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Old 24th July 2014, 10:16 PM   #1
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Default Guitar Level to Line Level Hi-Quality Preamp

Hi All,

So I've decided to build an instrument/guitar level (-20db) to line level (-10db) preamp for a recording interface. I want to use a high end op-amp to do the job, and through forum surfing came across the OPA134 as a potential candidate to do the job. I'm using a schematic for a TL071 as reference (pictured below) since this is my first preamp build. I have a couple questions regarding my design:

1. Does the supply voltage affect the circuit other than defining the range of the input/output voltage? Guitar signals use very low voltage (passive pickups are anywhere from 0.1V to 1.0V, active up to 1.75V), so I guess lowest supply I could use would be a 9V supply, since the pickup + gain shouldn't be more than (1.75V + (1+220k/100k) = 5V)?
2. Do I need a completely different design than the one pictured? Or is this a good reference to use? I know different op-amps call for different combinations and placements of resistors and capacitors, but I'm having trouble finding a similar project with an op amp in the OPA family.
3. If I wanted to put a gain knob on the op-amp, would this be achieved by putting a pot in place of R2, with the value of the pot being 220K?

I'm going to start putting together a schematic and order some parts to mess around with, but any guidance is greatly appreciated!


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Old 25th July 2014, 02:40 PM   #2
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Power supply voltage has nothing to do with signal (audio) voltage. The power supply only runs the amp, it doesn't interact with the audio signal. Or that is, it doesn't interact unless the audio input voltage approaches the power supply voltage, see next paragraph.

Broadly speaking, an audio input voltage can come within 2 volts of the power supply voltage. That is, if your power supply voltage is +/- 16 volts (for instance), then an audio input up to 14 volts is OK. Running a 9 volt single supply (still for instance), maximum input would be 2.5 volts.

(It's 2.5 volts because the 9 volts is actually divided into two voltages of 4.5 volts each to run the amp.)

If you play heavy metal, the output of a guitar pickup can be several volts--but probably not 14 volts. Same for piezo pickups.

The TL072 is not used for hi-fi. I suggest using the NE5532 (about 50 cents apiece on Bay). The NE5532 was the top of the line for many years, every studio in the world had and has dozens or hundreds of them in their boards and amps. Recently developed chips like the LM4562 (about $5), or OPA134, do have better specs on paper, but I personally don't hear that the paper specs make any real world difference.

Getting to cases, what you need is a buffer. A buffer has extremely high input impedance (100s of megohms), so in effect it "looks at" an audio input source (guitar pickup), but does not affect that source in any way. This is exactly what you want with audio inputs.

I happen to have a schematic on hand, so I'm posting it.

I'm also posting a representative (only that) dual power supply schematic. Please give serious thought to using a dual power supply, as opposed to a single supply, because dual power supplies make amp circuits simpler. The NE5532 runs happily at around +/- 16 volts, which you get from a 24 volt center tapped transformer.

Some circuit notes follow. Especially notice at the end for guitar pickups.

Hope this might be of some help.

===

Circuit notes:

R1 is overcurrent protection for the NE5532, and also guards against oscillation.

C1 shunts radio frequency energy to ground. Both stage and studio environments can be expected to have high levels of radio frequencies.

R2 is a load resistor, put there because you usually don't know what the input signal is going to come from. By providing a current path to ground R2 ensures that there will be voltage at the input side of C1. R2 in parallel with R3 sets the input impedance, which here is about 20k.

(If R2 is not present, then the value of R3 is the input impedance.)

C2 and R3: C2 blocks DC, and with R3 forms a high-pass filter, also called a bass rolloff filter. Here the rolloff starts at just over 20 Hz.

U1a (1/2 of a NE5532) is a buffer. It's characteristic of op amps that connected this way they have unity gain, extremely high input impedance, and extremely low output impedance, all of which is ideal for audio inputs.

Rvol is the volume control. The value can actually be anything from 2k (no lower) on up.

U2b (the other half of an NE5532) is the gain stage, which here is arbitrarily set at a gain of 11 (21 dB). Gain can be anything theoretically, but keep it to 20 or so in the real world. The formula is simply R4 divided by R5, and add 1. That is:

Gain = R4/R5 + 1

For preference, leave R5 at about 2k and adjust R4. Higher resistance for R4 = more gain. R5 no lower than 1k, if you must.

The output is low impedance, and should be suitable for the line-in input of a mixer, preamp, or amp.

