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Old 24th February 2008, 03:47 PM   #1
mpw01 is offline mpw01  United Kingdom
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I have decided to build my first pair of speakers. Im going for a small 2 way vented design using the Seas ER18RNX woofer and the 27TDFC tweeter much the same as Zaphs SR71.

I think it should be a better learning experience for me to calculate box and crossover design myself rather than go for the kit on madisound. I might be biting off a bit too much, but id sooner just jump in and see what happens.

I have an old pair of Quad 303's (with new power supply and rectifier boards) that I want to use to bi-amp the speakers. Not the best amps in the world, but I love them and more importantly they have already passed the wife acceptance test, although I suppose that could change after the next re-decoration(!).

Im only going to be using CD as source and Im building a passive volume control using a 24 step ladder attenuator.

As im still at this stage trying to visualise the whole setup my first question (first of hundreds probably) is with regards to the crossovers. Where is the best place in the chain to put them? Sould they go before the power amps so each amp is just dealing with its own range of frequencies or can they be fed straight from the CD outputs or is the best place after the amps?

I've read many posts on here regarding bi-amping and active XO's. Can I not just use the same passive XO's that I would use if I wasn't bi-amping with one amp using the low pass circuitry and the other using the high pass?

Any advice will be greatly appreciated and I'll try and keep a step by step account of how I get on. It might be useful to anybody in a similar postition.
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Old 24th February 2008, 11:33 PM   #2
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My personal opinion is that true biamping (i.e. line level crossover before the power amps) is superior to filtering at the speaker level. If you are using digital source and solid state amplification I see no reason not to just build or purchase a simple opamp based line level crossover as the cost is really trivial compared to the cost of the second amp.

mike
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Old 25th February 2008, 12:39 AM   #3
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Hi mpw01,

You said you are after a learning experience and use both Quad's . With that in mind, would a better solution be to build two pairs of SR71s for a 4.1 setup or run a study / bedroom system?

Choosing the SR71 certainly would give you a fallback position should your own attempt not satisfy. However I doubt you could do better than Zaph with what he has done with the SR71 given his experience in this.

What you probably mean is you like the drivers Zaph chose. Any change to the kit is another design and you can't blame / aportion any of it not working to Zaph - hope that is clear.

Cheers,
David.
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Old 25th February 2008, 09:00 AM   #4
mpw01 is offline mpw01  United Kingdom
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Quote:
Originally posted by mashaffer
My personal opinion is that true biamping (i.e. line level crossover before the power amps) is superior to filtering at the speaker level. If you are using digital source and solid state amplification I see no reason not to just build or purchase a simple opamp based line level crossover as the cost is really trivial compared to the cost of the second amp.

mike
Thanks Mike. I'll look into that. From what you're saying I assume then that there are big differences between XO's built for line livel and speaker level signals?

Quote:
Originally posted by Dave Bullet
You said you are after a learning experience and use both Quad's . With that in mind, would a better solution be to build two pairs of SR71s for a 4.1 setup or run a study / bedroom system?

Choosing the SR71 certainly would give you a fallback position should your own attempt not satisfy. However I doubt you could do better than Zaph with what he has done with the SR71 given his experience in this.

What you probably mean is you like the drivers Zaph chose. Any change to the kit is another design and you can't blame / aportion any of it not working to Zaph - hope that is clear.

Cheers,
David.
Hi David. I chose the drivers on Zaphs recommendation basically. Im fully aware that designing enclosure and XO means a completely different design to Zaphs and even more aware that I will not be able to better the SR71's, I just feel that there is more to be gained for me personally by having a go at it. If it all goes belly up then im left with some nice drivers and a fully worked out design in the SR71's as a (very nice!) get out as you say.

Thankyou all

Martin
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Old 25th February 2008, 02:34 PM   #5
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Yes, big differences between LL and SL. Using LL allows the power amp to buffer if you will the crossover from the wildly varying impedance of the loud speaker. Also since the LL is a low power situation parts can be smaller and shall we say more refined.

At LL there are two or three options.

