New DIY Mid High (90deg) - AKA PM90

Carl,

Congratulations on an awesome new tool!

I did the design with a single 1/4" straight flute end mill in mind way back in 2016. Be sure that you have either a stiff frame with powerful motors or that your spindle speed is pretty low. You can burn through bits pretty easily when cutting baltic birch if you're not careful.

As it turns out, I will again have access to a CNC router in the near future myself. Finally, I should be able to make flat packs for the first time in many years!

If you have any questions or want to compare notes, please let me know!
 
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OK I have tried the DCX 464 on the XT1464 ... it fitted but the driver was touching the B&C 12NDL75 (back of the DCX464 on the 12NDL75 frame and about 1mm clearance to the speaker terminals) ... so you will need a spacer under the horn 6-10mm. That means the HF950 will fit with the DCX464. I also tried the Ciare PR614 which will also fit with the DCX464.

DCX 464 in PM60.jpg
 
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Peter,

I suspect I have slightly more internal space in the drawings that I did than you do in your boxes. It would be interesting to know just how close we are to each other.

I can try and do a 3D model eventually to verify, but it may take a while.
 
Max Warasila said:
Peter,

I suspect I have slightly more internal space in the drawings that I did than you do in your boxes. It would be interesting to know just how close we are to each other.

I can try and do a 3D model eventually to verify, but it may take a while.
Click to expand...

I think my boxes are exactly to plan. That picture is the position required for the XT1464 horn. The HF950 is not as deep so it will have more clearance as shown on your recent drawings.

FWIW the first prototype PM90 was a bit smaller and the DCX464 does not fit with either the HF950 of XT1464 horn.
 
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Much has been written about the critical signal processing with the PM90s (and PM60s) but little regarding amplification. As a firm advocate of massive amplifier headroom, I would be most interested to hear what level of output power or make/model of amplifier with which users have been driving each of the three crossover bands, and any comments comparing digital with analogue output stages.
 
So I wanted to go back and take a look at the components and my notes before I said anything further.

Some assumptions that I'm making here:
  1. The expected design crest factor is 12 dB. This is mostly informed by Bennett Prescott's propaganda from over the years.
  2. The real power that a driver can dissipate effectively is much less than AES.
  3. Drivers are all the 8 ohm versions.
  4. The limits to concern ourselves with are:
    1. Peak voltage required to cause excursion limit.
    2. Peak voltage to handle for crest factor (12 dB above the higher AES). This is likely to exhaust the mechanical limits of the driver.
    3. RMS voltage for complex power short term thermal limit (AES rated power)
    4. Complex power short term delivery requirement (AES rated power)
    5. Real power long term thermal limit (adapted from Powersoft's True Power limiter table)
DriverVpk @ FrequencyVpk AES+12dbVrms for AES Power
(nominal, minimum)		AES PowerReal Power
B&C 12NDL7690 V @ 100 Hz320 V57 V @ 8 Ω, 49 @ 6 Ω400 W150 W
BMS 4594HE HFN/A196 V35 V @ 8 Ω, ? @ ? Ω150 W55 W
BMS 4594HE VHFN/A143 V25 V @ 8 Ω, ? @ ? Ω80 W30 W
B&C DCX464 MFN/A238 V42 V @ 8 Ω, 36 @ 6 Ω220 W60 W
B&C DCX464 HFN/A215 V36 V @ 8 Ω, 38 @ 9 Ω160 W45 W

As you can see, for this loudspeaker system, you don't need that much real power delivery, and should pretty much just get as much peak voltage as you can. Realistically, I think 200 V is more than adequate, really. 320 V at the 12" drivers is enough for the cones to slam into the baffle, and the CD's will start showing serious distortion at that point.

I still think my favorite option on the East coast of the US would have to be getting a used PLM20k from Clair. Otherwise your options are pretty much Powersoft, or Linea Research. I really like the tight integration of DSP into the amplification platform. I'm not sure I'd buy into the Crown offerings at the moment.
 
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Hi Max, you raise an interesting point about real / true power. What Powersoft is doing is measuring the actual current and multiplying it by the actual voltage, and then integrating this over a time period to anticipate the temperature rise in the VC. i.e. they are calculating against the real impedance, not the nominal impedance.

