Allae Crossover "Mod"
Posted by: Greg Beatty on 29 May 2004
My Allaes have sounded great and I have no regrets of the purchase. But...
...in my room, they don't quite do the "band is in the room" thing that some other speakers I home demmed were able to do. I believe this is a quantity of bass issue as the speakers that did the "live" trick were bassier.
To find out more, I have been exploring room modes, moving the speakers around, and finally got a Radio Shack analog sound pressure level (SPL) meter. Used in conjunction with the bass, midrange, and treble warble tones on Stereophile's Test CD3 and the published-on-the-net correction values for the meter, I measured the in-room response using my Sony ES series CD player and Panasonic DVD player as sources. Both sources measured the same.
The response graphs I got were nearly identical to what Stereophile reported for the NBL. Not saying Allaes are NBLs, but the Allaes definitly have the same frequency response signature. A drop off of 3db starting at 500hz on down and a rise in the mids and treble region.
A bit more research turned up the notion of baffle step. Baffle step is drop in forward radiation of bass frequencies that "wrap around" the baffle of the speaker. The frequency of the step is determined by the width of the baffle - narrower baffles beginning the step at a higher frequency than wider baffles. So I plugged the width of my Allaes into the equation and - wouldn't you know it - the baffle step starts at 500hz!!!
The usual remedy is for the speaker's crossover to include components to compensate for baffle step. The maximum decrease in bass is 6db, which is subjectively half as loud. Naim assumes (or so I have read - moderators feel free to correct if I am wrong here) that the baffle step bass drop will be addressed by placing the speaker near or flat against a *solid* wall and the lost bass will be redirected back into the room. This is the "boundary reinforcement" that we read about from time to time.
Well guess what. I moved my Allaes flat against the wall...took measurements in different places in the room...and even placed the Allaes *in the corners* trying to get additional boundary reinforcement from the side walls.
No dice. I moved some of the resonant frequencies around a bit, but the overall 3db drop remained. Oh, and I don't know if the Allaes have partial baffle step correction and so that is why I measure -3db instead of -6db or if they have no correction and my walls are partially reinforcing the bass. In either case, bass to match the mids and treble is impossible in my room without some adjustment.
A bit more research and I find this:
How To Correct Baffle Step
which explains a very simple crossover circuit to correct baffle step. And the "mod" is in quotes because the simple circuit is placed in between the speaker wire and the crossover - no need to actually change the Naim crossover, just need to do the baffle step correction prior to the speaker.
The "mod" consists of putting an inductor and a resistor in between the speaker wire and the speaker. Dead easy.
So I have ordered two 1.5mH inductors and I already have 8 ohm, high-wattage resistors. So for less than $20 including shipping I will be able to add baffle step correction to my Allaes. The inductors are probably not very good at $5 each, but I want to try the concept before shelling out for better inductors.
I see no reason why this couldn't be done with any speaker as the circuit is standard when building crossovers.
I will write back when the baffle step correction is up and running.
- GregB
Insert Witty Signature Line Here
...in my room, they don't quite do the "band is in the room" thing that some other speakers I home demmed were able to do. I believe this is a quantity of bass issue as the speakers that did the "live" trick were bassier.
To find out more, I have been exploring room modes, moving the speakers around, and finally got a Radio Shack analog sound pressure level (SPL) meter. Used in conjunction with the bass, midrange, and treble warble tones on Stereophile's Test CD3 and the published-on-the-net correction values for the meter, I measured the in-room response using my Sony ES series CD player and Panasonic DVD player as sources. Both sources measured the same.
The response graphs I got were nearly identical to what Stereophile reported for the NBL. Not saying Allaes are NBLs, but the Allaes definitly have the same frequency response signature. A drop off of 3db starting at 500hz on down and a rise in the mids and treble region.
A bit more research turned up the notion of baffle step. Baffle step is drop in forward radiation of bass frequencies that "wrap around" the baffle of the speaker. The frequency of the step is determined by the width of the baffle - narrower baffles beginning the step at a higher frequency than wider baffles. So I plugged the width of my Allaes into the equation and - wouldn't you know it - the baffle step starts at 500hz!!!
The usual remedy is for the speaker's crossover to include components to compensate for baffle step. The maximum decrease in bass is 6db, which is subjectively half as loud. Naim assumes (or so I have read - moderators feel free to correct if I am wrong here) that the baffle step bass drop will be addressed by placing the speaker near or flat against a *solid* wall and the lost bass will be redirected back into the room. This is the "boundary reinforcement" that we read about from time to time.
Well guess what. I moved my Allaes flat against the wall...took measurements in different places in the room...and even placed the Allaes *in the corners* trying to get additional boundary reinforcement from the side walls.
No dice. I moved some of the resonant frequencies around a bit, but the overall 3db drop remained. Oh, and I don't know if the Allaes have partial baffle step correction and so that is why I measure -3db instead of -6db or if they have no correction and my walls are partially reinforcing the bass. In either case, bass to match the mids and treble is impossible in my room without some adjustment.
A bit more research and I find this:
How To Correct Baffle Step
which explains a very simple crossover circuit to correct baffle step. And the "mod" is in quotes because the simple circuit is placed in between the speaker wire and the crossover - no need to actually change the Naim crossover, just need to do the baffle step correction prior to the speaker.
The "mod" consists of putting an inductor and a resistor in between the speaker wire and the speaker. Dead easy.
So I have ordered two 1.5mH inductors and I already have 8 ohm, high-wattage resistors. So for less than $20 including shipping I will be able to add baffle step correction to my Allaes. The inductors are probably not very good at $5 each, but I want to try the concept before shelling out for better inductors.
