Perceived volume vs. mathematical volume.

[billfitzmaurice]

Below clipping, the difference between the voltage into an 8-ohm load and into a 4-ohm load is negligible, typically less than 0.05 dB. For all practical purposes, it is the same.

Quite right. Run a sine wave or pink noise signal through an SS amp into a speaker, metering the voltage. Add a second cab, or a third or fourth. The voltage will remain constant so long as you don't exceed the amp's current capacity.

That is why an amp's 4-ohm power ratings are usually less than double its 8-ohm figure.

By a factor of about .77, which is a figure that crops up pretty regularly in speaker design as well.

 

[DavePlaysBass]

If everyone agrees that the 4-ohm rating is less than double an 8-ohm rating, how does the voltage applied to the load stay the same? If you agree that the voltage sags at 4-ohm, then it would stand to reason that there would be more voltage applied to the speaker load at 8-ohm.

 

[Bob Lee (QSC)]

If everyone agrees that the 4-ohm rating is less than double an 8-ohm rating, how does the voltage applied to the load stay the same? If you agree that the voltage sags at 4-ohm, then it would stand to reason that there would be more voltage applied to the speaker load at 8-ohm.

I explained all that in my posts.

The output voltage doesn't sag into 4 ohms. The supply rails sag compared to the same output voltage into an 8-ohm load, so the voltage where clipping starts is lower.

 

[Bob Lee (QSC)]

By a factor of about .77, which is a figure that crops up pretty regularly in speaker design as well.

It'll vary with the characteristics of the amp's power supply. If the amp has a cheap power transformer with high losses, for example, then the lower-impedance clipping point will be at a significantly lower voltage than into a high load impedance. Replace that transformer with a high-quality unit that has very low winding resistances, and the difference will tend to be much smaller.

 

[billfitzmaurice]

If everyone agrees that the 4-ohm rating is less than double an 8-ohm rating, how does the voltage applied to the load stay the same?

Bob not only has the requisite educational and professional experience to back up his statements, he's probably also actually done the measurements himself a thousand times. I have, and I can't imagine a single professional loudspeaker or amplifier designer who hasn't.

 

[Lelander]

This has been informative. Thanks, Bob and Bill. My favorite aspect of TalkBass is that there are at least a few experts around who are ready and willing to put my mind in the right place when I am having trouble understanding something.

 

[DavePlaysBass]

Yes.

I wonder, can anyone expound on this whole "wattage @ 8 ohms > 1/2 W @ 4 ohms" business. Granted, it's been a very long time since I took intro to circuit analysis, but it's my recollection that Ohm's law states that voltage is inversely proportional to resistance at constant current. That being the case shouln't twice the resistance always result in half the voltage, which (if I am understanding this correctly) means half the volume?

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This has been informative. Thanks, Bob and Bill. My favorite aspect of TalkBass is that there are at least a few experts around who are ready and willing to put my mind in the right place when I am having trouble understanding something.

You are a smarter man than I. As one who has worked in the field of Power Electronics for 20 years with patents, papers, a pair of degress, and thousands of successful products in field, I find the explanations vague. At any rate, the subject has been beat death.

 

[KJung]

Apologies for going off topic, but ...

On the contrary, Genz specs are about the most conservative I've ever encountered. The GBE600, which is rated at 425 watts into 4 ohms, is louder than some amps I've played which are rated at nearly twice the power.

Sorry for the off topic blurb ...

Cheers

That would be a separate issue, but as long as you bring it up, I never really got this 'Genz has louder watts' thing. I did not find the GBE600 into 4ohms to sound louder than its given volume rating, and those not aligned with the company actually give a slight volume and 'girth' edge, for example, between the Shuttle 6.0 and MB LMII to the 500 watt LMII in direct comparisons.

I still don't dig companies 'inflating' the perceived wattage of their heads by naming the amp for its maximum 2ohm output versus the more typical 4ohm usage for most of us (Genz is surely not alone in this). Just a personal 'pet peeve' of mine.

IMO of course!

I will again say, back on topic, that I would have either used an 8ohm internal speaker on the OP's Nemesis unit, or just eliminated the external speaker jack and not 'pretended' that you could add a second cab to that particular unit. I guess to be fair to Eden, the unusual series output jack would eliminate any problems with someone plugging in an external extension cab and pushing the total nominal impedance to under 4ohms, but still kind of a strange design to me.

