Ashdown preamp EQ

[lowell]

Anyone familiar with opamp circuits that have pots across the inputs and the pot wiper goes through caps to the gate of a jfet? There is also some feedback from the source of the jfet back to the gate through some caps. No signal is tapped off the drain. There are as many jfets as there are EQ controls IE bass, mid , treble. This seems as some sort of active filter maybe? I recently fixed a Mag 600 and I really like the way these amps sound. I'm curious about the preamp EQ and how it works and what this type of circuit is called. Should I post a schematic or is it a no no. I had to contact Ashdown for it and don't wanna upset anyone.

[Steve Conner]

I bet the JFET is being used as a gyrator: a circuit that simulates an inductor. Google it, you should find a lengthy article by RG Keen with example circuits.

Usually a BJT or op-amp is used, but perhaps Ashdown discovered that the JFET made some nice distortion reminiscent of the iron-cored inductors used in old EQs, before gyrators were invented.

[lowell]

Steve thanks! How's my ancestral homeland? At least you're up at a normal hour.

[lowell]

Actually just like this except with a jfet! Thanks to RG.

[Jazz P Bass]

Gyrator Circuit.
Good old Elliot has a well versed explanation.(sans jfet)
Link:Audio Designs With Opamps
(It starts about 1/2 way down the page.)

[exclamationmark]

I actually used to use a bc547 (I can't remember exactly... some sort of cheap plasticy transistor) as a gyrator to quiet my power supply down fed from a SMPS wall wart. Quite useful I imagine a gyrator is used because chokes/inductors in the henry range are ridiculously expensive, considering it doesn't actually handle any power.

[jpfamps]

I actually used to use a bc547 (I can't remember exactly... some sort of cheap plasticy transistor) as a gyrator to quiet my power supply down fed from a SMPS wall wart. Quite useful I imagine a gyrator is used because chokes/inductors in the henry range are ridiculously expensive, considering it doesn't actually handle any power.

Gyrators are indeed used due to their much lower cost.

Also inductors are very good at picking up hum, so may need additional shielding to achieve an acceptable noise performance.

[TransLucid]

There's a nice discussion of this circuit in "Small Signal Audio Design" by Douglas Self, a book I have found to be very useful and interesting.