Why 4 Ohm Loads Stress Your Amplifier

240 Volts said:
Yes!! :yes: (and maybe No!! :no: )

The "Yes" bit : Class A amplifiers get hottest when they are idling with no signal. So when a class AB amp would be almost stone cold the class A amp is stinking hot.

The "maybe No" bit : At maximum output power I'm not sure if the class A amp will be any hotter than the class AB. :scratch2:

- Richard B.
Quick comment on this...Class AB amps produce maximum waste heat at 1/3 of full power, whereas Class A produce maximum waste heat at idle (but you knew that).
 
240 Volts said:
Yes!! (and maybe No!! )

The "Yes" bit : Class A amplifiers get hottest when they are idling with no signal. So when a class AB amp would be almost stone cold the class A amp is stinking hot.

The "maybe No" bit : At maximum output power I'm not sure if the class A amp will be any hotter than the class AB.
EchoWars said:
Quick comment on this...Class AB amps produce maximum waste heat at 1/3 of full power, whereas Class A produce maximum waste heat at idle (but you knew that).
Nice catch EW. :ntwrthy: :ntwrthy:
I was trying to get over the idea that just because Class A amps get very hot at idle it doesn't necessarily follow that they run hotter than class AB amps under all operating conditions. Unfortunately I was very careless with my language - and to pick this of all threads to do it in!! :dammit:

- Richard B.
 
Well, I wasn't nit-picking at you so much as finding an opportunity to educate a few guys.
 
Series bad - parallel good...

Exactly which amps (or receivers) connect the speaker outputs in series - if you select two or more outputs?

I can't imagine hooking up two sets of speakers in series. Has everyone seen an impedance curve from a tuned port LF driver? The Z of the typical 8 ohm woofer will go as high as 60 to 100 ohms on either side of the port tuning freq, and will rise due to inductance at higher freq. I doubt a sealed driver would have a flat Z curve either. I have the "Woofer Tester II" from Parts Express, and now I have a nice printout of several woofer's test results. they're not flat, and it has to be dealt with.

Many speaker manufacturers (like vintage Klipsch) use auto-transformers in their crossovers - especially with compression drivers (horns) for midrange, and up. My Altecs have horns, and NO Lpads to help maintain Z above the crossover point. I'm surprised how well tube-type amps deal with this "nominal" speaker Impedance.

My point is: two unequal sets of speakers in series would give unequal drive to BOTH sets. Not something I want...

Richard C.
 
kenwood_lover said:
I'm pretty used to my noisey-as-hell liquid nitrogen fan in my PowerMac anyway so hopefully I won't notice the further increase in noise too much :D

lol I can relate with that building houses for 15 years kinda conditions your ears

I have a couple fans made for a monster ac/dc power supply run off ac and are quiet as a church mouse

I ran across them the other day and rembered the post EW had made about the noise

the fans made he installed on a amp he was repairing so i had to stop and plug them in ........... :scratch2:

they were damn quiet
 
Cool great work I understood maybe 10 % of it :O lol

thanks did make a few bulbs in my head turn on

thanks for the info

Brian
 
Nice timing, guys. I just picked up a pair of Phase Technology PC-800HO speakers, and panicked a little bit when I found out that they were 4ohm. They are the only 4ohm speakers I own.

I plan on driving them with my HK730 (40wpc) receiver or HK665 integrated (100wpc). I will only be listening at moderate levels for a couple of hours at a time. I doubt I'll get past 1/4 volume.

I assume no problems here. But stop me if I'm doing something dumb!

Jim
 
why 4 ohm loads stress your amp

at one time in my party building me and my buddy built(8' by 12' feet metal outbuilding with the walls and ceieling insulated and carpet on the floor) i had 4 8 ohm speakers on A nad 2 4 ohm spekers on B and the amp would get a little warm but the sound never diminished it never got hot to the touch (it was only played jammin for maybe 45 minutes at a time w/breaks in between) and it was f@#$ing lowwwwd. so i sat rock on (responsilbilly)
 
Well, it's been a month since I asked about anyone knowing of a receiver or power amp that internally connects speaker outputs A and B in SERIES. I sure would like to see a partial schematic of the output section. I wouldn't hook up my speakers in series because of induced colorations, and my headroom would be gone in any SS amp I have! Each speaker output (series connection) would decrease by 6dB. 1/4 power!

