Guitar preamp op amp tl072 input noise

[macboy]

If you want to try a lower noise, high quality FET input dual audio op-amp, look at TI OPA2134, OPA2132, OPA2604. These are Burr-Brown designs (TI acquired them).

[Ale]

Well, tonight I hadn't enough time to try all. But I build a simple power supply with an EI transformer I have here, without regulators, all over a breadboard, without crossed cable or shielding and... it works pretty well, there is some noise, but really low and acceptable. Now I'm eager to try with a toroidal one, with shielded cable, etc... but I have to buy one 

macboy, the OPA you post seems to be very very good, but they are really expensive. I will probably try one in the future

Meanwhile, thank you all, I learnt alot here and you are all very kind. Now I have to order some stuff, eatching the board and so on, it will take me days I will post the finished project as soon as possible.

[HoracioDos]

I will post the finished project as soon as possible.

Please post the final schematics when it's ready. I would like to see how it works in spice. Thanks!

[Zero999]

You raise some interesting points. Another alternative to regulator(s) is an emitter follower with a RC circuit. It drops some of the supply voltage but can work quite well. I've done it before for the TDA2003, which has a very poor supply rejection, of just 36dB and was a poor choice for a mains powered amplifier. It should be more stable and better at rejecting high frequencies, than a linear regulator.

In this circuit it is needed a resistor from the base of the transistor to ground, because if the capacitor is charged to the peak voltage then it can not do something and the ripple will pass from the collector to emitter ...

Another resistor may help, but it's not a necessity. There will be some voltage drop across R1, due to the base current.

There's a compromise between voltage loss and ripple rejection. The main reason why I added it to my circuit is because the ripple was noticeable on the output of the amplifier, especially at the higher harmonics. I didn't measure the ripple on the supply but estimate it was on the order of 100mV.

Well, tonight I hadn't enough time to try all. But I build a simple power supply with an EI transformer I have here, without regulators, all over a breadboard, without crossed cable or shielding and... it works pretty well, there is some noise, but really low and acceptable. Now I'm eager to try with a toroidal one, with shielded cable, etc... but I have to buy one 

macboy, the OPA you post seems to be very very good, but they are really expensive. I will probably try one in the future

Meanwhile, thank you all, I learnt alot here and you are all very kind. Now I have to order some stuff, eatching the board and so on, it will take me days I will post the finished project as soon as possible.

There are better op-amps around but I think the TL072 is good enough for this. It just needs to be configured correctly.

Here's another circuit. This one uses an op-amp as a buffer and a 1M biasing resistor, before the potentiometer.

EDIT:
DO NOT BUILD THE ATTACHED THIS CIRCUIT. IT WON'T WORK!

[Damianos]

What about the operating voltage range of the second amplifier?

[Audioguru]

The second opamp is biased WRONG at GND instead of being correctly biased at half the supply voltage like the first opamp.
HINT: add 1 coupling capacitor and one resistor to the half supply voltage divider.

[Zero999]

Yes, the common mode range of the second op-amp is exceeded. The lower side of RV1 should have been connected to half the supply voltage. Note that wouldn't be a good idea with a bipolar op-amp because the high bias currents would result in noise when the pot is adjusted, but it's a non-issue for a J-FET op-amp.

[HoracioDos]

Perhaps someone may find this interesting.

[Zero999]

Perhaps someone may find this interesting.

The amplifier is overdriven so it's clipping.  It may not behave exactly the same, in real life, if you conducted the same test with a signal generator, because this kind of behaviour might not be modelled accurately. Reduce either the input signal or the gain of the amplifier.

[HoracioDos]

The amplifier is overdriven so it's clipping.  It may not behave exactly the same, in real life, if you conducted the same test with a signal generator, because this kind of behaviour might not be modelled accurately. Reduce either the input signal or the gain of the amplifier.

Advice taken!! 

[Ale]

Sorry Hero999, I don't understand why to use the first amp as a buffer. Does it add more stability to sound? reduse noise? as far as I know it should change the impedance from high to low. Is it an important step to do befor amplify the signal?

thanks

[Damianos]

Sorry Hero999, I don't understand why to use the first amp as a buffer. Does it add more stability to sound? reduse noise? as far as I know it should change the impedance from high to low. Is it an important step to do befor amplify the signal?

thanks

In my opinion, the input buffer is a wrong choice. It degrades the signal to noise ratio, because it adds its own noise without amplifying the signal. Additionally, it drives a load that is of the same order of magnitude as is its input. So, there seems to be no reason to use it.
Also, high amplification, in one stage only, decreases the available bandwidth of the amplifier...
As there are two amplifiers, it is better to share the amplification work between them, as I have already shown.

By the way, what are the specifications of the pickup? Is there a datasheet? My memory does not help me to remember many things, from a project years ago!
Also what is expected from this amplifier?

[Zero999]

Sorry Hero999, I don't understand why to use the first amp as a buffer. Does it add more stability to sound? reduse noise? as far as I know it should change the impedance from high to low. Is it an important step to do befor amplify the signal?

thanks

In my opinion, the input buffer is a wrong choice. It degrades the signal to noise ratio, because it adds its own noise without amplifying the signal. Additionally, it drives a load that is of the same order of magnitude as is its input. So, there seems to be no reason to use it.
Also, high amplification, in one stage only, decreases the available bandwidth of the amplifier...
As there are two amplifiers, it is better to share the amplification work between them, as I have already shown.

By the way, what are the specifications of the pickup? Is there a datasheet? My memory does not help me to remember many things, from a project years ago!
Also what is expected from this amplifier?

