Hello
I am building a guitar preamplifier and am going to use op-amps. Old style (non active) guitar pickups have a typically high resistance, being bed though a 500KOhm pot into the pre-amp. The pre-amp therefore has to have high input impedance else it will load the guitar and the sound will lose high frequencies.
I have been using an old Fender amp which is build with TL072s and was thinking I may use these or use the OPA2134 or any other PDIP8 FET input op-amp. The Fender does have a lot of noise and we cannot mic it properly with all the hiss coming out of it. I was thinking the opa2134 may eliminate the noise in my circuits, I am thinking of using it just where I need high input impedance, and use the LM4562 elsewhere.
The places where I need high input resistance are the main input and the tone control circuitry recovery amp.
One question is can I just use the LM4562 throughout, and simply add a simple FET buffer to achieve the high input resistance eg the 2N5459?
Any other comments/suggestions?
Thanks
I am building a guitar preamplifier and am going to use op-amps. Old style (non active) guitar pickups have a typically high resistance, being bed though a 500KOhm pot into the pre-amp. The pre-amp therefore has to have high input impedance else it will load the guitar and the sound will lose high frequencies.
I have been using an old Fender amp which is build with TL072s and was thinking I may use these or use the OPA2134 or any other PDIP8 FET input op-amp. The Fender does have a lot of noise and we cannot mic it properly with all the hiss coming out of it. I was thinking the opa2134 may eliminate the noise in my circuits, I am thinking of using it just where I need high input impedance, and use the LM4562 elsewhere.
The places where I need high input resistance are the main input and the tone control circuitry recovery amp.
One question is can I just use the LM4562 throughout, and simply add a simple FET buffer to achieve the high input resistance eg the 2N5459?
Any other comments/suggestions?
Thanks
Suggest you post in the Live Sound section.
I doubt the TL072's are causing the noise. They are not noisy by guitar amp standards, just by audiophile standards.
Hi fi components like the LM4562 are pointless in electric guitar equipment.
I doubt the TL072's are causing the noise. They are not noisy by guitar amp standards, just by audiophile standards.
Hi fi components like the LM4562 are pointless in electric guitar equipment.
LM4562 also does not have the sound for guitar amp. It's sound is just too smooth and refined. In guitar amps j-fet opamps sound much more valve-like. Noise problem is more likely related to ground loops somewhere.
try 627 the high freq open sound... i didnt believe that the pre is an outstanding amp but after burnin and also dont forget to use highend input caps...
Using a bipolar input opamp in a circuit that was designed for a fet input opamp could result in a high d.c ofset, that may or may not be a problem depending on where the opamp is in the circuit 😉
for good info on guitar pickups' electrical parameters:
UIUC Physics 498POM Guitar Pickup Measurements
I doubt there is a lot better noise performance possible with the hi R pot as source
the TL072 input noise V ~ 16 kOhm equivalent ~ 3 dB noise figure
at higher source Z the V noise of the op amp is even less important and the noise figure can at best only asymptotically approach 0 dB (eventually limited by current noise equivalent R as high as GigaOhms)
paying extra for 1/2 the input Vnoise (OPA2134 vs TL072) won't move the noise figure at 100s of kOhm or higher noise source R by audibly perceptible amounts
directly buffering the pickup coil is a little different – the Rdc of many magnetic pickups is < ~ 10 kOhm and lower Vn fet op amps could improve on dynamic range at the fundamental frequencies and where the higher Z is from complex impedance instead of pure resistance
depending on coil Rdc you could see some improvements in noise with lower Vn fet op amps
so you need to examine the circuit, its capabilities, your expectations - maybe a specific TL072 has taken a esd hit - could cause increased noise while still basically functioning so swapping it out with a good TL072 could even help
maybe you could just filter – for guitar there's not much above ~4 kHz, the cable C in most setups will give a low pass with the pickup anyway
UIUC Physics 498POM Guitar Pickup Measurements
I doubt there is a lot better noise performance possible with the hi R pot as source
the TL072 input noise V ~ 16 kOhm equivalent ~ 3 dB noise figure
at higher source Z the V noise of the op amp is even less important and the noise figure can at best only asymptotically approach 0 dB (eventually limited by current noise equivalent R as high as GigaOhms)
paying extra for 1/2 the input Vnoise (OPA2134 vs TL072) won't move the noise figure at 100s of kOhm or higher noise source R by audibly perceptible amounts
directly buffering the pickup coil is a little different – the Rdc of many magnetic pickups is < ~ 10 kOhm and lower Vn fet op amps could improve on dynamic range at the fundamental frequencies and where the higher Z is from complex impedance instead of pure resistance
depending on coil Rdc you could see some improvements in noise with lower Vn fet op amps
so you need to examine the circuit, its capabilities, your expectations - maybe a specific TL072 has taken a esd hit - could cause increased noise while still basically functioning so swapping it out with a good TL072 could even help
maybe you could just filter – for guitar there's not much above ~4 kHz, the cable C in most setups will give a low pass with the pickup anyway
Just should add that though a guitar might not produce much over 4k, things like effects in the signal chain might.
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