What the AES measurement is doing is calculating the power handling based on a nominal fixed impedance (e.g. 4 or 8 ohms). As I understand you don’t need to use this 1/3 power suggestion unless you are using Powersoft’s True power limiter.

As an example my double 18 subs are nominally 4 ohms. I can remember looking at my Powersoft K10 when driving 2 of these. It told me the impedance the amplifier was seeing on average was just below 7 ohms, not the 2 ohm nominal impedance. In other words the real or true power was about 1/3 of the nominal power calculated from the applied voltage.

In this case my limiter was set to a voltage based on the driver’s 8 ohm 1800w rating.
 
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Peter

Yep! Powersoft is doing what I've always wanted to make a box for. It's far down on my to-do list. For setting limiters without using the True-Power limiters, AES power is usually fine.

The reason I bring it up is that it's the relevant number for choosing the power delivery capabilities of an amplifier.
 
I'm running my PM90 loaded with b&c12ndl76 and rcf ND950 1.4 with class A Chevin A1500. Headroom for days and very happy with the combination. I wonder if there is something i could upgrade with one day, preferably class A also 🤠
 
ivan cuic said:
I'm running my PM90 loaded with b&c12ndl76 and rcf ND950 1.4 with class A Chevin A1500. Headroom for days and very happy with the combination. I wonder if there is something i could upgrade with one day, preferably class A also 🤠
Click to expand...
I doubt that the Chevin has a Class A output stage; you are perhaps confusing this with its name: "A-Series". I suspect it is more likely to be Class AB or H.
 
Hey everyone + Peter, this thread is really inspiring and I've learnt a lot. Thanks so much Peter for this incredible project and build. I am intending to build 2x PM90's in the next month and have read through all 39 pages of this thread. Currently intending on the BMS4954HE + B+C 12NDL76's with a 16ohm BMS passive crossover. Here are my questions:

- Am I correct in thinking this will be appropriately amped with a single NL4 connector in the back? 1+2 12's parallel and the HF+VHF on pins 3 + 4 ?

- Is the B+C DXC464 a better choice now? If so - will the BMS passive crossover still work correctly? We definitely won't have a budget for FIR processing I think

- Noticing that the HF - VHF seems to work on a 1.49~1.64ms delay - the 0.2ms difference is negligible ? Or could you achieve this by installing a delay component onto the passive crosser ? (Is that even possible?)

- Can the entire build be achieved with 15mm birch ply ? Or is better to use a thicker ply for external/internal?

- Can the RCF horn flare be rotated 90 degrees so the PM90's can be positioned horizontally instead of vertically ?
(I am intending to sit each PM90 on a 4 leg pedestal type thing on top of 2x Martin Audio WLX 18's per side)

- Lastly, any recommendations on amplifier/processing for 4x 18" subs (B+C 18TBX100's) and 2x PM90's that won't break the bank after spending 4k on drivers?

Thank you all - some info on myself: I am a 26 year old audio engineer (bands/touring) + DJ/events background based in Brisbane, Australia. This will be my first DIY speaker build and it's humbling to know how much more there is to learn but I do learn very quickly and I'm really excited to put this project together.

Thank you again all! I really appreciate any insight, advice and expertise anyone can offer
 
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Hi Ethan,

- Yes and NL4 will be all you need with the passive crossover.
- B&C DCX464 its better than a BMS4594 but the 4594HE is about the same all things considered. The DCX464 needs a different crossover - FB-464 - USSpeakers have them.
1610160174324.png
- The BMS passive crossover is designed so that there is no need for different delays in the HF / VHF - its has 18/12 dB/oct slopes and the different phase response corrects for the time difference.
- 15mm is fine with bracing but you will need a spacer under the 12" drivers.
- its best to mount the box vertically - the horizontal pattern of the 2 x 12" drivers starts to collapse as the frequency increases because of the dipole action caused by the spacing of the drivers. At 400- 500 Hz its around 50 degrees vertical - below is a manufactures plot of a speaker using the HF950 horn - the dotted red line is my best guess (rough measurements) of the PM90's vertical directivity.
1610160889259.png
BTW I'm in South Australia - send me a PM if you want.
 
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