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Amp Cluebag

Discussion in 'Amps and Cabs [BG]' started by ZuluFunk, Jul 14, 2001.

  1. ZuluFunk

    ZuluFunk Supporting Member

    Apr 14, 2001
    I don't usually read the Amp stuff here. I couldn't find a thread though I'm sure one's out there. This type of question comes up from newbys all the time under Basses..."What should I buy?".

    I've been playing since 1981 but I know squat about amps. I've never needed anything for semi-serious gigging...until now. I always would have a decent combo for playing in my room. I had a Peavey, Crate, and now a Hartke Kickback 12". I got them for the convenience. I usually just played at home anyway. Whenever I played out, it was often on a house rig.

    Now I need to get a rig to fill medium sized rooms (bars & small clubs). I have an Ampeg that a buddy said he will loan me when I need it until I get something. I haven't tried it yet. I also use the rig at the studio which has 2 Hartke 4-by cabs and an SWR? head. The question is what to get and how much to pay.

    What's a good source for learning about what to consider for specifications (watts, ohms, blah blah blah)?

    Are there good combos out there that I can use and still get a big variable sound? I tried an Ampeg digital cube and one tube job. Problem is, they may sound great in the shop, but I don't know how they'll sound when I'm trying to play over drums, guitar, singer and horn section on a gig.

    Thanks for your patience.
  2. ZuluFunk

    ZuluFunk Supporting Member

    Apr 14, 2001
    For example, what do these terms mean and what do I need to know about them in terms of utility?

    Input Impedance: 1M Ohm
    Nominal Input Level: -20dBm
    Optimal Output Load: 4 Ohms
    Power Output: 600 watts RMS @ 4 Ohms (minimum load),
    400 watts RMS @ 8 Ohms
    Maximum Power Consumption: approx. 1KVA Forced-air cooled
    AC Input Power: 100V, 117V, 230V, 240V
    Cycles: 50/60 Hz

    Are there other specifications I need to look for?
  3. Chasarms

    Chasarms Casual Observer

    May 24, 2001
    Bettendorf, IA USA
    Input Impedance: 1M Ohm - This the maximum impedance that the preamp will recocnize. Not worth worrying over unless you have some sort of piezo element pickup or something else weird. Normal bass, this isn't really that big of a deal.

    Nominal Input Level: -20dBm - This deals with the output of your bass. Passive magnetic PUs are usually in the this range. Active Pickups have more output. This isn't that big of a deal unless you have a really weak passive bass or a very hot active bass. Most amps have a pad for a really hot bass. If your input is too hot, you'll overdrive the preamp. (Distortion)

    Optimal Output Load: 4 Ohms - This is ohm load that the offers the best power output for the amp. If you are using only one speaker cabinet, a four ohm is better. If you are going to use two, you need two eight ohm cabs.

    Power Output: 600 watts RMS @ 4 Ohms (minimum load), 400 watts RMS @ 8 Ohms - Just what it says. Use a 4 ohm cab, it'll push 600 watts, 8 ohms get you 400. You can get 4 ohms with one 4 ohm cabinet or two 8 ohm speaker cabinets.

    Maximum Power Consumption: approx. 1KVA - this is the amount of current it pulls from the wall.

    Forced-air cooled - It has a fan to cool the amp.

    AC Input Power: 100V, 117V, 230V, 240V
    Cycles: 50/60 Hz - This just means it works with various A/C power from different countries.

    Are there other specifications I need to look for?

    The only real specs that matter are output and minimun ohm load. Those really relate the performance.

    You could look at THD, but the almost all SS amps list harmonic distortion at .1

    s/n, signal to noise ratio gives you some idea of how noisy the amp is.

    Dynamic range is important.

    I think you see that most amps spec out the very similar.

    Features are more important.


    How are the EQ frequencies grouped and how flexible is it?

    Compressor, gate other tone shaping features?

    Overall quality.

  4. Great thread Zulu,you are asking all the questions I need answered too.Hope you don`t mind me asking one also?

    Chasarms:I recently got a new Ampeg B2R head and will be getting a cab soon(most likely a Carvin 2x10).I have to start out with only one cab but in the future I want to move up to 2 or more,what ohms rating should I get? 8,4?I don`t want to overdrive my amp or anything.

