Whats is your means to change level - your "volume control"? Do you have VU meters on your DSP system?
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I had the same symptoms prior to 2015, when I used Windows laptop (soundcard) output to drive the preamp (i.e., D-to-A in the laptop, then RCA analog to the input of a preamp), and a non-USB microphone (an ECM8000 that used a separate "mixer" that had multitudes of problems providing microphone level calibration and linearity of output).
When I switched to a USB microphone (i.e., self-calibrating output level A-to-D) and HDMI from my laptop to my preamp/processor, all those problems ceased.
Can you describe your microphone setup (i.e., are you using a separate mixer to provide phantom power and A-to-D), and your signal chain from REW from your computer, then out to your preamp, i.e., what kind of bus are you using)?
Chris
Hi, may I ask again how the 15"s are wired, and how connected to the amp?
Are they 4 ohm? (That's all I've seen looking at Crites images.)
Your response looks excellent with a mild mid dip of 5db with no large dips or peaks. Highs look good to 16khz which I can't hear now anyway. Excellent work.
My signal chain is analog preamp > MiniDSP 4x10HD analog input > 4x amps (sub, woofers, mids, highs). All of the amps have volume/gain knobs, and they are all max'ed out (except the sub, but it is turned off for these measurements). All level matching is happening in the MiniDSP, and global volume control is via the analog preamp, upstream of the DSP.
I am using a Umik-1 USB mic to measure (with calibration file). It is connected to a dedicated HTPC that I use for REW and to interface with the MiniDSP.
The 15's are wired series-parallel. They are 4ohm woofers. I have them wired parallel on each side, and then I have the 2 ohm loads wired in series to get back to 4 ohms. I have checked all drivers with a multimeter and they measure as you would expect.
I am still wondering if it is a power issue? I thought it was a 150 watt amp, but apparently, their marketing got the better of me. I just looked it up, and it is 75wpc in stereo, 150wpc bridged to mono. So I am only working with 75 watts per side. I still would think that should get me more than 105 dB, but I am using a lot of EQ down there. Maybe that is eating up all my power?
https://www.daytonaudio.com/product/783/apa150-150w-power-amplifier
I am using a Umik-1 USB mic to measure (with calibration file). It is connected to a dedicated HTPC that I use for REW and to interface with the MiniDSP.
The 15's are wired series-parallel. They are 4ohm woofers. I have them wired parallel on each side, and then I have the 2 ohm loads wired in series to get back to 4 ohms. I have checked all drivers with a multimeter and they measure as you would expect.
I am still wondering if it is a power issue? I thought it was a 150 watt amp, but apparently, their marketing got the better of me. I just looked it up, and it is 75wpc in stereo, 150wpc bridged to mono. So I am only working with 75 watts per side. I still would think that should get me more than 105 dB, but I am using a lot of EQ down there. Maybe that is eating up all my power?
https://www.daytonaudio.com/product/783/apa150-150w-power-amplifier
Thank you. It really does sound excellent at normal listening levels. I would have never even discovered this woofer issue if I had not tried some high volume testing.
Aah great, i was working on a reply when your post just hit...
Here is the reply i was about to make...
I ask about your wiring / amp hookup, because I think there is a very good chance you are correct about being underpowered.
If your drivers are 4 ohm, the only way to run them with the APA150 amp without presenting the amp with an impossible 2 ohm load, is two drivers in series to each amp channel.
Doing this gives an 8 ohm nominal to an amp that is loosely specified to be only 26-50 wpc @ 8 ohms.
If the 4 drivers were put into a two parallel two series config, the resultant nominal would be 4 ohms and not allow bride mode, as the amp will see that as 2 ohm loads on each channel.
So there's no way to get more than maybe 40 wpc out of the amp, using two drivers in series per ch. So about 15dB headroom vs 1 watt.
Whatever boost is being applied to raise the lower end of the 15"s response should be substracted from that 15 dB potential amp gain...in estimating potential SPL...
Via an HDMI output that goes to a HT receiver, from there to the analog input of the MiniDSP. I bypass the analog preamp when using the computer output.
The easy way to see if the problem is amp power, or line signal, or measurement setup, is to measure the voltage of sine sweeps.
Measure a line output, slow sine sweep from REW....should be constant voltage wrt frequency.
Do the same at the amp output / sub terminals....again should stay constant wrt freq.
If amp output voltage doesn't increase in lockstep with line level voltage increase, and stay constant voltage wrt...the amp be the issue.
http://www.sengpielaudio.com/calculatorVoltagePower.htm
I'll be surprised if you have a signal or measurement problem...but then again, i get surprised a lot ..
Good luck
Measure a line output, slow sine sweep from REW....should be constant voltage wrt frequency.
Do the same at the amp output / sub terminals....again should stay constant wrt freq.
If amp output voltage doesn't increase in lockstep with line level voltage increase, and stay constant voltage wrt...the amp be the issue.
http://www.sengpielaudio.com/calculatorVoltagePower.htm
I'll be surprised if you have a signal or measurement problem...but then again, i get surprised a lot ..
