amp suggestion for JBL 2235

[skudra]

I've got JBL 2235's in a modified Altec A-7 box with 4 ports per box, tuned to about 30hz.
The passive crossover has a brick wall cutoff at 250hz passing to an Altec 803B horn and an unobtanium compression driver.
Can you recommend suitable amps for the lows and highs and how I might handle compensating for the differences in gain of the two amps.
The horn is currently attenuated by about 13db to match the ~96db sensitivity of the 2235 with resistors off of the compression drivers.
I'm currently using a Rogue Atlas PP with KT-90's for both highs and lows. It sounds quite good now, but think I could get a bit more out of the 2235's with a dedicated amp for them.

[Mr. Widget]

The 2235H in a ported box tuned to ~30Hz will result in a fairly linear response. Placing it in such a box with a short horn on the front will undoubtedly result in a response that is significantly less linear. How did you come to this combination? What are your design goals?


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[skudra]

Placing it in such a box with a short horn on the front will undoubtedly result in a response that is significantly less linear.

So, you mean that the short horn of the A-7 box will result in a less linear response?
This was conceived by a designer friend.
Can you recommend a box that would be better suited to my situation?
The 803B is sitting on top of the box and the ports are in the bottom section of the box.

[Mr. Widget]

To shed more light on this, here is a verbal description of the bass response of the A7 along with a factory published frequency response graph.

Firstly we see what they mean by 35Hz performance and secondly it is apparent that the short front horn on the woofer really kicks in ~200Hz.


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[Mr. Widget]

Can you recommend a box that would be better suited to my situation

Hard to say... there are so many paths you may choose. That said, here is a JBL factory plot of JBL 4430 Studio Monitor with the 2235H in a simple ~5 cu ft box tuned to ~34Hz. Tuning it to ~30Hz flattens it out even more. The box may be actually a bit smaller and the tuning frequency slightly different, it has been years since I messed around with this stuff, but you get the idea.

I am not suggesting that a simple on-axis plot of a speaker's frequency response really tells you the whole picture, but it does give you an indication of what you are listening to. In this case it accurately shows us that the A7 enclosure will have markedly rolled off low frequencies and fairly nonlinear performance. In your case you can ignore the bit above the crossover region as this performance is that of an Altec driver on a Mantaray horn.


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[Loren42]

I've got JBL 2235's in a modified Altec A-7 box with 4 ports per box, tuned to about 30hz.
The passive crossover has a brick wall cutoff at 250hz passing to an Altec 803B horn and an unobtanium compression driver.
Can you recommend suitable amps for the lows and highs and how I might handle compensating for the differences in gain of the two amps.
The horn is currently attenuated by about 13db to match the ~96db sensitivity of the 2235 with resistors off of the compression drivers.
I'm currently using a Rogue Atlas PP with KT-90's for both highs and lows. It sounds quite good now, but think I could get a bit more out of the 2235's with a dedicated amp for them.

I am running my 2235Hs in a 6 cubic foot vented boxes tuned to 28 Hz.

If this is for home stereo, use what you want for the amp. If you install an active crossover, make sure that you have a cap in series with the tweeter. The rest of the passive crossover goes away.

When you bi-amp the relative SPLs for the drivers really doesn't matter because you can adjust the gain as desired.

I should add, from little I know about horns, the area of the terminal end of the horn is one component that determines the resonance of the horn. Also, the horn and the box resonance should be the same. If that is true, then the mouth of the horn would need to be very, very large to have a resonance at 30 Hz.

If the two resonances are not coincident, then the horn acts like a transmission line and you lose phase coherence and you end up with a non-linear frequency response.

The point of the horn is to acoustically couple the driver to the room. It is essentially an impedance matching device between driver and room. When done correctly the result is a net gain in SPL. However, in a living room, which has a relatively smale volume, a horn is probably not needed because the driver (i.e., 2235H) has more than enough SPL by itself to satisfy any kind of volume requirements the listener needs. My two 2235Hs are just loafing when I use them, which is one reason I employed them in my system over a more traditional 8" woofer.

The only gotcha is if you are using different amps for the woofers and tweeters, make sure that the phase relationships are the same. Some amps invert the input signals phase and others do not.

Other things to be aware of is baffle diffraction loss of the cabinet and where you set the crossover.

The face of my box is about 22", so I get a step at about 210 Hz, which is below the crossover point of 400 Hz.

Your box is horn loaded, so I don't know how that impacts the equation.

[pentictonklaus]

.That said, here is a JBL factory plot of JBL 4430 Studio Monitor with the 2235H in a simple ~5 cu ft box tuned to ~34Hz. Tuning it to ~30Hz flattens it out even more. The box may be actually a bit smaller and the tuning frequency slightly different, it has been years since I messed around with this stuff, but you get the idea.