  5. captainpabst


    Mar 18, 2001
    usul - the carvin 2x10 is rated at 4ohms. check the minimum output impedance of the b2r - if it's 4 ohms, using an additional cabinet (besides the carvin 2x10) will cause the load to be 2 or 2.67 ohms, and this could damage the amplifier. it works like this:

    one 8 ohm cab = 8 ohm load
    two 8 ohm cabs = 4 ohm load
    one 4 ohm cab = 4 ohm load
    two 4 ohm cabs = 2 ohm load
    one 8 ohm + one 4 ohm cab = 2.67 ohm load

    you probably want an 8 ohm cab to leave room for future expandability.
  6. captainpabst


    Mar 18, 2001
    The question is what to get and how much to pay.

    that is most definitely everybody's question! if you gave a $ range and an idea of the sound you wanted, we could make suggestions based on that.

    Are there good combos out there that I can use and still get a big variable sound?

    yes - the carvin cyclops would fit the bill - www.carvin.com

    nemesis 4x10 combo
    swr silverado

    man, i could start listing things, but it'd be like taking shots in the dark. you'll find out the most if you can make it to a big corporate music store (heh) and try out everything there. good luck.
  7. MikeyD


    Sep 9, 2000
    Generally you have a good post, but I have a few comments:
    Well, it's the actual input impedance. If you put 1 volt across it, 1 micro-ampere will flow. It is desirable that the input impedance be high so it doesn't excessively load down (draw too much current from) the signal source.
    Actually, it is the volt-ampere rating, which is like the power input rating, not current. Volt-amps results the simple (scalar) multiplication of current and voltage. Power is the real portion of a complex number (or phasor) multiplication - in other words, the component of the current and voltage that are in phase. For laypersons, 1kVA can be thought of as something like 1000 watts (if the current and voltage are 100% in phase).
    Not true. I've seen many different ratings for THD between different SS amps.
    In my experience, many amplifier specs are quite different if you look at them closely.

    - Mike
  8. rickbass

    rickbass Supporting Member

    You could just use my approach - keep trying amps till you find the one that gets the tones you need, the volume than you need, and fits your budget.

    If you aren't near someplace where you can try out many, make the drive. We're talking a substantial investment here, (for most of us anyway).

    I usually only really worry about spec's when it comes to something like bridging cabs or slaving heads.
  9. ZuluFunk

    ZuluFunk Supporting Member

    Apr 14, 2001
    That's been my approach in the past. However, I've only looked for amps that are suitable for lower volumes. Like my kickback 12 really does sound nice in my music room, but it's pretty brappy when I juice it up for rehearsal. I cut way back on my volume on the bass itself to keep distortion down to no avail. I just don't think the thing was built for heavy duty. Now I need a better rig for playing in a live situation, and I can't really scruitinize that in depth because that would mean spending a couple of hours in the shops at gig volume. I'm sure the staff and other customers wouldn't appreciate it.

    I like the SWR head w/ 2 Hartke 4X10s at the recording studio, but I'm not sure if that's really what's best for my needs. I want to narrow down my selection to spare the local music vendors my wrath and prevent a big mistake.

    I am digesting the feedback. I did look at the Carvin web page and saw what looked like a Combo/cab package (about $50 off purchasing separate). Warwick had the same thing but much more expensive. I also read that someone was using a Nemesis combo and running it through a 4-by cab. These ideas sound good rather than a head and cab rig because the size of the rooms will vary and maybe even some outdoor stuff. I could just take whatever I need for the gig, and leave the cab at home when not needed.

    I'm thinking anywhere from $800-$1,200 range.
    I play funk and ska and some radio type stuff so I need to vary my tone from tune to tune.

    Thanks for the input.
  10. rickbass

    rickbass Supporting Member

    Zulu - I totally understand that situation.

    Fortunately for me, some of the stores I hit have sound rooms where you can redline the amps.

    But some don't and what I do in those situations is put the onus on the salesperson/the owner/the manager. In other words, I tell them, "I really like the tone of this rig here in the store. But spec's don't tell me how it is going to sound when I crank it in the club. Can we work it where I take it to rehearsal and see how it performs when the other instruments are competing with it?" They know an Aguilar and a Fender aren't going to sound anywhere near alike when cranked. Otherwise, let me make it sweat some in the store.

