K1000 Notch Filter

[Fitz]

It's long overdue, but I finally decided to go and figure out exactly how the filter PCB in the K1000 affects the sound. As has already been established, it is a low Q notch filter in the treble region, with a noticeable effect on the sound. It consists of an inductor, a resistor and a capacitor in parallel with each other, and the filter is in series with the driver. The values I measured were 7.1mH, 88.9R and 0.168uF. The resistor markings clearly show it as an 88.7R 1% resistor, but the other two would require assumptions towards their nominal value.

Simulating the filter based on a 120 ohm impedance gave results that conflicted with the measurements I took of the K1000 with and without the filter installed. Measuring the impedance showed it varies from 130 ohms at 1KHz to over 140 ohms at 20KHz, so I put 135 ohms as a compromise and then the simulation and actual measurements agreed with each other.

Based on both the filter simulation and the actual driver measurements, it seems the notch filter is centered around 4.6KHz with 4.4dB attenuation, and has a bandwidth from approximately 2KHz to 11KHz.

[luvdunhill]

Interesting. Is the DCR of the inductor enough to be significant to mention?

[Fitz]

DCR is only 5.6 ohms.

[cetoole]

How about driver inductance?

[Fitz]

No idea, never really felt like trying to make a complete model of the driver. I don't think it has any significant amount of inductance, at least not relative to the filter, since the resistor model matched the acoustic measurements pretty closely.

[nikongod]

If anyone has an inductance meter handy (that can measure what may be a LOW value accurately) you are welcome to my K1000 PCB's in the name of science.

One of them didnt survive my re-recabling efforts... and I do the notch digitally now.

[luvdunhill]

Fitz, any inkling on if these values changed over the life of the design? I'm kinda tempted to bypass mine and build an external crossover with better components. Ari, how do you do your active filter?

[nikongod]

Ari, how do you do your active filter?

The Behringer DEQ2496 has parametric EQs. I set it based on this thread on the 'fi

link

I am also interested in building a standalone clone of the stock filter in case I ever sell these.

[Fitz]

Fitz, any inkling on if these values changed over the life of the design? I'm kinda tempted to bypass mine and build an external crossover with better components. Ari, how do you do your active filter?

I don't think so, at least not significantly. The sim doesn't match up as closely to the measurements I took of a very high serial # pair, but the difference is rather small and would make sense with 5-10% tolerances on the cap and inductor. The real question is what the nominal values are supposed to be. I suppose one could reasonably go with the nearest common value.

[Wmcmanus]

Interesting... so do you like the K1K sound better with or without the notch filter? Had my notch filter removed years ago and don't miss it myself.

[Fitz]

Removing it just boosts the treble, which isn't something I really want.

[kevin gilmore]

If anyone has an inductance meter handy (that can measure what may be a LOW value accurately)

I have such a meter...

[luvdunhill]

I don't think so, at least not significantly. The sim doesn't match up as closely to the measurements I took of a very high serial # pair, but the difference is rather small and would make sense with 5-10% tolerances on the cap and inductor. The real question is what the nominal values are supposed to be. I suppose one could reasonably go with the nearest common value.

I'd propose a custom wound inductor, polystyrene caps. It would be nice to fit them in a small project box. It would be cool if a switch could be fashioned that defeated or enabled in the internal crossover, but I suppose it would have to be on the headphones themselves.

Ari: As for measuring the actual values, I'd probably just generate a frequency response and phase graph of the filter itself and then reconstruct the filter. That way the inductance of the resistor (if any) and resistance of the inductor will be captured properly.

[Smeggy]

taking out the circuit board = moar extreme tweaking!!!!!