Power Amp/ Pre Amp

[Bob Lee (QSC)]

From what my buddy told me this amp puts out about 1350 watts bridged mono 2 ohms

No, the minimum load in bridged mono is 4 ohms. The Crown MicroTech amps can be run, as thepontif said, with the amp channels in parallel. This is more than just paralleling the inputs, as most amps do. The outputs also are paralleled (you must connect a wire jumper between the red binding post terminals) to double the current capacity over that of a single channel. This would allow you to drive a 2Ω load, which would amount to a 4Ω load on each channel.

In parallel with this amp, don't run it without the jumper, and don't use a jumper like this on any amp that isn't designed to operate that way.

 

[jokerjkny]

Is it really necessary to bother lugging around a 41-pound power amp just for 850 watts?

You can get close to that from some bass heads. Heck, your Tour 450 puts out half that much. The difference in volume from 2x the power is quite small.

If, of course, you're doing this because it seems cool, then by all means go ahead. At some point the lifting and lugging will get old.

i gotta be honest,

as much as i love my epifani 902 head's 18 lbs. weight, i've been going modular these days, and am currently crushing on my demeter hbp-1 / qsc1804 setup. at first, i wanted to use it primarily for my "louder than loud" setup, but have been diggin the tones i'm getting at home. rehearsal here we come!

 

[jokerjkny]

Definitely do a Talkbass search, as your question has been asked hundreds of times on Talkbass before. (...)

+1

course, this question has to be asked at least once a month to make sure we're all on our toes.

 

[HugoM]

. . . my buddy told me this amp puts out about 1350 watts bridged mono 2 ohms . . .

Bob Lee is correct: in bridge-mono mode, DO NOT go below 4 ohms.

The manual can be found here:
Invalid Link Removed

 

[xinsaneguitarzt]

maybe i am reading wrong, but you said you used the DI from your peavy to the input on the poweramp?

well, there is your preamp. all the DI does it send the same line signal that goes to the internal power amp of the peavy

i am actually running this exact setup (kinda), a peavy tmax head DI to a crown xti 1000

sounds blissful, until you turn the power up

Yea i understand that but im trying to build a whole nother rig. i want my peavey rig that i have now for small-meduim veniues then the crown amp and preamp for large venues and outdoor functions.

 

[xinsaneguitarzt]

No, the minimum load in bridged mono is 4 ohms. The Crown MicroTech amps can be run, as thepontif said, with the amp channels in parallel. This is more than just paralleling the inputs, as most amps do. The outputs also are paralleled (you must connect a wire jumper between the red binding post terminals) to double the current capacity over that of a single channel. This would allow you to drive a 2Ω load, which would amount to a 4Ω load on each channel.

In parallel with this amp, don't run it without the jumper, and don't use a jumper like this on any amp that isn't designed to operate that way.

So if i bridge this am keep the load 4+ ohms

 

[Bob Lee (QSC)]

So if i bridge this am keep the load 4+ ohms

Right, 4Ω or higher.

 

[thepontif]

Bob,
One thing I'm trying to wrap my head around in terms of amplifier power is the whole Voltage gain vs. power gain thing. If I understand correctly, bridging a stereo amplifier into a particular load (series bridging) will try to quadruple the power into the load, right? I'm basing this on the formula Vsquared/R=Power. So if 50V into a 4ohm load is 625 watts, then 100V into a 4 ohm load is 2500 watts. So does the clipping/danger come when the amplifier can't meet the current requirement Justice Ohm is demanding based on the voltage across the load?

 

[Bob Lee (QSC)]

All a power amp (a voltage source) provides is voltage gain. Power is totally dependent on the voltage applied and the load impedance (for example, if you have an amp producing signal but with the load disconnected, you will see voltage gain but absolutely no power delivered by the output). If you take a certain load impedance and double the voltage applied across it, you do quadruple the power, and that's what happens when you bridge an amp.