    That's been good enough for them in the past. For one thing, I'm not 17 years old by a long shot. (No dis to the younger guys, but music stores usually treat them very differently unless mom/dad's along with the Visa card). Hell, they're looking at moving a nice rig out of the showroom - that's how they make their LIVING!

    I've never had to resort to, "If it's damaged in any way so that you can't sell it as new, I'll be responsible for repairs or I'll buy the amp."
  11. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Just to add to Mike's post: The amp's input impedance forms a voltage divider with the bass's output impedance.

    For example, let's say your bass has an output impedance of 10,000 ohms and your amp's input impedance is 50,000 ohms.

    The actual voltage that the amp sees will be

    50,000/(50,000+10,000) = 0.833

    or 83.3% of the full output voltage of the bass. You've lost almost 17% of the signal voltage in the bass's impedance.

    Therefore, you want the input impedance to be much higher than the bass's output impedance, by say, a factor of 10. Passive instruments have high output impedances because of the long lengths of very thin wire in the pickup windings. Active instruments have low output impedances because the circuitry acts as a buffer to isolate the pickups from the effects of the cable and amp input and anything else on the bass's output.

    So a high input impedance is good. But high-Z inputs also are more sensitive to noise, so amp designers have to be careful not to overdo it. That's why an input impedance of 20,000 megohms would not be a good idea.

    On THD:
    When looking at THD specs, don't mistake the amp's the THD reference in the power spec for the actual, normal distortion measurement. The THD referred to in a power spec is simply to define the onset of clipping on a sine wave. Clipping begins gradually at the peaks, so there's no fixed point at which you can say it starts. An amp might be doing maybe 0.01% THD on a sine wave as you increase its level, and then as you increase it through the point where clipping begins, the THD starts to rise dramatically due to the flattening of the peaks.

    So if you're an amp manufacturer, you have to pick an arbitrary THD level to use for determining your maximum power output. A scrupulous manufacturer might tend to use a relatively low THD figure to define the clipping point, while those that are driven by bigger, bigger power numbers might tend to use THD as high as 10%.

  12. MikeyD


    Sep 9, 2000
    Another excellent post, Bob! I agree with everything you said. I have two further questions which you may be able to answer regarding power ratings. I have looked at a number of ratings from different manufacturers, including your company's. I often see the THD reference point increased when driving a low impedance such as 2 ohms, or bridging a stereo amp into a mono load. This does not surprise me, but I'm not sure of the reason. Can you enlighten me on this?

    The second question is, what do you think influences a solid state amplifier's ability to deliver rated power across the entire audio spectrum (20-20 kHz) vs. delivering it at one frequency in midband (e.g. 1 kHz)? In other words, is (as I believe) an amp's ability to deliver narrowband power at 1 kHz generally much greater than broadband? Maybe you can throw in some "gain-bandwidth product" reasoning here. If so, by how much do the corresponding ratings often differ? If you could give us your perspective on this, it would be very educational! Thanks.

    - Mike
  13. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Hi Mike,

    The 1 kHz specs are measured using the EIA (Electronic Industries Association) standard, while the broadband specs are FTC (Federal Trade Commission, a US government consumer protection agency).

    The EIA specs are just calculated by measuring the RMS voltage with a stated midband sine wave at a stated clipping point (% THD) across a stated resistive load. The EIA method allows the measure of one channel at a time, but this could hide a design defect, like a wimpy power supply that can't supply anough power to both or all channels. Unless it's otherwise stated, QSC does its EIA tests on all channels simultaneously.

    The FTC method is far more rigorous. The amp has to pass a preconditioning procedure of 1 kHz (sine) at 1/3 power on all channels for 60 minutes, then 5 minutes at full power, without shutting down, current limiting, etc. Then the power measurement is over a stated frequency range, usually 20 Hz - 20 kHz, at a stated clipping point (% THD). And that THD point chosen has to be valid all the way down to 250 milliwatts, too, which is where you'd expect to find any crossover distortion. The preconditioning thing is a tough nut to crack at low load impedances, which is why of all the hundreds of power amplifier models on the market, there are only a couple that I know of with FTC specs at 2 ohms/channel; it doesn't mean the amp won't perform well at that load, though.

