SevenSixTwo - InDIYana 2018 Coax Design

[KEtheredge87]

Can you post the transfer function of the tweeter?

R2 concerns me, you'll be putting ALOT power through it. And I'm not sure why you would want to pad a woofer anyway. There must be a way to get rid of it.

Looking at those off axis measurements you can see why it was recommended to model off axis too. That energy from 7-10khz almost has to be audible, it is only down directly on axis, and room reflections will send all that energy to the listener. More generally, looking at the 45 degree measurement, the peak at 2.5khz, dip 2.5khz-7khz, and peak 7-10khz will *probably* be audible. The only way to know is make a crossover smoothing that area out and listen to both. Time for a MiniDSP

HI Brandon, thanks for the feedback! You are right about the power on R2... pretty high dissipation is predicted from XSim, which is why I spec'd out a 25W "Zister" for that spot, as well as a few others in the mid/tweet circuits. The reason I stuck that resistor in place was to deal with inductor bump I was getting from the 4.5mH coil. WIthout it, PCD has me up another 1.3 dB at 100 Hz. If I decide to get rid of it, I'll probably be adjusting the capacitor on that woofer section too... which in turn will cause me to retool the mid and tweeter sections too

With your comments regarding the 7-10kHz energy... maybe I need to listen to the speaker again while sitting off at 45 degrees. I could use a mini-DSP for listening tests, but I'll would need to convert it back to a passive version for the competition. Rules are rules

Here's the tweeter transfer function you requested. I had to get it from XSim since I used all my PCD spaces on the tweeter and couldn't add that final capacitor.

[isaeagle4031]

Even at 25W, that resistor is going to cook. Might try an inductor with a higher dcr or maybe a 50 ohm in parallel

[rpb]

Can you post the transfer function of the tweeter?

R2 concerns me, you'll be putting ALOT power through it. And I'm not sure why you would want to pad a woofer anyway. There must be a way to get rid of it.

Looking at those off axis measurements you can see why it was recommended to model off axis too. That energy from 7-10khz almost has to be audible, it is only down directly on axis, and room reflections will send all that energy to the listener. More generally, looking at the 45 degree measurement, the peak at 2.5khz, dip 2.5khz-7khz, and peak 7-10khz will *probably* be audible. The only way to know is make a crossover smoothing that area out and listen to both. Time for a MiniDSP

That's 2 dB scales, not 5dB.

[augerpro]

That's 2 dB scales, not 5dB.

Very true. Still worth playing with though, and gives him another perspective about what metric to design around.

Yeah I hate inductor humps. The parallel resistor mentioned above can work well. Sometimes you can play with box tuning to get the response you want too.

[KEtheredge87]

Hi Guys, Thanks again for all the suggestions. I did a bit more fiddling in XSim and it looks like I could probably replace the 4.5 mH iron core inductor with a 5.0 mH air core with higher DCR like Chuck suggested. That would let me get rid of the two resistors in the woofer path. Beyond that, I haven't been able to get rid of any other resistors like that paralleled set of 6.2 ohm zisters in the HF section. When I try getting those out, either the response sucks, or the response is pretty good but the system impedance dips to 3 ohm or worse with high electrical phase.

I did a quick poll in XSim with all the resistors I have (minus the woofer ones). I set the XSim amp to 100W at 4 ohms (not a level I expect to play at continuously) and the highest power dissipation I see is ~30 W on the coaxial midrange "R3". Assuming a fudge factor for music being dynamic rather than a constant impulse at all frequencies, I think I should be within thermal limits. All the same, I intend to measure the external temperature of these 25W "zisters" with an infrared thermometer after playing them for a while (thanks JavadS for the idea!) For what it's worth, these haven't even gotten warm when playing some music at 80ish dB for a while. The aluminum heat sinks built into the body of these resistors does a great job!

[Wolf]

Thing to also know is that panel mount resistors derate when not using a heatsink. Those 25W are closer to 10W without a heatsink. Granted, that would be with continuous sine waves, but it's of concern. As long as you're not going as loud as Pellegrene did during a Queen DVD, you'll likely be okay. I've used many of those style resistors, and have not had any problems.

Later,
Wolf

[KEtheredge87]

...As long as you're not going as loud as Pellegrene did during a Queen DVD, you'll likely be okay...

I feel like there's a good story attached to this one that you'll have to share sometime!

Based on the concerns folks have shared here (Thanks a million!) I think I'll pick up a few of those 5mH / higher DCR air coils and see what life looks like without the 25W resistors in the woofer circuit.

[Wolf]

I second the 50 ohm across the woofer if the higher DCR does not suffice.
Later,
Wolf

[KEtheredge87]

Hi Guys, I appreciate the comments and help today. Although the more I look at it, the more I find myself stumped by resistors of all things... I feel like the safest bet is to combine resistors to get the values I need while still getting the XSim predicted power handling for each resistor under the rated values. That approach is leading me to unattractive ideas like splitting up a few single resistors in favor of series and/or paralleled groups to achieve the power handling side of things. That could actually lead me to break up one 10 ohm / 25W panel mount resister into a pile of five 2 ohm / 10W sand cast jobbies. That doesn't seem like an elegant solution to me.

I'm sure there's a realistic / "happy medium" approach here since we keep talking about music vs. pure sine wave inputs, but it's not clear to me what I can, and cannot get away with. Can someone try (once again) to set me straight? I think I get the concerns on the woofer circuit, but shouldn't I have the same concern about cooked resistors in the mid and tweeter path? Those two drivers needed a significant amount of padding to bring them down to the level of the Esoteric driver.

Check this mess... is this really how I ought to be doing things (Better organized on the diagram, of course)

[PWR RYD]

Don't you do have the crossover prototyped externally? Just crank'm up as loud as you ever expect them to play. Let them play all sorts of different music that loud for an hour. Then check all the parts with your trusty fingers to see if anything is too hot to touch. If

[KEtheredge87]

Don't you do have the crossover prototyped externally? Just crank'm up as loud as you ever expect them to play. Let them play all sorts of different music that loud for an hour. Then check all the parts with your trusty fingers to see if anything is too hot to touch. If

Thanks Craig! Yep. I do have everything prototyped externally. My engineering OCD is likely getting the best of me here. I'm always trying to reconcile what I see from predictions like XSim vs. real world experience from experienced folks like yourself and others on the forum. At the end of the day, I just want to do all I can to avoid burning up any resistors (or worst case, somehow start a fire!)

I'm sure I'll end up doing what you recommend with playing music for an hour with high volume.

[PWR RYD]

Check out this thread (it's what Wolf was referring to):

http://techtalk.parts-express.com/forum/tech-talk-forum/56544-do-crossovers-get-hot

[jhollander]

+1, with more practical experience you can temper the data. Same thought goes with all the parts you have in the crossover.

[KEtheredge87]

+1, with more practical experience you can temper the data. Same thought goes with all the parts you have in the crossover.

I'll take that as a vote of confidence that one day I won't be showing a 30+ part XO design for a 3 way speaker

I'm sure there's better ways to do what I've done... This HF section required everything I knew how to throw at it just to calm all the fidgets up to 20K. I should probably A/B Test with and without those LCRs just to see if I notice them missing or not.

[KEtheredge87]

+1, with more practical experience you can temper the data. Same thought goes with all the parts you have in the crossover.

Alright John... You've got me thinking again. Thanks for the push (intended or not!) I'll be experimenting more tonight. More details later! -- Sent from my SM-N900V using Tapatalk