>"Do the math. 8.9 W into 8 ohms is 8.5 Vrms, 12 Vpk and 24 Vpp, thus8.9 W is for a bridged amplifier. " You beat me to it.
Speaker Impedance
Started by ●December 15, 2017
Reply by ●December 23, 20172017-12-23
Reply by ●December 23, 20172017-12-23
On Fri, 22 Dec 2017 18:24:01 +0200, upsidedown@downunder.com wrote:>On Fri, 22 Dec 2017 08:56:05 -0500, krw@notreal.com wrote: > >>On Fri, 22 Dec 2017 09:50:27 +0200, upsidedown@downunder.com wrote: >> >>>On Thu, 21 Dec 2017 19:18:21 -0500, krw@notreal.com wrote: >>> >>>>On Thu, 21 Dec 2017 03:46:46 -0800 (PST), jurb6006@gmail.com wrote: >>>> >>>>>>"BTW, with the increased use of hybrids and full electric cars with >>>>>high battery voltages for the electric motors, why not use the motor >>>>>battery for also driving the audio amplifiers :-) " >>>>> >>>>>That would make them incompatible. >>>>> >>>>>Also in 24 volt systems there is usually a battery balancer if there is to be any 12 equipment run off it. >>>>> >>>>>In a nutshell, a nominal 12 volt supply will yield about 8.9 watts into 8 ohms. Divide and multiply as needed, like 17.9 into 4 ohms, and so forth. >>> >>>Do the math. 8.9 W into 8 ohms is 8.5 Vrms, 12 Vpk and 24 Vpp, thus >>>8.9 W is for a bridged amplifier. >> >>You assume a sine wave (i.e. 0% distortion). > > >Of course, that has been the standard practice for nearly a century: >Connect a known resistor to the amplifier and an oscilloscope across >the resistor. Turn up the sine signal until the waveform starts >visually to distort. Read peak to peak voltage and calculate the >power. It is to visually detect a few percent distortion (far before >reaching 10 % THD). > >Now the fucking PMPO people try to inflate the power readings for >marketing purposes. Why don't the use a square wave, to get bigger >numbers :-).It seems that some manufacturers prefer to advertise PMPO watts instead of continuous power, they usually have a too small mains transformer with thin secondary wire (high resistance) and small reservoir capacitors. At startup with mo load, the secondary voltage is directly proportional too the turns ratio. The capacitor charges to the secondary idle peak voltage. Applying a short signal burst to the amplifier for a few ms, the power is taking from the capacitor. For a longer burst (more than 10 ms), the flimsy transformer is no longer capable of resupplying the original capacitor voltage. --- A much more useful way of expressing amplifier power is the use of dBW (dB compared to 1 W). If one manufacturer claims 10 Front (10 dBl) and an other 15 Wpm po (11.8 dBW), the difference is only 1.8 dB, which sounds much less in an advertisement less a 50 % change in Watts.
Reply by ●December 23, 20172017-12-23
On Sat, 23 Dec 2017 12:38:25 +0200, upsidedown@downunder.com wrote:>On Fri, 22 Dec 2017 18:24:01 +0200, upsidedown@downunder.com wrote: > >>On Fri, 22 Dec 2017 08:56:05 -0500, krw@notreal.com wrote: >> >>>On Fri, 22 Dec 2017 09:50:27 +0200, upsidedown@downunder.com wrote: >>> >>>>On Thu, 21 Dec 2017 19:18:21 -0500, krw@notreal.com wrote: >>>> >>>>>On Thu, 21 Dec 2017 03:46:46 -0800 (PST), jurb6006@gmail.com wrote: >>>>> >>>>>>>"BTW, with the increased use of hybrids and full electric cars with >>>>>>high battery voltages for the electric motors, why not use the motor >>>>>>battery for also driving the audio amplifiers :-) " >>>>>> >>>>>>That would make them incompatible. >>>>>> >>>>>>Also in 24 volt systems there is usually a battery balancer if there is to be any 12 equipment run off it. >>>>>> >>>>>>In a nutshell, a nominal 12 volt supply will yield about 8.9 watts into 8 ohms. Divide and multiply as needed, like 17.9 into 4 ohms, and so forth. >>>> >>>>Do the math. 8.9 W into 8 ohms is 8.5 Vrms, 12 Vpk and 24 Vpp, thus >>>>8.9 W is for a bridged amplifier. >>> >>>You assume a sine wave (i.e. 0% distortion). >> >> >>Of course, that has been the standard practice for nearly a century: >>Connect a known resistor to the