• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

Ohms FAQ

#61
kyo said:
okay i have read this entire thread and the other one linked, i THINK i know the answer to this but i really would feel much better if someone set me straight.

i have a head thats 200w at 4 ohms (i know, i know) and 100w at 8. i've got an 8ohm 115. i thought that i was safer than if i had a 4 ohm 115 when i bought it, but is this not the case? would i be better off if i bought an additional 8 ohm cab?

and also if it is a problem running what i do play loud, what would happen? would there be damage to the head or cab? and also, is it possible for there to be a problem even if i dont hear it distort?
Click to expand...
as long as you have a solid state head rated for a minimum 8 ohm load you can run it all day with an 8 ohm cabinet (as long as you arent distorting the cabinet). YOu will notice an appreciable increase in volume with a second 8 ohm cabinet, not ony due to getting the full 200 watts, but because you will increase the amount of air you are moving with two cabinets. In general, and as long as the sensitivities of the cabinets in question are about the same,you'll get more bang for the buck volume wise with two 8 ohm 15" cabinets than with one 4 ohm 15" cabinet. Of course, that means you have to carry two cabinets. :p
 
#62
Ivanmike, I've just become your biggest fan. In a similiar vein. I've been doing a bit of research on slew rates, damping factors, and actual wattage at given frequencies. I've seen and heard the best specs with the QSC PLX series and the Yorkville "A" series, and was quite disappointed with my old Carver PM 900-damping factor of >100. I also notice that switching designs like Walter Woods while rated at 800 watts for a 1K signal, went down to 25watts for a 40 hertz. We compared that to an old 50lb Trace-Elliot AH500, that stay between 225-250watts going from 40-1K. Having said all this, what are some great power amps that are lighter in weight, but can really handle the low frequencies? :confused:
 
#63
IvanMike said:
as long as you have a solid state head rated for a minimum 8 ohm load you can run it all day with an 8 ohm cabinet (as long as you arent distorting the cabinet). YOu will notice an appreciable increase in volume with a second 8 ohm cabinet, not ony due to getting the full 200 watts, but because you will increase the amount of air you are moving with two cabinets. In general, and as long as the sensitivities of the cabinets in question are about the same,you'll get more bang for the buck volume wise with two 8 ohm 15" cabinets than with one 4 ohm 15" cabinet. Of course, that means you have to carry two cabinets. :p
Click to expand...

yeah, ive been thinking of another cab, but dreading actually carrying it :) oh and btw, its a hybrid, has a tube pre and a SS pre. does that make a difference vs if it was a pure SS?
 
#64
brooklynbassguy said:
I've seen and heard the best specs with the QSC PLX series and the Yorkville "A" series, and was quite disappointed with my old Carver PM 900-damping factor of >100. I also notice that switching designs like Walter Woods while rated at 800 watts for a 1K signal, went down to 25watts for a 40 hertz. We compared that to an old 50lb Trace-Elliot AH500, that stay between 225-250watts going from 40-1K. Having said all this, what are some great power amps that are lighter in weight, but can really handle the low frequencies? :confused:
Click to expand...
that's a subject i can't answer fully. hardly any amp manufacturer is going to give you the full wattage "curve" dependent upon frequency. It is true that trace elliot heads were loud as heck. :p I like the plx and stewart amps, but your taste may differ
 
#65
kyo said:
yeah, ive been thinking of another cab, but dreading actually carrying it :) oh and btw, its a hybrid, has a tube pre and a SS pre. does that make a difference vs if it was a pure SS?
Click to expand...
when it comes to hybrid amplifiers you should treat it the same as a fully solid state amplifier in terms of determining what sort of speaker loads it can handle. It helps to think of an amp head as a preamp and a poweramp. Only the poweramp matters when determining what speaker combinations you can use with the amp.
 
#69
Great thread, thanks for posting!
I don't need a lot of volume/power and am looking for a 200 watt head. I was thinking I could get a head rated for 300watts at 4 ohms and run it with and 8 ohm cab for a result in the area of 200 watts. I guess the 8/200 won't be much less loud than the 300/4 setup?
 
#70
Tom P said:
Great thread, thanks for posting!
I don't need a lot of volume/power and am looking for a 200 watt head. I was thinking I could get a head rated for 300watts at 4 ohms and run it with and 8 ohm cab for a result in the area of 200 watts. I guess the 8/200 won't be much less loud than the 300/4 setup?
Click to expand...
probably not dependign on what speakers you use. definitely get specific info from the manufacturer of the head and try a bunch of stuff before you buy. if you get an 8 ohm cabinet, look into the possibility of getting a second one in the future for bigger gigs.
 
#71
SS amps are NOT constant voltage. Tube amps are NOT constant current. Both are (inperfect) voltage SOURCES, with the only exception to this I can think of being a pentode amp with no feedback, which would approximate a current source. To save a long tech explanation here's an analogy: if a SS amp was constant voltage, it would be like a battery, and putting one of those across your speaker terminals doesn't generate much sound: cone moves out and stops, and heat is generated in the voice coil, that's all.

