Author Topic: Amplifier gain (Read 104 times)

Bradley Young · « **on:** December 26, 2007, 03:13:54 PM »

I have a QSC PLX 3402 power amp, and two Bag End S15D cabinets (one is an X-D, but that should be immaterial).

The S15D cabinets are 8 Ohm, and I'm feeding each of them from a separate channel of the amplifier (i.e. the amp is seeing 8 ohms per side). These cabinets are rated at 200 Watts.

The PLX has 40x (32 dB) gain per channel.

My questions are:

1) What is the maximum safe setting for the gain knobs (I've always heard to put 2x rated speaker capacity, i.e. a 200 Watt cabinet should be powered by 400 Watt amp)?
2) What would be the setting to limit output to around 200 Watts per channel?
3) Any comments on the 2x rule appreciated; where is this from?

Bradley

keith_h · « **Reply #1 on:** December 26, 2007, 04:29:56 PM »

Here is a recent question from the QSC site about gain knob setting. In a nutshell it all depends upon how much voltage the input is being driven with.

The 2X rule is continuous or program power is roughly 2X RMS. Basically you spec to amp to the program power.

Keith

David Houck · « **Reply #2 on:** December 26, 2007, 05:22:57 PM »

The 3402 is 700 watts into 8 ohms per channel.

Bradley, read this thread from four years ago this week. It might not help, but, in its own way, it's entertaining, and possibly even thought provoking.

[edit: I was on the phone for over an hour and didn't see Keith's post.]

(Message edited by davehouck on December 26, 2007)

briant · « **Reply #3 on:** December 26, 2007, 06:42:02 PM »

That thread that Dave linked above should answer your questions.

I'm curious why you have such an obscenely huge amp for driving those (or any) bass cabs. Did you use it previously in a PA system or something? Or get some sort of smokin' good deal on it? Here I thought my Stewart World 1.6 was overkill.

Bradley Young · « **Reply #4 on:** December 26, 2007, 11:13:23 PM »

I did get a killer deal on the power amp.

I'm beginning to understand the gain knobs, so my next question is: what is the output voltage for an f1x? I'd rather run that hard (to release the inner tubey goodness) and cut the input gain at the amp.

jazzyvee · « **Reply #5 on:** December 27, 2007, 12:37:12 AM »

byoung, i got a great deal on the same QSC amp last year and I'm driving 3 Mesa boogie cabs with it. I'll be reading this thread with interest as my desire is the opposite to you I want the cleanest sound possible and the best way to get it at any volume. So knowing where things start to get all tube would be good to know.
Jazzyvee

811952 · « **Reply #6 on:** December 27, 2007, 07:15:46 AM »

I've only ever blown speakers when I've run less power than I should have. I'm much more comfortable running gobs more power than I need, and listening carefully for any hint of distortion resultant from the speakers reaching the end of their travel. Really, if you are reasonable about your volume and aren't trying to fill the room from your bass amp, then the extra power will get you lots of clean for your sound that isn't available from a smaller amp. A bigger amp won't make your cabinet louder, but it will make it cleaner, in my experience.

John

edwin · « **Reply #7 on:** December 27, 2007, 02:30:25 PM »

Running the F1X won't release any tubey goodness as the tube is used for initial gain and buffer purposes in this design. The rest of the gain is all solid state. For an all tube signal path, check the F2B, but even that can put out a huge amount of clean gain.

It's been discussed around here before but just because something has a tube in it, doesn't mean that it's not going to be clean. In fact, clean tube gain sounds better to me than clean solid state gain.

I would run the amplifier wide open and then use the F1X to give you the volume level you want. I think this has the best possibility of keeping the noise level low.

Just my 02c, Happy New Year all!
Edwin