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Article: How to choose a power amp

Discussion in 'Amps and Cabs [BG]' started by bongomania, Feb 3, 2013.

  1. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    We get posts nearly every day asking which power amp to buy, which is the "best", whether to buy one model versus another, etc. So I wrote this article as a way of helping beginners (and maybe some experienced players too) figure out which power amp will be a good purchase.


    Again it is mainly written with the non-expert in mind, so I have kept some things simplified. If you see anything in there that could be explained more simply or elegantly (while remaining factual), please do let me know, and offer suggestions! My aim is to make it all more understandable.

    I'll also welcome technical corrections, if you see any--but be advised that I am skeptical about what some people will consider "corrections". ;) You know how we argue here in Amps.

  2. Floyd Eye

    Floyd Eye Banned

    Feb 21, 2010
    St. Louis
    As is the case with your compressor reviews, this is an invaluable resource. Great job my friend.
  3. jastacey


    Feb 8, 2009
    Good read ... enjoyed it
  4. nolezmaj

    nolezmaj Supporting Member

    Sep 22, 2011
    Great article, enjoyed reading it as much as the rest of your site.
  5. Jim C

    Jim C Spector#496:More curves than Sophia + better sound Supporting Member

    Nov 29, 2008
    Another nice piece Cyrus; well done.
    Great that you mention input sensitivity as it is rarely discussed but important in a MI application.
  6. Supertanker

    Supertanker Watch the dog! He is trained to bite!

    Jun 23, 2005
    Good you kept it simple, because when you boil it down it is simple (as it should be).
    The average bass player does not have an EE degree.
    Fortunately in this day and age it is not required thanks to the excellent power amps and cabs being produced today.

    Good job!
  7. Interceptor

    Interceptor Supporting Member

    Mar 29, 2005
    Madison, WI
    Nice work!

    I really liked the way you suggested that reliability is the spec to spend one's research on.

    I could argue that the way to look at PA amps these days is to talk to your favorite retailer, and ask them which model that satisfies your (the purchaser) goals for price, weight and power that has fewest returns. Buy that amp.
  8. 1958Bassman


    Oct 20, 2007
    One thing I would mention about highly sensitive input sections regards the noise level and distortion that can result from using a noisy preamp/long effects chain with several gain and level controls- as long as everything is at unity gain without strong emphasis in the upper-mid/high frequencies, noise and distortion won't be a problem. Needing a higher input level allows full power output but lower noise when low signal levels are present. This is the reason car audio uses 2V+ for the head unit's line output, sometimes 4Volts- the input sensitivity controls can be set to maximize signal to noise ratio and in a 500W-1000W+ car audio system, this is extremely important because of the small volume of space in a vehicle but in a bass/PA rig, there's not as much need to have extremely high S/N ratio, but nobody wants to hear this noise coming through the PA (regardless of whether the signal comes in as DI or is mic'd.

    The distortion issue, OTOH, is a bog problem, if all of the "I blew my speakers" threads is any indication. If there's no provision for defeating an effect pedal or rack unit, the user will usually end up adding gain to the signal when it's already capable of causing the signal to clip once it gets to the power amp because, you know, "louder is better", right? I don't know how many times I have seen graphic equalizers with all of the bands shoved to the maximum point and many of the users didn't have any idea why they blew speakers or heard a wash of noise coming from the system. They often thought of the equalizer as a "power booster", which it clearly is not, nor was it ever designed to be.

    I would like to see green and red LEDs on every piece of rack equipment, for showing that the input level is at the maximum for clean input. I have seen input level indications on some equipment, but I think it would be helpful if they came up with a way to know without needing to use test equipment and having a degree. I think this would eliminate a lot of damaged equipment, blown gigs, disappointed crowds & club owners and frustrated musicians. Sales and repairs would lag, but what's really the goal, here?
  9. Baird6869

    Baird6869 RIP Gord Downey. A True Canadian Icon. Supporting Member

    Great job! Well written. This is sticky-worthy.
  10. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    Another good one, BM. This along with the info in a couple of the other articles should set most people on the right path. I always laugh at the 'heavier sounds better' thing. Especially in 'the field'. I've been using a 7lb power amp for almost 2 years and sharing my rig liberally. At no point has anyone after playing it 'blind' said anything about it not having enough horsepower or 'weight' to the tone. And usually I get comments about how great it is to play a big 'lead sled' power amp. And I just smile and nod.
  11. jnewmark

    jnewmark Just wanna play the groove. Supporting Member

    Aug 31, 2006
    Stax 1966
    Play guitar.
    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.
  12. Primakurtz

