May Queen High Filter Issue

[StickMan]

I've built a May Queen, and I put the optional high filter on a 5 way rotary switch with a selection of smallish caps.

Here's the schematic, to save some time:



Everything works perfectly, except the high end filter cap.  I've checked the voltages at all the points listed, and everything is bang on. I've checked the continuity on the capacitor switch connections, and everything is fine.  I've checked the resistance around the trim pot, and it looks good too.

Nothing I do with the capacitor switch makes ANY difference to the tone.  I've even left one of the switch positions empty, so it's an open switch, and there is no difference.  Nothing.  Nada.  Zilch.

Any thoughts?

I thought maybe I'd put a second trimpot, with the capacitor switch in parallel and in series with the final .1uF cap.  That would really make a difference.

[John Lyons]

What is the range of caps you have in there?
Sometimes you can hear a difference easier when you turn up the amp and listen to the hiss switching back between caps.
That cap is cutting a little of the very high end or "Air" ,so it's not a night and day change.
Put in a .01 or something larger than what you will end up with to hear a drastic change...then start scaling back until you hear what you like.

John

[StickMan]

I can try that, but remember that I have an empty position on the switch, and I don't hear anything different even with that.

[brett]

Hi
empty position = no filtering (which is what you're hearing)

According to my calcs, it should be kicking in at about 0.01 uF (and larger).  If not, check the connections again.
cheers

[StickMan]

The original schematic called for 2.2nF in that spot, which is substantially smaller than 0.01uF.  Without doing tedious math, I figured that filtering aspects were going to involve multiplying, so I decided to try powers of two for options on the switch.  The 2.2nF went in the middle, and I put a 4.7nF and a 8.6nF on one side and a 1.9nF on the other side, with one switch position unpopulated.  The 1.9nF wasn't really small enough, but I didn't have 1.0nF or thereabouts cap lying around.

Anyways, it doesn't appear to matter.  I figure I'd hear the biggest difference between the 1.9nF and the unpopulated one.  Nothing.

[slacker]

You should hear the biggest difference comparing the biggest cap with no cap. The larger the cap the lower the rolloff frequency will be, smaller caps will cut less.
The value the trim pot is set to will affect the frequency the cut happens at as well, the smaller the value the trim is set to the bigger the cap you'll need to hear any difference.

I'd try getting a big cap like a 0.1uF or bigger and put that temporarily in the empty switch position, if you still can't hear any difference it's possible you've got the switch wired up wrong.

[StickMan]

This is a high pass filter, guys.  A bigger cap will pass more lows.  So the biggest sound change should be between the smallest filter and no filter.

Also, and this is what I don't really understand, but this cap is between the +9V and the source of the JFET, and isn't really in the signal path at all.  So I'd kind of expect it to have a somewhat subtle effect, but what I'm seeing is extreme.

And to add to that, I've also tried cranking the trimpot way down, which should increase the effect of the capacitor as well.  It definitely changes the gain of the circuit, especially as notes are allowed to ring and fade out, but still doesn't make the capacitor noticeable.

[slacker]

This is a high pass filter, guys.  A bigger cap will pass more lows.  So the biggest sound change should be between the smallest filter and no filter.

If it was in series with the signal path then the cap and the trim would form a high pass filter and would cut bass. It this case it's connected from the output to ground, 9 volts is ground as far as AC signals are concerned. This means that it passes high frequencies to ground, so a larger cap means more treble cut.

Have you tried just putting a cap straight across the trim to see if that works, just to rule out a problem with the switch?

[John Lyons]

No offense, but when you ask for suggestions and then ignore them or at least don't try them out then what's the point?
I'm no electronics wiz but I do have some experience with the problem here and that's what I based my suggestion on.
Try it out.

John

[Paul Perry (Frostwave)]

The output impedance of the Q3 stage is pretty low.
Certainly below 10K at least.
So you will need a pretty big cap to make a difference.
If It was me, I'd put a .1 for a start, just to see, like stickman said.
And note, whatever tone effect you get, it is going to vary if you adjust the trim.

[brett]

Hi

Also, and this is what I don't really understand, but this cap is between the +9V and the source of the JFET, and isn't really in the signal path at all.  So I'd kind of expect it to have a somewhat subtle effect, but what I'm seeing is extreme.

Passive filters such as this one usually work by passing some of the signal to AC ground.  A quirk of nature says that +9V is AC ground (or close to it.  There's some resistance in the battery.  Maybe a few tens of ohms?).  In this case, its a cap that sends signal to ground (+9V). 

Looking at different frequencies for this filter - when the frequency is high enough for the impedance of the cap to be equal to the resistance of the resistor, it is clear that the overall impedance has been halved.  This is the half-power (or -3dB) point.

In practice it is all a bit confusing because circuits such as the Treble booster, which contains (i) a filter that *reduces bass*, and (ii) an amplifier that boosts what's left.

The maths for all of these types of filters are :

fc = 1/(2.pi.R.C)

where fc is the "cutoff" frequency (poorly named because it is the -3dB point, where the frequency cut might *barely* be heard).  The roll-off beyond this point is -6dB per octave, so the effect is quite strong about one octave below or above the fc.  (ie 0.5 x fc or 2 x fc)

pi = 3.14
R is resistance in ohms
C is capacitance in Farads

Assuming that your trimpot is set to 8 k ohms, and for fc = 6 kHz (audible to someone older or anyone with "rock n roll" ears), C should be around 0.0033 uF in this circuit.

If that isn't working, check that the JFET is working (audioprobe the input and output  The output sould be much louder.)  After that, I'd ensure that your supply is at AC ground (put a 100uF cap from +9 V to ground).

If all that doesn't work, ask again.
cheers

[StickMan]

John,

I am absolutely not ignoring the advice.  I posted the question last night, and I simply haven't had a chance to get a soldering iron to the circuit since then.  I'm going to try out the suggestions, for sure.

In the meantime, I'm just trying to understand the suggestions.  The most confusing thing is that what you guys are saying is 180 degrees the opposite from the notes from Runoffgroove that are right on the schematic:  "Larger values will yield less highs".

This is the way I look at that section of the circuit:  The drain of the JFET is trying to suck power out of the battery +ve proportional to the gate voltage, but it has to overcome the resistance of the trimpot.  The presence of the capacitor serves to lower the resistance between the battery and the JFET drain for higher frequencies.  The output signal is tapped right off the drain, so it's going to reflect the current that the JFET is causing to flow through the cap/trimpot filter, which is going to emphasize highs.  Hence a smaller cap equals more highs at the output.

[John Lyons]

The schematic notes are correct. A larger cap will = less high end.
This is the same thing you are thinking and the same thing everyone has posted about.
So we're all on the same page 
The only thing is that a .002 cap will really not effect the sound much.
I think what the ROG folks were thinking is that .002 was a "starting place" meaning that anything bigger than this will start cutting highs.

What I would do is take a .01 cap and just touch it to board across the trim pot and listen to the difference. Try the hiss test I mentioned above if it's subtle. Try going larger until you hear a clear difference. Then you can add that cap and larger caps to your rotary selector switch and play with the different values without having to hold them to the board.

In high gain amps the same cap trick is used to keep oscillations out so the caps are a little subtle and just barly in the audio range, mainly cutting some of the fizz out of the signal.

Let us know what you find.

John