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Old 1st January 2009, 05:02 AM   #1
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Default BSC for MLTL FE207e

Quick question - am putting together the bsc for my new mltls. They will be powered by a pair of peter daniel LM3875 gainclone monoblocks.

What damping factor should I work on?
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Old 6th January 2009, 05:10 AM   #2
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Default I would really like some help on this...

Ok, I guess this must be a dumb question. I've done a lot of reading around, and it seems that gainclones have a reasonably low damping factor, although I don't really understand what that means, or how to measure it. I am also building an F5 clone (easy to get hooked on this DIY business - I figure the only way to learn is to get stuck in)

I have built the MLTL cabinets to MJKs spec apart from one error - the wood was cut wrongly due to a communication error with the cantonese-only speaking timber shop (for which I am taking responsibility). The enclosures now have a 9.5" internal square section, instead of the 9.5" x 11" specified.

The drivers have had a couple of hundred hours run-in as I wanted to have a listen to my new amplifiers. I am impressed with the clarity of the speakers and they are already an improvement on what they replaced, but I am conscious of the lack of bass and the midrange shout is fatiguing.

What I need help with is the maths - I will stuff the boxes as per Martin's instructions unless the changed enclosure section/volume means I need to shorten or lengthen the stuffing tube.

I also need a suggestion for the values for the bsc circuit.

The speakers will be placed within a few inches of the wall, but I still feel that a bsc/zoebel circuit would help - the ohmite rheostats that Martin uses in the variable circuit are available locally - would these help make the speakers more flexible with different room placements and amps.

Apologies for the long post and demands - some day I would like to be of use to the community.

Matt
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Old 6th January 2009, 12:59 PM   #3
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Default Re: I would really like some help on this...

Quote:
Originally posted by afunkymonk
I also need a suggestion for the values for the bsc circuit... would these help make the speakers more flexible with different room placements and amps.
Yes definitely. But I don't think you need to re-calc the BSC because of the cabinet under-volume. I would use exactly what Martin uses for the circuit (speaking as a newb).
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Old 6th January 2009, 01:02 PM   #4
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BSC normally assumes that the speakers are free standing -- at least 2-3' from the wall. If the speaker is pushed up against the wall -- in your case with the baffle ~1' from the wall, the baffle step becomes a notch rather than a step down. That can be hard to work with. You will need to measure the in-room response and then design a notch filter accordingly.

Since you don't know the DF of your amp, assume "high". 2mH is probably a good start and using a variable resistor will get you what you want.

The FE207E is definitely NOT shouty. You just have no bass. Once you get your new cabinets built, all will be well. Your cabinets will be slightly too small, but not devastatingly so. Lengthen the port to 7" and you should be alright.

The FE207E is short on the top end. I do not use a zobel with my BSC filters for this driver. Eliminating the zobel brings up the top end a few dB. You still may want a tweeter crossed in ~10kHz.

Bob
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Old 7th January 2009, 06:37 AM   #5
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Thanks Bob

Leaving out the zobel will make things simpler. On Martin's circuit for the variable bsc, he has the 15R rheostat in parallel with a 3.0mH inductor and a 2uF capacitor.

Does the cap need to be there. and if I use a 2.0mH inductor, will that alter the value of the cap?

Thanks for your reassurance on the cabinet size. I have cut 6 inch ports, but can easily make 7 inch ones if you think it will make a difference.

Cheers
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Old 7th January 2009, 12:20 PM   #6
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The unfortunate fact is that if you are going to do your filter by ear rather than by measurement and cross over modeling, you need inductors in 1/2 mH increments from 1.5 to 3mH and a variable resistor. Your ears hear differently from mine and your room is different. All I can tell you is what works for me. I suspect that since you are placing your speakers closer than normal to the wall, you will need less BSC.

The capacitor bypasses the BSC filter at high frequencies. The larger the cap, the lower the frequencies bypassed. 2uF seems a bit large to me. Again, if you ar doing this by ear, you need series of test cap in ~1/2uF increments from .68 to 2.0uF. Not using a zobel has the same effect. FWIW, I do not use a bypass cap with 167's and 207's, but do bypass 206's and Lowther DXx's because I wind up with a rather large resistor in the BSC filter.

Bob
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Old 30th January 2009, 09:02 AM   #7
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Can I start by just adding the series resistance, and add the correct inductor when I have set the resistance correctly, or are they mutually dependent?
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Old 30th January 2009, 11:51 AM   #8
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No. Simply adding series resistance will screw up the cabinet alignment and will not do what you need to do -- bring down the treble to match the bass. Yes, it is the resistor that brings down the treble, but it is the inductor that bypasses the resistor for the bass. Also note that the resistance of the inductor (DCR) is important. You need as low a DCR as you can afford. I use Erse Super-Q 16ga laminate core inductors which have a DCR of ~0.15ohm for a 2.0mH value.

Start with 2mH||10ohm. Possibly a little agressive, particularly for a small room, but you will surely hear the affect.

Bob
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Old 30th January 2009, 11:53 AM   #9
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Got it. Thanks again Bob
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