Just got some new PA speakers to play around with - "something old, and something new!"

[neRok]

I've been eyeing off some PA speakers to try out for home duties, and managed to snag some QSC CP8's for a fantastic price. Their EASE / GLL data looks pretty decent compared to similarly priced rivals (for example, they are generally a bit cheaper than Yamaha DXR10's, but I got them for only a little bit more than DBR10's ). So it's going to be interesting to see how they perform.

But the really cool thing is these old Yamaha PA speakers I found on FB marketplace, model PS-100B. What a strange design! Yamaha website has the manual, and the last page suggests they are from about 1975. They look pretty good condition, all things considered. The guy I bought them off said he imports a couple of sea containers worth of car parts etc from Japan each year, and a friend of his happened to be clearing out or renovating an old house or something, and these were in it. So the friend gave him the speakers, he imported them to use in his shed, decided he didn't want to, and now I have them I'm keen to see how they go. The lights on the amps turn on, so that's a good start.

Edit: Just properly read those specs. It says the 100B has 5x8" woofers and weighs 43kg, whilst the 75B has 8x8" and weighs 28kg. But that's not what I have in front me, as the tag on the back of the speakers definitely says PS-100B, and there is obviously 8 speakers in the photo. So I think Yamaha must have written the x5 and x8 in the wrong columns, lol.




Edit2: Was just on the QSC website registering for the extended warranty, and as it turns out, they just got bought out QSC, LLC to be Acquired by Acuity Brands

 

[neRok]

I was just giving the Yamaha's a clean and noticed a few leaves and stuff in the bottom of one, which suggested a mouse had been living inside. So I opened it up, and yep, there had been a mouse. But luckily all the hardware seems okay to me, so I just cleaned it all out (including the insulation, coz it had poo and cockroach eggs stuck in it).


This gives me a chance to look at the amp, which would help if I knew what I was looking at Does anything about it stand out to anyone? Perhaps someone like @restorer-john can chime in? At the very least, could someone confirm that it is a balanced/TRS input?


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About the QSC CP8's - I unplugged my Kali IN-8 and put the CP8's in their spot, however their alignment wasn't perfect because I had slightly moved 1 speaker and my desk since taking the IN-8 measurement a couple of weeks ago. Anyway, both were set to 0 gain on their back panels, but the CP8 were so much louder! I had to sweep them -16dBfs lower to get the same measured SPL.

They also have a fair bit of hiss. Not a crazy amount, and they may be okay nearfield when actually playing music, but when sitting idle it is noticable. I tried unplugging the signal cable and lowering the gain, but it made minimal difference - they just hiss.

So after all that, the measurements turned out broadly similar. The IN8 go lower, but room modes dominate the 80-400Hz range of both speakers (which have pretty much the same response in that area). The speakers are relatively equal above 2kHz I feel, which leaves 400-2kHz to discuss. Like I said, I didn't put any effort in to aligning the speakers or the stands, or even my desk/the-mic, so that might be the cause of waviness in the CP8. Or maybe the fact they have offset HF drivers is playing a part too? I didn't have time to test, so can't answer that.



You might ask how did they sound, and I would say "just as crap as the Kali" That room mode at ~93Hz just destroys the sound of either pair of speakers, and I can't listen to them when they sound like that. I did take them out to my lounge room though, along with my 15" PA sub, and did a quick measure to get them in alignment (I set the speakers to "default + ext sub", and the sub need 90° phase shift, and that was all it took to get great impulse alignment! It was heaps easy). It's been a number of years since I listened to speakers in my lounge room, and I thoroughly enjoyed it! The CP8 did their part very well.

 

[neRok]

I was just looking at the Yamaha manual, which also shows a matching mixer. It shows the RCA outputs going from the mixer to the speakers, and both the mixer outputs and speaker inputs are "-6dBm (0.39v)", and the speakers additionally say "Input power of -6dBm provides maximum output". So I take it that means I should feed them a very low level unbalanced signal?

I was also wondering if the speakers were wired in multiple "active circuits", so that perhaps some speakers were doing lows+mids, and others mids+highs? But that is not the case, because there was only 1x 2-wire connection between the speakers and amp. So it's basically just a bunch of 8" speakers playing full range

 

[mhardy6647]

Too bad you didn't get a truly odd Yamaha PA/MI loudspeaker such as the (slightly domesticated variant) NS-15.

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[AnalogSteph]

This gives me a chance to look at the amp, which would help if I knew what I was looking at Does anything about it stand out to anyone? Perhaps someone like @restorer-john can chime in? At the very least, could someone confirm that it is a balanced/TRS input?

It doesn't matter as the unit is floating and the jack uses R+S on TRS or T+S on TS, very clever.

A -6 dBu input sensitivity is quite low and approaching consumer line level, so with a 100 W amp gain has to be fairly high. That array of speaker drivers probably makes the whole thing quite sensitive, so coupled with the standards of mid-1970s amplifiers (PA no less) I'm not surprised it's a bit hissy. This could probably be improved upon but would require a bit of an amplifier reverse engineering session since it doesn't appear there's a schematic out there.

BTW, please don't measure L+R unless you want your results to look even more messy than individual ones. It might work out when using MMM (highly recommended) but I wouldn't bet my life on it.

 

[neRok]

It doesn't matter as the unit is floating and the jack uses R+S on TRS or T+S on TS, very clever.

A -6 dBu input sensitivity is quite low and approaching consumer line level, so with a 100 W amp gain has to be fairly high.

Cool, thanks for the info.

I'm not surprised it's a bit hissy

Just to keep everyone on their toes - there's 2 discussions taking place in this thread. The hiss was about the new QSC CP8 speakers I also purchased. I haven't sent a signal through the vintage Yamaha's yet, but I probably will tonight...

 

[AnalogSteph]

The hiss was about the new QSC CP8 speakers I also purchased.

Oh. I'm kinda not surprised... 500 W RMS, and 124 dB max @1 m. This will definitely be hissy when throwing an average Class D amp at it (103-105 dB SNR), and even with a better than average one (110 dB). You'd pretty much need a Hypex / Purifi grade amp, and that's assuming the circuitry before that is up to the task.