Archive for September, 2008

Three way speaker using Dayton DC380-8_Eminemce Alpha 6 and Hi-Vi Research RT 2H-A

Monday, September 15th, 2008

This speaker is modeled at the request of Loren42.

The woofer is the Dayton DC380-8.  This driver is available from Parts Xpress.  This has an F3 of 30 Hz in a 5.5 cu.ft cabinet tuned with a single 4? diameter port 9.2 inches long flared at both ends.  Spl is 111 db.  The speaker is slightly limited by xmax between 30 and 55Hz, however overall performance is excellent.  The vent air speed velocity is 18 m/sec so there should be no issues with vent chuffing.

The mid range cabinet should have all walls lined with mineral fiber such as Rockwool.  The cavity should be filled with Polyfill without compression.

The bass enclosure should have 50% of each panel lined with 4? thick Rockwool.  Special attention should be payed to placing Rockwool on the panel behind the driver.

Here is the box alignment.

The Crossover is a three way all-pass Linkwitz Riley second order.  The composite electrical and driver slopes are third order 18db per octave in the low pass/band pass filter and fourth order 24 db per octave in the band pass/high pass filter.

Zobel networks have been provided to equalize the impedances of the LF and band pass drivers.

The woofer has a sharp cone break up peak at 1500Hz which has had to be dealt with in the woofer compensation network.

The mid band response with this network is excellent and unusually smooth for inexpensive drivers with this network.  With the polarity of the band pass driver inverted, the phase response is excellent.  Group delay is very acceptable.  The impedance curve has been equalized and this driver should be a very easy amplifier load, and not cause any problems.

Allowing for insertion loss of the network, the sensitivity of these speakers should be 92 db 1 watt I meter.  This accounts for the high spl.

As is is typical of three ways using a large woofer, the crossover network is complex with a part count of 20 components per crossover.  Because the low pass/band pass crossover has to be at 400Hz,some inductor and capacitor values are high.  This cost of the components for these crossovers will be significant.

The ports and all crossover parts are available from Madisound.

Even though  the drive units are all in the lower price range, with a properly braced  and damped enclosure and this crossover network, this should be a superior speaker.

Here is the alignment for a Peavey 15 black widow in a 6 cuft box.  It requires 4, 4inch diameter vents flared at both ends, 11.4″ long.  Optimal box id 2.2 cuft.

The Bemidji mini

Tuesday, September 2nd, 2008

Here is a simple simple cost effective bookshelf speaker design that will yield superior results compared to commercial offering from manufacturers at similar or greater price tags.  The performance of this speaker should be superior to the sum of the parts.

 

I chose the above drivers carefully to transition well together with a low part count in the crossover.  I have selected a small Vifa polypropylene woofer.  The on and off axis response is excellent, and there is nothing troublesome with the out of band response.  There is a small I KHz peak and as is the norm with small polypropylene bass/midranges a step response.  Here is the manufacturer’s specification.

 

The tweeter is by Morel.  This is one of their cheaper offerings.  The roll off and dispersion characteristics are excellent.  Fs is 900 Hz.  This is the full specification.

 

The crossover needs to make a smooth transition at crossover without ripple, correct the step response, as far as possible take care of response irregularities.  It should have a respectable, impedance, phase, and time response.  It should also be the most elegant solution with the smallest part count compatible with achieving goals.  These crossovers have five components on each board.

 

Now this crossover has a combined fourth order acoustic and electronic slopes.

The crossover is very symmetrical without ripple.  As you work with a design you find the design begins to tell you where the crossover needs to be.  In this case it is 3 KHz.  This is a very good place for a two way crossover point by the way.  There is no zobel impedance equalization.   This is for a good reason.  I took advantage of the rising impedance to correct the step response.  The low pass filter is in fact first order over most of it operative range.  It changes to second order.  Now please note that the program does not let me write in and show R1.  If you look at the component values you will see that C2 shows an internal resistance of 4 ohms.  No cap has this value, so you need to put a 4 ohm resistor in series with C2.  This is VERY important.  This resistor is operative in the model.

 

The HF filter has the Q of the filter aligned to combine with the acoustic roll off of the tweeter and make for symmetry with the woofer response and make a very smooth transition.  This crossover also optimizes the off axis response of the speaker.  The tweeter is 33db down at F3, which is excellent.   This filter also switches order.

