DBX DriveRack PX

[JCJR]

Thanks Ausdesciple and camsr.

I don't have a good flat omni like an earthworks. Always wanted one. The old omni that fits the old rta (assuming it still works after all these years) only works plugged into the rta and nowhere else.

I have a couple of small diaphragm cardioid condensers that are "close to flat" according to the spec sheets, but directional.

And then an old ev PL9 dynamic omni that should be decently flat unless age has caught up to it, except the PL9 rolls off a bit in the top and bottom octaves.

If those were your only choices to ring thru the puter, which would you choose?

[camsr]

You will want to use an omnidirectional mic obviously. Directional mics are no good for room measurement. I don't think any of those mics will give a clear picture. Maybe the RTA you have has an out?

[JCJR]

Thanks, will try to dig it out and check. The old pl9 wasn't too shabby for flat, for an omni dynamic, and was near impossible to overload, but droops a bit on the edges--

[AusDisciple]

There's a function in TrueRTA that allows you to import a cal file if you know the frequency response of the mic. Keep in mind though, some mics vary from the manufacturer plotted data if that plot isn't specifically for the exact, individual mic.
More often than not though, the plots are usually pretty good.
If you used TrueRTA and created a cal file using that frequency response plot, you could probably use it to get some useful ideas of your room response at various positions.

I haven't really played around with REW very much but I'm pretty sure it also has the capability of loading a cal file.

[JCJR]

Thanks. Noticed today that sweetwater dropped the price of that driverack px from $200 down to $150, and can't find hardly anybody online loudly complaining about the box, so finally decided to order one.

It comes with a mic. All the driveracks have presumably phantom power xlr front panel input, so maybe the m2 mic will also be usable plugged into my focusrite saffire pro 24 to also experiment with puter analysis.

I don't expect much out of the included mic except maybe it is fairly flat. Always wanted an earthworks for recording as well as analysis, but never wanted one bad enough to pay the price. It looks like most of the current cheap omni analysis mics in the $150 and lower price range, are intended to be suitable for analysis and nothing else. They don't bother to list specs for s/n, sensitivity, etc.

In the old days, except for real expensive rta's, it seemed the same. They would often design the rta to work good with specific cheap omni electret capsules that worked good enough connected to the rta, but were too crappy to use as a recording mic.

Had got that pl9 in the mid 70's for as flat as I could afford for analysis, but it sounded real good recorded and I used it a lot. I wanted an rta bad, but couldn't afford one. So built a one channel analyzer that was real slow to use, but worked pretty good for a penniless musician.

The box contained a function generator, sin/tri/square. And a digital pink noise generator, and a single LED ladder meter.

Built a state variable adjustable q filter, adjusting the filter frequency using a pair of radio button latching decade switches. One decade switch selected capacitors for octaves, and the other decade switch selected 1 percent resistors for tenth octaves.

After construction, used the oscilloscope to select every switch combination, tune in the peak with signal gen, measure the frequency on the scope. Made a 10x10 hand drawn frequency grid on the face of the box below the switches.

So for ringing a room, crank the pink noise, then push the radio buttons while watching the LED meter for the level of each band of interest. It was slow and tedious, but at that time rta's costed thousands of dollars.

I'd also use it to tediously analyze sounds of interest. Record the sound, then play the tape back over and over, selecting different frequencies with the decade switches, watching the meter, writing the measurements on grid paper.

Compared to that, $150 for a driverack px is just luxurious and nearly free!

[AusDisciple]

That's VERY cool JCJR!! I love hearing such stories of DIY projects!
...and hand drawn frequency grid. Now you're talkin'

Reminds me of my earlier years before I started work as a repair tech. I used to build things in cardboard boxes and your mention of the LED meter brings back memories of my first home built LED level meter... built in a cardboard box with holes punched in the front for the LEDs with a pair of scissors! It was real rough but it worked and I thought it was pretty cool at the time! lol

[camsr]

What works better to hole out cardboard is a broken car antenna. Don't ask, they just exist in my environment.

150$ for the driverack is The mic DBX puts out is pretty good. I have the rta-m and it costed me 70$ new.

[JCJR]

150$ for the driverack is The mic DBX puts out is pretty good. I have the rta-m and it costed me 70$ new.

