I’ve been looking into building a new amp and thought of the idea of running a helix rack unit into a power amp to power my Ampeg PN-410 HLF. It’s rated at 850w at 8 ohms, but I’ve never used a preamp/poweramp setup before. I’ve read that I should look for a power amp in the neighborhood of double what the cab is rated at, but everything I’m finding is rated at 4 ohms. Is this information correct and do you guys have any suggestions? Thanks in advance.
Speakers are often rated with a 6dB crest factor, so you might get a rating like 300W RMS, 600W Program, 1200W Peak. This is partly where the idea to use a power amp with double the power rating comes from. Basically this sort of rating means the speaker can handle 300 watts of average power with short transients up to 1200W. The problem is the power rating is usually related more to the thermal rating of the voice coil than the drivers mechanical excursion. FYI the excursion limits are determined by a combination of the drivers Thiele/Small characteristics and the cab design.
As the frequency goes below about 100hz, driver excursion tends to increase drastically. Despite the increasing excursion, the output of the drivers will eventually start to decrease. With a ported design, the cab is tuned so the output from the port starts picking up as output from the driver starts decreasing. An advantage of this, is that as the port becomes resonant, it actually controls driver excursion. At Fb, the port tuning frequency, most of the cabs output is coming from the port and the driver is barely moving. But as the frequency goes lower, the port unloads and looses control over the driver. At this point driver excursion begins to increase very quickly. So if you feed the cab a lot of low frequencies you can easily push the driver beyond its excursion limit. Unfortunately, ported cabs often reach there low frequency excursion limits before they reach even their RMS power rating. So no, I don't think it's necessarily a good idea to double the cabs power rating when you buy an amp.
As proof of this take a look at the Eminence S2012 datasheet and suggested cab designs. This driver is rated for 150W RMS 300W Program. Review the Medium Vented Bass Guitar Cabinet. Note Fb is 47hz. Take a look at the Cone Displacement graph, and note diver excursion descreases towards 47hz, and then rapidly increases. The design hits Xmax, 5.2mm, at 125W just below 40hz. The design is also fairly close to it's excursion limit at 70hz. Next, review the Frequency Response charts and note the output is down by about 6dB at 40hz.
These documents came from PartsExpress. They have cab design docs for most of the eminence woofers they stock. I just picked the S2012 randomly, but you can review designs for other Eminence drivers if you like. Most will demonstrate the same problem.
I did go down the helix rabbit hole but I took your advice and it worked out well for my needs. Thank you.