 

[billfitzmaurice]

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The figure illustrates the effect of high source resistance within the amplifier, or high resistance within the cable between the amp and speaker. For illustrative purposes the source resistance is wildly exaggerated and is not a real world case. It's very safe to say that no product coming out of Bob's R&D with a one ohm source resistance value would ever make it into production. Using a hundred foot run of 20 gauge wire between the amp and cab would give this result, though.
But the real fly in your ointment is that loudspeakers do not present a uniform load. Their impedance varies with frequency. The average 8 ohm rated speaker shows an actual load anywhere between 5 and 50 ohms within their operational bandwidth. Despite that fact amplifiers still deliver a uniform voltage across the bandwidth, which is quite serendipitous, for if this was not the case the speakers simply would not work.

 

[hasbeen]

we don't "inflate". We can build you a 750 watt at 4 ohm if that's what was called for. The fact is, most of our target customer for heads over 500 watts (excluding our light weight amps), will be running two, 4 ohm cabs on many gigs.

With our powerflex feature, you can get almost full wattage at 4 ohm anyway. Lastly, our amp design mentality comes very much from the serious, pro audio world where 2 ohms is the standard.

We don't disclose a 2 ohm rating hoping you won't use it, use it hard and use it often. Some companies may but that isn't our situation.

That would be a separate issue, but as long as you bring it up, I never really got this 'Genz has louder watts' thing. I did not find the GBE600 into 4ohms to sound louder than its given volume rating, and those not aligned with the company actually give a slight volume and 'girth' edge, for example, between the Shuttle 6.0 and MB LMII to the 500 watt LMII in direct comparisons.

I still don't dig companies 'inflating' the perceived wattage of their heads by naming the amp for its maximum 2ohm output versus the more typical 4ohm usage for most of us (Genz is surely not alone in this). Just a personal 'pet peeve' of mine.

IMO of course!

I will again say, back on topic, that I would have either used an 8ohm internal speaker on the OP's Nemesis unit, or just eliminated the external speaker jack and not 'pretended' that you could add a second cab to that particular unit. I guess to be fair to Eden, the unusual series output jack would eliminate any problems with someone plugging in an external extension cab and pushing the total nominal impedance to under 4ohms, but still kind of a strange design to me.

 

[KJung]

we don't "inflate". We can build you a 750 watt at 4 ohm if that's what was called for. The fact is, most of our target customer for heads over 500 watts (excluding our light weight amps), will be running two, 4 ohm cabs on many gigs.

With our powerflex feature, you can get almost full wattage at 4 ohm anyway. Lastly, our amp design mentality comes very much from the serious, pro audio world where 2 ohms is the standard.

We don't disclose a 2 ohm rating hoping you won't use it, use it hard and use it often. Some companies may but that isn't our situation.

We've had this discussion before I was mainly commenting on the 600's 425 watts somehow sounding like more than 425 watts. It doesn't to me.... that's not necessarily a bad thing (it's a very nice moderately powered head to my ear), but an endorsee stating that it sounds louder than 'most amps he's heard with 'double the wattage' was misleading and ridiculous IMO and IME.

Just trying to provide some 'real world' view based on not being aligned with any company... just what my ears tell me with no politics, friendships, or 'endorsements' impacting what I might be hearing.

 

[hasbeen]

Or at least as much output as you would need. I have a 100W combo for rehearsal and have never heard it distort or clip at any volume I use.

You know, I would really expect companies like Genz to market with a smaller BS factor. Damn my naiveté.

nope...can't do it. I'm in charge of the marketing for GB and let me assure you, my ex-wife will tell you I'm full of MUCH more BS than probably anyone working for one of our competitors.

 

[hasbeen]

We've had this discussion before

yes.....saw that post after your first post.

Give me a minute to catch up....still having my first cup of coffee.

 

[KJung]

yes.....saw that post after your first post.

Give me a minute to catch up....still having my first cup of coffee.

As you know, I'm a fan of most of your stuff. I just enjoy these discussions, etc. We've gone OT, but it seems the OP has his answer, so what the heck I actually very much appreciate the fact that your heads are designed for 2ohm operation, versus the maddening 'it's safe but we don't recommend it' approach by many other manufacturers.

 

[hasbeen]

As you know, I'm a fan of most of your stuff. I just enjoy these discussions, etc. We've gone OT, but it seems the OP has his answer, so what the heck I actually very much appreciate the fact that your heads are designed for 2ohm operation, versus the maddening 'it's safe but we don't recommend it' approach by many other manufacturers.