 

The step response is corrected.  A small remnant of the small woofer peak remains, however the Q of the peak is changed.  It is lower in magnitude and moved down to 750 Hz.  It is 400 Hz broad from 600 to 1000Hz, and less now about 2db.  I have made the judgment that in that range and at that magnitude it will do no arm.  Also I suspect that if the speaker is around 10 inches from room boundaries that the response below the slight peak will be lifted by that small amount and help compensate for cabinet diffraction loss.

 

A word about sensitivity,  note that the average sensitivity is now 86db 1 watt one meter.  That is because with any passive network does not allow for boosting a signal, only attenuating it.  This reduction in sensitivity is a consequence of correction of the step response.

 

Now impedance.   Now the bass curve will be the one shown in the woofer alignment.  The published spec is sealed, and has one impedance hump.  This woofer requires reflex loading and there will be two impedance humps.  One is at 20 Hz the other at 73Hz.  There is a dip to 6 ohms between the peaks.  Now although these are 8 ohm drivers, correcting the step response, which in my view is essential, always drops the impedance.  The impedance is 6 ohms from 200 Hz to around 650 Hz and then rises.  This is where a good deal of the power is, so regard these speakers as 6 ohm.  There is an impedance rise to 14 ohms in the crossover region.

 

These speakers should be an easy amp drive.  Any amp the over heats driving those, should never have seen the light of day.

 

Now to phase, this crossover puts woofer and tweeter out of phase at crossover.  There is an unusually good phase and time response.  The shift at crossover is 2.2 inches or 0.17msec.  Now a woofer?s acoustic center is the geometric center of the cone, which for me is impossible to visualize and certainly calculate.  However the tweeter will be ahead of the woofers acoustic center, so will modify this.  I would start with the wiring as shown, but when the speaker is constructed see which polarity has the greatest output at crossover.  I have a hunch that with the drivers lined up on a flat panel recessed flush, the polarity switch will not change the output.  If that is the case, I would wire the tweeter in phase.  This is something that is hard to predict from modeling.

 

For the crossover use good quality air cored inductors and mount them as far away from each other as possible, with one horizontal and the other vertical.  This will minimize cross inductance.  Please select polypropylene caps.

 

This should be an easy low budget speaker to construct.  I would welcome comments from anyone who builds a pair.

Polk RT3 speaker mod.

Monday, September 1st, 2008

This is a mod for the Polk RT 3 speaker.  It uses this woofer.  This is a pdf. of its specification sheet.

It uses this tweeter.   Here is the spec. sheet.

This is the woofer alignment.

This is the crossover pdf and simulations.

The crossover is at 3 KHz.  However the component values have been significantly modified. There is 2 the 3 db of diffraction compensation and the response irregularities of the woofer between 200 and 2000 Hz have been significantly blunted.

L1 and C1 form the high pass filter.  This starts to attenuate first order at around 6.5 KHz and switches to second order around 1200 Hz.  The tweeter is down 33db at resonance.  This should mitigate against any tweeter roughness.  Rp1 and Rp2 pad the tweeter output.

L2 and C2 form the low pass filter.  This starts as a first order filter around 400 Hz and starts to perform diffraction compensation.  It changes to second order just before crossover at 3 KHz.   Req and Ce perfrom impedance compensation.  Without it there is significant rise in response above crossover.

Note that providing diffraction compensation has sacrificed about 3 to 4 db of driver sensitivity.  Although the sensitivity is around 87db 1 watt 1 meter which is good for a small speaker like this.

There are impedance peaks of 50 and 40 ohms as a result of reflex loading at 25 and 100Hz respectively.  Because of the equalization of diffraction and  smoothing of the response the impedance is 6 ohms over most of the operating range.  There is a rise to 12 ohms at crossover.

The phase response with the polarity of the tweeter reversed is excellent. with the drivers having a maximum pahse angle of 45 degrees in the crossover region.  Driver offset of the front baffle will modify this a little.  As  result group delay is only significant on the driver tuning range.

The existing vents should be made 7.62 inches each.

The crossover can be mounted in and out of the cabinet.

This mod to the RT 3 should be able to be accomplished with minimal cabinet modification.

If a bigger box were to be built, then F3 could be lowered to 59 Hz.