The reason for the buffer was to increase the input impedance from 100k to 1M. It will make a negligible contribution to the noise of the amplifier, because it's configured for unity gain. Splitting the gain between two amplifiers, will not make any difference to the noise. For low noise, the amplifiers should be connected in parallel but I doubt noise is much of a concern. The thermal voltage noise, in a pick up with a DC resistance of 15k, at room temperature will be 15.6nV/Hz^0.5, which will be added to some thermal acoustic noise, in the air and strings, as all matter above zero temperature vibrates randomly. There will be more noise in the volume control (maximum when set to 50%) which could reduced in value, if it's a concern.

I agree about spreading the gain across both amplifiers, to increase the bandwidth, although this would require more components. I don't know what the highest frequency of interest is for a guitar amplifier. I don't think it's 20kHz. The TL072 has a gain of around 300 at 10kHz, so a gain of 100 is doable, although more negative feedback is good, so reducing the gain and splitting it may help.

[Ale]

The guitar frequency range is about from 80hz to 1200hz

Anyway it seems to have also some harmonics above


What I had in my mind is to build a first pedalboard stage with one input for the guitar and one for the voice (with differential op-amp configuration to drive signal from balanced to unbalanced). Both the signal, guitar and voice should be slighly amplified and should have overdrive and fuzz (I will add that with a couple of fast switching diode). After this stage the unbalanced signals will pass through the other pedals, modulation, delay, then one output to the guitar amplifier and the voice to a DI Box (to rebalance the signal) and to the mixer. I don't exclude to add some tone control or a 3 band equalizer in the middle.

[Zero999]

Going by those graphs, most of the energy is below 6KHz, so a bandwidth of 10kHz is more than sufficient.

[alexanderbrevig]

I don't know about those values..?

Here's a random pinch harmonic from a DI recording of my guitar. I play a C5 (523 odd hz) with a pinch harmonic on G5 (783 odd).
You can see that there's a lot of content above the theoretical sine max.

There's pinch harmonics, natural harmonics and other fun abuse. It's a fun challenge, next time I'm recording I'll check what my max freq is

Band-pass just added for reference.

[Richard Crowley]

Going by those graphs, most of the energy is below 6KHz, so a bandwidth of 10kHz is more than sufficient.

According to the pitch chart, a guitar highest fundamental is only ~1.5KHz and virtually no instruments reach ~5K fundamental.

Note, however, that it is the HARMONICS ("overtones" in musicspeak) that distinguish one instrument playing A 440Hz from another playing the same pitch.  Else everything would sound like a boring sine-wave from an oscillator.  The standard for "conventional" audio bandwidth is the 10 octaves from 20Hz to 20KHz.  However there are fewer and fewer listeners who can hear over 10KHz, and MPEG compression does no great benefit to very high frequency response.


https://en.wikipedia.org/wiki/Template:Vocal_and_instrumental_pitch_ranges

[Audioguru]

Most musical instruments have harmonics that reach at least 15kHz. But an electric guitar has a magnetic pickup that resonates with the cable capacitance at 5kHz and makes a pretty high peak in the response that the cheap speaker reduces in level to be fairly flat to about 6kHz.

The magnetic pickup produces a level so high that this low gain preamp is used:

[HoracioDos]

What about this?

[Zero999]

What about this?

Much better.

Most musical instruments have harmonics that reach at least 15kHz. But an electric guitar has a magnetic pickup that resonates with the cable capacitance at 5kHz and makes a pretty high peak in the response that the cheap speaker reduces in level to be fairly flat to about 6kHz.

The magnetic pickup produces a level so high that this low gain preamp is used:

So you think the gain is too high?

Wouldn't it be better to put a pre-amplifier inside the guitar, right next to the pick-up? That would avoid any resonance or noise pick-up from the cable.

[Ale]

@HoracioDos I don't understand the node RV1 and RV2, one is to control the volume which comes from the first amp to the input of the second one, but the other?

[Zero999]

@HoracioDos I don't understand the node RV1 and RV2, one is to control the volume which comes from the first amp to the input of the second one, but the other?

It represents the potentiometer, which is split in to two resistors because LTSpice doesn't have a potentiometer model.

[Audioguru]

Why are the values for C6 and C7 so high? They will cause gain at earthquake rumbling frequencies at 0.8Hz and lower. Use 220nF film capacitors for each cutoff at 36Hz then both create a cutoff at 72Hz.
EDIT: Why is the output capacitor value so high that it will take weeks to charge? Calculate a suitable value for the load resistance it will have (100k? Then the capacitor will be 100nF).

The gain required by the opamps depends on the sensitivity of the pickup and how hard you strum the strings.
High frequency resonance is used and is needed because guitar speakers have poor high frequency response like an old telephone or AM radio.

[Zero999]

Why are the values for C6 and C7 so high? They will cause gain at earthquake rumbling frequencies at 0.8Hz and lower. Use 220nF film capacitors for each cutoff at 36Hz then both create a cutoff at 72Hz.
EDIT: Why is the output capacitor value so high that it will take weeks to charge? Calculate a suitable value for the load resistance it will have (100k? Then the capacitor will be 100nF).

The gain required by the opamps depends on the sensitivity of the pickup and how hard you strum the strings.
High frequency resonance is used and is needed because guitar speakers have poor high frequency response like an old telephone or AM radio.

The high value capacitors are presumably relics from my design, which had much lower value gain setting resistors.

The value of the output capacitor is unknown because no one knows what load it is going to be driving and it's better it being way too big, than too small.

[HoracioDos]

Few more changes.