    If you look at a plot of at what output power the THD reaches a certain point, that point will be a bit lower at 20 kHz than at mid- and low-band. For example, an amp might reach 0.1% THD at 300W @ 8 ohms at 20 Hz, 295 watts at 1 kHz, 290 watts at 10 kHz, and 280 watts at 20 kHz. For a specified clipping point of 0.1% THD, its FTC spec would be 280 watts at 8 ohms, 20 Hz - 20 kHz, at 0.1% THD. It might hit 1% THD at 1 kHz at 310 watts, so it could have an EIA spec of 310 watts @ 8 ohms, 1 kHz, at 1% THD, or 295 watts at 0.1% THD, or other levels depending on what the manufacturer chooses as a clipping point.

    The EIA spec is easy and simple, and even someone with a modest test bench can do it. The FTC spec is tougher, and obviously with the preconditioning requirement, you have to have an idea of what the amp can do beforehand so you know what the 1/3- and full-power points are. So it takes at least a couple go-rounds and is best done with automated test equipment.

    QSC and some other manufacturers offer both specs wherever possible so you can do apple-apple and orange-orange comparisons, because some manufacturers only offer EIA specs. Also, many manufacturers (including QSC) round down their measurements when producing their specs so that the product you buy will always do at least as good as the specs say. But some manufacturers don't.

  14. MikeyD


    Sep 9, 2000
    Thank you for your detailed and informative reply, Bob. This thread is of great value to me. I have saved it as an html file on my PC.

    With the exception of the power output being better at 20 Hz. than at 1 kHz, everything you said was sensible to me. And depending on the amp design, I guess I could see it being capable of higher output at 20 Hz. than at higher frequencies.

    The net results of the two different testing methods correspond well to my own experience with amps, and QSC's amp specs also reflect them. There is a lot of consistency. Though I'm not going to advocate promotion of one's employer here, I will say that my experience with studying QSC's specs on power amplifiers has been a very good one. There are others here in Talkbass who have lamented the generally poor state of specifications across the bass amplification industry and, like me, would really like better detail and more consistency in specs between products and brands.

    I think your point about EIA requiring only one channel driven is excellent. I suspect there are many stereo power amps out there using a single power supply that is not quite capable of providing firm rail voltage when both sides are drawing maximally. Even in my own rig, I can see the effect of heavy transients on light bulbs in the room! The current draw is high enough that the line voltage sags. I would expect that one thing for people to consider is if the line voltage sags by 5% under heavy load, the power amp may be able to attain only 90% of its normal output (.95 squared). When I bought my second (larger) Carvin amp head, first thing I did was buy a much heavier extension cord for it.

    Can you discuss the importance of having a solid line voltage under all loads, and what effect sagging voltage has on amp output capability (for many common designs)? I'm aware of energy storage considerations (e.g., larger filter capacitors) for transients, but let's assume continuous output for the time being.

    - Mike
  15. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Hi Mike,

    It's not really that the amp's power output is higher at 20 Hz, but the distortion is usually inherently lower, so you reach 0.1% THD or whatever point at a higher output level than at higher frequencies.

    Thanks. One of the big things that drew me to QSC was that they're conscientious in how they operate.

    A sagging AC line will affect the amp's maximum power capability, because for the most part, the supply rail voltages will sag somewhat proportionally to the AC line. Below clipping, the amp won't be any softer or louder, but the clipping point will be at a lower voltage. The only way it will affect the sonic quality of the amp would be on peaks and transients that actually reach the point where the amp clips.

    Some amps, like the PLX and PowerLight lines, have switching supplies with soft regulation, so their power capabilities aren't as sensitive to line variations. But the supplies are considerably more complex.

    Ironically, at low AC line voltage, an amp is more efficient, but that often comes at the expense of gross inefficiencies in the AC line itself. That's not a good trade-off, IMHO.

  16. MikeyD


    Sep 9, 2000
    Hi, Bob - thanks for your reply. This makes sense to me.
    So it looks as though the clipping point (wattage) would move down by about 10% under a line voltage sag of 5%. This is what I was surmising.
    Interesting. Yes, if the amp can draw more current from the power outlet, there's more I*R drop. I wonder if some switching supplies put it into a positive feedback situation: a somewhat lossy power feed drops a bit under load, so the power supply tries to compensate by switching differently to draw more current and keep the net power draw constant. The added current draw further drives the power line voltage down, etc. Of course, this situation would be more likely on a pretty lame power feed.
    - Mike

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