    Primakurtz Registered Nihilist Supporting Member

    Nov 23, 2011
    Denver, Colorado
    Well done! Many thanks.
  13. rpsands


    Jul 6, 2007
    Phoenix, AZ
    re: but they do not limit the output of that stage

    The input stages on power amps will overdrive, absolutely, and so the attenuator will functionally limit the output -- the maximum the input stage can take combined with the level of the attenuator is the maximum amount of voltage that can hit the power amp and be multiplied.

    * Check the manual on a QSC PLX 1804 for example, and you'll find this spec:
    Input Clipping 5.5 Vrms (+15 dBu)

    That means once your input signal gets over 5.5VRMS or so you're clipping the input stage and will wind up being hard limited at some point thereafter.
  14. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    This is the same debate as on the pre/power of an integrated bass amp. It depends on what you're trying to do. I never run my power amp full out because I want to lessen the chance that the amp will send through dangerous transients to my speakers, or that the signal will clip the input of the power amp.

    On the other side, I like to run my preamp levels fairly hot, but without the chance of clipping either the input of the preamp nor the output stage of the preamp. So it becomes a balancing act. If you get a preamp that's 'clean' to start with, you really don't have to worry about a noise floor either way.
  15. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    That would be a reasonable interpretation except for two things:
    1) The attenuators are located after the input buffer of the power amp, so the question of whether the signal-noise ratio, or levels in general, are ideal has to be dealt with before the input jacks of the power amp (before its input buffer stage).
    2) There is no master volume knob, the only controls are the gain knobs, so it is like an old guitar amp. If the best tone comes only at the highest volume setting, then the usefulness of the amp is limited to high-volume gigs. So it's not constructive to treat it in those terms. Better to match up the preamp output with the known sensitivity, usually having the preamp output at (or near) maximum, and then use only however much power amp gain is needed for volume.
    Actually the attenuators have no control over that, because again they are actually after the input stage of the power amp. As you say, it is a balancing act--sending as hot of a preamp signal as you can, without clipping that input stage, and the master volume knob of the preamp is the tool for controlling that.
  16. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    That's a very misleading argument, because the gain knobs have nothing directly to do with that limitation. Yes, if you drive the amp at maximum gain you will run out of headroom, and hit a limit of how much wattage is possible to produce with that amp. But turning down the gain knobs doesn't control that limit in a direct linear (or even logarithmic) way. For every increment you turn down the gain knob, you could just turn up the master volume of the preamp, or send a stronger signal into the preamp. You could argue that once the input stage of the power amp is driven into hard clipping, and cannot increase any further, then sending a stronger input would not compensate for turning down the attenuators; but that would be such an outlandishly unrealistic and inappropriate use of the power amp as to render the argument nonsensical. In other words, even though the gain knobs have a definite impact on the output levels, they do not limit the output in any practical sense.
  17. so whats the best power amp for metal?
  18. Oobly


    Jun 19, 2008
    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...

    Nice article, bongomania! Well written and should make it easier for many to decide. Wish I had read something like this before I bought mine, ended up with a low sensitivity power amp and a lowish output pre... had to make a booster.
  19. Scarred


    Feb 7, 2013
    I was wondering if the tubes do play a role. Since I'm a big fan of the sound of the Ampeg Svt-vr withe the 6550 tubes but I wanted to replace it with a power amp that I could place into a rack and runs in combination with my sansamp rbi (who I use currently bypassing the ampeg preamp)
  20. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    The tubes DO play a role, as does the output transformer of a typical tube power section. However it's very hard to make any general statements about those roles, because each tube amp is a little different. Where solid state PA-type power amps are designed with transparency as a goal, tube power amps are designed with a broad range of goals, from high fidelity to heavily colored distortion, and everything in between. Also the ones that are designed for more tone color often change their sound in dynamic response to the incoming signal peaks--but again, how much they do that will differ from one amp to the next.

    I should write a paragraph about tube power amps on that page, but I'm not sure what to say that's helpful and not wishy-washy.

    In your particular case, I'd say just experiment. You might love the sound with a regular power amp, or you might find the Ampeg power section was the key to your tone. It's up to your ears only.