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Making a passive low shelf filter?

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#1
I've got a pair of headphones which I want to EQ passively to make them sound neutral. The entire EQ is a single low shelf attenuation filter:
1555211153762.png
I would like to replicate this with a small circuit I could put between the headphone and amp. However, I cannot find any easy to understand or complete information on how to make this. If I had an explanation of the circuit design, what values to change, and how to calculate them, I could perhaps get somewhere and attempt to build something with some components. But everything I am finding online makes me think I'd need an electrical engineering degree to even begin to understand half of it.

The effect of the EQ is shown here:
1555211308698.png
The headphone luckily has a simple linear slope down from the mids that is trivial to correct with EQ. It is a bit too warm for my tastes without, but with it could replace my HD600 as my main headphone.
 

RayDunzl

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#2
I would like to replicate this with a small circuit I could put between the headphone and amp.
Are you listening to digital sources, or tape/vinyl?
 

RayDunzl

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#4
I'm not finding an easy u-build-it shelf filter either, so I don't have the answer you want.

Generally, EQ is applied to the signal before power amplification.

I use a miniDSP unit in front of my DAC. All my source digits go through it.
 

solderdude

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#5
Is very easy to build passively between DAC out and amp in.
In your case (DT250/250 ?) it can even be done passively in the headphone cable chain when you have an amp that can drive lower impedances.

Let me know if you want passive filter in line-level or headphone cable and what in and output resistances it concerns (or what devices)

Do you feel the DT250 needs more treble ? Below is what it sounds like to me.

 
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#6
Is very easy to build passively between DAC out and amp in.
In your case (DT250/250 ?) it can even be done passively in the headphone cable chain when you have an amp that can drive lower impedances.

Do you feel the DT250 needs more treble ? Below is what it sounds like to me.

Dt250/80.

1555230542355.png
1555230639591.png
1555230461307.png

I created the EQ based on the first measurement, and tuned it by ear. My yoga block finally arrived today, so I redid the measurement and got a flatter response, though I still prefer the brighter more airy sound with the EQ as I come from HD600. It'd be nice to have the option to switch between both tunings without any software.

This pair of DT250 was modded to fix the seal around the connector and correct channel imbalance, and part of the PCB in the cup was also removed.

I actually did manage to find some information I needed:
1555230875098.png
with Rz being the headphone driver (80 ohms). Would this be the route to go here? I do want something between the headphone and amp, since I will be using this with portable sources.
 

solderdude

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#7
What amplifier is used.. or do you know the output resistance and max. output voltage ?
You will be loosing 5dB in max. output level.
Should be possible with 80 Ohm version.

Can your amp drive 32 Ohm HP's ?
 
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#8
What amplifier is used.. or do you know the output resistance and max. output voltage ?
You will be loosing 5dB in max. output level.
Should be possible with 80 Ohm version.

Can your amp drive 32 Ohm HP's ?
Yeah, it can. Power isn't really a concern, these are fairly easy to drive as is and -5dB isn't a problem since my 1Vrms dongle can easily run them with the EQ enabled. Would a non-sub-1ohm output impedance make a significant difference in the filter?

So far I've come up with this with the help of someone else:
1555234616246.png

EDIT: adding 5 ohms resistance between source and filter increases the effect by .5dB so that's not too bad I guess.. Just won't want to be using it out of my laptop with its ~45 ohms impedance :p
 
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solderdude

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#9
You need to add a resistor in parallel to the 80 Ohm.

I suggest to calculate the capacitor again but with R1 = 16.5 Ohm (2x 33 Ohm in parallel) and instead of R2 being 80 Ohm use 20.5 Ohm.
This 20.5 Ohm will be formed by a 27 Ohm in parallel with the DT250 which has an impedance between 80 and 90 Ohm.

The load on the amp will be around 35 Ohm and the HP will see an output R (for lower freq.) of about 10 Ohm instead of 64 Ohm so less of a midbass bump (which isn't sizable anyway in this case)

The capacitance will be around 2uF so 2x 1uF in parallel or 1x 2.2uF

You will need to drive it from a low output R amplifier <3 Ohm otherwise the filter will become less effective.
As an experiment (by ear) you could increase the capacitance to 3.3uF or even 4.7uF
 
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#10
You need to add a resistor in parallel to the 80 Ohm.

I suggest to calculate the capacitor again but with R1 = 16.5 Ohm (2x 33 Ohm in parallel) and instead of R2 being 80 Ohm use 20.5 Ohm.
This 20.5 Ohm will be formed by a 27 Ohm in parallel with the DT250 which has an impedance between 80 and 90 Ohm.

The load on the amp will be around 35 Ohm and the HP will see an output R (for lower freq.) of about 10 Ohm instead of 64 Ohm so less of a midbass bump (which isn't sizable anyway in this case)

The capacitance will be around 2uF so 2x 1uF in parallel or 1x 2.2uF

You will need to drive it from a low output R amplifier <3 Ohm otherwise the filter will become less effective.
As an experiment (by ear) you could increase the capacitance to 3.3uF or even 4.7uF
Alright, thanks. I'll try stuff out.

1555242989767.png

This is what you are talking about, right? Excuse the messy diagram.
 
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solderdude

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#11
yes, it does the same as the other filter but this one has a much lower source resistance.
It is also evident that it is no 100% substitution for the digital filter you listened to.
 
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RayDunzl

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#12
Ok, so the above is a high-pass with a shortcut (the resistor in parallel with the capacitor) to keep the low frequencies from disappearing.

Or, it's a resistive voltage divider with a shortcut for the high frequencies.

What would an analog shelf filter look like?

Or, does it require active parts to make it work?

(I looked but haven't found much about shelving filters)
 

solderdude

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#13
Ok, so the above is a high-pass with a shortcut (the resistor in parallel with the capacitor) to keep the low frequencies from disappearing.

Or, it's a resistive voltage divider with a shortcut for the high frequencies.
Its 'and' not 'or'. ;)

What would an analog shelf filter look like?
like the circuit above or another circuit entirely.
There are many ways to make a filter but in this case it is the cheapest and easiest solution when you want to solve it in the analog domain without using many parts and not compromise too much.

does it require active parts to make it work?
Only if you want to make the filter slope steeper or you want the filter not to attenuate the lower frequencies.

You could also make the passive filter steeper by using inductors as well.
 
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