Article: How to choose a power amp

[Mark Reccord]

Originally Posted by Oobly

A big, heavy, metal one! Everyone knows that. The heavier the metal, the heavier the amp. Of course the attenuators need to go to at least 11. And it's fun to watch the roadies trying to carry the amp, too...

The old QSC mx1500s had attenuators that went to 11. I always assumed it was a Spinal Tap reference.

[ERIC31]

That is a great article! Thank you for writing that. It filled in some gaps in my knowledge and blew up some lies that people here at TB have spouted as gospel truth. LOL!


This should be a sticky for sure!!!

[1958Bassman]

Originally Posted by jnewmark

As usual, Bongo, great stuff. Here's a quote I'm curious about:

This is because the volume knobs turn down the level of the signal going into the actual power gain stage, but they do not limit the output of that stage. That output stage is always at its full potential, ready to multiply whatever you feed it, up to its peak ability. Turning the knob down 50% just means you turned the preamp signal down by some amount before it gets amplified

So, if I understand this correctly, in order to get the strongest preamp signal to the power amp, the volume or attenuator controls on the power amp should be turned all the way up ? There is always a debate about where these controls should be set here on TB, but this quote seems to indicate that they should be turned up to get the best signal.

Look at it this way- the power amp section takes a signal and amplifies it by a certain, fixed amount. This is called a 'differential' and is why amplifier sections are sometimes referred to as 'differential amplifiers'. There's nothing anyone can do to change this amount of gain. The input signal, large or small, it increased by that amount. The amplifier's differential is designed for typical input voltage levels, so the range of the volume control works within this parameter. If the minimum voltage that causes rated output (reached by turning the input level control "up" is .5VAC, sending more will cause clipping. If the minimum input voltage that will cause rated output is 3VAC, sending 1VAC will never cause clipping, but it also won't result in full rated output, either.

Stereo equipment is designed the same way and the volume control often has -0dB at the highest setting and - infinity at the lowest. The volume control is usually called the 'attenuator' and they do this because adding gain in the power amp AND several stages before (preamp, tone control section and buffer) will cause unacceptable noise.

[jnewmark]

This is kinda OT, but does anyone know if an Ipod can drive a power amp ?

[bongomania]

I'll test it out this weekend and get back to you.

[jnewmark]

Originally Posted by bongomania

I'll test it out this weekend and get back to you.

Thanks. FYI, it would be a Peavey IPR 1600, with an input sensitivity of .775.

[tekdiver500ft]

I didn't read all the comments here, so it may have been covered already. Overall, I thought it was a great article. I did find a couple minor grammar errors and one technical "error." I use quotes because there is some debate about it, but I believe that I have solid science behind me. While you can't harm a speaker by providing it with less power than it can handle, you can easily blow it by using an amp that is rated for less power (even significantly less) than the speaker/cab.

[tekdiver500ft]

Originally Posted by jnewmark

As usual, Bongo, great stuff. Here's a quote I'm curious about:

This is because the volume knobs turn down the level of the signal going into the actual power gain stage, but they do not limit the output of that stage. That output stage is always at its full potential, ready to multiply whatever you feed it, up to its peak ability. Turning the knob down 50% just means you turned the preamp signal down by some amount before it gets amplified

So, if I understand this correctly, in order to get the strongest preamp signal to the power amp, the volume or attenuator controls on the power amp should be turned all the way up ? There is always a debate about where these controls should be set here on TB, but this quote seems to indicate that they should be turned up to get the best signal.

Sort of. In order to get the best sound quality from a PA or mixer, you want everything prior to the power stage to be at unity gain. So, what I do is turn the input attenuators all the way down, set all channel gain to the appropriate level (as hot as possible with no clipping/feedback), then run all channel strips, subgroups, and the master up to unity. At this point, I turn the attenuators up until I am just slightly louder than my max for the evening. This gives me the best sound the rig can provide, IME. If running a pre/power combo just for bass, then I leave the attenuators wide open.

[bongomania]

Originally Posted by tekdiver500ft

I did find a couple minor grammar errors

Please point them out! If they are questions of colloquial usage, sometimes I intentionally choose the manner less correct but more commonly understood; but otherwise I do want correct grammar.

Originally Posted by tekdiver500ft

...and one technical "error." I use quotes because there is some debate about it, but I believe that I have solid science behind me. While you can't harm a speaker by providing it with less power than it can handle, you can easily blow it by using an amp that is rated for less power (even significantly less) than the speaker/cab.

OK, are you taking the "clipping blows speakers" stance? If so, then NO. That has been proven false, and no science supports it.

OTOH if you are talking about how an amp driven into hard clipping can produce higher than its rated wattage, that's true; however the problem is in trying to make that idea useful and usable to a "layman" reader trying to choose a power amp. If you have a clever way of explaining it that makes it easier to understand what to buy, then I'm all ears.

[tekdiver500ft]

Second paragraph, "independent" should be "independently," as it's an adverb. There were one or two others (none were about colloquialisms), but I don't recall what they were at the moment. I'll read it again when I have a few minutes, though.

I'm talking about an amp driven into heavy clipping producing more than rated power. I'll think about an easy way to say that.

[bongomania]

Fixed the "independently", thanks. The thing about the power rating is that the only way I see to present the point as purchasing advice is to say that the power amp should always be rated for less than half the wattage ratings of the cabs. And even though that is one valid approach, it seems likely most readers would scoff at it as extreme and unnecessary. I'm not denying the validity, just looking for an effective dosage.

[tekdiver500ft]

My advice has always been to purchase them both matched (or have the amp rated at more watts than the cab), but never drive them into clipping. I submit that if you use your ears, the actual combination doesn't matter - you'll never damage it.

[cosmonautsix]

Pros vs. cons of using a pedal pre-amp (tone hammer or BDDI) and external power amp vs. head/cab or combo? I'm seriously thinking of just picking up a poweramp off CL to drive a 4x10 cab...

[JimmyM]

Originally Posted by tekdiver500ft

My advice has always been to purchase them both matched (or have the amp rated at more watts than the cab), but never drive them into clipping. I submit that if you use your ears, the actual combination doesn't matter - you'll never damage it.

Kind of rules out cranking tube amps, but I guess not many bassists use tube power amps these days, though I do know a couple guitarists who do. Metalheads, of course.

[1958Bassman]

Originally Posted by jnewmark

This is kinda OT, but does anyone know if an Ipod can drive a power amp ?

It can, but using the headphone jack won't give full output because it's really designed to drive headphones, not a line input.

[jnewmark]

Originally Posted by 1958Bassman

It can, but using the headphone jack won't give full output because it's really designed to drive headphones, not a line input.

The Ipod dock has a direct out. Would that make a difference ?

[bongomania]

I don't know about the dock's direct out, but the average output of the ipod itself is about 1 v. So it should be able to drive a power amp with an input sensitivity of 0.77 v just fine.

[tekdiver500ft]

We run an ipod through our board at unity gain throughout with no makeup gain, so running one direct should be fine.

[jnewmark]

Originally Posted by bongomania

I don't know about the dock's direct out, but the average output of the ipod itself is about 1 v. So it should be able to drive a power amp with an input sensitivity of 0.77 v just fine.

That input sensitivity is at 4 ohms, though.

[bongomania]

That's the speaker impedance, not the output impedance of the device going into the amp.

Now, it is true that whatever the output impedance of the ipod IS, it will affect how well the amp is driven. Voltage isn't the only factor. But all you need is for the input impedance of the power amp to be at least 10x the output impedance of the ipod, which is a good possibility.