Need some help with active speaker conversion

[gfinlays]

Hi All, I'm no expert in speaker design, though I've learned quite a few things from lurking around here. Early this year, I finally finished a build of Troels Gravesen's Poor Man's Stradivari.

Full build photos here.

I'm in the process of converting from the passive XO to a fully active setup.

I have all of the hardware (Earthworks M23r, Focusrite Scarlett 2i2, fanless PC, 2 x AHB2s, 2 x Hypex Nilai 500 monos, Motu Ultralite Mk 5 and a Lundahl VC2361) and software (Acourate and Hang Loose Convolver) to do the active XOs and drive the speakers, but I have a few questions.

I've read a lot of discissions about directivity and waveguides. Without a tweeter waveguide, it seems the off-axis response tends to drop off significantly at higher frequencies. Also, from my reading relating to XO frequencies and acoustic centres, the mid/tweeter centre to centre should dictate the XO frequency.

Given that with Acourate-derived XOs, I have more or less carte-blanche, should I consider waveguiding the tweeter (I'm open to alternatives to the Seas 27TFFC) and reducing the mid/tweeter XO frequency from 3000 Hz to ~2400 Hz to take account of the 145 mm acoustic centre spacing between the mid and tweeter?

Thanks in advance.

 

[staticV3]

Afaik, crossover frequency should be set at a point at which the two drivers exhibit similar directivity, so long as both drivers are comfortable playing up/down to that frequency, irrespective of the driver spacing.

 

[gfinlays]

Midrange FR:

Tweeter FR:

Looking at directivity for the mid (top) and tweeter (bottom), I suspect nearer 2000 Hz rather than 3000 Hz for XO frequency?

 

[Salt]

Mid-Range should mean it literally, so from 1 to 4 Khz. Measurements above would mean that the tweater would do the job as well or better, not?

 

[staticV3]

Looking at directivity for the mid (top) and tweeter (bottom), I suspect nearer 2000 Hz rather than 3000 Hz for XO frequency?

Here are the relevant graphs:


And crossed at 1.5k/2k/3k:


I'd say do some nearfield THD and MD measurements of the 27TFFC and if it's comfortable playing down to 2k or even 1.5k, cross it there.

If crossing it higher would give you lower MD and more headroom, consider putting it in an appropriate waveguide to better match the MCA15RCY's off-axis response at that crossover frequency.

 

[Rick Sykora]

While you have done some nice woodworking, not sure what your redesign goal(s) are here? Before can help, helps to know what you are hoping to improve about the speaker.

For that matter, do you have any other constraints (budget, cabinet, etc.) A contemporary redesign needs consistent off-axis measurements of the drivers on the intended baffle. Are you equipped to make such measurements?

 

[gfinlays]

While you have done some nice woodworking, not sure what your redesign goal(s) are here? Before can help, helps to know what you are hoping to improve about the speaker.

For that matter, do you have any other constraints (budget, cabinet, etc.) A contemporary redesign needs consistent off-axis measurments of the drivers on the intended baffle. Are you equipped to make such measurements?

Hi, the original plan was to build a pair of Troels' PMS, which I have, and they work very nicely.

My next step is to go fully active with filters produced using Acourate.

Rather than just replicating the existing crossovers at 375 Hz and 3000 Hz, I figured while I was there, I could optimise the design and improve the overall directivity. Directivity is probably not ideal with the mid and tweeter crossed at 3000 Hz.

I'd rather not redesign the cabinet - a lot of time, effort and cost went into it. But I'm not averse to remachining the tweeter cutout for a waveguide, for instance.

I have Acourate, REW, a Focusrite 2i2 and an Earthworks M23r at my disposal.

 

[Rick Sykora]

Ok, but active crossovers or waveguides are means to the end goal. Let me state it differently. Rather than particular features you want to change, what sound benefit(s) are you hoping to accomplish?

As for directivity, do not really know until you have baselined the current design and then would know the current behavior of the speaker. Lacking a solid spin, really just guessing at how wide, consistent, etc. the current design really is. No point in measuring drivers for a redesign until you know what you might want to improve. @staticV3 is also correct and that there may be other considerations. If the Seas 22TFFC is just like the TFF, the distortion will rise significantly as you lower the crossover frequency. This may be acceptable or you may want a better tweeter.

