John Sayers' Recording Studio Design Forum Archive

here's a pic of my killatones, made from hardwood and $200 worth of fostex fx-120s... More than pocket change here in india, so i'm taking it a little slow...

right now problem is, they're completely sealed!!! theres no cables or connectors... the cabinet is ready but not the electronics or anything...

three basic questions...

1. can anyone explain the components used in the prescribed circuit? What part of it is the crossover and what is the BCF? (this is my first project and i know nothing of electronics...) Is there any real downside to the passive electronics... i hear a lot of guys complaining about passive electronics reducing audio fidelity...

2. do the electronics change in any way due to the change in the driver... i've used the the much better fostex fx120 and not the fe127e.

3. can the following "active" BFC be used with the speaker?

http://sound.westhost.com/bafflestep.htm

I have bought the following lm 3875 kit to power the speakers, and would like to know if its a good idea for me to integrate the amp kit with the above active BCF.

http://www.audiosector.com/lm3875.shtml

thanks,

jai shankar.

1. can anyone explain the components used in the prescribed circuit? What part of it is the crossover and what is the BCF? (this is my first project and i know nothing of electronics...) Is there any real downside to the passive electronics... i hear a lot of guys complaining about passive electronics reducing audio fidelity...

There is no crossover in the circuit (upper drawing), nor is any needed, unless there is a tweeter hidden somewhere I'm not seeing. A crossover is normally used to route high frequencies to a tweeter, and low frequencies to a woofer. Since this is a single widerange speaker, all frequencies are routed to just the one speaker.

The Baffle Step Filter is a PASSIVE circuit, because there are no active components. An active component is one that amplifies. All that's happening here is a simple RC filter, consisting of VR1, R1, and C1 in series. The combination of series resistance and series capacitance forms a frequency-dependent voltage divider. The adjustment of VR-1 controls how much of the signal is passed to the output, and the total resistance and capacitance determines at what frequency the roll-off happens.

Increasing the capacitance (with no other changes) will cause more of the low frequencies to be shunted to ground, (by lowering the frequency where this shunting effect starts) - changing this capacitor value is what you use to adjust the correct frequency for your size baffle.

Adjusting the potentiometer, as mentioned on your link, determines how much attenuation of the highs will take place.

When people talk about passive vs. active, generally it's about crossovers in 2 or 3-way speakers (passive) vs. separate, active amplifiers driving each element in a 2 or 3-way speaker. One of these arguments I don't agree with totally is that the amplifiers will be better matched to the speakers; while this is definitely possible, there's no guarantee that a marginal company would even know what "optimal" is, much less supply it.

2. do the electronics change in any way due to the change in the driver... i've used the the much better fostex fx120 and not the fe127e.

Only if the characteristic impedance is different in one speaker compared to the other. That might change things a bit, but since this is a passive circuit and can't drive a speaker anyway, it shouldn't make any difference. This is NOT to say that different speakers won't sound different; this is more a result of speaker cone stiffness, magnet strength, etc.

3. can the following "active" BFC be used with the speaker?
http://sound.westhost.com/bafflestep.htm

See above; this isn't an active circuit. Considering the amp kit mentioned below, that passive filter should have no problem interfacing with the input of the LM3875. You will, however, probably need a buffer stage at the INPUT to the passive (baffle step) filter to keep from loading down its input. See the information in section 4.3 of the LM3875 manual

http://www.audiosector.com/nigc_kit-users_guide.pdf

for one possibility.

I have bought the following lm 3875 kit to power the speakers, and would like to know if its a good idea for me to integrate the amp kit with the above active BCF.

http://www.audiosector.com/lm3875.shtml

I don't see any reason this wouldn't work; you are aware that this amp is MONO, right? In other words, you would need TWO of them for a stereo amp (refer to the PDF above if you've not already done so)

HTH... Steve

hello steve,

as usual, an incredibly patient reply from you. Thanks a ton.

Firstly, by "prescribed circuit" i meant the one thomas barefoot recommended in his original design (pic attached).

thanks for the definition of "passive". I guess i was confused by the fact that the said circuit is to be used between source/preamp and amp and not inside the speaker...

w.r.t. the circuit i had posted...

It is essential that the compensation circuit be driven from a low impedance source, and the load impedance should be reasonably high. There will be little error with loading above 20k, but basically the higher the impedance, the better. Opamp buffers at the input and output may be used if you cannot ensure that the source impedance is 100 ohms or less, and that the load impedance of the following stage is greater than 20k. My recommendation would be to use a buffer stage at the output with an input impedance of about 100k.

and

You will, however, probably need a buffer stage at the INPUT to the passive (baffle step) filter to keep from loading down its input.

...this is what worries me... what opamp should i use to match impedances...? what can be the negative effects of the same...?

as regards "crossover", it was with reference to thomas' original input... "The "crossover" consists of a parallel impedance compensation network (R2, C1) along with a baffle step compensation filter (R1, L1)."


well, i certainly understand that it manages the distribution of the main signal between two (or more) drivers... in this case, i understand it to be the "high pass filter" which is optimised to the single driver's response... so that low frquencies that the driver cannot handle are not forced upon it...

you are aware that this amp is MONO, right?

as i understand, its a dual mono kit.

thanks for any additional input.

jai shankar.

Jai, this is all you should need to do; Barefoot designed the baffle step filter to be driven at speaker level. This is obvious because (a) the circuit is shown connected directly to the speaker terminals, and (b) if the circuit could only handle line level signals, the resistors wouldn't be spec'd at 10 watts and the inductor wouldn't be spec'd at 16 gauge wire.

Also, Barefoot apparently spent the time to choose component values to complement the prescribed size of the front baffle, so no need to change those. The LM-3875 can drive the speaker/filter direct, and has a high enough input impedance NOT to load down a typical line level output, most of which can drive a 10 k-ohm input with no problem.

The less components in any circuit (while still maintaining functionality), generally the cleaner the sound. So my combined sketch below is what I'd start with; if for some reason it doesn't work well together, we can go from there.

Keep in mind though, that these are NOT full-range speakers; don't expect any serious bass from them, even when loaded in a corner... Steve

audiothings wrote: 1. can anyone explain the components used in the prescribed circuit? What part of it is the crossover and what is the BCF? (this is my first project and i know nothing of electronics...) Is there any real downside to the passive electronics... i hear a lot of guys complaining about passive electronics reducing audio fidelity...

R1 and L1 form a -6dB high-shelf filter. At low frequencies the impedance of L1 is very low, so it acts as a short circuit across R1. At high frequencies the impedance of L1 gets very large, acting like and open circuit. This creates a voltage divider between R1 and the speaker, lowering the speaker output by -6dB at higher frequencies. However, the speaker impedance is not flat at high frequencies. Its impedance rises due to the voice coil inductance. We compensate for this rising impedance by putting the circuit R2 and C1 in parallel with the driver. As the voice coil impedance rises C1's impedance falls, so the high-shelf filter always looks into a constant impedance.

2. do the electronics change in any way due to the change in the driver... i've used the the much better fostex fx120 and not the fe127e.

No changes are necessary. The FX120 is a drop in replacement.

3. can the following "active" BFC be used with the speaker?

Yes, this will work just fine. Just set Wb to 8" to determine the value of C1. You don't really need a potentiometer. A 10k resistor will work, unless you really want to be able to adjust the HF response.

Looks great!!!

Thomas

Thomas, you're back?? Hopefully it's more than "just passing thru", 'cause John's been real mean to me while you've been "goofing off"... Steve