At this year's Burning Amp, Jack Hidley very generously gifted many of us several Peerless 850397 6.25 inch 8 ohm mid-woofers (NHT part npt-11-078-1). Since I am between 'proper' loudspeakers, I wondered what quick and dirty little speakers I could make that would be better than listening to my old Cambridge Ambiance surrounds. The question was which cheap tweeters from Zaph's list to combine with my new drivers. My leading candidate had been the Vifa DQ25SC16-04 Ti tweeter. But a friend had insisted I hold onto an old pair of Boston A70 (1983 particle board with dark woodgrain vinyl monkey coffin with sealed 8 inch and 25mm copolymer ferrofluid tweeter) with rotten foam, so I dragged one inside thinking I could just replace the old woofer with one of the new Peerlesses.
As the cabinets were on the floor and I was holding up a Peerless to see how it would look in place of the 8 inch woofer, it occurred to me that these new mid-woofers fit perfectly onto the sides of the cabinets. And if I used two in parallel, I might get a close match with the existing 6 ohm 90dB tweeter and have enough low end in the 0.94 cu ft sealed box. (According to the online ISD calculator: F3 ~80 Hz sealed and a slightly messy ported F3 ~50 Hz with Fb = 43 Hz.) I expected the crossover to be a simple first order high pass and low pass (based on a 6 uF bipolar electrolytic cap, unknown inductor, and two 1 ohm resistors), but when I traced the actual connections, it turned out they were running the old woofer full range and the tweeter second order high pass. Based on a tweeter DCR of 5.3 ohms, plus the existing 1 ohm resistor directly in series, and cap of 6 uF, this gives a first order high pass crossover of about 4200 Hz. (I assume the inductor in parallel with the tweeter and resistor is sized to give a second order HP at about the same 4200 Hz. The second 1 ohm resistor is before the cap.) With two new mid-woofers and the old tweeter and crossover connected together on the floor, it didn't sound too mismatched in the midrange and at least gave me a good starting point.
I had already created a CAD model of the new arrangement, so I borrowed a jigsaw and started making a mess in the kitchen. I mounted all the drivers in the new holes (with the original holes still open) and listened - no bass obviously, but not awful. I shorted out the resistors to play with the tweeter level but kept returning to the original. When I plugged off the original holes, the whole thing came alive. I really couldn't believe my ears at how smooth the midrange was and how punchy the upper bass was and kept looking for flaws with my best test tracks. (One of my favorites is Ride Across the River by Dire Straights: it has everything!) I could live with these for the next few months while I decide what to build next. Since I can't stand the look of dark woodgrain vinyl, I painted them with my favorite standby: gray primer in a spray can. This always comes out a perfect even medium gray satin finish with minimum effort, plus its sandable if needed and dries fast. (And it apparently sticks to vinyl just fine.) And I added some surplus handles as bumpers and that purposeful mission control room appearance.
While the components were out, I replaced the electrolytic cap with a combination of two surplus film caps to get back the original 6 uF. I usually do this on principle, but it really improved the high end. I'm very pleased with the whole setup, especially since I have about $10 in them including paint. I expect I'll eventually add a port to get some more bass but make it reversible. By the way, can anyone point me to a frequency response graph of these Peerless mid-woofers?
Here are the T/S parameters from Jack Hidley's Excel spreadsheet:
Peerless 850397 6.25 inch 8 ohm
Fs 51.7Hz
Qts 0.48
Vas 16.1 l
Xmax 4.9mm
87.6 dB
Re 4.84 ohm
Qms 2.74
Qes 0.58
Cms 0.000543m/N
Mms 17.4 g
BL 6.85
As the cabinets were on the floor and I was holding up a Peerless to see how it would look in place of the 8 inch woofer, it occurred to me that these new mid-woofers fit perfectly onto the sides of the cabinets. And if I used two in parallel, I might get a close match with the existing 6 ohm 90dB tweeter and have enough low end in the 0.94 cu ft sealed box. (According to the online ISD calculator: F3 ~80 Hz sealed and a slightly messy ported F3 ~50 Hz with Fb = 43 Hz.) I expected the crossover to be a simple first order high pass and low pass (based on a 6 uF bipolar electrolytic cap, unknown inductor, and two 1 ohm resistors), but when I traced the actual connections, it turned out they were running the old woofer full range and the tweeter second order high pass. Based on a tweeter DCR of 5.3 ohms, plus the existing 1 ohm resistor directly in series, and cap of 6 uF, this gives a first order high pass crossover of about 4200 Hz. (I assume the inductor in parallel with the tweeter and resistor is sized to give a second order HP at about the same 4200 Hz. The second 1 ohm resistor is before the cap.) With two new mid-woofers and the old tweeter and crossover connected together on the floor, it didn't sound too mismatched in the midrange and at least gave me a good starting point.
I had already created a CAD model of the new arrangement, so I borrowed a jigsaw and started making a mess in the kitchen. I mounted all the drivers in the new holes (with the original holes still open) and listened - no bass obviously, but not awful. I shorted out the resistors to play with the tweeter level but kept returning to the original. When I plugged off the original holes, the whole thing came alive. I really couldn't believe my ears at how smooth the midrange was and how punchy the upper bass was and kept looking for flaws with my best test tracks. (One of my favorites is Ride Across the River by Dire Straights: it has everything!) I could live with these for the next few months while I decide what to build next. Since I can't stand the look of dark woodgrain vinyl, I painted them with my favorite standby: gray primer in a spray can. This always comes out a perfect even medium gray satin finish with minimum effort, plus its sandable if needed and dries fast. (And it apparently sticks to vinyl just fine.) And I added some surplus handles as bumpers and that purposeful mission control room appearance.
While the components were out, I replaced the electrolytic cap with a combination of two surplus film caps to get back the original 6 uF. I usually do this on principle, but it really improved the high end. I'm very pleased with the whole setup, especially since I have about $10 in them including paint. I expect I'll eventually add a port to get some more bass but make it reversible. By the way, can anyone point me to a frequency response graph of these Peerless mid-woofers?
Here are the T/S parameters from Jack Hidley's Excel spreadsheet:
Peerless 850397 6.25 inch 8 ohm
Fs 51.7Hz
Qts 0.48
Vas 16.1 l
Xmax 4.9mm
87.6 dB
Re 4.84 ohm
Qms 2.74
Qes 0.58
Cms 0.000543m/N
Mms 17.4 g
BL 6.85
Attachments
You might think about adding some small amount of series resistance ahead of the woofers. As it now stands, if they are paralleled, your amp is seeing under 3 ohms from them. If you don't mind the tweeter, build a cheap measurement mic (ala Linkwitz) and his cheap mic preamp, download Arta, and see how it really turned out. 🙂
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