LM3875 and speaker impedance questions

[Nigel Goodwin]

I had a big argument about speakers in series with the moderator on another electronics chat forum.
I said it ruins the excellent damping of speaker resonances provided by a modern amplifier.
He said that Geetar speakers are frequently connected in series.
I said that the Geetar speakers sound terrible, have the back of their enclosure open and the resonance is an effect.

I prefer to listen to a clear acoustic guitar, not a distorted electric Geetar.

All I will say is try it and listen to the results! - true, it does ruin the damping factor, but this makes far less difference than you might suspect!.

And PA speakers, as well as guitar speakers, are often wired in series as well.

 

[audioguru]

The response of that woofer causes trouble between 300Hz to 6kHz.
The response curves of both speakers must have been smoothed with a filter.

 

[hentai]

Hi, audioguru are u refering to the drivers curves or to the charts i posted?
the freq response charts that i posted are the expected electrical response of the xover for the woofer's region, tw's region and the 2 responses summed. the attenuation from 300 Hz was deliberately designed to compensate somehow the woofer's natural rise in spl.

 

[audioguru]

Hi Hentai,
I talked about the drivers curves.
The woofer's cutoff in your crossover network occurs at a frequency too low.
The step in the woofer's response is not necessary.
The response of the woofer and tweeter don't match. I think the woofer is too big to go smoothly up to 4kHz. They need a mid-range speaker in a 3-way speaker system.

 

[hentai]

hmm 3 way system would be much better but what i tried was to get the response of the system as flat as posible with minimum components. I disagree about that step. If its not tamed it will sound terrible in a very sensible region. The woofer i dont think its cut too low it may seem so because of the step but it will make transition of the sound from woofer to tweeter much smoother.

 

[whiz115]

@Audioguru I'm too newbie to all these and i can't see the things you're referring to, my opinion is that Hentai has done a very good job...but if there any mistakes (always exist...) and can be corrected please explain farther...

@Hentai what can i say? you're great!
as i said before i want to use my LM3875 with the specific speakers...so from what Audioguru and Nigel Goodwin said i don't know if i can connect them in series so i can make them 8 ohm without loosing sound quality...i guess i should try it...what's your advice?

P.S the calculator didn't helped me much...maybe i don't understand it...it requires Coil Diameter/Coil Length which i don't know... can you explain how it works?

 

[hentai]

hi
first a crossover is mostly design by ear u must play with it until it sounds better. tell u what if u plan on building the xover first build it without C5 R10 and R6 this will give u ordinary 3rd order butterworth with 2200Hz for woofer and 2700 Hz for tw (connect the tw in normal polarity this way, the + of the tw goes to where R7 and R8 join) listen how it sounds then add C5 R10 and R6 and reverse the polarity of the tw.
2nd about ur LM3875 problem my advice is this get the speakers out of their currently boxes and build a new box with 2 woofers and 2 tw the xover also needs modification for 8 ohm.
3rd about the coils.. i suggest u use 1mm wire u need something to turn this wire onto preferably a cilinder that will support the coil . At the coil diameter u input the cilinders diameter and at the lenght ...its lenght. im not good with english i hope u understand if not we'll keep trying
let me know what u decide about the lm3875 issue

 

[Nigel Goodwin]

so from what Audioguru and Nigel Goodwin said i don't know if i can connect them in series so i can make them 8 ohm without loosing sound quality...i guess i should try it...what's your advice?

Try it and see! - like I said, in theory you will lose the full effect of the damping factor of the amp - in practice this may, or may not, make any audible difference. Lot's of people spend stupid amounts of money on valve amps - valve amps have rubbish damping as well, yet some people prefer them!.

You've already said you're using 'el cheapo' speakers, so it's likely to have no effect at all, and messing with crossovers won't make poor speakers sound good!.

 

[audioguru]

I made many 2-way speakers with 3rd-order Butterworth crossover filters. I selected woofers and tweeters that match so there is not a "hole" in the important midrange, the woofer is filtered from its problematic high frequencies and the tweeter goes low enough to fill the hole without being damaged.

I worked with many 2-way high power speakers that used an active crossover with complicated EQ to make the sound just about perfect.
I also worked with a single driver speaker with a high frequencies diffuser that used a complicated EQ circuit to make it sound fantastic.

