QSC PLX 3002 Issue/Question

[warrenstuart]

Hi

I picked up a used QSC PLX 3002 amplifier to drive my EV ZX4 speakers, amp runs 550w RMS into 8 ohms per channel, 9.5kg in weight. This has replaced my old C-MARK MR2450 that runs 400w RMS into 8 ohms per channel, 26kg in weight!! The idea being mainly to cut the weight but also to give me a bit more headroom for the speakers.

I have run it for about 10 gigs now (small mobile disco) and the sound just didn't seem very punchy and no depth or "guts" if you like to the sound, also I'm just constantly up to 0db and even the odd flash of the clip limiter in quite small venues that I have regularly played over the years with my old less powerful amp and not really run much more than -3db to be more than loud enough with no sign of clipping.

So on Saturday I took the old amp out with me too and put the old amp on one channel/speaker and the QSC on the other... Oh dear, oh dear, oh dear. what a difference! Compared with the QSC the speaker on my old C-MARK amp was louder, brighter and deeper, it was very noticeable. I am confident all of my leads etc are wired correctly and there is no double-terminating etc when wired to the QSC causing a drop in output. During the test I also turned off the HPF and clip limiter on the QSC as the C-MARK doesn't have these to make the test more fair, however turning on the clip limiter and 30hz HPF didn't make a noticeable difference, turning on the 50hz HPF caused a slight reduction in bass as expected on the QSC.

So I ended up playing the gig with the old C-MARK amp, my old sound was back, it was brighter, louder and deeper.

 

My question here really is from anybody with experience, is this normal for amplifiers with switch-mode power supplies, is it this model or could there be a fault??

 

Thanks.

[ChazHS]

The PLX is not QSC's budget range and should beat the C-Mark hands down. How old is the PLX and did you buy it from new?

[AV Ian]

Strangely I also find the the PLX series a little 'dull'. I don't struggle with level like you seem to, but in a comparison between equivalent spec RMX and PLX amps on the Mid/Hi section (250hz and up) of my TMS4's I much prefer the sound of the RMX. I can't explain it, but I'll be sticking with my heavy weights. I've used a PLX3402 on subs (50-160HZ) on quite a few occassions and can't fault it. In fact, I've heard them on many speakers with no complaints, I just don't like them on mine. Horses for courses perhaps?

[Doug Siddons]

The QSC PLX are great amps but I'm not surprised that the "heavyweight" gives a better sound running full range like this. Switchers are getting better all the time but I'm still not convinced that unless you go for the mega expensive ones that a switcher can pass more volts than a torroidal rated at the same power.

[mackerr]

So on Saturday I took the old amp out with me too and put the old amp on one channel/speaker and the QSC on the other... Oh dear, oh dear, oh dear. what a difference! Compared with the QSC the speaker on my old C-MARK amp was louder, brighter and deeper, it was very noticeable.

 

Do you know what the gain of the C-Mark amplifier is? The PLX3002 has a gain of 32dB, and an input sensitivity of 1.7V for full output. If the C-Mark has more gain, or a lower voltage for full output, of course it will be louder than the QSC with the same input, and will seem to sound better because it is louder. To compare the amps fairly you will have to match the gains, probably by turning down the C-Mark.

 

Using the clip lights to determine amp output is at best very flawed. Generally the clip light comes on somewhat before actual clipping as a warning, and there is no standard as to where that happens. Some amps actually do indicate clipping, Crown comes to mind, but since the manufacturer knows at what point the amp runs out of rail voltage a warning light just before that is easy to do.

 

Mac

[warrenstuart]

So on Saturday I took the old amp out with me too and put the old amp on one channel/speaker and the QSC on the other... Oh dear, oh dear, oh dear. what a difference! Compared with the QSC the speaker on my old C-MARK amp was louder, brighter and deeper, it was very noticeable.

 

Do you know what the gain of the C-Mark amplifier is? The PLX3002 has a gain of 32dB, and an input sensitivity of 1.7V for full output. If the C-Mark has more gain, or a lower voltage for full output, of course it will be louder than the QSC with the same input, and will seem to sound better because it is louder. To compare the amps fairly you will have to match the gains, probably by turning down the C-Mark.

 

 

 

Part of the testing I did was to turn the C-MARK down so that both sides were running at the same volume and the C-MARK just sounded like it had a wider bandwidth, especially at the low end frequency response.

