I am working on a McIntosh 6200 amp with L driver board problems, so far 2 transistors. I am replacing this LM201 with an NE5534. The compensation capacitor (1.8pF) on the 201 is bad so I cannot re use it and the smallest cap in my stash is 47pF.
Can I safely leave the compensation cap off with the NE5534 ?
Can I safely leave the compensation cap off with the NE5534 ?
1.8pF is super low. There is likely at least as much stray capacitance on the PCB between pin-1 and pin-8. If you have access to an wide-band O-scope, you can check for oscillation or signs of instability, but I should you won’t find any, especially if op-amp isn’t set for above unity gain.
Based on the schematic fragment you provided, it's impossible to say if this will work with an NE5534. Could you show a bit more, particularly the whole feedback network?
In my experience 22pF to 47pF between pin5 and pin8 is needed for NE5534N, depending on the opamp topology. I experienced oscillation or ringing without it. No harm of using one.
An NE5534 is stable without external compensation capacitor when the closed-loop gain is at least three. That condition is met. Its higher input bias current will cause an offset across the 1.8 Mohm, but that will be blocked by the electrolytic capacitor.
My best guess is that it will work without external compensation capacitor with the improved decoupling recommended by @ejp. It's only a guess because I don't really understand the circuit.
My best guess is that it will work without external compensation capacitor with the improved decoupling recommended by @ejp. It's only a guess because I don't really understand the circuit.
It's the 10 pF capacitor C806 (?) that confused me. It's drawn with a straight and a curved plate, which made me think it was a 10 uF electrolytic capacitor. In fact, all capacitors are drawn with a straight and a curved plate and all capacitances are in pF unless otherwise noted.
If I understand it correctly, the circuit monitors the error voltage between the bases of the input differential pair of the main amplifier. The op-amp circuit (differential amplifier with different impedances for the voltage dividers driving the positive and negative op-amp inputs) senses and amplifies this error voltage. When the amplifier is driven into clipping, the error signal gets large. It's amplified, rectified and drives an optocoupler with LDR output that reduces the gain. The part where the gain reduction takes place (presumably in the feedback network of the main amplifier) is not shown.
If I understand it correctly, the circuit monitors the error voltage between the bases of the input differential pair of the main amplifier. The op-amp circuit (differential amplifier with different impedances for the voltage dividers driving the positive and negative op-amp inputs) senses and amplifies this error voltage. When the amplifier is driven into clipping, the error signal gets large. It's amplified, rectified and drives an optocoupler with LDR output that reduces the gain. The part where the gain reduction takes place (presumably in the feedback network of the main amplifier) is not shown.
C806 is 18pF, the compensation cap is C818, 1.8pF. I attached the material list with values that is easier to read.