When an amp clips, it's almost always because it's run out of voltage; rarely is it because of current except in the case of protective current limiting. Power supplies, though, have internal impedances due to transformer windings, rectifiers, resistance of copper wires and traces, etc., and heavy current demand will thus cause a corresponding voltage drop in those impedances, so the supply rail voltages sag. And that means the amp will clip--or, run out of voltage--at a lower output voltage when driving low load impedances (which draw more current at a given output voltage) than with higher impedances.

Power supplies with lower internal impedances exhibit less loss and less voltage sag, and conversely those with higher internal Z exhibit greater loss and sag. This phenomenon may be depicted in a spec called "dynamic headroom." An amp with a really high dynamic headroom spec is telling you in a gentle way that it has a lossy and maybe deficient power supply.

 

[thepontif]

So then would it be safe to say that the output impedance of a bridged stereo amp twice that of one of the channels? And also that, practically speaking, 100V from a bridge configuration into 4 ohms behaves more like 2 50V sources into 2 ohms each than a single 100V source into 4 ohms?

 

[Rick Auricchio]

...i've been going modular these days, and am currently crushing on my demeter hbp-1 / qsc1804 setup.

Yes, but your QSC weighs only 13 pounds, not 41 like the OP's power amp. That's a huge difference.

 

[gerryjazzman]

No, the minimum load in bridged mono is 4 ohms. The Crown MicroTech amps can be run, as thepontif said, with the amp channels in parallel. This is more than just paralleling the inputs, as most amps do. The outputs also are paralleled (you must connect a wire jumper between the red binding post terminals) to double the current capacity over that of a single channel. This would allow you to drive a 2Ω load, which would amount to a 4Ω load on each channel.

In parallel with this amp, don't run it without the jumper, and don't use a jumper like this on any amp that isn't designed to operate that way.

Don't want to get into a techfest here, but wow, I didn't think any commercial amps could do this, Bob. Don't the gains between the two channels have to be matched EXTREMELY closely so that you don't have circulating current between the two outputs that could destroy the output devices? I've seen this done with some of the chipamps but they use 0.1% gain setting resistors to accomplish this.

 

[Bob Lee (QSC)]

So then would it be safe to say that the output impedance of a bridged stereo amp twice that of one of the channels? And also that, practically speaking, 100V from a bridge configuration into 4 ohms behaves more like 2 50V sources into 2 ohms each than a single 100V source into 4 ohms?

If the output impedance of a single amp channel is some milliohms, then the output impedance of two channels bridged is 2× some milliohms. Both are not worth much consideration.

A 100V source into 4 ohms, versus two 50V sources in series into two 2-ohm loads in series, is really no difference at all.

 

[Bob Lee (QSC)]

Don't want to get into a techfest here, but wow, I didn't think any commercial amps could do this, Bob. Don't the gains between the two channels have to be matched EXTREMELY closely so that you don't have circulating current between the two outputs that could destroy the output devices? I've seen this done with some of the chipamps but they use 0.1% gain setting resistors to accomplish this.

In the Crown amps with that type of parallel operation, the mode switch disables the feedback loop on one channel, and the jumper between the output terminals ties the two channel outputs together. They end up sharing one feedback loop, so their gains are regulated to match. That's why you must use a jumper across the outputs if you use one of those amps in parallel. Without it, the channel whose feedback loop is open may do weird things.

I may be wrong, but I don't think any recent Crown amps have this feature any more. As pro amps have become more accessible downmarket, a feature like this would probably become very expensive to support (more technical assistance, warranty claims, et al) when the amps are in the hands of less knowledgeable users.

 

[RONQUITO]

I´m suscribing to this thread, lotsa tech info that´s priceless.

TB is the best.

 

[sleepy_monkey]

There are several in the TB classifieds right now. Ampeg SVP-CL (I own one and love it) and also Bongomania is selling a preamp that was made for Bootsy Collins...I think it is $400.

 

[thepontif]

If the output impedance of a single amp channel is some milliohms, then the output impedance of two channels bridged is 2× some milliohms. Both are not worth much consideration.

A 100V source into 4 ohms, versus two 50V sources in series into two 2-ohm loads in series, is really no difference at all.

Got it. I see where I screwed my math up.