Question of crossover

Dear ALC/others,

Attached is the crossover at my woofer, it is a band pass filter right?

If that's the case, when use <6.8uF the bass will be cut off more, if use >6.8uf, the bass can go lower right?

The bass would not go lower, because the amount of input energy is fixed. The passive crossover network can do is selecting the band going to either woofer or tweeter. Altering the values would shift the crossover point (lossing part of the band). Stronger bass probably achieves by attenuatting tweeter energy (adding series resister).

by looking at this circuit, the 6.8uF is reponsible for a low pass crossover point?

i a bit confuse whether this circuit is a low pass filter or bandpass filter??

as band pass filter will has 2 crossover point (1 at HF) 1 at LF.


i had replace my 7.3uf cap with 6.8uf, the bass is still strong... but i feel taht the lower Lf seems like a bit cut off as the HF now like slightly dominant....i'm not sure whether it is due to the changes of crossover point or the cap still not run in....

In passive circuitry, every component, including resistor and speaker driver characteristic, plays an role in the crossover frequency formula.

The analysis is as follow: From the woofer point of view, within the woofer band (assuming 20Hz to 400Hz), the woofer can be considered as resistive. The woofer crossover network is dominant as a first order low-pass function, since 1.03mH is inductor and woofer itself is resistor. The effect on 6.8uF and 3R3 resistor branch is small in this frequency range.

However, this woofer also acts as midrange to cover the frequency maybe up to 3kHz. The woofer impedance would rise as frequency increasing due to both coil inductance and mechanical compilance of cone. Also the sensitivity of woofer at low and high frequency is different. The 6.8uF and 3R3 branch forms a parallelling impedance with the woofer to control the woofer sensitivity approaching crossover point. Since there is 3R3 in the 6.8uF branch, in theory it is a zero (in terms of circuit theory), the overall system is not entirely a second order network, but still a low-pass function.

Additionally, even there would be much higher impedance at the woofer resonance frequency around 100Hz, the impedance is still resistive.

The 6.8uF certainly be part of formula in controlling crossover frequency. Only 7% change in capacitance value would not cause so much the crossover frequency shifting. The "capacitor sound" may be the issue.

Dear windwss,

The filter you show is a 6 db lowpassfilter with a impedance correction network. The L is the lowpassfilter. The RC is for impedance correction network. Other value will change the sound but if the builder done his homework right the value's are right. Why whould you change the parts?

Wow, Koifarm is here again.

Welcome you back. We miss you here  :wink:

I still don't have the chance to bring the CD 33 Power Supply board back to China for checkup because of terrible workload.

However, I managed to DIY a Pass Labs 1.7 .. ha.ha.

Alfcat, all th time i have read this forum. But had nothing to ad. And i had a new job which take a lot of energy.

Keep up the great work. I like the way you build and write about it.

Sorry for this off topic reply.

Thank you for all your input!!

Dear Koifarm,

In fact the original 6.8uF(written on cap) Alcap bi-polar cap actually read 7.3uF, 7.3 is odd so i just put in a 6.8uf film cap as i prefer using a single cap if possible.

Anyway after using the 6.8uf film (measure value is 6.9) cap i feel that the balance seems had been shifted, where the tweeter/HF become more dominant....

So last night i have no choice but try to add/ parrallel a 0.33uf to make it to ~7.2uf then the sound quality/balance come back..... initially i thought differences of 0.33 which is ~5% won't make much differences, it seems i'm mistake....

I decided to change the cap as i feel that the bi-polar cap is blur

If you change the value from 7.3 uf to 6.8 uf as you did, the crossover frequentie of the woofer goes down with 700 Hz ( asuming 8 ohm woofer). So this will sound different because there will be a 700Hz hole between woofer and tweeter.

dear Koifarm,

How u calculate the Frequency? still using F=1/2piRC???

undamped natural frequency fo = 1/{(2pi)sqrt(LC)} = 1.9kHz

resistance of speaker (8ohm) makes fo < 1.9kHz due to lowering the system damping factor, crossover frequency < 1.9kHz.

however the zero f' = 1/{(2pi)(6.8uF)(3R3)} = 7kHz brings fo up a little bit. Hopefully resultant crossover point around 1.9kHz.

[quote:e9d296f6c9="WINDWSS"]dear Koifarm,

How u calculate the Frequency? still using F=1/2piRC???[/quote]

Yes.

The impedance correction network ( CR) has a crossover frequentie of 2300 Hz. Below 2300 Hz the 3.3 ohm has no influence but above 2300Hz it lowers the impendance of the woofer.

wow..still a lot to learn... thank you brother...

Just receive my 1mH foil indcutor last night, the impedance seems ~0.05-0.1R higher than my original iron inductor... i hope itwont reduce much my bass energy... wish me good luck this round!

Dear ALC & accphoto,

Kindly refer picture attached, do you know what kind of resistor is this? Is it good for hifi system?


Dear ALC,

Finally i hoke my iron core inductor to foil inductor, and the bass enery really reduce.... so i use your method by adding series resistor in tweeter to do the compesation and it really works.

My question is by adding resistor in tweeter, it will reduce the sensitivity of the speaker right? (i need to add ~1R-->From 3.3R to about 4.3R

Besides, would it change the crossover point of the tweeter?

I never seen this kind of component before. But i guess it is resistor of temperature compensation or something like resetable fuse. Well, I'm still not sure.

I think the crossover shift is not much. Othewise you could hear the dip or peak at 2kHz which is around the crossover point.

The sensitivity drop is only at tweeter side. And main music power is addressed in region below 2kHz. Therefore, the woofer/midrange handle most of the music power. The slight energy demand increase in tweeter does not make the amplifier running hard.

the whole pics of the resistor seems made of grafit/carbon. And most of them for low resistant ~1-10R.... wondering wheter carbon composite type of resistor will sound good in speaker crossover??

After listen to several days, i decided to put back the original iron core inductor....as i like the colouring/taste of the iron core inductor.


So if i were made use of the foil inductor (1mH) to make a sub woofer LPF with xover point of 100hZ, is it possible?

what is the xover point with 1mH & 20uF cap for a 4R woofer?

The natural undamped frequency is fo = 1/{(2pi)sqrt(LC)}
= 1/{(2pi)sqrt(1mH)(20u)} = 1.1kHz
woofer impedance 4 ohm will make it lower than 1.1kHz
100Hz crossover may need 20mH and 68uF which may be impractical.
so subwoofer usually use active crossover

TQ.