I think the input sensitivity of the C-MARK is quite high compared to other makes as I borrowed an EV CP2200 (I think this was the model number!) rated at 500w per channel into 8 ohms and again the C-MARK was louder but I couldn't tell any difference in sound quality between the two.

As soon as I have some time I will do a frequency sweep of the QSC to see what it is like, I did this on the C-MARK a while ago and it is totally flat from 10hz - 25khz.

In answer to the other question I don't know what the gain of the C-MARK is without measuring it as there is very little information on these amps... don't we love the Chinease!

[jonathanhill]

Unfortunately a lack of bass is a characteristic of an amplifier with a switch-mode power supply, rather than a classic toroidal transformer. Lots of bass requires large currents that switch-mode PSU's struggle to supply. As with most things in life, a compromise happens between weight and bass response with regards to switch-mode versus toroidal.

[Simon Lewis]

Unfortunately a lack of bass is a characteristic of an amplifier with a switch-mode power supply, rather than a classic toroidal transformer.

 

I'm not an amp or PSU designer, but I understand the "lack of bass" comment to originate from some of the early SMPS based amplifiers where either through lack of knowledge or cost constraints the smps was not up to the task and would indeed curent limit when pushed hard.

 

A lot of amplifier designs have come and gone since the early Carvers etc., and I would suggest that an amplifier's ability to deliver high power low frequency amplification is a function of its design and component specification, and not solely its power supply or output stage topology. If the lack of bass sentiment was universally true, I doubt if the hire and installation companies would embrace the high end SMPS equipped amplifiers as warmly as they do.

 

Granted, there are technical challenges and compromises in all of these areas, and also the potential to believe that a system which is not matched in terms of gain, sensitivity, etc., is better / worse than an apparently similar product.

 

In the OP's situation, I wouldn't want to dismiss the observations, but would suggest that if a truly level playing field is achieved, the apparent difference between the two offerings may not be as apparent as first thought.

 

Simon

[warrenstuart]

The PLX is not QSC's budget range and should beat the C-Mark hands down. How old is the PLX and did you buy it from new?

 

 

Sorry I didn't answer this one earlier, I didn't buy it from new so I don't know how old it is but could possibly find out from the serial number I guess if I contacted QSC??

Sadly it doesn't seem to beat the C-MARK other than on weight where there's no contest... maybe as suggested the compromise I may have to live with.

[mervaka]

saying SMPS cant deliver large current is quite a sweeping generalisation. PKN amplifiers I've been told are very good at delivering sustained power. I'm not surprised either, PKN are first and foremost an industrial power supply manufacturer.

 

EDIT: Here's a link

[mackerr]

saying SMPS cant deliver large current is quite a sweeping generalisation. PKN amplifiers I've been told are very good at delivering sustained power. I'm not surprised either, PKN are first and foremost an industrial power supply manufacturer.

 

EDIT: Here's a link

 

Here is another link. The tests were done at 4Ω, so the current isn't as high as it would be at 2Ω, but the very high power levels are testament to the capability of all the components in the amps being tested.

 

Mac

[dbuckley]

There was an interesting thread a while ago (which I now cant find!) about the power delivery capabilities of MC2 amplifiers, which illustrates what the manufacturers mean when they describe the power outoput of modern lightweight amplifiers.

[KevinE]

The Cmark does have clip limiters, they are permanently engaged and you cant turn them off. They are optical and work well to prevent the clip lights flashing, I suppose you might be riding the limiters leading to compression which may make the sound seem meatier.

 

The PLX are a worthy amp and simply in a different league to the cmark.

[Nid]

warrenstuart,

 

Just in case you overlooked...

Did you check the LF dipswitches on the back of the PLX (eliminating the filters...)?

 

B-)

[warrenstuart]

warrenstuart,

 

Just in case you overlooked...

Did you check the LF dipswitches on the back of the PLX (eliminating the filters...)?

 

B-)

 

 

Hi Nid

Yes I did turn off the filters etc on the QSC as a fair test if you see my original post.

 

Also I think maybe some of the replies on here are of the impression that I'm only noticing this when driving the amp hard but no, at talking volume my original observation of sound quality are just as noticable... so no real strain or demand on the power supply for lots of current draw for punchy bass response at low volume levels.

I'm hoping to get the time to do a frequency sweep of the QSC tomorrow and see what that shows up, if it's as flat as the C-MARK then I guess it's a dynamic range thing but we'll see.

Post my findings on here as soon as I have them.

 

Thanks everyone for all of the comments so far :-)