The 2235 is crying for a 5 cu box tuned to around 30. I got both, 2235 in 5 cu ft and a 2235 in 4.0 cu ft. There is a big difference in the lower bass. The reason for the 4.0 cu ft is that I bought a housing from John W a couple of years ago. John built this housing for a diffrent driver. The 5 cu ft box is a proven concept that is easy to do. The 2235 sounds very good in the smaller enclosure, just does not reach the lowest tones as good as the bigger one. Since it is a 2 way, I will try a 2234 soon. The 2234 might be better in the 1000 hz crossover range.

Klaus

[skudra]

Thank you for all of the informative responses.
Others were wary of the A7 box from the beginning. I do think that I'm getting better bass than the one graph depicts. Placing an SPL meter one foot in front of the woofer and playing Stereophile's test cd with spot frequencies I get about 5db down at 31.5hz and 10db down at 25hz and 20db down at 20hz.
How would I go about designing a proper box for the 2235's? It looks like the JBL monitor is a full-range speaker. I just need the enclosure for the woofers.
I kind of like the idea of 2 2235's per box, which would give me a few more db of efficiency.
I guess ultimately, I'm not married to either the A7 box or the 2235. I wouldn't mind a driver that's more efficient that wouldn't require me to bi-amp. The compression driver is a good one however.

[Loren42]

Thank you for all of the informative responses.
Others were wary of the A7 box from the beginning. I do think that I'm getting better bass than the one graph depicts. Placing an SPL meter one foot in front of the woofer and playing Stereophile's test cd with spot frequencies I get about 5db down at 31.5hz and 10db down at 25hz and 20db down at 20hz.
How would I go about designing a proper box for the 2235's? It looks like the JBL monitor is a full-range speaker. I just need the enclosure for the woofers.
I kind of like the idea of 2 2235's per box, which would give me a few more db of efficiency.
I guess ultimately, I'm not married to either the A7 box or the 2235. I wouldn't mind a driver that's more efficient that wouldn't require me to bi-amp. The compression driver is a good one however.

You can use this as a data point, but here is my 3-way design. I am using 2235Hs, but no horns. Follow the links for the 2235H crossovers for more details.

I need to update the site because I have since gone with an active tri-amp system using an Ashly XR2001 and Harmon Kardon amps.

My box is 186 liters, which seemed to work well with the BassBox Pro plots. I will not dispute the 5 cubic foot box mentioned by an earlier poster, The smaller box will flatten out the response curve, but costs you bottom end. My alignment is more an extended vented alignment.

Remember, a vented box rolls off at 24 dB per octave below the tuning point, so I tend to try to set the tuned point a little lower and adjust the slump with EQ.

Once you are below the resonance point the roll off is too steep to EQ. You just end up exceeding the excursion limit very quickly and blow drivers.

I don't have accurate data on the 2235H, so I don't know where the cone breakup point is. Anyone know that data?

Lastly, even though you are trying to get frequency plots with a mic, room EQ will severely corrupt any data you collect. I know my plots are a mess, but I don't have a chamber to test in. You have to take the data a little skeptically.

If you have the ability to actively bi-amp, then that wins hands down over any passive crossover, period. If you have the equipment I would not even consider using a passive crossover for a whole host of reasons.

[cvengr]

Mark.

[Zilch]

4507, retuned.

[jerry_rig]

The original post was about amp suggestions but strayed into cabinet issues. For my 2235s, I am using a Krell KSA 300s, a high powered class A amp connected directly to the drivers, with very good results. I feed the Krell with the sub channels of a top-of-the-line Denon AV receiver.

Based on my experience, the large the amp, the better the driver control at the lowest frequencies. For the same reason, I would stick with a solid state amp, at least for bass.

Best of luck!

[Loren42]

The original post was about amp suggestions but strayed into cabinet issues. For my 2235s, I am using a Krell KSA 300s, a high powered class A amp connected directly to the drivers, with very good results. I feed the Krell with the sub channels of a top-of-the-line Denon AV receiver.

Based on my experience, the large the amp, the better the driver control at the lowest frequencies. For the same reason, I would stick with a solid state amp, at least for bass.

Best of luck!

Good points!

So, what makes a good amp for the 2235? Would that amp be different if he was driving a Seas or JL Audio woofer?

I would guess not, but I would be interested in hearing opinions and why if it does.

This could simply devolve into a contest for which amp is better and why.

Maybe a better question to the original poster would be, "What's your budget for a new amp?"

Got money? How about a Quad 909? Actually, they are about $1500 if memory serves.

I agree with you that a solid state amp is probably a better choice.

[Mr. Widget]

The original post was about amp suggestions but strayed into cabinet issues.

I thought it wise to discuss the larger issues. Sure the amp may have a big impact, but the cabinet is orders of magnitude greater in importance especially when you use what most would consider the wrong woofer for a particular application.

We can discuss which brand of gasoline will make a certain engine run it's best, but if there is no transmission it doesn't make much of a difference.


Widget

[Allanvh5150]

I would have to agree Widget. You gotta get the cabinet right first. Too much power will overextend a 2235 quite easily if it is in the wrong box. 5 cubic feet at around 30 Hz is a good start. Once it is in the right box I would run 300-400 watts per driver.

Allan.