old ross phaser volume loss problem, mark hammer

[rhinson]

hello there,  what in the ross phaser circuit would account for vol. loss when the effect is engaged?  i'm looking at the schematic supplied with the tonepad ropez clone,  and would prehaps increasing the 15k feedback resistor in the second 4558 section give me more volume when the effect is on?  just a thought---are there any others?  i would appreciate any help from those of you (mark hammer and others) who have dinked with this circuit before and have way more understanding than i do.   thanks very much          rh

[Mark Hammer]

The effect certainly is designed around volume drop so I imagine it has something to do with the output impedance of what you're feeding it.

You are correct in assuming that increasing that 15k feedback resistor is your ticket to maintaining volume balance.  Indeed, I would suggest replacing it with a 27k - 39k resistor, and replacing the 150k fixed resistor on the output with a 100k volume pot and 47k resistor from the ground side of that pot to ground.  It will let you achieve effect output levels from higher than to lower than bypass levels.

[rhinson]

thanks for the reply and the good info mark.  btw, would this part of the circuit benefit (both in gain and noise) from a better grade opamp, say like a 5532 or opa2134?  thanks again for your help.   rh

[Mark Hammer]

Sure.  I don't see anything critical in the circuit that needs either the 4558 or a bifet.  The resistances involved will likely bring out the best in an NE5532.  Whether that will be noticeable in the face of whatever the 13600's contribute is another thing.  It's a question of popping something in a socket, right?  So let your ears guide you.  

You can always calculate a suitable feedback cap value for the final op-amp feedback path if you want want to declare an all-out "war on hiss".  For example, with a 39k feedback resistor, a 330pf cap will roll off around 12.4khz, and a 390pf around 10.5khz, which is probably enough bandwidth for guitar.  With a 22k feedback resistor, a 560pf cap will roll off around 12.9khz, and a 690pf cap around 10.6khz.