a power amp question (QSC compared to db750)

BiigM · #1 08-24-2010, 01:32 PM

Hi guys.

I'm thinking of getting a poweramp for my Demeter vtb-201s.

I've been running it into the poweramp in on my Aggie DB750, and that's loud enough for me.

I was thinking about a QSC RMX amp. I had a crown XTI, but I didn't like it. It didn't seem to deliver nearly as well as the db750 poweramp in the low frequencies, even though the specs had a way higher wattage. more the 2000W bridged!!

Personally I think it's because of the switchmode powrsupplys. Hence the reason for wanting the RMX instead.

But what kind of wattage should i go for, for it to be on the same level as the Aggie? The aggie does seem to have amazing headroom for a 750W amp.

Thx :-)

rpsands · #2 08-24-2010, 01:37 PM

If you still have it: try the crown xti again but apply a steep high pass (24db/oct) @ 40hz or so.

Also, do not make the mistake most people do: assuming that you need to only turn up your power amp gain attenuator half way or so. How high it is only determines how much the input is attenuated, it is not like a power section dial or anything. If you're not clipping the power amp on the XTI, you can just keep turning the knob until just before the clip lights come on (and even a little after, since the Crown has a nice limiter).

People often switch from bass heads to power amps and expect the knob to work the same way as the volume taper on bass heads. The knob is usually a smooth taper, unlike bass heads which usually peak in volume by the time you get half way up.

BiigM · #3 08-24-2010, 01:41 PM

Hi.

No the xti is long gone. And i had the power amp turned up to 100%. And it was clipping! And was not as loud as the db750.

Anyway. What kind of wattage in a rmx to match the aggie?

koobie · #4 08-24-2010, 07:35 PM

Were you using just the power amp section of the DB750 when you made your comparison? Or as a complete amp head?

Zooberwerx · #5 08-24-2010, 07:42 PM

Quote:

Originally Posted by BiigM

Hi.

No the xti is long gone. And i had the power amp turned up to 100%. And it was clipping! And was not as loud as the db750.

Anyway. What kind of wattage in a rmx to match the aggie?

An RMX 1450 will give you ~850 watts / bridged mono / 8 ohms. I use either that or an RMX 2450. Both are great values on the used market.

Sidenote: why do you have two "i"s in your name?

Riis

dmusic148 · #6 08-24-2010, 07:43 PM

Quote:

Originally Posted by koobie

Were you using just the power amp section of the DB750 when you made your comparison? Or as a complete amp head?

He already said...

Quote:

Originally Posted by BiigM

I've been running it into the poweramp in on my Aggie DB750, ...

**xed** · #7 08-24-2010, 07:49 PM

I use a RMX 1850HD and I like it just fine. It's not the lightest thing in the world but I put it in it's own 2 space SKB rack so it can be carried solo.

Zooberwerx · #8 08-24-2010, 07:52 PM

This link will allow you to compare and contrast the various RMX offerings:

http://www.qscaudio.com/products/amps/rmx/rmx.htm

Riis

rpsands · #9 08-24-2010, 08:38 PM

There is absolutely no way that you were clipping a bridged XTI 2000 into a cabinet and anything was louder than it. You could easily push all four of your cabinets listed in signature to the maximum volume they can attain with that much wattage, much less just an NV412.

It is very distantly possible that some compression is built into the power section of the DB750 that makes it seem louder, but you could achieve the same thing by using a compressor if that is the case.

Bottom line: If a clipped XTI2000 bridged is not getting you enough volume, there is NO power amp that will get you more. None.

The other possibility is that you were failing to connect it with a bridge cable (1+/2+ instead of 1+/1-) or a banana adapter. Unfortunately very few power amps offer an easy bridge mode hookup, and most require special speakon wiring or a banana cable adapter.

BiigM · #10 08-24-2010, 11:20 PM

Hi.

As far as I remember the XTi is bridged electronically via the control panel. Anyway. It was bridged just fine., I'm 100% sure on that.

And wether it's theoretically possible or not to clip it I don't know. All I can say that giving the B string on a fiver a wack made it light up as a cristmastree. Up top it was fine, but in the deep frequencies it had problems. By the way, searching the forum shows that I'm not the only one with the same feelings about the lightweight poweramp offerings.

Thanks for the link. I actually already checked it. It's just that IMO specs are just specs. Real world performance is something different.

And about the two I's in my name. I use the same nick different places. And Usually BigM is taken, but BiigM is not :-) That simple. It's not that I'm so big I really need two I's, hehe

Doug Parent · #11 08-25-2010, 12:15 AM

I have a RMX QSC 2450 on the classifieds and it is ridiculous. QSC lead sled slams like a bad boy and just laughs at you.

BiigM · #12 08-25-2010, 01:05 AM

I think it's gonna be either the rmx 2450 or the 1850HD.

