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Amateur guitar effects builder.......seeking enlightenment!

gophert

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#21
Many pedals used Germanium transistors and each Transistor had different gain and leakage issues so each pedal sounds different. Some pedals sounded like nothing or like a mess unless the germanium had just the right properties.
 

audioguru

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#23
I use 1N34A's in my distortion pedals, along with TL082s / TL084.
Never had a complaint. :)
TL08x and low noise TL07x opamps produce a unique kind of distortion. If an input voltage gets within a few volts from the negative supply voltage (which is usually 0V in a pedal circuit) then the output suddenly goes as high as it can. It is called Opamp Phase Inversion.
 

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#24
TL08x and low noise TL07x opamps produce a unique kind of distortion. If an input voltage gets within a few volts from the negative supply voltage (which is usually 0V in a pedal circuit) then the output suddenly goes as high as it can. It is called Opamp Phase Inversion.
I don't use a single supply for my pedals. I use a TC7660S, which synthesizes the negative rail. :)

But I can't help but wonder what it would sound like. :D
 

gophert

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#25
I don't use a single supply for my pedals. I use a TC7660S, which synthesizes the negative rail. :)

But I can't help but wonder what it would sound like. :D
Do the TC7660s add any 10kHz buzz to your pedals. I think they operate at fairly low frequency (audible).
 

audioguru

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#26
A 7660 produces a 10kHz whistle that most deaf electric geetar players cannot hear. If you use the boost-frequency pin then the battery runs down sooner. But the opamp phase inversion can still happen if the input gets within a few volts from the 10kHz modulated negative supply voltage. I think the sound has raspy even harmonics (frequency doubling).
 

unclejed613

Well-Known Member
#27
TL08x and low noise TL07x opamps produce a unique kind of distortion. If an input voltage gets within a few volts from the negative supply voltage (which is usually 0V in a pedal circuit) then the output suddenly goes as high as it can. It is called Opamp Phase Inversion.
that's always a nasty surprise... according to the data sheet, this happens when the input is within 1 volt of the negative rail [data sheet] it's on pgs 29 and 30. there's additional app notes [here] and a complete tutorial about op amps (originally published by Burr-Brown) [here].
 

audioguru

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#29
that's always a nasty surprise... according to the data sheet, this happens when the input is within 1 volt of the negative rail [data sheet] it's on pgs 29 and 30.
1V is nothing, the datasheet shows 4V.
Page 29 of the datasheet you linked and the Recommended common mode input voltage range is 4V above the negative supply up to the positive supply.
 

unclejed613

Well-Known Member
#29
these curves show the phase inversion as being at just under 1 volt from the (-) rail.



tl072-phase-inv.png

and this is what it says about the curves:
tl072-v-in.png
 

audioguru

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#30
All graphs on a datasheet are for a "typical" device that you cannot buy. Some of them are better than typical and some are much worse with the range shown in the printed spec's. The recommended common mode input range is 4V more positive than the negative supply which is ground in that circuit.

Don't electric geetars blast the levels too high producing clipping (fuzz) which causes the input of the TL07x opamps to go close to the negative rail and produce the phase inversion no matter if the opamp common mode input range is minimum or maximum?
 

unclejed613

Well-Known Member
#31
that depends on the sensitivity of the pickups, playing style, how the string height is set up. i used a TL074 to build an internal preamp in a bass i built, and i never had a problem with phase inversion, even when the batteries got low (i used a 9V battery and one quarter of the TL074 as a virtual ground, so the rails were +/-4.5V with a fresh battery)... and i know that +/-4.5V is less than the minimum on the data sheet, but it did work quite well... i might have pretested the TL074 and picked one that worked reliably at that low rail voltage...
 
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audioguru

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#32
I notice that the new (2017) datasheet for the TL07x opamps shows a recommended minimum supply of 10V (it was 7V years ago), maybe due to the phase inversion problem on some of them. Your 9V battery will drop to 6V over its life.

I never use an opamp to provide a low impedance Virtual Ground since the (+) input of these opamps can be biased with very low current from a resistor divider (the same divider for your opamp virtual ground).
 
#33
Don't electric geetars blast the levels too high producing clipping (fuzz) which causes the input of the TL07x opamps to go close to the negative rail and produce the phase inversion no matter if the opamp common mode input range is minimum or maximum?
Not these days. We use a log amp, which gives really nice distortion with really long sustain. (What 'dem geetar players really like :) )

Oh, and IIRC, fuzz and distortion are actually two different animals.
 

audioguru

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#34
I think the sustain that I hear from electric guitars is feedback. The speaker keeps the guitar strings vibrating forever.
I have a simulation of a Fuzz Face circuit that produces even and odd harmonics clipping distortion. It sounds as bad as its waveform looks.
 

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