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Old 22nd July 2014, 02:36 AM   #1
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Default Getting Started on Preamp Design

Hi All!

First post on this forum, I've been a huge fan of all the info that's available on here. Basically I want to get started on building a pre-amp circuit. I want to integrate it into an ADC/DAC design (specifically, something similar to the XMOS reference designs, if anyone's familiar with their products). I'm looking to design a clean, low-noise amplification for instrument level inputs to line level outputs. I'm new to the audio design community, and while I've done a good amount of research on different pre-amp circuits, I was wondering if any of you had some suggestions on documentation that helped you build pre-amp designs. I'm not sure what type of op-amp to use, if I need multiple op-amps (if I need none?), if it should have an inverting or non-inverting input, etc, so any help is much appreciated! Thanks in advance,

Pat
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Old 22nd July 2014, 02:46 AM   #2
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Hi Pat, and welcome to diyAudio! I've moved your thread to the analog line forum as I think that is a more appropriate place for it

Tony.
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Old 22nd July 2014, 02:47 AM   #3
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Noted, thanks!
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Old 22nd July 2014, 11:28 AM   #4
udok is offline udok  Austria
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What inputs do you need?
What is the voltage level of the inputs?
Do you need equalizer circuits or other features?
Do you need outputs for an power amp besides line level outputs?

NE5532 is a good choise for starting.
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Old 22nd July 2014, 06:24 PM   #5
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- I need two inputs for instrument level signals.
- Isn't this standard across instrument level signals? I looked it up quickly, and it says 0.1V to 1.0V for passive signals, up to 1.75V for active, and I'd like the whole range, so I guess 0.1V to 1.75V?
- I don't need any features other than gain control.
- No levels for power amps, just the line level output.

I'll check out the NE5532 for now, thanks for the reply!

Pat
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Old 23rd July 2014, 08:47 AM   #6
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This site has a lot of info you might find useful...(sound .westhost .com)
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Old 24th July 2014, 05:43 PM   #7
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<< I need two inputs for instrument level signals - Isn't this standard across instrument level signals? I looked it up quickly, and it says 0.1V to 1.0V for passive signals, up to 1.75V for active, and I'd like the whole range, so I guess 0.1V to 1.75V? >>

You generally stick with the non-inverting input of an op amp because that's the high impedance side.

Broadly speaking, the input of an op amp can be within 2 volts of its power supply voltage. That is, if you power an NE5532 with +/- 15 volts, then any input up to 13 volts is OK. Data sheets give exact numbers, of course.

There is no lower limit of input if you use a buffer. Within reason, of course. Nuclear submarines have special requirements.

Back in the day "line" meant one volt. But today there are at least 3 "line" standards (if they are standards) that I know of. Still you'll be OK if you consider line to be the classic one volt.

"Instrument" can mean a lot of different things around here...or anywhere. Electret mics put out maybe 2 millivolts, piezo transducers might put out several volts, as might magnetic pickups on guitars (instruments). Might be best to specify.

I'm posting a schematic that's something of a do-all. Not that you should solder it up right away (although you could, and it would work), but as a starting point, or an illustration of what you seem to be saying.
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File Type: jpg NE5532-buffer-preamp_01_b.jpg (74.0 KB, 133 views)

Last edited by bentsnake; 24th July 2014 at 05:55 PM. Reason: clarity
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Old 25th July 2014, 03:23 PM   #8
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bentsnake, thanks so much for your response, your provided schematic especially helps visualize everything you said. Firstly, I'd like to note (since I haven't flat out said it) that I'm using this preamp for an audio recording interface for musical instruments, specifically direct-in guitars and possibly expanding to microphones. I used the vague term "instrument level" because thats what the -10db level in consumer recording electronics is referred as, which specifically is the range of most passive guitar pickups. So to sum it all up, this project is for signals coming from a guitar pickup.

Before I go on, I've done quite a bit more research, and while the NE5532 seems to be a great, low cost op-amp that's been used in so many audio applications, I've begun looking into the OPA series by TI, specifically the OPA134. It seems like this op-amp and the others in the series are considered some of the highest quality for audio recording, and while the difference between the two may be small, I'd definitely prefer to build my project off of the best quality op amp off the bat, rather than do it later. Any opinions on this decision would be appreciated. But for now, I'll go off of the information you've provided so far!

Quote:
Originally Posted by bentsnake View Post
.

Broadly speaking, the input of an op amp can be within 2 volts of its power supply voltage. That is, if you power an NE5532 with +/- 15 volts, then any input up to 13 volts is OK. Data sheets give exact numbers, of course.

.
So this is the first time I've noticed that it's the input of the op amp that is restricted to the supply voltage. I assumed it was the output that was restricted to the supply voltage. Does this mean that since my guitar levels will never exceed 2 volts, I could run my op amp off of a 5V supply?

Quote:
Originally Posted by bentsnake View Post
.

There is no lower limit of input if you use a buffer. Within reason, of course. Nuclear submarines have special requirements.

.
Forgive my basic questions, but I'm still struggling to understand the applications for a buffer vs the gain stage. What does each do, and why cant they be done in one stage?

Quote:
Originally Posted by bentsnake View Post
.

Back in the day "line" meant one volt. But today there are at least 3 "line" standards (if they are standards) that I know of. Still you'll be OK if you consider line to be the classic one volt.

.
Yes, just the standard 0db (1V) is what I'm looking to output.

Thanks again for the help!!

Pat
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Old 25th July 2014, 03:47 PM   #9
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Another two questions upon further inspection of your schematic:

There isn't much resistance on the input of the circuit... Guitar signals are high impedance, so it was my understanding that the resistances should be high on the input. Most schematics have at least 100k on the input, with the norm being 1M ohm of impedance.

Second, can I have any value pot to control the volume? I have a bunch of 10k pots lying around, I feel as if they could get the job done.

Thanks again!

Pat
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Old 25th July 2014, 04:04 PM   #10
Mooly is offline Mooly  United Kingdom
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A fet opamp like the OPA series is better suited to a high input impedance. In the circuit above that means R3 (and R2 because its in parallel at AC) can be 1meg (or higher if needed). 10K pots are fine too.

The output of any opamp is limited to the rail voltage (less a small amount), the inputs need to be limited because if they exceed the rails they will begin to force current into the various semiconductor junctions within the IC, possibly destructively if there is enough current behind it.
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