Bypass Caps with Source Resistors

[WGTP]

I'm not sure how to calculate the frequency and curve for these.  Are they 6db/oct. or more like shelving?  I saw a chart for 12ax7 tubes and would like to make one for BS170's, J201's, etc.   It would help with tuning our favorite gain stages.   

[R.G.]

I'm not sure how to calculate the frequency and curve for these.  Are they 6db/oct. or more like shelving?  I saw a chart for 12ax7 tubes and would like to make one for BS170's, J201's, etc.   It would help with tuning our favorite gain stages.   

For a single resistor and single cap, the governing equation is always F = 1/(2*pi*R*C), that being the frequency where Xc = R. Away from that, the combined impedance either changed not at all (on the resistor side of the turnover frequency) or linearly with frequency (that is, -6db/oct), that being what capacitors always do.

Shelving happens when there is a resistor in series with the cap that limits the effect of the cap declining toward zero, or parallel to that which limits the cap's effect of increasing to infinity as frequency drops.

One R, one C is always single pole. More Rs, more Cs gets more complicated.

[PRR]

> For a single resistor and single cap

Yes, but this is a two-resistor network.

There's the resistor, and the cathode impedance.

In vacuum tubes, very often the resistor and cathode are similar impedance. Voltage gain doubles when cathode is well bypassed; conversely as the cap lets-go the gain falls 5db-7db and then shelves.

In most BJT stages the "cathode" (emitter) impedance is MUCH less than the resistor under it. Like 1mA, 1K resistor, 28 ohms looking up the emitter. Gain falls 35:1 or 31db then shelves. Since we don't care what happens 31db down, the value we really want is the emitter impedance (Shockley's Law). However we also see the input-source impedance via hFE. And that often includes yet another capacitor! So BJT "cathode tuning" is more often "slap in a big cap, we'll shave bass some other place".

JFETs come closer to vacuum tubes. In fact the 12AX7 curves will get you in the right half-octave, and further tuning always has to be by-ear on actual music.

MOSFETs.... I have not seen useful data for MOSFETs at audio-typical currents. Anybody?

[WGTP]

Yeah, Paul's got what I'm talking about.  Thanks to both.  I didn't know how to explain it.  I got the basic RC stuff figured out, but the resistor and bypass cap don't seem to match that, based on what I hear.  Also, for example the Mu/Srrp, you see from 2.2k to 470 ohm resistors with the bypass cap, whats up with that?  The different amp sims around etc., probably need some adjustment.  

[PRR]

> example the Mu/Srrp, you see from 2.2k to 470 ohm resistors

The bias resistor goes by the desired DC conditions and the device parameters. If you need high gain and have infinite load impedance (always limited by stray capacitance), you bias as low-current as possible which tends to be a higher bias resistor. OTOH if you need to drive 600 ohms and use a low-Vto device the resistor will be small.

> the resistor and bypass cap don't seem to match that

When un-bypassed gain is not much less than bypassed gain, the shelf is close and the slant slope is small. On a bass-heavy source like steel string through blunt pickup, a mild shelf may not sound like "low-cut".

> The different amp sims around

Well, that's all make-believe anyway, rough approximations of the larger differences between the Real Thing and what you are doing. It isn't a Marshall Stack without eight air-hammers beating your chest; real power output stages have complex actions that can't be faked with a few FETs. In that you can get the general slope and droop of a Famous Amp and some roughly similar breakup, it may be fun.

[WGTP]

Rather than using the familiar 470k/470pf circuit after the first stage of a distortion, I'm want to use the first stage as a treble boost.  I have found using smaller cap for by-pass around .1uf seem to work.  Guess I'll have to do it with old fashion way by ear, but I wish I could figure out a way to sim it in LT Spice.  

[WGTP]

Started the experiment with a Srpp type stage with a BS170 mosfet in the bottom, 1M resistors and a 1k resistor on the source.  Switching between different caps with a single coil guitar sitting in front of the sealed cabinet speaker to hear the low frequency hum.  The 10uF had slightly more bass than the 4.7uF, so I'm thinking the roll off is around 100Hz.  22uF could not hear a difference between it and the 10uF.  On the other end, couldn't hear much effect from 47n cap but could hear 100n. 

Pretty crude method, and it will differ on different gain stages, but I guess this was what I was after.

From there it's a matter of 1/2 cap values and doubling the roll off frequency.  So, a .47uF cap should but things around 1000Hz.  Next, I'll try a J201 jfet.