A good speaker driver costs only a little more than a lousy one so I only use good ones.

 

[whiz115]

ok...i'll try connecting them in series! at least it would be better for my amp! if not for my ears! right?

Hentai i think i must modify the crossovers you've made for me?


P.S about the coil winding...i have two bobbins 2,24cm diameter, center hole is 1,2cm in diameter, radius to the center hole is 0,6cm what else do i need to know to enter the inputs to the calculator?
(yes i'm not good at math...as well as my English... )

All coils are mandatory if i work it at 8 ohms? (i'm asking this because i don't have a third bobbin and i might have difficulties to source one more...)

 

[hentai]

check this out for ur coils https://colomar.com/Shavano/inductor_info.html
the xover must be modified for 8 ohm only if u build a new loudspeaker with 2 woofers and 2 tweeters as i sugested. this way u only need 1 xover. If u plan to use ur current speakers in series than u need 2 of those xovers (1 for each box) with no modification. Yes in that configuration all the coils are mandatory.

 

[whiz115]

i think there is a misunderstanding and it's my fault.

i have a pair of 2 way speakers...every speaker has 2 drivers (a tweeter and a woofer) and they're connected the one to the left channel of the amp and the other to the right.

When i say to connect them in series...i mean the drivers and not the hole speakers! (i use to call speakers what you call drivers sorry...)

btw the second site you gave me to calculate my coils is very good but from a quick look...I think my bobbins are not suitable for the coils needed! and probably i must find new ones... and from what i see the coils are to big to wind them...the two of the three is over 9m!

any ideas for that too?

things are getting difficult...but i must somehow make those speakers decent!

 

[hentai]

this confusion happens frequently because both ways are correct... i think
in this case the xover i gave u can not be used
i will modify it for 8 ohms

 

[whiz115]

most appreciated and sorry that i haven't made it clear from the start...
do you think that is possible to modify the bobbins i already have and make them respond to the frequencies you have calculated the crossover diagram?

 

[hentai]

dont worry!
the values in that diagram are calculated for 4 ohms dont use them.
for the next xover u can use those coils only if their value is higher than those on the xover. that way u can cut some of the wire so the value will drop. anyway u can use that plastic frame (assuming its plastic).

 

[audioguru]

At the beginning of this thread you said the amplifier sounded fine driving 4 ohm speakers. It needs 8 ohm speakers only for max output power. It is fine driving 4 ohm speakers if the power supply voltage is not its max. and if its volume is turned down a little.

Don't connect the second pair of speakers in parallel with the first pair because then the load on the amp will be only 2 ohms per channel.

Try connecting two speakers in series for each channel and hear how they sound. If they are all in phase then the missing damping will probably make them sound "boomy" like a drum. Also the woofers might make whistling sounds like a piezo smoke alarm.
Maybe you like boomy whistling sounds.

 

[Nigel Goodwin]

LIke I've said before - try it and see, presumably Audioguru has never actually tried it? - and is just talking theoretically!. But as he says, it's ESSENTIAL to connect them in series, or bye, bye, bass!.

BTW, a crossover in the speaker drops the damping factor considerably as well!.

 

[whiz115]

u can use those coils only if their value is higher than those on the xover. that way u can cut some of the wire so the value will drop. anyway u can use that plastic frame (assuming its plastic).

yes exactly! that is what i wanted to say...and i hope it is possible!
or else i must go to one of the stores that wind motors to wind my coils....

take a look....this is how my bobbins look like...the transparent one is the closest much to what i have.

**broken link removed**

 

[audioguru]

I connected speakers in series when I made a Hafler simple surround sound system. I didn't like the boomy bass so I used an opamp to extract the difference signal and feed the speakers with their own amp. Then they sounded perfect.

The bass disappears when speakers are connected out-of-phase with each other like they are in appliance stores.

 

[whiz115]

Try connecting two speakers in series for each channel and hear how they sound.

I have only one pair of speaker boxes and i'll connect the drivers of each speaker box in series..and each speaker box will be connected to the corresponding channel of my amp. i hope we are saying
the same thing..

At the beginning of this thread you said the amplifier sounded fine driving 4 ohm speakers.

I know what i said at the begging but obviously the setup between my amp and my speakers was not correct...and i lost too much from power output, quality and maybe my amp had hard time to give me what i asked to give.