TL072C upgrade for audio interface

Hi,

I am modifying the PCB of a GuitarPort audio interface. It's a pretty old product and they go for penny's on eBay. I got this one for 99p, so just fancied hacking about.

I've disassembled it, and it's got a tl072c right next to the guitar jack input, so I'm assuming that's the preamp and therefore will be the most important component to replace if I want to upgrade this interface? I've not modified circuits like this before, but I've got a lot of experience in SMD rework (component replacement etc).

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Whatever you do, don't replace it with an op-amp with a bipolar input stage, especially not an ultralow voltage noise type with base current compensation. They have ultralow voltage noise, but also ultrahigh current noise, and for a high-impedance source like an electric guitar, current noise matters more than voltage noise.
 
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And what do you think replacing a 5p chip with a £30 "special" chip will acheive, apart from an empty pocket.
It will not change the response of the circuit, the components around it do that.
Understood. I'll concede that I'm not an expert in this field (audio amp circuitry), just an amateur interested in hacking with the right level of electronics skills to actually pull off the mods.

Perhaps I should state what I'm looking to do... These GuitarPorts are USB interfaces with excellent driver support. I own a few and I've used various other iterations including the Toneports. I also know they're an older piece of tech and there are newer USB interfaces that I can get... Albeit if I'm willing to invest good money.

Is there anything obvious that I can do to improve this interface? I am wondering what components I could replace to improve the input signal.
 
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In case the electrolytic caps are older than a decade or 2, the only improvement worth considering is changing those and leave the rest as is.
Old tl072 have the advantage of beeing original. If you buy new stuff there is a high risk of getting fakes.
 
There is nothing wrong with the tl072, especially in that application. I'd leave it. Guaranteed you'd here little if any improvement if you change it, especially considering all the other effects that are likely to follow before reaching anyone's ears.
 
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In case the electrolytic caps are older than a decade or 2, the only improvement worth considering is changing those and leave the rest as is.
Old tl072 have the advantage of beeing original. If you buy new stuff there is a high risk of getting fakes.
These products were manufactured around 2003. None of them look bad but I'll definitely replace. Easy job :)

When you say fakes? As in an fake amplifier IC?
 
The TLE2072 is basically a straight upgrade in this scenario. Is there any particular problem you are looking to solve? In an application like this, the TL072's only problem is its relatively high voltage noise, and maybe the rail to rail voltage drop if its being starved for voltage. (Measure the voltage on the + and - pin)
 
Whether you get a significant noise improvement by upgrading the TL072 will depend on its gain in the circuit. The DS says it has 4uV noise, your ADC (AK4528) has about 11uV noise. So if the TL072 is running under 6dB (X2) gain I'd leave it in, if its above that then it might be worth upgrading to a lower noise part.
 
I've been told that electric guitars typically have a 250 kohm or 500 kohm volume potmeter built in. When that is set near half volume, it will dominate over the 18 nV/√Hz of the TL072.
Yep. You may really need low noise from an guitar amp input only at very high gain settings which are typically used with the guitar volume full open. The guitar output impedance is then basically that of the pickups, which is a few kOhms at DC and large inductive component, very similar to MM pickup, just that the resonance is much lower at a few kHz.

Other noises (especially hum/buzz, even with humbucking pickups) will dwarf the resistive noise, so the whole point is questionable. For peace of mind I'd use OPA1642.
 
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I used OPA1641 in DAC for I/V and tried it in line preamp. The sound is cleaner and much better than the TL071. It will just be a lot brighter, the question is will he like it?
OPA1642=2xOPA1641
OPA1644=4xOPA1641
Also with FET inputs the OPA1655/1656 sound very good.
OPA1656=2xOPA1655
 
The TLE2072 is basically a straight upgrade in this scenario. Is there any particular problem you are looking to solve? In an application like this, the TL072's only problem is its relatively high voltage noise, and maybe the rail to rail voltage drop if its being starved for voltage. (Measure the voltage on the + and - pin)
There's no specific problem with this interface, it works OK, but I know that if I spent a few hundred pounds I could buy a better (sounding) one. Lower noise floor, better head room etc. However I'd prefer not to contribute to the e-waste problem :)
This interface has excellent low latency USB qualities (one of the best considering how old it is) and works great on Windows 10, despite it's old age. I just want to see if I can get any more out of it before I end up biting the bullet and buying a Focusrite like every other guitarist in the world!
 
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Yep. You may really need low noise from an guitar amp input only at very high gain settings which are typically used with the guitar volume full open. The guitar output impedance is then basically that of the pickups, which is a few kOhms at DC and large inductive component, very similar to MM pickup, just that the resonance is much lower at a few kHz.

Other noises (especially hum/buzz, even with humbucking pickups) will dwarf the resistive noise, so the whole point is questionable. For peace of mind I'd use OPA1642.
Are there any other components I could replace or am I likely just grasping at straws?

If it's a pointless operation then I'll more than happily post afterwards to say so, save someone else doing the same work.
The OPA1642 was the chip I had in mind though!
 
Understood. I'll concede that I'm not an expert in this field (audio amp circuitry), just an amateur interested in hacking with the right level of electronics skills to actually pull off the mods.

Perhaps I should state what I'm looking to do... These GuitarPorts are USB interfaces with excellent driver support. I own a few and I've used various other iterations including the Toneports. I also know they're an older piece of tech and there are newer USB interfaces that I can get... Albeit if I'm willing to invest good money.

Is there anything obvious that I can do to improve this interface? I am wondering what components I could replace to improve the input signal.
In a nutshell: improving "components" will do nothing.
If you feel up to it, improve design.
 
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Whatever you do, don't replace it with an op-amp with a bipolar input stage, especially not an ultralow voltage noise type with base current compensation. They have ultralow voltage noise, but also ultrahigh current noise, and for a high-impedance source like an electric guitar, current noise matters more than voltage noise.
Agree. JFET inputs have pico-amp range gate current leakage.