LM3875 and speaker impedance questions

[Nigel Goodwin]

What are you hoping to achieve?, didn't you say they were cheap and nasty speakers? - you can't really change cheap and nasty to good by playing with the crossovers.

 

[audioguru]

Yes, speakers in series sound "boomy". Also most woofers have trouble around 5kHz which might make a "shriek" sound if the woofers are in series and therefore don't have good damping from the anplifier.

 

[whiz115]

i'm trying to fix the part which is my fault...i'm not trying to totally change the audio performance of the speakers, i know it's imposible...

For the history...
i have changed 2 pairs of crossovers which are at 8ohm... the first one
is 1st order and the other (which i have posted the schematic) is 2nd order...

i was very disappointed so i replaced them with just a filter capacitor for the
tweeter and i left them as is for over a year until i've made my LM3875 amp and then i discovered that my speakers aren't appropriate for the amp and that's why i created this topic...none of crossovers sound good because i was very ignorant...now i believe i have learned 5 things...i've made my tests with parallel and series connection etc...without any good result, so believe it is time to correct my mistakes!

As i said to my last post...i just want to modify my 2nd order crossover so they can work with my speakers and leave them that way until i buy something better.

 

[hentai]

whiz those filters that u have are not good for ur drivers. with that woofer's response 1st order is not enough, not to mention the tw (u kill the speaker's dynamics). If u use 8ohm xover for 4 ohms drivers the xover frequencies will be cut in half. But guess what, without a zobel network like in the xover i gave u the woofer's impedance wont be 4 ohm at the xover freq (dont remember exactly ~2700). it will be much higher !!! this will give a higher xover cut point for the woofer. (the tw impedance is usually quite linear). This means the woofer will enter in its cone breakup region and specially in 2nd order there will be big phase cancelation between woofer and tweeter specially in the sensitive mid range. Have u tried the xover i gave u?

 

[whiz115]

Hi hentai!

i had difficulty to source more bobbins locally...and i thought that i should try to modify my current crossover bobbins and in the case that is a total failure i could use them and buy some more from RS components... and re-wind them for the second schematic you gave me...for that option i still have to find a shop because i don't think there's not a way to wind 9+ meters of wire all by my self accurately! i this case i hope a shop that winds transformers can handle my unusual request...

at the moment i want to try the easy way...hopping it can save me from trouble and money.

do you know if there is any equivalence between mH and ohm? so i can cut my bobbins to lower values?

 

[hentai]

well if ur still experimenting try to put the Zobel in parallel with the woofer (a 4.7 ohm resistor in series with a 33u or 47u capacitor) and see how things change with ur 2nd order xover. When using 2nd order 2 way xovers remember to put the tweeter in reverse polarity!!!
for ur coils give me the coils dimensions ( not the frame) and the wire diameter.

 

[whiz115]

well if ur still experimenting try to put the Zobel in parallel with the woofer (a 4.7 ohm resistor in series with a 33u or 47u capacitor) and see how things change with ur 2nd order xover.

ok i'll try it! but the only cheap way at this values is electrolytics...and guess what! it's very hard to source NP capacitors close to these values locally in Greece i was searching for a 22uF when i was building my amplifier and i couldn't find one! so i've made myself one NP from two 47uF electrolytics anyway i'll try it....................

When using 2nd order 2 way xovers remember to put the tweeter in reverse polarity!!!
for ur coils give me the coils dimensions ( not the frame) and the wire diameter.

yes...i'll try that too!!! (i haven't try it until now!)

the wire diameter is probably 0,6mm
General dimensions for the coils are in previous page...

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[hentai]

i'll try that too!!! (i haven't try it until now!)

its a must otherwise u'll have phase cancelation and certain frequency ranges will be very attenuated. No wonder ur sound is bad.

i'll try to determine the coil's value, or if u want to, there is way u could find out the value urself. take this little program put the left channel from ur soundcard's output to the xover input. at the xover output coresponding the coil u want to determine, put a 8 ohm (8.2 ohms)/1W resistor. the connect this output (with the 8ohm load) to the left channel of the line in input of ur sound card. Select from ur windows mixer line in as input source and run the program. U will see the response of the filter. note the frequency where the amplitude starts droping (in theory if its a butterworth the corner freq is at -3db). after u noted this freq use this formulae to determine the value:
L=1.8/F aprox. put the freq in Hz and u get L in H this formulae works for butterworth xovers, if its a LR xover then L=2.54/F. if u dont know the type use the butterworth.

 

[whiz115]

i forgot to ask you...how many watt the resistor for the zobel network?
i have a spare from the crossovers that say 7 ohm/5W is it possible to be
used?!

 

[hentai]

yes u can use it but when u can replace it with a 10W.
as for the coil i think its something between 2.2mH and 2.4mH but i cant be certain. i suggest u verify this with the method i described above. Good luck

 

[whiz115]

hentai you're unbelievable!!!!

thank you for the very useful tips!!! i have just connected my tweeter
in reverse polarity with the crossover on the one speaker and the sound
has changed dramaticaly! it's more balanced! much better sound!

the other speaker which has my hand made 1st order crossover...it justs
screams! and the bass is awful... (i've tasted them with the same song)

i'll try to make the tests with the program you gave me...but i have to screw both crossovers so i can get the resistors....i don't have any others at high watt...

 

[hentai]

,well if ur pleased with the sound just forget about Zobel network. When u'll have the components available u can try it. Put 2nd order xover ( u have a pair of 2nd order xovers, dont u?) to both speakers with the drivers connected properly and enjoy the music. Take care.

 

[whiz115]

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click to enlarge...

the measurements are at the same sound level with a ~7ohm resistor in series with the (+) woofer/tweeter output from the crossover (i hope i did it correct)

 

[hentai]

hmm something is wrong . u should put the resistor like u put a driver and the line in from the soundcard the same ( must measure the voltage across the 7 ohm resistor). the soundcard output should be connected like the power amplifier. Also if ur using the left channel from the line in declick R and L-R from the top bar it will give u a cleaner view

 

[whiz115]

ok heres an update...

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if i choose L-R the waveform is a little different! especially for the tweeter which
falls at the end of the waveform.

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[hentai]

hmm they still dont look right. Dont use L-R use only L, for the tw is ok as u can see it starts droping below 2000. U can use the cursor to determine the slope of the filter. thus the order. This way u can check if its ok. For the woofer the graph should fall after a frequency in the mid range. If u could make a drawing of ur setup i could see if something is wrong. Anyway here is the how it should be done.

 

[whiz115]

Hi...

i don't know what is wrong, the cable setup is ok..this is the best i can do
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i have changed from L to R channel and also the cables. Probably this is the frequency response of the crossover

P.S i've done the tests with Sound Blaster Live and i've noticed that when i loopback tested the card with a simple cable i had ripple in highs...i could use the onboard sound card...but i haven't installed any drivers

 

[hentai]

i'm sure this software has flaws of its own.... i mean its under 1meg what can we expect but its able to make a picture of whats going on. Now i see your Y scale .
Try adjusting the Y scale i see 1 div is like 50dB thats too much. The graph might be good. Make it show 0 - 50 db. Ur soundcards noise floor could also be blamed. Also use the cursor to determine the slope!!!

 

[whiz115]

if i keep posting photos i'll probably see my nickname to the top 10 banned users!

sorry moderators! a newbie is a newbie!

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Hentai please remind me something...the slope is where the frequency has rapid
fall or raise?

 

[Nigel Goodwin]

if i keep posting photos i'll probably see my nickname to the top 10 banned users!

The thought never crossed my mind, there's no problem with relevent pictures - however, why don't you upload them here rather than providing external links?.