Good luck
At the risk of displaying my ignorance. I also wonder if the wood structure/design is distorting (playing along) at high volumes. In particular I wonder what is the effect of having the woofer ports completely off to one edge/side of the woofer cone as opposed to a more centered port or ports (e.g. ChrisA's K402 MEH design). Note: I'm thinking of the edge orientation vs centering of the port relative to the woofer cone not their edge/center placement on the horn face. Specifically would an edge placement tend to make the woofer/wood structure system distort at lower volumes than a more centered placement? I know little about Synergy Horns/ MEHs beyond Chris' design.
Someone correct me if I am wrong.
My understanding of offset ports is that they lower the top-end response of the driver. Since I am only using my woofers up to ~150Hz, I have no need for high end extension in my woofers. Therefore the offset ports would have no negative impact and may even be desirable.
My understanding of offset ports is that they lower the top-end response of the driver. Since I am only using my woofers up to ~150Hz, I have no need for high end extension in my woofers. Therefore the offset ports would have no negative impact and may even be desirable.
I should have said first that this is a really exciting design and build and many thanks for posting about it! This is from my perspective of using very large horns (K402s) with tons of help from Chris (Cask05) but so far rather cowardly when it comes to cutting one or two of them up and following him into MEH land. So, it's awesome to see you build one even larger than the K402!
That said, I did find this with a quick search on ports and MEHs:
https://www.diyaudio.com/community/threads/how-do-we-determine-synergy-entry-port-size-shape.377469/
"Centering the port to the cone will make it’s pressure forces more symmetrical, which may sound “cleaner” at high excursion, due to less rocking motion."
If I'm understanding you...if by offset ports, you mean ports that are not centered under the cone, then my experiments agree that they lower high-end response some compared to centered ports. (This is without regard to where the ports are placed in the synergy horn itself, like in center vs corners etc)
The size (area) of the ports doesn't seem to effect high-end response much (which makes sense).
But I did find that as port area is decreased, low freq response decreases too....both level and extension...
I like testing this off of a horn...just make a sealed box where you can put a plate on top the driver, with ports of various sizes, and move the plate around...
That agrees with my research as well. Since I was using the woofers all the way down to 20Hz, and I did not need much HF extension from them, I made the ports as large as possible and offset them. I went for approximately 4:1 compression ratio.
I called the shop yesterday and it seems my old Yamaha P2200 is beyond repair. The tech cannot find the parts needed to fix the hum in one channel. Rats. Those VU meters were really cool! Oh well, it still has one good channel. Maybe I can use it as a sub amp.
Anyway, that puts me in the market for a powerful SS amplifier to drive these beasts. Any suggestions? I was initially thinking something like this:
https://www.parts-express.com/QSC-GX5-Power-Amplifier-500-Watts-ch-at-8-ohms-245-652
However, I am open to recommendations. Looking for something that can feed 8 hungry woofers with all they can eat!
Bummer about the old Yamaha amp.
Hey, how do figure compression ratio? I use area of the port divided by driver Sd.
Looking at your picts, trying to eyeball the area of the circle segments vs the full circle, i was thinking your compression ratio is 10:1 or higher. ????
(If your amp showed linear output on the sine sweep test i proposed, my next thoughts were going to be about compression maybe being too high...but i didn't want to go there given the difficulty of changing it.....until all else is exhausted...)
I've had good luck with used QSC CX amps off ebay. They are install sound workhorses, robust. https://www.qsc.com/resource-files/productresources/amp/cx/cx_series_2-ch/q_amp_cx_2ch_specs.pdf
I'd look for at least the CX702, with the 1102 being best for your needs.( Oh, NOT the CX V models...they are optimized for 70V systems.)
Reasons I'd get these over the GX5 from PE, is price, and perhaps more importantly they match the load you have better.
The 4 ohms drivers you have are perfect for when one driver goes to one amp channel, as most higher power sub amps are optimized for 4 ohms.
But when you need to put multiple 4 ohm sub drivers on a channel, it becomes a pain in the butt.
Either two drivers go in parallel....needing an amp that's good at 2 ohms (the CX amps are, the GX amps are not).
Or two drivers go in series..needing an amp with huge 8 ohm balls. (which inevitably are the same amps that can handle 2 ohm loads)
Or last you can series / parallel four drivers together for a 4 ohm load...but yikes, the power that's needed then !!!
You've kinda got me fired up about making a bid, non-portable synergy....i was studying last night what it would take to add two 18"s in the way i like to build them...😀
I calculated the woofer compression ratio by using the area of a segment of a circle formula. .I can't easily type it here as it contains sine symbols, etc. The ratio might have been 5:1 instead of 4:1, but it was definitely not anywhere near 10:1. I used 8:1 for the mids, and wanted a lower ratio for the woofers. I can measure and re-calculate it when I get home.
I have my woofers wired series-parallel for a 4 ohm load, if that makes a difference.