is not a particularly sensitive mic, but it can put-out nearly 1V if you stick it directly in front of a kick drum...

All I got was just a bunch of noise, no signal. I didn't have the first cap on hand, so I didn't use it... Could that have caused my problem?

Yes, that could be a problem, especially with a single-supply. The mic will kill the bias (half the supply voltage) and you are putting DC voltage into the mic. It's unlikely, but you can potentially damage the mic by doing that! (Phantom power used by studio condenser mics is applied common-mode between the signal lines and ground, so that the mic won't get damaged, even if you connect a dynamic mic, which doesn't use phantom power.)

Today I was using an SM58.

I assume that has an XLR connector? It's also possible that your mic is connected incorrectly. All studio/performance mics use a balanced (3-wire) connection. A proper microphone preamp will have a balanced [u[/u]](Operational amplifier applications - Wikipedia) input. (Sometimes, a transformer is used in place of a differential amplifier.) If you need high-quality audio, you should use a proper mic preamp with the proper low-impedance balanced input.

If you are simply running the audio into the Arduino and you don't need a clean low-noise signal, you can get-away with connecting one of the signal lines to ground. In any case, you need to connect all 3 wires coming from the mic.

P.S.
If you are running the preamp-output into an Arduino, you need to protect the Arduino input from negative voltages and/or positive voltages greater than 5V. (You can do that with a resistor and a pair of diodes.)

12

I always try and ask in posting like this, what is it that you are expecting to do with the audio signal once it is properly interfaced to an arduino analog input pin? There are real limits on a AVR 8 bit controller as far as processing speed and SRAM space to do much with audio signal information. Or is this not an arduino application posting, which would still be fine as this is the 'general electronics' section?

Lefty

13

DVDdoug:

I tried a really quick build with the 5534 before I had to leave for work, and I'm sure I'm doing something wrong somewhere, maybe too much gain?? I set the resistors at 1k for the Input resistor and 100k for the feedback resistor to try to get some more voltage.

A gain of 100 seems about right, but with acoustic audio you always need a gain control or a volume control. From what I've read, an SM58 is not a particularly sensitive mic, but it can put-out nearly 1V if you stick it directly in front of a kick drum...

It usually has a decent output, not one of the strongest I've used, but it's not bad either!

All I got was just a bunch of noise, no signal. I didn't have the first cap on hand, so I didn't use it... Could that have caused my problem?

Yes, that could be a problem, especially with a single-supply. The mic will kill the bias (half the supply voltage) and you are putting DC voltage into the mic. It's unlikely, but you can potentially damage the mic by doing that! (Phantom power used by studio condenser mics is applied common-mode between the signal lines and ground, so that the mic won't get damaged, even if you connect a dynamic mic, which doesn't use phantom power.)

I'm actually using a bipolar supply with +/- 15V. I wasn't using any type of phantom power. I'll have to get the cap tomorrow..

Today I was using an SM58.

I assume that has an XLR connector? It's also possible that your mic is connected incorrectly. All studio/performance mics use a balanced (3-wire) connection. A proper microphone preamp will have a balanced [u[/u]](Operational amplifier applications - Wikipedia) input. (Sometimes, a transformer is used in place of a differential amplifier.) If you need high-quality audio, you should use a proper mic preamp with the proper low-impedance balanced input.

Today I was just using it unbalanced, and I didn't use a differential amp, which I'll try to do tomorrow as well. I'm fairly sure I connected the -terminal to ground. I usually do...

This is going to be used for communication, not for anything that's mission critical where sound quality would be an issue. All of it will be off-air stuff.

If you are simply running the audio into the Arduino and you don't need a clean low-noise signal, you can get-away with connecting one of the signal lines to ground. In any case, you need to connect all 3 wires coming from the mic.

14

Lefty...

This is not a specific Arduino project, although I'm trying to get my Arduino to interface with this.

This will be a pre-amp that will be mixed with another signal and end up in headphones after some switching (where the Arduino might come in). The rest of the circuit works great, but I'm trying to build my own pre-amp as it will significantly help keep the space in my enclosure at a manageable level, and limit the number of power supplies going into this box which is a huge concern since I have to travel this thing across the country while being checked as luggage on an airline, and you all know how they can be with luggage and weight!