Yeah, crazy price. List on the pa+ is $750, street $400. List on the px is $600, street $150. 25 percent of retail.

Can't find out how similar is the rta-m mic, vs what they list as an m2 mic shipped with the px. In some web pictures, the mics look similar, but maybe one is metal but the other is made of crispy taco shell or whatever.

[JCJR]

[Double post]

[camsr]

Plastic is untrustworthy over time, so it's a really good idea to keep it cool and dry as much as possible.

[AusDisciple]

What works better to hole out cardboard is a broken car antenna. Don't ask, they just exist in my environment.

lol

150$ for the driverack is The mic DBX puts out is pretty good. I have the rta-m and it costed me 70$ new.

Agreed about the price. Can't say I remember much about the mics though. We never used them for our installations and nearly always tuned PA's by ear.

[JCJR]

Plastic is untrustworthy over time, so it's a really good idea to keep it cool and dry as much as possible.

Is your rta-m metal or plastic? The px is sposed to arrive today, so will know about the m2 mic.

In reading online reviews of the jbl 4300 series monitors that ship with calibration mics, a few users have complained that the mics look cheap and flimsy, but fewer have claimed them unfit for purpose.

In the old days you had to be fabulous wealthy to buy an rta, and obviously if you could afford the rta, you had no right to complain about the outrageous cost of the calibrated mic.

Then more models of affordable rta began arriving in the years around 1980. They still were not cheap but at least you didn't have to rob a bank.

A lot of the less expensive units at the time (but still costing hundreds of dollars) would use tiny panasonic omni electret mic capsules the size of pencil erasers. Sometimes built into fancy looking mic shells. Commonly used back then in such as $30 consumer hand held cassette recorders. Available from mouser or digikey for a dollar or two apiece. Maybe the exact same model numbers are sold to this day.

Perhaps the bigger manufacturers would get panasonic to make a slightly better version, or selected versions for a few cents more per piece, dunno.

The tiny little cheap elements were "near perfect" for an rta if your customers couldn't pay more for a measurement mic than the cost of the analyzer. The elements were very omni, with very flat freq response. Unfortunately were not suitable for much else except recording 10 percent distortion on a cheap handheld cassette deck. The signal to noise ratio on the little mics was not good, but for "high quality" recording you couldn't just swamp the self noise with spl, because the tiny diaphragm would overload easily, and as best I recall usually ran on something like 3 or 5 volts, so there wasn't any voltage swing available for a wider dynamic range.

But the capsules just accidentally happened to be fairly fit for purpose for an affordable analyzer, because of being purt dang flat.

Maybe some of the current less expensive analysis mics have a similar heritage? Dunno. Put a $2 element in a plastic mic shell molded in the shape of an earthworks? As long as it works, no big deal.

[AusDisciple]

Ah, the Panasonic WM-60 and WM-61a. There's a lot of tricks one can do to these to get them up to par for recording (and I'm not talking about the Linkwitz mod which I am not really a fan of for recording purposes). I'm actually using the 61a capsules for my PZM designs and they actually work surprisingly well. The PZM prinicple combined with a floating power supply (mic capsule not earthed and floating between the 0 and 48v rails) and a proper compensation filter give these a decent signal to noise ratio, overcome some of the SPL limits and provide a surprising amount of detail.
I'll be posting some example recordings online soon once I'm happy with the initial tweaks.

[JCJR]

Thanks that is fascinating. Didn't know. Curious to hear your results.

I vaguely recall reading an article long ago about using the capsule for recording, that maybe included some of those strategies, but didn't pay attention. Assumed at the time (perhaps wrongly) that the author back then was probably just so deaf that he didn't mind low spl handling and s/n worse than 50 dB.

My narrow mindedness was not completely without cause. A lot of do it yourself articles of the time described very low fidelity gadgets. (Cough-- PAIA--)

[camsr]

Plastic is untrustworthy over time, so it's a really good idea to keep it cool and dry as much as possible.

Is your rta-m metal or plastic?

It's metal. But I think the condenser itself is plastic (pretty sure). Keep plastic out of direct sunlight and UV light, or you know what'll happen.