amplifier and an oscilloscope across >>the resistor. Turn up the sine signal until the waveform starts >>visually to distort. Read peak to peak voltage and calculate the >>power. It is to visually detect a few percent distortion (far before >>reaching 10 % THD). >> >>Now the fucking PMPO people try to inflate the power readings for >>marketing purposes. Why don't the use a square wave, to get bigger >>numbers :-). > >It seems that some manufacturers prefer to advertise PMPO watts >instead of continuous power, they usually have a too small mains >transformer with thin secondary wire (high resistance) and small >reservoir capacitors.Reservoir capacitors? LOL!>At startup with mo load, the secondary voltage is directly >proportional too the turns ratio. The capacitor charges to the >secondary idle peak voltage. Applying a short signal burst to the >amplifier for a few ms, the power is taking from the capacitor. For a >longer burst (more than 10 ms), the flimsy transformer is no longer >capable of resupplying the original capacitor voltage.Automotive amplifier secondary voltage? <snicker> If you want to learn something about amplifier power, think about the crest factor of typical music. Hint: people don't play sine waves into resistors to heat their rooms. PMPO, my ass.>--- > > A much more useful way of expressing amplifier power is the use of >dBW (dB compared to 1 W). If one manufacturer claims 10 Front (10 dBl) >and an other 15 Wpm po (11.8 dBW), the difference is only 1.8 dB, >which sounds much less in an advertisement less a 50 % change in >Watts.Clueless.
Reply by ●December 23, 20172017-12-23
krw@notreal.com wrote:> > If you want to learn something about amplifier power, think about the > crest factor of typical music. Hint: people don't play sine waves into > resistors to heat their rooms. PMPO, my ass.A few years ago I saw a set of computer speakers marked 1200W PMPO. It was powered by a 9V half amp wall wart. I almost bought it, just as a gag gift for an engineer I worked with.
Reply by ●December 23, 20172017-12-23
On Sat, 23 Dec 2017 00:48:22 -0800 (PST), jurb6006@gmail.com wrote:>>">In a nutshell, a nominal 12 volt supply will yield about 8.9 watts into 8 ohms. Divide and multiply as needed, like 17.9 into 4 ohms, and so forth. > >Double that, again, for a full bridge output. " > >No, that is for a full bridge. That's right, half bridge only puts out 2.25 watts/8 ohms. > >>">Looking at the DC convertor in a standard cat amp, it doesn't add that much cost on a manufacturing level. The transformer is cheap, you need a uPc type chip for the drive/control. The pound of silicon is cheap these days. Just a bunch of MOSFETs in parallel. the heatsinks cost more. heatsinks you got a tradeoff, either alot of aluminum or cheat with a high efficiency design, which is a more expensive solution, or maybe even one of those berylium heat pipes. (which doesn't really lend itself well to this application)" > >"Wrong." > >Which part ?All of it.
Reply by ●December 23, 20172017-12-23
On Sat, 23 Dec 2017 10:06:08 -0500, Michael A Terrell <mike.terrell@earthlink.net> wrote:>krw@notreal.com wrote: >> >> If you want to learn something about amplifier power, think about the >> crest factor of typical music. Hint: people don't play sine waves into >> resistors to heat their rooms. PMPO, my ass. > > > A few years ago I saw a set of computer speakers marked 1200W PMPO. >It was powered by a 9V half amp wall wart. I almost bought it, just as a >gag gift for an engineer I worked with.There's always fraud. Sears (and they're certainly not alone) advertises 6.5HP shop vacs that have 16Ga. cords with NEMA 5-15P plugs.
Reply by ●December 23, 20172017-12-23
>" A few years ago I saw a set of computer speakers marked 1200W PMPO.It was powered by a 9V half amp wall wart. I almost bought it, just as a gag gift for an engineer I worked with. " THE HOLY GRAIL AT LAST ! Our troubles is over. Almost 1,000 % efficiency.