I see no reason why this couldn't be done with any speaker as the circuit is standard when building crossovers.
I will write back when the baffle step correction is up and running.
- GregB
Insert Witty Signature Line Here
Alex -
"Somehow my answer was put beneath yours but I didn't see yours."
It took me quite some time to compose the post. My guess is the forum software takes the time stamp from when the Reply window is opened rather than when the Post Now button is pressed. So you posted while I was writing
"Strange. OK, last questions: Why the positive lead? Doesn't the negative lead carry the actual signal?"
I was just following directions in using the positive lead. As far as I know, the signal is AC (alternating current) so the idea of the signal coming from the positive or negative lead is misleading (pun intended). That is, if you hooked up your speakers backwards they would work just fine as long as both speakers were wired the same way. Some people say they can hear the difference…I doubt I could.
"Last but not least, where is the dB in the equation? More resistance is a higher compensation?"
The reference I have states that 6 db of compensation will result when the resistance equals the impedance of the speaker. The mids and highs are going through the resistors and are depressed by the resistors...so decreasing the resistance will lessen the baffle step compensation and vice versa.
"Meaning more Ohms is more bass but less treble and mids?" - Yes!
"And when calculating, don't you have to adjust your inductor to whatever changes you make to your resistor?"
I've wondered about this...but have not found it to make a significant practical difference. I have only used 1.5 mH inductors.
"Or are you not trying to do this by formula and just listening?"
I measured the frequency response of the system, inserted the circuits (one for each speaker of course) and measured again. Adjustments to the resistance are based on listening. I give it a few days so I have time to hear the tele, hear a few CDs and DVDs...
Then I decide if it is too thick sounding or still too thin and adjust the resistance. I measure after changing the resistance as well just to quantify the effect. Something I like to do (I'm a measurement freak) but that is not necessary. As they say, if it sounds good...
And, really, and equalizer would be an easier way to do this. I just like the challenge and believe that the end result may be better by using fewer components and using higher grade components than one would find in a cheap EQ.
- GregB
Insert Witty Signature Line Here
"Somehow my answer was put beneath yours but I didn't see yours."
It took me quite some time to compose the post. My guess is the forum software takes the time stamp from when the Reply window is opened rather than when the Post Now button is pressed. So you posted while I was writing
"Strange. OK, last questions: Why the positive lead? Doesn't the negative lead carry the actual signal?"
I was just following directions in using the positive lead. As far as I know, the signal is AC (alternating current) so the idea of the signal coming from the positive or negative lead is misleading (pun intended). That is, if you hooked up your speakers backwards they would work just fine as long as both speakers were wired the same way. Some people say they can hear the difference…I doubt I could.
"Last but not least, where is the dB in the equation? More resistance is a higher compensation?"
The reference I have states that 6 db of compensation will result when the resistance equals the impedance of the speaker. The mids and highs are going through the resistors and are depressed by the resistors...so decreasing the resistance will lessen the baffle step compensation and vice versa.
"Meaning more Ohms is more bass but less treble and mids?" - Yes!
"And when calculating, don't you have to adjust your inductor to whatever changes you make to your resistor?"
I've wondered about this...but have not found it to make a significant practical difference. I have only used 1.5 mH inductors.
"Or are you not trying to do this by formula and just listening?"
I measured the frequency response of the system, inserted the circuits (one for each speaker of course) and measured again. Adjustments to the resistance are based on listening. I give it a few days so I have time to hear the tele, hear a few CDs and DVDs...
Then I decide if it is too thick sounding or still too thin and adjust the resistance. I measure after changing the resistance as well just to quantify the effect. Something I like to do (I'm a measurement freak) but that is not necessary. As they say, if it sounds good...
And, really, and equalizer would be an easier way to do this. I just like the challenge and believe that the end result may be better by using fewer components and using higher grade components than one would find in a cheap EQ.
- GregB
Insert Witty Signature Line Here
Alex -
Thank you for the link. The Edge suggests that the resistor be equal to the impedence of the speaker (set R1 = 7 ohms and the recommended values for R2 will be 7 ohms for 6db of compensation). I am running at 7 ohms now. My earlier tests with 8 ohms were not correct due to one speaker being wired up improperly. I'm likely to end up at 6 or 7 ohms.
The Edge suggests a higher value inductor to have the bass lift coming in at a lower frequency. The value I use is based on 500hz as the -3db point.
You can always experiment or just get an equalizer and set it that way. It is nice, either way, to be able to measure what is happening. You can use the Radio Shack meter with test tones and a table of correction values (I have a spreadsheet for this that graphs the results as well) or an equalizer with a spectrum analyzer built in.
Let me know how it goes!
- Greg
Insert Witty Signature Line Here
Thank you for the link. The Edge suggests that the resistor be equal to the impedence of the speaker (set R1 = 7 ohms and the recommended values for R2 will be 7 ohms for 6db of compensation). I am running at 7 ohms now. My earlier tests with 8 ohms were not correct due to one speaker being wired up improperly. I'm likely to end up at 6 or 7 ohms.
The Edge suggests a higher value inductor to have the bass lift coming in at a lower frequency. The value I use is based on 500hz as the -3db point.
You can always experiment
or just get an equalizer and set it that way. It is nice, either way, to be able to measure what is happening. You can use the Radio Shack meter with test tones and a table of correction values (I have a spreadsheet for this that graphs the results as well) or an equalizer with a spectrum analyzer built in.Let me know how it goes!
- Greg
Insert Witty Signature Line Here