Bottom line, until you have measured and set some target goals, it is much more difficult to give meaningful advice.

 

[gfinlays]

Ok, but active crossovers or waveguides are means to the end goal. Let me state it differently. Rather than particular features you want to change, what sound benefit(s) are you hoping to accomplish?

As for directivity, do not really know until you have baselined the current design and then would know the current behavior of the speaker. Lacking a solid spin, really just guessing at how wide, consistent, etc. the current design really is. No point in measuring drivers for a redesign until you know what you might want to improve. @staticV3 is also correct and that there may be other considerations. If the Seas 22TFFC is just like the TFF, the distortion will rise significantly as you lower the crossover frequency. This may be acceptable or you may want a better tweeter.

Bottom line, until you have measured and set some target goals, much more difficult to give meaningful advice.

I'm looking to properly time-align and linearise the drivers using Acourate, and do some room correction to get the best stereo image that I can. I'd also like to get a wider "sweet spot" than the current centre of the sofa position.

I guess the best place to start is with the current driver setup and see where its limitations are.

I'm not averse to a different tweeter to achieve a lower xover point. For instance, the Scanspeak D3004/660000 I know is quite happy being crossed over around 1.5 to 1.7 kHz as I already have it in a 2-way at 1.7 kHz with a 6 1/2" woofer. Another option might be the Seas Excel T35C002 35 mm midrange tweeter which is designed to go down to 1.5 kHz.

 

[AudioJester]

@Keith_W

 

[Rick Sykora]

I'm looking to properly time-align and linearise the drivers using Acourate, and do some room correction to get the best stereo image that I can. I'd also like to get a wider "sweet spot" than the current centre of the sofa position.

I guess the best place to start is with the current driver setup and see where its limitations are.

I'm not averse to a different tweeter to achieve a lower xover point. For instance, the Scanspeak D3004/660000 I know is quite happy being crossed over around 1.5 to 1.7 kHz as I already have it in a 2-way at 1.7 kHz with a 6 1/2" woofer. Another option might be the Seas Excel T35C002 35 mm midrange tweeter which is designed to go down to 1.5 kHz.

Based on your desire for wider dispersion, agree the directivity likely needs some work.

Gather you are a Graham McTavish fan?

 

[gfinlays]

Gather you are a Graham McTavish fan?

The avatar?

That's actually a self-portrait I shot a few years ago for my CV when I lived in Austria.

Though now that you mention it, there is an uncanny resemblance.

We both hail from the same country, he's a Graham and I, a Graeme.

I do like his work.

 

[LSPhil]

I believe your issue is not the crossover frequency between the midrange and tweeter being too high, but rather the lack of steep filtering in the bass and midrange regions. For example, a violin playing on the left channel is reproduced by both the bass driver and the midrange driver, while the brilliance comes from the tweeter. To minimize interference between the bass and midrange drivers, the overlap between their frequency ranges needs to be very small. Reducing this overlap significantly improves spatial imaging.

I’ve tested this myself, and the results confirm it works. My recommendation would be to use an active crossover, which will allow you to experiment with various settings. For now, avoid implementing a waveguide, as it might compromise the integrity of your speaker enclosure. Instead, focus on achieving steeper crossover slopes first.

 

[gfinlays]

I believe your issue is not the crossover frequency between the midrange and tweeter being too high, but rather the lack of steep filtering in the bass and midrange regions. For example, a violin playing on the left channel is reproduced by both the bass driver and the midrange driver, while the brilliance comes from the tweeter. To minimize interference between the bass and midrange drivers, the overlap between their frequency ranges needs to be very small. Reducing this overlap significantly improves spatial imaging.

I’ve tested this myself, and the results confirm it works. My recommendation would be to use an active crossover, which will allow you to experiment with various settings. For now, avoid implementing a waveguide, as it might compromise the integrity of your speaker enclosure. Instead, focus on achieving steeper crossover slopes first.

Many thanks. Acourate does allow some very steep filter slopes, so I'll see what it turns out.