Tube amps generally have no problems with higher impedances, in terms of failure modes, but the sonics can vary a bit depending on the amp's topology. If you look at the impedance curve of most speakers, say a 15" bass cab, you'll see one or two resonant peaks that can easily exceed several times it's nominal impedance at near the box's tuning frequency, and the driver's inductance will increase the impedance with frequency. If in doubt, connect the speaker to the tap which is nearest in value to the speaker's nominal impedance.

The only time that you might damage a tube amp in use with too high an impedance , is if you have nothing connected to the secondary of the output transformer (effectively infinite impedance) when you turn it on, plug in an instrument or wind the volume up. Then it's possible under some conditions for the transformer to be damaged, and not something I'd recommend. I've had few issues doing this with tube amps, and none with SS amps.

Loudspeaker's vary in their impedance wih frequecy, as I mentioned above. Looking at the diagram below, what would be the nominal impedance of this speaker (black line)?
Invalid Link Removed

It's not unusual for many speakers to have a curve like this, as the impedance just represents the airload on the speaker (LF spikes and wobbles), it's DC resistance (relatively even bit in the midrange) and the rising impdance because of the voice coil inductance (the bit above 1kHz). And that's before you add in a crossover and other drivers.

Simply stating that you can parallel Brand X speaker (8 ohms) and Brand Y speaker (also 8 ohms) and you'll have a 4 ohm net load on the amp is misleading. If they were both the same brand and model, there's a greater likelihood of no problems, as the curves should be close to identical and add together, ending with half the impedance (assuming the amp can drive this net load). Two dissimilar boxes can have curves that are very different and at some frequencies may be a lot lower than the nominal parallel load. Lots of amps don't like really low loads; modern PA designs from reputable companies will likely have the least problems, but other? Who knows? Most of the time it works out fine, but I would strongly advise caution, starting at a lower volume and increasing, noting any sonic peculiarities or amps running a lot hotter than usual.
 
#72
well, i didn't get into variable impedance as related to frequency as i thought it would be unessesarily confusing. I've found using the nominal impedance of professional cabinets to be fine when using them with professional heads and amps. When amp manufacturers are honoring warranties and such i think most of them take the variable impedance curves into account before stating that their product can be safley run into a 2 ohm load, knowing full well that at some frequencies the load may fall below that. I'm sure they are aware of the full capability of their product and wouldn't take chances with having to do tons of free repairs due to the common practice of mixing and matching speaker cabinets with stated nominal impedances.

i cant comment on psychobassguy's input, but i respect his opinon highly and i hope he'll take the time to respond.
 
#73
You seem to really know your stuff IvMi, your posts surely standout. I am fairly new here on TB and though a veteran bassplayer, an admitted novice concerning the technical side of "bassdom", I know shame on me indeed. But happily it is never too late to learn, and learning I am. Just wish I'd discovered these sites a few years ago, might have saved me big bucks on alot of equip purchases. Better later than never. Also, I should have searched the FAQ's before I posted my question/reply on a couple other threads hear a moment ago about these same subjects, but it is very late and I was being lazy. From now on I suggest everyone search the FAQ sites to avoid redundancy and esp to get their info quicker and with more detail and correctness. I know I will. Thanks again.
 
#74
Dharmabass said:
SS amps are NOT constant voltage. Tube amps are NOT constant current.
Click to expand...

Sorry, but they are. You have amplitude and frequency confused.

Both are (inperfect) voltage SOURCES,
Click to expand...

What is a "perfect" voltage source? While that may sound technical and impressive, it has no actual electronics significance relevant to the topic.

with the only exception to this I can think of being a pentode amp with no feedback, which would approximate a current source.
Click to expand...

Pentode, triode, tetrode, ect has NO bearing over any kind of "redefining" what an amp puts out. Output tube configuration controls amplifier class and power amount. Again, you're using terms you don't understand to try and support a nonsensical "argument."

To save a long tech explanation here's an analogy: if a SS amp was constant voltage, it would be like a battery, and putting one of those across your speaker terminals doesn't generate much sound: cone moves out and stops, and heat is generated in the voice coil, that's all.
Click to expand...

As I've said before, you've confused frequency with amplitude. As recent as the 80's, solid state amplifiers were rated by output voltage with actual power ratings only derived using Ohm's Law for different loads. (It was a handy trick to get around having to guarantee output power at low impedances since no amplifier is 100% efficient and most put out significantly less power into lower impedance loads than they should have.)

I'm VERY interested in your "long tech explanantion." If the amp were putting out DC (which incidentally, IS constant current) voltage then it would actually be analogous to a battery, but because an amp's output signal varies in frequency (the sound), the cone moves. You have no idea what you're talking about.

Tube amps generally have no problems with higher impedances, in terms of failure modes,
Click to expand...