I do know you "support" our stuff....greatly appreciated.

I will someday, I WILL, I WILL get you playing GB!!!


Moderators- I'm only playing here...not spamming. KJ is my personal challenge.

Seriously, I do appreciate your non-biased contributions to this forum.

 

[KJung]

I do know you "support" our stuff....greatly appreciated.

I will someday, I WILL, I WILL get you playing GB!!!


Moderators- I'm only playing here...not spamming. KJ is my personal challenge.

Seriously, I do appreciate your non-biased contributions to this forum.

I would have thought I would have already spent some time with the 6.0, but I can't seem to pry it out of Mikebass's hands

 

[Lia_G]

an endorsee stating that it sounds louder than 'most amps he's heard with 'double the wattage' was misleading and ridiculous IMO and IME.

Just trying to provide some 'real world' view based on not being aligned with any company... just what my ears tell me with no politics, friendships, or 'endorsements' impacting what I might be hearing.

For the record ... I thought the same as I do now long before I became an endorser. It could be, though, that my experience of Genz's ratings may actually be that they are accurate where certain other companies I have experience with tend to exaggerate.

And please, Ken, if you're going to insult me, at least get my statement right. I never said 'most amps I've heard with double the wattage.' I said some amps I've heard.

I highly value my integrity and my word. An endorsement is most definitely *not* something that will make me lie about a product. I'm trying very hard not to feel this way, but I find that I do strongly resent your implications to the contrary.

Liam

 

[DavePlaysBass]

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The figure illustrates the effect of high source resistance within the amplifier, or high resistance within the cable between the amp and speaker. For illustrative purposes the source resistance is wildly exaggerated and is not a real world case. It's very safe to say that no product coming out of Bob's R&D with a one ohm source resistance value would ever make it into production. Using a hundred foot run of 20 gauge wire between the amp and cab would give this result, though.
But the real fly in your ointment is that loudspeakers do not present a uniform load. Their impedance varies with frequency. The average 8 ohm rated speaker shows an actual load anywhere between 5 and 50 ohms within their operational bandwidth. Despite that fact amplifiers still deliver a uniform voltage across the bandwidth, which is quite serendipitous, for if this was not the case the speakers simply would not work.

The illustration is simplified model. This would account for transistor losses, power supply droop, etc. No one is putting an actual 1-ohm resistor in the amp. In fact the resistance is most likely non linear fuction of current like most things in this world.

As one making high density, high current, class D amps for motors and engines that go into vehicular and industrial enviroments without fans for years of contiuous operation at 105 deg C ambients, it is voltage bus or power supply droop that gets you at high loads as stated by Bob which is a voltage decrease. As I believe Bob indicated, this can be from the input magnetics exhibiting drop and IME non ideal capacitor behavior.

Also for the sake of power rating and comparison, something needs to held constant. Sure impedances move around as a function of frequency. And I imagine there is variation from speaker to speaker ona design basis. But we usually do not let these types of variations keep us from analyzing what's going on and looking for the trends and sensitivities.

But can you anser this simple question:

Eden state 200W for an 8-ohm load, 320W for a 4-ohm load. What is the voltage across each respective load? Does you answer violate ohm's law?

BTW, I have provided a simple mathematical explanation to a 1.75 dB increase based on the orignal posters numbers. If you disagree with me, can you show where there is an error in the math?

 

[fdeck]

I think that this diagram approximately illustrates the situation.

Here, the power supply is shown in series with an effective resistance, which covers filter ripple, transformer losses, etc. Like you say, this "resistor" is not really a linear resistive element. Q1 represents the amplifier circuit, and R1 the load.

In my view, we use power as a figure of merit for amplifiers, because it allows us to compare systems that have different impedance loads. Also it may date back to when engineers routinely computed the thermodynamic efficiency of systems.

But I think that when discussions start getting technical, then it makes much more sense to talk in terms of voltage and current, for many reasons. Perhaps most importantly, an amplifier is a voltage source. But also, a speaker is generally modeled assuming a voltage source input. When we talk about power, we realize that we are entering into a territory populated by many approximations and a few unknowns such as the actual load impedance.

At least, I have found in my own amateur tinkering with amplifiers, that I have to think in terms of voltage and current in order to make any sort of accurate predictions.

 

[DavePlaysBass]

I agree. Nice way of showing it.