NOTE: The 20k input impedance shown here is much too low for instrument pickups, such as on a guitar or bass. It will work, but not really well. To be strictly correct with an instrument pickup, change both R2 and R3 to 1meg, for a 0.5meg input impedance. Change C2 to 0.068uF

Caution: 1meg is a very high value in audio terms, and noise problems can multiply. Keep leads short, use shielded wire, and ground everything, including the body of pots (Rvol).
.
Attached Images
File Type: jpg NE5532-buffer-preamp_01_b.jpg (74.0 KB, 158 views)
File Type: jpg power_supply_dual_small_B.jpg (62.6 KB, 153 views)

Last edited by bentsnake; 25th July 2014 at 03:08 PM.
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Old 25th July 2014, 03:01 PM   #3
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To be honest, I suggest the first schematic using TL071 is much better as guitar buffer. NE5534 is a superior choice when driven by lo impedance source - but they are a no-go as input stage of hi-impedance sources like magnetic guitar pickup. Due to the high noise current, the TL071 will by far outperform the NE5534 in this application.
Furthermore it is no good idea to load the PU with an input impedance of about 23kOhm, which kills any overtones yielding a muffling sound.
All in all, there is absolutely nothing bad with that simple TL071 input stage.

Just my 2c
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Old 25th July 2014, 03:27 PM   #4
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voltwide, you didn't read the circuit notes, did you? Nooo, you didn't. They might be of interest.

<< I suggest the first schematic using TL071 is much better as guitar buffer >>

The TL071 circuit shown is not a buffer, it's an amp. Two different animals entirely.
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Old 25th July 2014, 03:46 PM   #5
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Well...wait...hold everything.

I have to semi-agree with voltwide. The TL071 circuit shown would certainly work. Bolt it together just as shown, don't change a thing, and it will do the job for you. Just be sure to keep connecting leads as short as possible, those resistors are waaay up into funny-noises territory.

Those should have been the first words out of my mouth...keyboard. Then I should have said something like, "But you might not have the audio quality you want for a studio setup, and if not here's an alternative," or to that effect.

So questions of impedance (not a problem) aside, maybe I did overreact and make things complicated. My bad.
.

Last edited by bentsnake; 25th July 2014 at 03:50 PM.
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Old 25th July 2014, 06:42 PM   #6
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Quote:
Originally Posted by bentsnake View Post
.
voltwide, you didn't read the circuit notes, did you? Nooo, you didn't. They might be of interest.

<< I suggest the first schematic using TL071 is much better as guitar buffer >>

The TL071 circuit shown is not a buffer, it's an amp. Two different animals entirely.
.
Whether you run the input amp at V=1 or V=3 does not really make the difference, g=3 is a good starter for a guitar input amp when powered by 9V.
I am in that business for decades. And your circuitry is much more complicated compared to the TL071 version, including the split supply, all that to no advantage, but with totally wrong input impedance and sub-optimal noise performance.
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Old 25th July 2014, 06:51 PM   #7
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Quote:
Originally Posted by bentsnake View Post
.
Well...wait...hold everything.

I have to semi-agree with voltwide. The TL071 circuit shown would certainly work. Bolt it together just as shown, don't change a thing, and it will do the job for you.
.
Thats it - don't waste your time with fiddling about with pre-Amps,
the sound of the guitar is at your fingertips
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Old 25th July 2014, 08:55 PM   #8
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Thanks to the both of you for your help! Since this preamp is a smaller piece in a bigger picture, it's probably best that I not do too much fiddling and stick to the working schematic I have. That being said, if anyone else has a preamp schematic for an op amp in the OPA series, I'm all ears! One question I do have is what's the difference between a buffer and a preamp? If the buffer really does just "look at" the signal, then why's it used? To get high impedance on the input? Why can't I just do that with resistors? Sorry if these are dumb questions, I'm fairly new to op amps. And if this is just a buffer, and it's all I need to get a good clean guitar signal into my ADC, then what would a preamp be used for in place in another scenario? For tone?

Secondly, bentsnake mentioned that if I play a lot of metal, I'd be looking at a higher voltage. My understanding was that passive and active pickups have a collective range of about 0.1V to 1.75V. If that's the case, I should be okay in using a 5V line, correct? The reason I'm asking is because my project currently runs off of a 5V dc supply, and while I could definitely use an external 9V supply and then later integrate a 9V supply line into my project, it'd definitely be easier to just use 5V. Obviously I want a full range for guitar input, so if 5V wont cut it, then I'll use 9.

Thanks again to the both of you!

Pat
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Old 25th July 2014, 10:34 PM   #9
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Guitar output level might vary over a wide range of voltage.
It depends on
-the pickups
-distance between pickups and strings
-thickness of strings
-the way you strum

I measured my strat and the most I could get out of it was about 2Vpeak-to-peak.
A humbucker will deliver even more.

If your supply is 5V, a buffer with FET-input and rail-to-rail input and output may be the best choice.

Anyway it is a good idea to measure with an oscilloscope the voltage you get out of your instrument.
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Old 25th July 2014, 10:47 PM   #10
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<< bentsnake mentioned that if I play a lot of metal, I'd be looking at a higher voltage. My understanding was that passive and active pickups have a collective range of about 0.1V to 1.75V >>

My mouth ran faster than my brain, sorry. I don't know the actual output of your pickups. I do know that the harder you hit it, the higher the output voltage, but I can't quote a real-world value.
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