"Failure modes????" How many more "technical" terms are you going to make up? That statement is DEAD wrong. Here's the equation for output power derived across a transformer:

Po = Zo'*(gm*Vg)^2

Power out= secondary impedance (transconductance x voltage) squared

It's plain to see that power increases with rising impedance, which is what destabilizes a tube amp.

but the sonics can vary a bit depending on the amp's topology. If you look at the impedance curve of most speakers, say a 15" bass cab, you'll see one or two resonant peaks that can easily exceed several times it's nominal impedance at near the box's tuning frequency, and the driver's inductance will increase the impedance with frequency. If in doubt, connect the speaker to the tap which is nearest in value to the speaker's nominal impedance.
Click to expand...

You're forgetting (or don't know) that speaker resonance has to be paralleled against amplifier output impedance. It doesn't matter what a speaker's impedance peaks are. There's an amp involved, and ignoring that aspect gives no useful information.

The only time that you might damage a tube amp in use with too high an impedance , is if you have nothing connected to the secondary of the output transformer (effectively infinite impedance) when you turn it on, plug in an instrument or wind the volume up. Then it's possible under some conditions for the transformer to be damaged, and not something I'd recommend. I've had few issues doing this with tube amps, and none with SS amps.
Click to expand...

Please reconcile that with the equation I've provided.
 
#75
Reading this::
'Also, if you hook up a 4 ohm cabinet and an 8 ohm cabinet to an amp that can power a 2 ohm load, the 4 ohm cabinet will get twice the wattage as the 8 ohm one'

I was wondering if anyone manufactures something to adjust
the volume between 2 cabs. I have 4 ohm 1x12 and a 8ohm 1x15 and would like to even out their volume levels.

Any advice??
 
#76
hillbillyfunk said:
Reading this::
'Also, if you hook up a 4 ohm cabinet and an 8 ohm cabinet to an amp that can power a 2 ohm load, the 4 ohm cabinet will get twice the wattage as the 8 ohm one'

I was wondering if anyone manufactures something to adjust
the volume between 2 cabs. I have 4 ohm 1x12 and a 8ohm 1x15 and would like to even out their volume levels.

Any advice??
Click to expand...
no, not really, except to get yourself a stereo amp, or use two amp heads and adjust the volume to each cab independently.
 
#77
hillbillyfunk said:
Reading this::
'Also, if you hook up a 4 ohm cabinet and an 8 ohm cabinet to an amp that can power a 2 ohm load, the 4 ohm cabinet will get twice the wattage as the 8 ohm one'

I was wondering if anyone manufactures something to adjust
the volume between 2 cabs. I have 4 ohm 1x12 and a 8ohm 1x15 and would like to even out their volume levels.
Click to expand...

The 4 ohm cab may draw twice the power, but if it is 3dB less efficient than the 8 ohm cab, then both cabs will be the same loudness (all other things equal). You have to factor sensitivity into this scenario.

I don't know what brand cabs you have (care to tell us?) but I suspect the 1x15 is more efficient than the 1x12. If that is the case, there may end up being no real difference in loudness.

Your best bet is to put a cab on each channel of your power amp and balance them that way. I wouldn't worry about the electrical differences that much. Use your ears and balance the sound that way.
 
#78
great point nightbass. i've used my 8 ohm bag end 1x15 and 4 ohm bag end 2x10 (the 2x10 coincidentally purchased from nightbass) on the same channel of a poweramp capable of driving a 2 ohm load, (or in this case, 2.67 ohms). The 1x15 is rated at 103 dB and the 210 is i think 100 dB. The two cabinets were almost identical in volume, and in fact the 1x15 might have been a bit louder.

if it aint broke, don't fix it.
 
#79
Wow, I came here looking for a simple answer and now I'm more confused. Not to the fault of IvanMike or anyone else who's posted. But more to mine for not being able to wrap my head around this stuff. Perhaps IvanMike would be kind enough to give me a simple answer here? A simple yes or no, and perhaps a short explanation as to why with as few syllables as possible?

I have a Traynor YBA200. (4/8 Ohm switch) Invalid Link Removed

I'm getting a yorkville XC808, but for now I NEED a cab to play through. My friend said I can borrow his really old Traynor YT-15 which says 16 Ohms on the Yorkville site. Invalid Link Removed

Is there a way I can do this safely? Low volumes? Faith?

Thanks.
 
#80
well actually your question was addressed in the original post although there has been some bantering in the thread itself. As a tube amp your head wants to see the impedance load that it's set for. Higher impedances in particular can damage a tube amp, but its generally somewhat safe to be off a bit. set at 8 ohms, using that 16 ohm cabinet would be as high of an impedance that you'd ever even want to consider using with your head. Even if it works out, it's my understanding that you'll be shortening the life of your power tubes by doing that. I would contact traynor and ask to speak to someone who knows their stuff and give them your situation. I would also find out if you would void your warranty by doing this. If traynor gives you the thumbs up, cool. I dont belive low volume will make any difference in this particular instance.
 

Latest posts

Top Bottom
Back