Reply by ●December 23, 20172017-12-23
In article <0kss3dlimt5fpptjv94n8mn2ahlq8161kb@4ax.com>, krw@notreal.com says...> > On Sat, 23 Dec 2017 10:06:08 -0500, Michael A Terrell > <mike.terrell@earthlink.net> wrote: > > >krw@notreal.com wrote: > >> > >> If you want to learn something about amplifier power, think about the > >> crest factor of typical music. Hint: people don't play sine waves into > >> resistors to heat their rooms. PMPO, my ass. > > > > > > A few years ago I saw a set of computer speakers marked 1200W PMPO. > >It was powered by a 9V half amp wall wart. I almost bought it, just as a > >gag gift for an engineer I worked with. > > There's always fraud. Sears (and they're certainly not alone) > advertises 6.5HP shop vacs that have 16Ga. cords with NEMA 5-15P > plugs.Maybe a slip up? The media gets away with it ;) Lets see... There is 550 foot pounds in 1 horse power, so does that mean it can pick up 3575 ibs? I think not, more likely that is the expected life cycle of the vac before it crokes ;) Then there is the electrical horse power, I don't see it drawing 40 amps !. Of course you do have inrush with is ~ 4 (curve b) times, so that being the case, I would say maybe its ~18 amps but that surely is going to get the cord hot..! Jamie
Reply by ●December 23, 20172017-12-23
On Sat, 23 Dec 2017 10:42:26 -0500, M Philbrook <jamie_ka1lpa@charter.net> wrote:>In article <0kss3dlimt5fpptjv94n8mn2ahlq8161kb@4ax.com>, krw@notreal.com >says... >> >> On Sat, 23 Dec 2017 10:06:08 -0500, Michael A Terrell >> <mike.terrell@earthlink.net> wrote: >> >> >krw@notreal.com wrote: >> >> >> >> If you want to learn something about amplifier power, think about the >> >> crest factor of typical music. Hint: people don't play sine waves into >> >> resistors to heat their rooms. PMPO, my ass. >> > >> > >> > A few years ago I saw a set of computer speakers marked 1200W PMPO. >> >It was powered by a 9V half amp wall wart. I almost bought it, just as a >> >gag gift for an engineer I worked with. >> >> There's always fraud. Sears (and they're certainly not alone) >> advertises 6.5HP shop vacs that have 16Ga. cords with NEMA 5-15P >> plugs. > >Maybe a slip up? The media gets away with it ;)Not accidental: <http://www.sears.com/craftsman-xsp-16-gallon-6.5-peak-hp-wet-dry/p-A011399822?plpSellerId=Sears&prdNo=5&blockNo=5&blockType=G5> <https://www.shopvac.com/product/shop-vac-16-gallon-6-5-peak-hp-wet-dry-vacuum-with-svx2-motor-technology-catnum:5801611> <https://www.stanleytools.com/en-us/products/hand-tools/other-tools/wet-dry-vacuums/8g-6hp-industrial-stainless-steel-vacuum/sl18401-8b> <https://www.homedepot.com/p/RIDGID-16-Gal-6-5-Peak-HP-Wet-Dry-Vac-WD1851/100638360>> > Lets see... > > There is 550 foot pounds in 1 horse power, so does that mean >it can pick up 3575 ibs? I think not, more likely that is the expected >life cycle of the vac before it crokes ;) > > Then there is the electrical horse power, I don't see it drawing >40 amps !. Of course you do have inrush with is ~ 4 (curve b) times, so >that being the case, I would say maybe its ~18 amps but that surely is >going to get the cord hot..!Unloaded (without blower) RPM times locked rotor torque? 125V X locked rotor current? Who knows?
Reply by ●December 23, 20172017-12-23
OGee wrote:> I have one test instruments to test with, a VOM. > > I have several old speakers that I want to hook up to a amp I am > trying to build. > > How do I determine the impedance of the speakers with only a VOM ? > > What value of resistance would I measure across the speaker coil for > different speaker impedance of say 4 or 8 or ? and other ohm speaker > impedance ? > > Is there a table someplace ? > > TIAIf there are plain on conde drivers (woofers, midrange, tweeters) and you are lonly looking to drive a "nominal" impedance with the amp your are building so you roughtly know the load. I agree with Phil 1.2 times the DCR is a good estimate for Nominal. It will only be near the DCR at frequencies below the responate peek (FS) and also about an octave above. Then the inductance takes over and it rises. For crossover design you need to account for the impedance in the range you are crossover over the drivers. One way is the use a R-C shunt across the coil to cancel the impedance rise with frequency and make the impedance look more flat and resistive. These are commonly called Zobels. Once you identify the nominal impedance. Normal resistive parallel/series conbination rules apply for speakers operating on the same frequency range. If you put a properly desiged crossover between the woofer, mid and tweeter, then even though you have three "8 ohm" speakers in parallel, the total impedance remains 8 ohms since they do no operate in the same frequency range.