I don't think this thread is dead until we discuss this.
 
Richard C. said:
Well, it's been a month since I asked about anyone knowing of a receiver or power amp that internally connects speaker outputs A and B in SERIES. I sure would like to see a partial schematic of the output section. I wouldn't hook up my speakers in series because of induced colorations, and my headroom would be gone in any SS amp I have! Each speaker output (series connection) would decrease by 6dB. 1/4 power!

I don't think this thread is dead until we discuss this.
I've never seen an amp that connects speaker outputs "A" and "B" in series. To be honest it never crossed my mind that any manufacturer would do such a thing - it could only make any kind of sense if all 4 speakers were identical (otherwise the different impedance/frequency characteristics of the different speakers would cause all sorts of strange "colouration" effects). Even with 4 identical speakers the dampling factor would be shot to hell.

- Richard B.
 
Richard C. said:
Well, it's been a month since I asked about anyone knowing of a receiver or power amp that internally connects speaker outputs A and B in SERIES. I sure would like to see a partial schematic of the output section. I wouldn't hook up my speakers in series because of induced colorations, and my headroom would be gone in any SS amp I have! Each speaker output (series connection) would decrease by 6dB. 1/4 power!

I don't think this thread is dead until we discuss this.



My Kenwood KR-9600 puts A & B speakers in series when A+B is selected...

Lefty
 
240 Volts, it looks clear to me, too...

Lefty,

Is there any way you can post a scan .jpg, .pdf, or whatever, (or point me to a link) of that portion of your Kenwood KR-9600 "output selector switch" schematic?

I know this is kinda like a two subject thread, but it was brought up in post #14 that some amps internally connect the speaker outputs in series.

I think this has been a great discussion on thermal ratings, observations, etc. on what happens on a reduced Z load. I have learned much, and I think I will stay with fan cooled heatsinks! I like the idea of variable speed fans, too.
 
A few quick comments:

1) The data at the start of the thread is fine, but the comparison is not -= you end up cpmparing apples and oranges. What needs to be done is a heat generated vs power generated graph for 4 and 8 ohms, with a limit drawn in for a typical limit on maximum power dissipation and maximum current - just pick an amp for an example.

2) Unless fully regulated, power supply rails drop under load, and the voltage actually varies with a frequency which is a mix of full wave rectified mains, and half-wave rectified output (talking class AB/B amps here, which pretty much covers 99% of all of them). This makes instantaneous power dissipation quite difficult to calculate. Also, as a result, the transistor generated heat is reduced, at the expense of increased power transformer and rectifier heat.
Although this complicates matters, it is important to mention because it has relevance to real world equipment. In fact, any amp that has less than twice the 8 ohm power rating at 4 ohms, is likely to make use of this fact.
Many manufacturers deliberately make the power supply 'soft', i.e. the power rail voltages are made to 'sag' under load (by careful selection of transformer size and winding, and filter capacitors), in order to keep heat generation down for complex/low loads. Some include a 8/4 ohm switch, which selects different taps on the power transformer. Often, the difference in rail voltage between the two will be small, AT IDLE. What must be taken nto account is that the fuilter caps in the power supply stay the same, so for (theoretically) double increase in output current with a 4 ohm load WRT an 8 ohm load, the rail voltage 'sags' more. This is taken into account when selecting the alternate rail voltage for 4 ohm operation, in order to keep the output power the same.

3) Designing around a transistor SOA (=Safe Operating Area) is a very difficult endevour. In theory, were it not for a phenomenon called 'second breakdown' (and indeed MOSFETs and VFETs don't have this phenomenon), one could use a transistor as long as its power dissipation is less or equal to whatever heats the heatsink to some maximum permissible temperature. If the heatsink was 'infinite', then this power limit would be the one found in transistor data sheets. In reality, the figure is 'derated' using the heatsink characteristics. It is also worth noting that because SS amps are typically symetrical, having at least one transistor that operates during half of each period of the output signal, the actual heat generated by the amp (and therefore power dissipation) is divided amongst two transistors.
A few details regarding the SOA:
Datasheet maximum current is guiven as a DC value. Because of SOA considerations, the actual current the transistor can pass reliably, can be lower, as well as higher! In general, the shorter the time of the current 'peak', and the lower the temperature, and the voltage across the transistor, the higher the current can be. It should be obvious that all of this points to instantaneous heat generation in the transistor - as current through transistor, times voltage across transitor equals dissipated power (= what produces the heat). Secondary breakdown in BJTs additionally reduces this, depending on voltage across the transistor, temperature, and duration.