The 1850HD is more expensive but i guess they are built better or something. It might also be better suited wattage wise?

Maybe someone who knows the amps well could fill me in on what the differences between the HD and the normal rmx series are?

I do wish that I could trust the PLX series to be as solid in the low Freq, but after my XTi experience I don't wanna risk it.

Zooberwerx · #13 08-25-2010, 04:44 AM

Quote:

Originally Posted by BiigM

I think it's gonna be either the rmx 2450 or the 1850HD.

The 1850HD is more expensive but i guess they are built better or something. It might also be better suited wattage wise?

Maybe someone who knows the amps well could fill me in on what the differences between the HD and the normal rmx series are?

I do wish that I could trust the PLX series to be as solid in the low Freq, but after my XTi experience I don't wanna risk it.

Excellent choices. You may want to PM our friend Bob Lee (QSC)....I'm sure he can offer some addt'l insight.

Riis

BiigM · #14 08-25-2010, 05:13 AM

Hi.

I actually already pm'ed Mr. Lee :-)

But in the meantime, I have an offer on a used Crest CA9 from the uk. It will cost me slightly less than an RMX2450.

People seem to think those are the perfect choice, so I'm leaning towards that right now.

Zooberwerx · #15 08-25-2010, 05:49 AM

Quote:

Originally Posted by BiigM

Hi.

I actually already pm'ed Mr. Lee :-)

But in the meantime, I have an offer on a used Crest CA9 from the uk. It will cost me slightly less than an RMX2450.

People seem to think those are the perfect choice, so I'm leaning towards that right now.

CA-9? You're serious about this power issue! Again, another great choice. I have the CA-6 in my back-up rig. My only gripe is that it only offers binding post / banana plug speaker outs...no Speakons. I understand this may vary model-to-model.

Do you own a handtruck?

Riis

zombywoof5050 · #16 08-25-2010, 07:26 AM

Quote:

Originally Posted by BiigM

But in the meantime, I have an offer on a used Crest CA9 from the uk. It will cost me slightly less than an RMX2450.

The CA9 has an input sensitivity of 0.775V (unless it's been changed internally by a previous owner) compared to the RMX2450's 1.23V, and this will make it easier to drive the CA9 to full power.

rpsands · #17 08-25-2010, 09:34 AM

No, the XTI is not bridged electronically through the control panel. You do have to tell it to use bridged mode, but according to the manual you still need a special speakon cable wired 1+/2+.

Either that or you need to use the banana plugs with an adapter to 1/4" or speakon.

You may want to check with Crown support to be sure but it sounds like you were comparing one channel of the Crown to the DB750.

BiigM · #18 08-25-2010, 10:57 AM

Quote:

Originally Posted by rpsands

No, the XTI is not bridged electronically through the control panel. You do have to tell it to use bridged mode, but according to the manual you still need a special speakon cable wired 1+/2+.

Either that or you need to use the banana plugs with an adapter to 1/4" or speakon.

You may want to check with Crown support to be sure but it sounds like you were comparing one channel of the Crown to the DB750.

woops. I just checked and I see you are right. Not a fair comparison then.

But now I'm even more confused as I was pretty sure before, that I need a conventional amp.

Any thoughts would be appreciated.

Thx

KJung · #19 08-25-2010, 11:00 AM

Quote:

Originally Posted by BiigM

Hi guys.

I'm thinking of getting a poweramp for my Demeter vtb-201s.

I've been running it into the poweramp in on my Aggie DB750, and that's loud enough for me.

I was thinking about a QSC RMX amp. I had a crown XTI, but I didn't like it. It didn't seem to deliver nearly as well as the db750 poweramp in the low frequencies, even though the specs had a way higher wattage. more the 2000W bridged!!

Personally I think it's because of the switchmode powrsupplys. Hence the reason for wanting the RMX instead.

But what kind of wattage should i go for, for it to be on the same level as the Aggie? The aggie does seem to have amazing headroom for a 750W amp.

Thx :-)

These days, power supply topology has little to do with anything. It's all about gain matching, and also the way the hi pass filtering is set. Wattage (once you get above 500 watts or so into 4ohms) is also somewhat secondary to voicing, gain matching, and cabinet voicing/SPL.

You never want to use a 'PA power amp' with bass that doesn't have some sort of hi pass filtering IMO.

BiigM · #20 08-25-2010, 11:13 AM

Hi Ken.

Based on those things, what would you suggest? I know that I like the voicing of the demeter vtb-201s into my NV412.

But I need as much kick as the power in my DB750 gives me. If there's any difference in sound between heavy and not heavy i'll pick heavy.

I'm not the kind of player that can get by with a markbass lightweight amp :-) I have the aggie sc500 too and that one is not powerful enough for all my needs.