I think my problem was answered, but I have to try it tomorrow... I realize that I didn't use it as a differential amp. DVDdoug, thanks for pointing that out. It had nothing to reference the signal against.

15

card9inal:
Lefty...

This is not a specific Arduino project, although I'm trying to get my Arduino to interface with this.

This will be a pre-amp that will be mixed with another signal and end up in headphones after some switching (where the Arduino might come in). The rest of the circuit works great, but I'm trying to build my own pre-amp as it will significantly help keep the space in my enclosure at a manageable level, and limit the number of power supplies going into this box which is a huge concern since I have to travel this thing across the country while being checked as luggage on an airline, and you all know how they can be with luggage and weight!

I think my problem was answered, but I have to try it tomorrow... I realize that I didn't use it as a differential amp. DVDdoug, thanks for pointing that out. It had nothing to reference the signal against.

I spent quite a few years collecting and repair vintage 1970s classic solid state hi-fi equipment. Anyway while in the hobby I found a great site for audio circuits, most have outstanding performance and very easy to duplicate. Elliott's site has tons of good information and is great for just general electronics. See the section covering mike amps:

http://sound.westhost.com/projects-0.htm#mic

There is even a 5532 based one.

Lefty

16

Lefty,

I'm building that circuit, figure it might be the easiest way to go about this... But I'm not sure what the Cb is on the 100nF cap between the +12V and the -12V terminals.

What does Cb mean? I've never seen that designation. Is it just a ceramic disc?

Please help!

Thanks!!

17

Bypass Capacitor (Capacitore bipasse ]:smiley: )

18

card9inal:
Lefty,

I'm building that circuit, figure it might be the easiest way to go about this... But I'm not sure what the Cb is on the 100nF cap between the +12V and the -12V terminals.

What does Cb mean? I've never seen that designation. Is it just a ceramic disc?

Please help!

Thanks!!

Yes, just a bypass wired across the two power voltage rails placed as close to the op-amp as practical. Any small ceramic cap in any package style is fine. And yes Cb is not a common symbol, maybe it's just an Aussie thing?
Lefty

19

Must be!

It took me a while, as I kept screwing up, but I finally got that circuit working! There's not a whole lot of gain on it, so I increased the feedback resistor, using a 100kOhm pot, and then the output of that goes into a TL074 where it gets amped again. However, there seems to be more noise than there should be. It's really pretty noisy.

Where would I start to look to lower the noise?

Other than that, it works pretty well!

I'm going to try to build the 5534 again, thinking that might be less noise, but unbalanced.

20

The 5534 is very good for an opamp, 3.5nV/root-Hz, but for cutting edge noise performance you want more like 1nV/root-Hz, and there are only a handful of op-amps that can manage that - individual low-noise transistor differential input stage would be cheaper. Make the input stage have lots of gain so that the next (noisey) stage can't add more effective noise to the signal (30x voltage gain perhaps).

And no resistors in the input signal path, they just add noise (all resistors generate noise). Feedback network needs to be very low impedance to reduce noise (perhaps 1k / 33ohm resistor divider) - the noise in the 33 ohm resistor will add to the signal but 33 is low enough (noise voltage proportional to square root of R, in fact about 0.13 sqrt(R) in nV/root-Hz, thus 33 ohms is 0.75nV, 1k is 4.1nV)

To get best low-noise performance from a transistor you have to set the collector current at the sweet spot for the device in question, often around 0.1mA.

Google for suitable circuits and look for noise performance near the 1nV/root-Hz range.

Incidentally 1nV/root-Hz is about 0.14 uV RMS across the audio band. 4nV/root-Hz is 0.56uV RMS... For 60dB S/N ratio the Vrms of the signal needs to be 1000 times the noise Vrms.

If you do want op-amp design I can recommend the AD8656 dual opamp - 2.7nV/root-Hz (better than NE5534 and low-voltage rail-to-rail so can use from single 5V supply).