4) Not all 8 ohm or 4 ohm loads are the same! Purely resistive (non-reactive) loads are the easyest to drive. For all the rest, the relationship of current and voltage called 'ohms law' applies, but with a lag, or lead. What this means in practise, is that at 0V out, there may not be 0A current. Because of this, at least one of the output transistor(s) in the amp has more voltage over it at a given current, than would be the case for a purely resistove load, and more heat is generated. Less reactive impedances of a given magnitude, are less difficult to drive.

5) Series connected speakers: BIG NO, unless the speakers to be connected in series are EXACTLY the same. Impedance variations with frequency woudl otherwise produce uneven division of power depending on frequency, and usually completely screw up the frequency response.
 
ilimzn said:
A few quick comments:
Wow!!! If that's your idea of "quick comments" then I'd really like to see your "in depth" analysis. :D

Generally agree with pretty much all of your points. I avoided going into detail of the effect of reactive loads, transient thermal impedance, SOA etc. because I didn't want things to get too complicated.

Interesting comments you made regarding the power supply design. Have you ever tried using a "SPICE" simulator to model power supplies, transistor power dissipation etc? There are various free downloads of "evaluation" versions available on the internet - usually these are limited to the number of "nodes" they can handle, but would be adequate for our purposes.
I see threads on AK talking about increasing the supply filter capacitors of some item of vintage equipment, and all anyone worries about is the increase in inrush current. I think I'm going to have to run some SPICE simulations to explain to everyone that increasing the capacitor values also increases the RMS current in the mains transformer and rectifier diodes during normal operation (not just the inrush).

ilimzn said:
1) The data at the start of the thread is fine, but the comparison is not -= you end up cpmparing apples and oranges. What needs to be done is a heat generated vs power generated graph for 4 and 8 ohms, with a limit drawn in for a typical limit on maximum power dissipation and maximum current - just pick an amp for an example.
Hmmm.... :scratch2: Seems like an interesting idea - I'll try to find some time to do the maths. :yes:

- Richard B.
 
240 Volts said:
Interesting comments you made regarding the power supply design. Have you ever tried using a "SPICE" simulator to model power supplies... I think I'm going to have to run some SPICE simulations to explain to everyone that increasing the capacitor values also increases the RMS current in the mains transformer and rectifier diodes during normal operation (not just the inrush).

I routinely use a simulator, but it is not easy to model a real world transformer. In particular, core saturation is a problem.
Regarding RMS current with bigger caps, if all things are equal (same power demand) the RMS current stays the same. The peak current does not, and this gives rise to further things NOT being equal. For one, rectifier diode reverse recovery becomes a larger problem, and increases rectifier and transofrmer dissipation (as well as saturation and stray foelds). Dependion on how all of it is dimensioned, this can become a signifficant problem.
Further, minimum voltage of power rails increases, so both power reserve, as well as power dissipation increases - obviously, if you can get more power out of an amp, it will take more power out of the power supply, hence, any increase you get, increases rectifier and transformer RMS current, and obviously heat generation in the power supply.
 
Gotcha, but isn't the solution to this a good impedance matching speaker switch, a al ADC or Niles, the high fidelity answer to the physics involved? Do these devices not successfully make multiple speaker loads digestible for the amplifier? Or are they smoke and mirrors?
 
Grey.Coupe said:
Gotcha, but isn't the solution to this a good impedance matching speaker switch, a al ADC or Niles, the high fidelity answer to the physics involved? Do these devices not successfully make multiple speaker loads digestible for the amplifier? Or are they smoke and mirrors?


Yes they do the job and it's why they cost more then just a simple switch would. I have a nice compact all metal Russound unit that handles two amp sources and 4 speakers. Inside there are lots of power resistors that handle the impedence problem assosiated with powering multi-speakers. Keep in mind that one problem these type speaker switches don't solve is the difference in effeicency of the speakers being driven. That's not a problem if the speakers are located in different rooms, however if driving multi-pairs in the same room using different model speakers then one has a problem trying to balance the volume of the different pairs due to different SPL sensitivity ratings.

