Is this a reasonable way to go with a plate amp.

Chris v

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I have 3 way open baffles with 15" bass drivers, 8" midrangers and 4" tweeters. Playing just the mid and tweeter together, sounds beautiful but lack much below 100Hz as expected.

The mid is an exceptional driver (SEAS Exotic W8) which I run full range without filters, and the bass driver is good but not great (Eminence Alpha). If asked to work at just very low frequencies it is perfectly acceptable, having a high Qts which is perfect for OBs.

Unfortunately the 'lower quality' 15" driver with passive low pass filter has a long 'tail' into the higher frequencies and this is muddying the sound of the midrange. My aim is to low pass the bass at 100Hz with a fairly steep slope and perhaps add a bit of amplification.

Throughout the process I am using REW to confirm what I am hearing.

So, does the following sound logical.

Divert the 'high level' signal from the bass driver into a plate amp with high level input. There I can adjust high pass, phase, slope and volume and from then back to the bass driver.
 

Shonver

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What you plan to do will improve matters, due to having better control over the woofer. However, your overall approach is flawed.

Presumably, your reason for going open baffle is to achieve superior sound quality to what an enclosed speaker does. From what I can see, you are doing One Good Thing but neglecting more important matters. You can reduce distortion in your midrange driver by giving it a high-pass filter, perhaps 100Hz is a good starting point. To achieve the optimal benefit from your design choice, you should include acoustic measurements for individual driver and have a go at a proper crossover. (It is possible to achieve a good-enough sound with simple filters and ear-tuning, but this is an unnecessarily frustrating process).

Depending on how technically-minded you are, you could look up Siegfried Linkwitz's development work on open baffles. Even so, it should give you an idea of the factors at play.

Good luck with your project.
 

Chris v

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Thanks Shonver. I am not that gifted at the electronics side of it, but happy to experiment.

Since my first posting I have added a second output to my passive pre amp which is being fed to a dedicated amp for the 15" drivers. I would prefer not to add passive components to the mid range driver, It is an exceptional driver and when played alone I can hear no evidence at all of it struggling as it gets around 100 Hz. BUT, I am happy to do so with the bass. At present the low pass on the bass is a series Jantzen C coil inductor 3.9 mH and about 160 uF of parallel PIO capacitance. I think this gives a low pass of about 120 Hz

Experiments so far include changes of amplification comparing a 30wpc Rotel with a 1000wpc Special ICE amp. Probably not surprising the ICE amp gives massively more controlled bass and when the bass is played in isolation is much more musical and easy to listen to the musical detail.

Secondly I have the option of two sets of 15" bass drivers at present, though considering the AV Nivana drivers.

I have the Eminance Alpha 15s which are OK, but most importantly are able to take being driven hard.
Secondly I have vintage Altec 416b alnico paper cone drivers. These have fabulous sound quality particularly in the 'tail' region above the crossover, BUT cant be driven hard in open baffles. I can spent many happy hours just listening to this driver in this configuration.

Any guidance as where to go next would be very much appreciated, particularly whether the AV drivers could get me lower bass at a good quality.

I attach a picture of my OBs 120cm tall and 50cm wide. They do have some additional baffle with the triangular supports to the rear, but perhaps a further option is something like an H Frame around the bass driver.
 

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Shonver

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As a DIYer you are free to take whatever approach you choose. I can only advise according to my own beliefs. FWIW, I believe that to measure is to know; so my advice for a next step is to get a measurement mic. At the least you will be able to see graphically the effects that your tweaks are having. It will also give you a feel for how what you hear relates to what the system is doing.
 

Chris v

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As a DIYer you are free to take whatever approach you choose. I can only advise according to my own beliefs. FWIW, I believe that to measure is to know; so my advice for a next step is to get a measurement mic. At the least you will be able to see graphically the effects that your tweaks are having. It will also give you a feel for how what you hear relates to what the system is doing.
Thanks, I do have a measurement mic and REW. :)

I have measured most things and it just confirms that aside from room resonances there is an ever decreasing output from the 15" drivers below 100Hz. I know we are talking open baffles with cancellation, but on the forums there are people talking about worthwhile output to say 40 Hz. Are they to be believed, and if so, would the AV NIRVANA drivers go some way to getting this extra extension rather than my Alpha 15s or Altec 416s. I don't want to throw a significant amount of money away on a set of drivers I have no way of auditioning (in UK) if there is a better route to explore.
 

Shonver

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You might want to consider response shaping, (A.K.A. equalisation). Besides replacing your woofers you could also look at applying bass boost. If your 8 inch mid is handing a full-range signal with no problem, it implies that your listening levels are benign. This furthermore implies that your woofer has spare headroom for handling some bass boost. I still recommend Linkwitz's web site, simply for his discussion of the factors affecting dipole speaker performance.

Linkwitz Lab web site
 

Chris v

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Thanks Shonver. I appreciate your responses. I have read through the Linkwitz stuff. He was something of a genius.

You're right, that my listening levels are benign. I listen only about 3m from the speakers and very rarely at greater than 75dB. I have remembered that my Rotel amp has a bass knob so that's worth turning up to see if its going in the right direction. I do have a miniDSP which I used for an earlier experiment. I can see how it could be used to push up the bass in the low frequencies without trampling on the low end of the mid range driver. I guess I remove the low pass filter and let the miniDSP do the work of low pass and curve shaping.

I do remember that the miniDSP allowed me to set the order of the low pass filter and that it could be very steep. Are there reasons to avoid really steep cut offs?
 
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