Pioneer HPM series crossover upgrade?

[Racket Scientist]

I have a pair of vintage Pioneer HPM-900s that I think sound pretty good. They're in very good condition. I ran across the Janzen crossover upgrade for the JBL Century (which was the launching point for the Pioneer HPM series) and it got me wondering if anyone has done something similar for the HPMs. I have too many other projects to fool with it right now, but someday it might be nice to see how good they could sound with some updated electronics.

 

[Racket Scientist]

Not on my short list for projects right now, but I guess just rebuilding the current crossovers with nicer parts is probably the best plan. Anybody got a schematic for these ancient speakers they care to share?

 

[Kozlvm]

Any member of Audiokarma can easily find HPM900 Xover scheme:
HPM-900 Crossover Schematic - AudioKarma.org Home Audio Stereo Discussion Forums
From further discussion at the reference it looks like the HPM900 woofer is connected directly to the input without any inductance. Although HPM900 woofer made of polymer grafite may be pure ande free from any big resonances, in this case new Xover schematic can improve the sound for sure. Can anybody place the original scheme at the forum or mail it?

 

[Racket Scientist]

Thanks for the input. I'm not a member at AudioKarma, but I probably should join - this is the second time a thread over there has been of interest to me (the first was the big Econowave thread). I'll probably join just for the schematic if no one posts it here or sends me a PM.

 

[Racket Scientist]

Well, I have seen the schematic and sure enough, the woofer runs "wide open". I think this is a cool speaker that could be greatly improved with an updated crossover design. I'm hoping to run into someone who has already gone through the process of redesigning the crossover for this, as my skill set is questionable for the task.

 

[head_unit]

Some designers run woofers wide open, because it avoids series resistance from inductors.

And, due to the impedance rise of woofers, a series inductor often doesn't really do very much. Maybe a gentle -3 dB per octave, quite possibly with unevenness due to impedance interaction.

 

[Racket Scientist]

I'm thinking I'll probably just rebuild the current crossovers with a change to air core inductors and polypropylene caps using the stock values. I might also add a low pass to the woofer - probably a simple LC low pass filter. I'm hoping that just changing to the air core from ferrite and better caps plus adding the low pass will give a significant improvement. Optimally I'd get a better crossover design, but I suspect that Mr. Locanthi's ancient design is better than something I'd cook up myself without having any measurements or other data on the drivers.

 

[Kozlvm]

Changing original capacitors for polypropylen seems to be a good idea, changing inductors is an option. Adding low pass isn't a good idea, as it will require rebuilding midrange part of filter too.

 

[head_unit]

I'm thinking I'll probably just rebuild the current crossovers...I might also add a low pass to the woofer - probably a simple LC low pass filter...I suspect that Mr. Locanthi's ancient design is better than something I'd cook up myself without having any measurements or other data on the drivers.

Er, you do realize you're contradicting yourself?

Definitely if you can't measure the driver responses including phase, you could just as well ruin the sound unless you get very lucky. Bart Locanthi was a smart guy; even without today's modern simulation tools the crossover is probably quite good.

I'm thinking I'll probably just rebuild the current crossovers with a change to air core inductors...

Just watch the DCR. You may need multiple air cores in series. Really, you might be better off with low-loss high quality iron cores like you can get from Zalytron or Madisound etc.

 

[Teti]

Look at my restoration of HPM-700

Pioneer HPM-700 loudspeaker refurbishment – part 1 | Audio Tweak
Pioneer HPM-700 loudspeaker refurbishment – part 2 | Audio Tweak
Pioneer HPM-700 refurbishment – all done! | Audio Tweak

An externally hosted image should be here but it was not working when we last tested it.

 

[Ron E]

If I am reading the fuzzy schematic correctly, the cap in the midrange is 4.7uF, this means the "woofer" is covering up to at least 1kHz, if not higher. This was IME typical in 70's and 80's monkey coffins with sealed back midranges, like Cerwin Vegas, etc... To improve them, I'd start with putting heavy felt on the baffle, to help mitigate the non-flush mounted drivers, then replace electrolytic caps. To go any further requires measurement and possibly a mid replacement.