I use mine just for the ease of switching to different speakers and two different amp/receivers, not to drive multi-pair speakers at the same time so I'm not really utilizing the impedence protection feature of the switching box.

Lefty
 
Richard C. said:
Well, it's been a month since I asked about anyone knowing of a receiver or power amp that internally connects speaker outputs A and B in SERIES. I sure would like to see a partial schematic of the output section. I wouldn't hook up my speakers in series because of induced colorations, and my headroom would be gone in any SS amp I have! Each speaker output (series connection) would decrease by 6dB. 1/4 power!

I don't think this thread is dead until we discuss this.

Some Onkyo amps/receivers put A & B in series.
 
somebody 'splain to me why this won't work

The whole shooting match described below is hooked to my home theater stereo and controlled by it. I have a set of 4 small speakers and sub in a second listening room. I also have two sets of Polk outdoors speakers in my backyard. Hooked up a Niles HPS4 selector to switch between indoor and outdoor speakers. It is supposed to handle a high powered amp output. I bought a Crown D150 to power all these speakers independently using the selector and, sometimes, to power them all at once. I want enough power to piss off the neighbors every once in a while. I have read that these old Crowns are made to power a 4 amp load and bought the selector specifically to prevent a drop below 4 amps. I bought slow burn fuses and connected them between the amp and the selector.

Now. . . why won't this work? After all this trouble am I going to burn the amp or speakers anyway? Is the amp a bad choice for this application? Better amp? Is there a better set up? Some of you guys 'splain this to a diletante.

-j
 
True Observations.

With my Yammie Gear, M-2 pair and M-4, I have observed the following:

1) Amps get their hottest at idle. Are cool at wide open volumes.

2) Have pushed 2 Ohm Subs in parallel with 8 Ohm Speakers Doin it right now and for over two years!

3) M-2 aren't rated by manufacturer into less than 8 Ohms; although similar (in appearance), the Professional models are rated at lower impedances.

4) Blew a friends outputs in seconds after hooking up just the two ohm sub!

___________________________________________________________________

Haven't read nearly all the first post, and only scanned some others. So, please don't tell me to read all that stuff! I may even tread over some.

Many have argued that amplifier current reserves and specifically capability arent' very important. They have always been important in my book. I have tried to find amplifiers that would do the "ideal" doubling trick with halving of impedence, automotive or home or professional. Mind you, it's easier in the automotive world.

I have a Rockford Fosgate 50.1 & two 50.2 examples. They DO the doubling trick. The .1 is mono and the .2 examples are 25w X 2. When bridged they will push 0.5 Ohm. When the .1 is used, it will push reactive loads!

**Use some common sense here. If speakers outputs vary frequency vs IMPEDENCE, and frequency vs. sensitivity, and an amp's output impedence vs. power (*current really for the scope of this reply), an amp that is less efficient at the different impedences or varies wildly ....

You see where I'm going with this. Although diminishing returns when referring to ALL of the possible parameters to maximize for best (nearing ideal) performance of an amplifier, there are the cumulative effects of the lackluster attributes of the speakers, amplifiers, source, interconnects, ....

So, making the current capability of an amplifier near ideal DOES aid. How much, depends.

In cars, one has to use the 4 Ohm loads. Ask yourself how much current can be developed in a 12 V (12-->13.8-->14.6V) environment? Ah switching supplies and ultra low loads. Think distortion! NOTTTTTTTT! When done correctly, amplifiers can be optimized for lower impedences.

Rockford techs and engineers say that the 250.1 and 50.1 are essentially the same, with mostly power supply nuances. Yup my, 50.1 is the same size and weight as the 250! The set of 50.2s are just as heavy and expansive.

At relatively higher loads, the 250 will deliver the goods but won't venture down into reactivity or near shorting outputs. The 50s will supply a lil voltage at relatively high impedences but will shine in the abyss. They deliver the same maximum RMS Watts at different loads!

One day I will build my own Class A amplifiers with heavily paralleled outputs--enough in number supply ample current and quality transistors that will deliver high peak voltages. I'm moving down in class or would that be up.

Sorry, jsut had to post something, been missing for awhile enjoying and learning more in the hobby, craft, science, job and not being able to post.

M(y)-2 cents conceptually.
 
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