Passive where an RC network is placed between the pre and power amps. This can be very clean and no additional noise to speak of is added by the network but you will lose some gain (volume) due to losses in the network and you are limited in the amount of boost or cut per octave as a practical matter. Can work nicely in some cases though.

Buffered passive. This is like the passive except that there are buffer amplifiers added around the passive circuit to nail down the impedance seen by the network and the outside amplifiers. Nice approach in my opinion.

Active where the network is built into a line level amplifier usually via. the feedback network. This is very flexible but I think more susceptible to negative effects from overload. This can be as simple a an operational amplifier and a couple of caps and a couple of resistors per LP or HP channel.

mike
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Old 25th February 2008, 05:31 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

You can build the SR71 as specified with biwirable inputs and then
you can common the 303 inputs to drive each speaker separately.

Apparently the 303 has a fully regulated supply with limited current
delivery, and this way of using they 303 is optimum, the much lower
current for the tweeter allowing more current for the bass unit.

IMO fully active crossovers for a two way are a bit of a waste of time.
given the above arrangement :
A) you could reduce the tweeter amplifer input coupling capacitor
to roll off some bass below the operating range.
B) you could redesign the tweeter passive c/o not to include the
attenuating resistor and top end shaping, including these in the
303 by changing the feedback loop. (only really worth doing with
a treble amplifier far less powerful than the bass amplifier).
Keep C1,L2. (new values)
C)you could redesign the the bass unit passive c/o to have no baffle
step correction, in fact the overall correction is near 10dB. You then
include this correction in the bass amplifiers feedback loop. This will
give you far higher dynamic range in the midrange - good design.
Keep L7, C9. (new values)

If you want to go completely active, getting rid of the 4 passives,
then you could implement these passive line level in the feed to
the amplifiers.
To do it actively is possibly easier but you need an active box.

All this will need cross checking with some sim tools :
http://www.rjbaudio.com/Audiofiles/FRDtools.html
http://www.geocities.com/woove99/Spkrbldg/

Personally I'd only investigate the active Baffle Step / Mid Correction.
Easiest to do and the most practically justifiable / effective.
With loads of treble power available I'd leave the tweeter section as is.
Then to go to a standard speaker you would just fit the designed L7,C9.

/sreten.
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Old 26th February 2008, 12:37 AM   #7
mpw01 is offline mpw01  United Kingdom
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Thankyou both for the pointers.

With my limited knowledge of electronics I feel much happier going with a passive RC XO design, as a starter anyway. I dont listen at levels anywhere near what the amps can output so a loss in gain is not that much of a problem to me.

Quote:
Originally posted by sreten
...you can common the 303 inputs to drive each speaker separately.

Apparently the 303 has a fully regulated supply with limited current
delivery, and this way of using they 303 is optimum, the much lower
current for the tweeter allowing more current for the bass unit.
Thanks for the time you spent on that sreten. If I understand the above quote correctly, you're saying I should run both amps full range and have one for left channel and one for right. I can see that this would give me better usage of the available power but it also contradicts what i've read in most other posts where people seem to be saying that bi-amping is a waste of time without a line level crossover. Is it the regulated supply/ limited current design specific to the 303 that makes this a better option, or would this be the best method regardless of type of amps?

martin
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Old 4th March 2008, 03:35 PM   #8
sreten is offline sreten  United Kingdom
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Join Date: Nov 2003
Location: Brighton UK
Quote:
Originally posted by mpw01

Is it the regulated supply/ limited current design specific to the 303 that makes
this a better option, or would this be the best method regardless of type of amps?
martin

Quote:
Apparently the 303 has a fully regulated supply with limited current
delivery, and this way of using they 303 is optimum, the much lower
current for the tweeter allowing more current for the bass unit.
Hi,

It is not generally applicable, in other words I agree that most of
the time simply bi-amping is extremely cost ineffective. It is effective
for amplifiers with limited current delivery and ideally with identical
amplifiers you would bi-amp a 3-way speaker as bass | mid+treble.

/sreten.
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