Series Cap for High Pass filter

[RSRecords]

Would adding a smaller cap in series with a larger decoupling cap raise the cut off frequency? Try to find an easy way to add (and reliably calculate) a simple passive filter. Would something like this work? Also curious about the single supply configuration.(sorry for the quick sketch) I was originally going to add it to the L pad but I thought this may be a simpler way.

 

[JohnRoberts]

Yes, but I would advise against driving the output pot with 12VDC.

JR

 

[merlin]

Yes to the input cap, but you also need an output cap before the pot. Otherwise you have DC on your pot.

 

[RSRecords]

Oh, duh. Thank you. What about the cap from the inverting input to ground? Trying to sort out a value there. Also not sure why it’s needed but I have seen it on single supply configurations so thought to include it.

 

[JohnRoberts]

Oh, duh. Thank you. What about the cap from the inverting input to ground? Trying to sort out a value there. Also not sure why it’s needed but I have seen it on single supply configurations so thought to include it.

that cap is needed to properly bias the op amp output at V/2.

The RC HPF formed by that cap and the 5K resistor should be calculated for probably at least an octave below your LF cut off.

JR

 

[merlin]

What about the cap from the inverting input to ground? Trying to sort out a value there.

22uF or more would be fine. Unless you want some bass cut, in which case smaller.

 

[Matador]

What about the cap from the inverting input to ground?

If the cap was not there, think about the DC bias conditions. If the bottom of the 5K was grounded, and the non-inverting input is sitting at 12V, then the feedback loop will try to make the inverting input 12V as well. Since there is a 50% voltage divider between the output and the inverting input, the only way to make the inverting input 12V is to drive 24V on the output (which is your supply rail).

At DC, the cap is an open, so there is no voltage drop across (either) 5K resistor, so the output can sit at 12V and make the inverting input 12V so everything is good. It is essentially setting the DC gain of the opamp to 0dB.

 

[Script]

Caps in series, yes. Could also put several caps on a rotary in place of the 220. Easier to calculate, more filter settings.

Or put lowest value into circuit (highest filter setting) and switch higher values in parallel to it on a make-before break rotary.

 

[JohnRoberts]

Note if you are going for a precision HPF tuning, the output impedance of your signal source will be added to 5k (10k in parallel with 10k) circuit input impedance. Some consumer and semi pro gear can have output impedances from hundreds to thousands of ohms, so enough to be a factor.

JR

 

[RSRecords]

Note if you are going for a precision HPF tuning, the output impedance of your signal source will be added to 5k (10k in parallel with 10k) circuit input impedance. Some consumer and semi pro gear can have output impedances from hundreds to thousands of ohms, so enough to be a factor.

JR

perhaps it would be more consistent if the biasing cap on the inverting input was used instead of the input cap?

 

[ruffrecords]

I would advise against biasing the + input using a pair of 10K resistors across the supply, You throw away nearly all the ICs PSRR. Connect the + input via a 100K resistor to the junction of the two 10K resistors and slap a 100uF capacitor across the bottom 10K to keep PSU noise out of the input.

Cheers

Ian

 

[RSRecords]

I would advise against biasing the + input using a pair of 10K resistors across the supply, You throw away nearly all the ICs PSRR. Connect the + input via a 100K resistor to the junction of the two 10K resistors and slap a 100uF capacitor across the bottom 10K to keep PSU noise out of the input.

Cheers

Ian

Hey Ian,
Something like this?

 

[ruffrecords]

Just like that.

Cheers

Ian

 

[RSRecords]

Ok so my design has shifted to move the high pass elsewhere. I think I'll implement after a fader buffer. I should have a extra opamp and this would allow me to isolate it from the rest of the circuit impedances.
But does anyone see an issue with this? There will be 8 of these feeding 8 relays. Relay chooses mic/line to mixer and direct outs. I'm concerned about crosstalk, not that it has to be especially low, but I know things can get dicey when adding channels.

opa2134 is kind of a place holder but I have a few in stock as well as 5532s. Open to other options as well.

edit: added pdf for better viewing
and transformer secondary should have pin 8 to 0V

Thanks for any help!

 

[RSRecords]

Fader buffer High pass filter with single supply. Not sure if this will work?

 

[RSRecords]

Any thoughts on the design of this buffer and filter?
I used the 100k resistor that ties to Vref as part of the calculation.

After the filter switch the signal will feed a line amp. The line amp is mounted in a card cage so there will be about 2' of wire. The electrodyne line amp has 15k input impedance uses the single +24v. There's a 10-15dB before the line amp (maybe 15k ohm series 4k7 shunt).

Any considerations with this aspect? Is this an ok configuration?

Note: Yes fader is shown wired backwards..

 

[ccaudle]

In miclineinput20240320.pdf it appears that C3 is reversed (unless whatever connects to MPO1 has more than 12V DC bias).
Similar in the FADER_HI PASS schematic, C1 has the negative terminal connected to a node with +12V present.
C20 is a little more complicated, it will have +12V on both sides, so is not really fully biased. Should be OK, but some electrolytic capacitors will slowly degrade over a period of multiple years with no DC voltage across them. I think that is less of a problem with modern capacitors so is likely nothing to worry about (as opposed to the capacitors which are definitely reversed).

 

[RSRecords]

Ok thanks. I was planning that all of the electrolytics will be bi-polar types. I just need to adjust the schematic/footprint to reflect this. I usually like bipolar for coupling caps anyway.

 

[RSRecords]

Ok ran into a little snag with the reference of the fader potentiometer. Should pin 1 of the fader connect to 0v or the vREF? Or should it could reference the gnd of the incoming signal as well. I also added drain resistors for the coupling caps. 22k seems like a common resistances (for Douglas Shelf anyway). And should these be referencing 0V?I don't know why single supplies trip me up so bad.

 

[JohnRoberts]

If the fader is referenced to 0V the polarity of C1 is backwards.

JR