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Below is a circuit I'm working on. It's a guitar input buffer featuring transformer isolation. The transformer I'm using is pretty expensive and should be fairly linear, but I decided to add a graphic equaliser to the circuit to correct any small non-linearities in the transformer.

In an ideal world there would be no 'volume' control, only 5 eq faders. But that would probably involve some form of compression, which I don't want. So instead I decided to add an overvoltage indicator which will flash when the output goes above 1V. The user can tweek the eq, and then attenuate the output to keep the volume under 1V using the indicator for guidance. Does that sound wise? Is it necessary?

I probably don't need the input cap and pop resistor on the eq input. But is it safe to remove the cap? Can I put the output of the transformer directly into the opamp?

The warning indicator opamp comparator output is normally at -9v but the output is grounded, meaning under normal conditions there's 9V across that LED arrangement. Is that ok?

Thank you in advance good people.

EDIT : I'm aware that both sides of the circuit share the same power supply. That's just for ease of simulation In fact, placing the EQ before the transformer gives exactly the same simulation response, and requires only one power source.

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  • \$\begingroup\$ but I decided to add a graphic equaliser to the circuit to correct any small non-linearities in the transformer. I hope you realize that an equalizer attenuates signals depending on their frequency. It cannot compensate for non-linearities (distortion). It can attenuate some distortions by filtering but compensating for them is impossible. \$\endgroup\$ – Bimpelrekkie Aug 16 '18 at 12:26
  • \$\begingroup\$ @Bimpelrekkie The problem with peak detection is where would I take the input from. It would ideally come from the raw input, but that is on the 'bad' side of the isolation, so I'd have to use an optoisolator to pass that voltage through.... complicated! \$\endgroup\$ – Richard Aug 16 '18 at 12:36
  • \$\begingroup\$ @Bimpelrekkie My simulation shows this particular transformer is loosing in the high frequencies. I could.... boost the input signal, send it though a high pass, and then mix it with the original, and do away with the EQ altogether. I did that and it worked. The problem is I'm not sure the actual transformer will perform like the model (even though the model is as close as I think I can get it). \$\endgroup\$ – Richard Aug 16 '18 at 12:39
  • \$\begingroup\$ No you don't need to take the signal from the transformer's primary, the transformer is only slightly non linear so the signal at the secondary (safe) side is nearly identical. \$\endgroup\$ – Bimpelrekkie Aug 16 '18 at 12:39
  • \$\begingroup\$ The transformer uses 70 H coils !?!? That's unrealistic. It would need to be the size of a refrigerator. With inductances stay below 0.1 H for a realistic value. \$\endgroup\$ – Bimpelrekkie Aug 16 '18 at 12:42
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You don't really need the oversignal indicator as when the circuit hits it's limits you'd need to be stone deaf not to hear that as the opamps will clip. For a much better oversignal detector, build a peak-detector circuit. Like it is now the LED will hardly light up visibly for nasty overload peaks.

Indeed LED's don't like reverse voltages, even 9V could be a problem. Either connect the kathode side of the LED to -9V ) assuming the opamp can drive it's output down to about -7 V or lower) or add a diode in series with the LED so that the diode will handle the 9V reverse voltage and not the LED.

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