filtering gain in discrete stages

RnFR · Post by **RnFR** » 22 Jan 2009, 20:31

recently i was turned on to the Rat style gain filtering in Soulsonic's Hunny Bunny design. it seems like a good way to do some manipulation of frequencies in a different way besides other RC filtering methods.

i was wondering if it is possible to do this with a discrete gain stage, and if so, how. anybody have any examples of circuits that use this in a discrete fashion? attached is the Rat schematic for reference. the parts in question are the 560/4.7uF and 47/2.2uF to ground.

earthtonesaudio · Post by **earthtonesaudio** » 22 Jan 2009, 21:10

The Hunny Bunny uses that technique twice. Once with the op-amp and again with the MOSFET (the "Honey" control is part of it) so yes you can use it in discrete stages.

See also BSIAB2, first gain stage.

RnFR · Post by **RnFR** » 23 Jan 2009, 01:03

i'm an idiot. i even asked SS about this in the HB thread and totally forgot about it. i think it kinda was late when i posted this topic.

RnFR · Post by **RnFR** » 23 Jan 2009, 03:03

ok, i do have a question regarding this.

how does the order of the resistor and cap off the emitter come in to it? i tried this the other night, and the transistor wouldn't bias with the resistors attached to the transistor and then the caps to ground. i see in the hunny bunny there is one RC combo set up like this, but the one with the pot has the cap attached to the transistor and the pot to ground.

is there a rule regarding the order that comes into effect here in order for the transistor to bias up?

do you have to bias the transistor first and then do the filtering on the gain?

earthtonesaudio · Post by **earthtonesaudio** » 23 Jan 2009, 15:21

do you have to bias the transistor first and then do the filtering on the gain?

Yes. DC bias has to be established before the transistor can operate predictably. In the Hunny Bunny, that's taken care of by 3 resistors and 3 separate DC sources. First, the 10k from drain to V+, then the 4k7 from source to GND, and finally the 1k to the op-amp output (which is presumably at 4.5V plus or minus the offset of the amplifier).

Once the "steady state" DC points are established, you can go hog wild with AC filtering. Adding AC paths from the source to ground will reduce "degenerative feedback" for those frequencies, which has the effect of increasing gain at those frequencies. They call this emitter or source "bypassing."

Fuzzer · Post by **Fuzzer** » 23 Jan 2009, 16:50

I hope this is a useful addition to Earthtonesaudio contribution;

The reason your set up didn't work was because it left the transistor's emitter hanging, because the cap doesn't exist for the DC analysis, and it is like an open circuit. The order doesn't matter, for any case (Unless you have some node coming out of the R-C junction).

briggs · Post by **briggs** » 23 Jan 2009, 17:42

If you check out my recent overdrive project that I posted the structure control in that circuit is another example of this.

RnFR · Post by **RnFR** » 23 Jan 2009, 21:58

Fuzzer wrote:I hope this is a useful addition to Earthtonesaudio contribution;

The reason your set up didn't work was because it left the transistor's emitter hanging, because the cap doesn't exist for the DC analysis, and it is like an open circuit. The order doesn't matter, for any case (Unless you have some node coming out of the R-C junction).

yeah i figured that out.

got it all working nice. will post the schematic soon. sounds freakin fantastic.

thanks for the clarification guys.