 

[ThatSoundsGood]

A good way to deal with directivity is to get a turn table for the speaker and measure on-axis then off-axis every 5 degrees. You can see how the drivers are behaving off-axis and make changes to the crossover based on that. With active filtering, it's easy and quick to change the filters then take measurements. You can do this with one driver at a time to see where the directivity is a good match and therefore where the crossover can be set (as long as this frequency is within the playing range of the drivers distortion limits). Steeper filters are great in many cases but can definitely affect the directivity off-axis. Takes lots of measurements.

I use this platform for it:

Rotating Platform - Arbor Scientific

Conservation of rotational momentum is a key principle that can be personally experienced when standing with both feet on the Rotating Platform. Demonstrate how a skater can spin faster or slower by extending or tucking arms. use hand weights to study rotational inertia, conservation of angular...

www.arborsci.com

 

[JanRSmit]

Midrange FR:

View attachment 408955

Tweeter FR:

View attachment 408956Looking at directivity for the mid (top) and tweeter (bottom), I suspect nearer 2000 Hz rather than 3000 Hz for XO frequency?

The tweeter directivity can be improved with a waveguide to get it closer to the directivity of the midrange at the frequency range of say 2k to 3k.
I assume these measurements are made with the drivers mounted in your beautiful cabinet.

 

[gfinlays]

The tweeter directivity can be improved with a waveguide to get it closer to the directivity of the midrange at the frequency range of say 2k to 3k.
I assume these measurements are made with the drivers mounted in your beautiful cabinet.

The on and off-axis measurement graphs are from the Seas data sheets (MCA15RCY in a 20 litre closed box and 27TFFC on a 600 mm x 800 mm baffle). At the frequencies we're talking about, the 15cm midrange is operating well above the baffle diffraction and the tweeter test baffle dimensions are not too dissimilar to my cabinet's dimensions (~600 mm x 1000 mm), so I don't expect the measured responses from my speaker will differ much.

I should be able to measure them later this week.

Thanks for the compliment on the cabinet.

 

[ThatSoundsGood]

The on and off-axis measurement graphs are from the Seas data sheets (MCA15RCY in a 20 litre closed box and 27TFFC on a 600 mm x 800 mm baffle). At the frequencies we're talking about, the 15cm midrange is operating well above the baffle diffraction and the tweeter test baffle dimensions are not too dissimilar to my cabinet's dimensions (~600 mm x 1000 mm), so I don't expect the measured responses from my speaker will differ much.

I should be able to measure them later this week.

Thanks for the compliment on the cabinet.

This is a good plan. While manufacturers measurements are sometimes accurate, it's very important to do your own because there are so many factors that effect them. It's also a fun learning process.

 

[gfinlays]

I finally got around to doing the first set of measurements.

Initially, the directivities looked great, in fact too great. Then on checking my wiring, I noticed I had done the midrange measurements at midrange height with the tweeter playing

Repeated the midrange measurements with the correct driver connected and got this:

Very similar to the graphs on the Seas data sheets.

3 kHz crossover point is really not good for these 2 drivers.

Crossing down below 2 kHz doesn't look much like an option either. The tweeter's distortion, especially 3rd harmonic, takes off below 3 kHz. At 1.5 kHz, it's only 30 dB down on the fundamental:

I think the next step is a test baffle with a waveguide (the Visaton WG 148R looks like it might be a good fit) on the tweeter to see if I can get a better directivity integration around 3 kHz.

 

[Rick Sykora]

I finally got around to doing the first set of measurements.

Initially, the directivities looked great, in fact too great. Then on checking my wiring, I noticed I had done the midrange measurements at midrange height with the tweeter playing

Repeated the midrange measurements with the correct driver connected and got this:


View attachment 412629

Very similar to the graphs on the Seas data sheets.

60 degrees looks suspect so may want to review source of wiggling. What is your gating?

3 kHz crossover point is really not good for these 2 drivers.

Crossing down below 2 kHz doesn't look much like an option either. The tweeter's distortion, especially 3rd harmonic, takes off below 3 kHz. At 1.5 kHz, it's only 30 dB down on the fundamental:

Yes, noticed this looking at Troels design. You might have some wiggle room if higher order slopes are used but otherwise need a different tweeter.

Did you do a spin of the existing speaker (using the Troels crossover)? Would help establish a baseline and identify areas for improvement.