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toto34: This depends on your bias setting, use ohm's law: Voltage / I (current) = Resistance (of speaker load).
have a look at what PRR had to say about this just a short while ago
If you're looking to drive a 4R load, you should be thinking about lower voltage levels and higher current than as specified in Mike's write up.
In general: More power into an 8R load calls for higher voltage with less current. More power into a 4R load calls for lower voltage with higher current. All of this needs to be balanced so you don't cook your output mosfet. I've been running a pair of MoFo amps for over a year where each mosfet bakes off about 60w of heat with a 30c temp rise on the sink. This is about as high as you can take things without needing forced air to keep things cool.
have a look at what PRR had to say about this just a short while ago
If you're looking to drive a 4R load, you should be thinking about lower voltage levels and higher current than as specified in Mike's write up.
In general: More power into an 8R load calls for higher voltage with less current. More power into a 4R load calls for lower voltage with higher current. All of this needs to be balanced so you don't cook your output mosfet. I've been running a pair of MoFo amps for over a year where each mosfet bakes off about 60w of heat with a 30c temp rise on the sink. This is about as high as you can take things without needing forced air to keep things cool.
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Your inductors are probably measuring correctly. Inductance depends highly on frequency of measurement. One meter that I have will measure at either 120Hz or 1kHz and various components sometimes measure differently depending on which frequency I choose.
I've never had an inductor actually measure what the spec sheet says... Mine have tended to measure a bit less than the specs show. Sorry that I don't have one of these to compare for you.
I've never had an inductor actually measure what the spec sheet says... Mine have tended to measure a bit less than the specs show. Sorry that I don't have one of these to compare for you.
Thanks Eric. I mostly test inductors for speaker crossovers. They do measure as quoted by the manufacture. I guess my meters might be ideal for that application.
I am still hoping someone can measure their Hammonds and see it they are off like I am reading.
They are both marked 159ZE. I have a very expensive when new ESI impedance meter and a cheap multimeter with inductor test capability. Can someone put a meter across their hammond inductors they are using for the MOFO and see if they read the exact value stated on the inductor. Any model will help me. That is my simple request. Thanks
I did some measurements with a PeakTech 2175 LCR Meter on a few chokes I have lying around.
Hammond 195T, 100mH
1: 64,3mH
2. 69,6mH
3. 68,7mH
4. 68,5mH
TRIAD C-59U, .01HY
1: 8,8mH
2: 8,7mH
Inductors for crossovers have very huge gap or are air cored, so they change inductance very little with frequency. Iron cored parts, with little or no gap, changes with frequency (like Eric mentioned), amplitude of measuring signal and DC standing current, so for measuring the manufacturer value is needed to measure with same parameters. I presume that chokes are measured with some intended DC bias current.
Thank you! Your test of the Hammond tend to support my lower readings. That is all I am trying to verify. Anyone else have one to measure?
There are some comments on some Hammond spec sheets that is in line
with what DIYBras is saying:
https://www.mouser.ca/datasheet/2/177/193-736896.pdf
https://www.mouser.ca/datasheet/2/177/153-159-736947.pdf
"Inductances measured at rated D.C. current."
"Proper Henry testing requires an LCR meter in junction with a 25A power supply that
does allow for DC bias as well a higher applied AC drive voltage to meet the test
specifications."
with what DIYBras is saying:
https://www.mouser.ca/datasheet/2/177/193-736896.pdf
https://www.mouser.ca/datasheet/2/177/153-159-736947.pdf
"Inductances measured at rated D.C. current."
"Proper Henry testing requires an LCR meter in junction with a 25A power supply that
does allow for DC bias as well a higher applied AC drive voltage to meet the test
specifications."
> the units I have measure just under 15 mH
As others say: the inductance of an iron core coil varies with coil current. On a large "DC Choke" it typically comes up significantly from zero DC current to a peak somewhere below rated DC current, should be near its spec AT rated DC current, and then falls at over-current.
The inductance also varies with frequency, being most at very low F and falling off above 300Hz due to eddy infesting the lams. (The fall-off is less than the rise of F so this does not compromise response.)
In principle you could measure L in-circuit with the amp live. But you have to block DC without adding much impedance in the meter loop. And in this case, looking-into the Source of a big hot MOSFET, it will be very hard to measure the shunting effect of the L. (Which is why it hardly matters what L you get.)
That's a DC Choke. For wave-filters (crossovers), variation of inductance is very bad. Iron-core speaker chokes are made with much larger air-gaps to swamp-out the variation in the iron. Therefore they cost more per H and conductivity than DC chokes.
As others say: the inductance of an iron core coil varies with coil current. On a large "DC Choke" it typically comes up significantly from zero DC current to a peak somewhere below rated DC current, should be near its spec AT rated DC current, and then falls at over-current.
The inductance also varies with frequency, being most at very low F and falling off above 300Hz due to eddy infesting the lams. (The fall-off is less than the rise of F so this does not compromise response.)
In principle you could measure L in-circuit with the amp live. But you have to block DC without adding much impedance in the meter loop. And in this case, looking-into the Source of a big hot MOSFET, it will be very hard to measure the shunting effect of the L. (Which is why it hardly matters what L you get.)
That's a DC Choke. For wave-filters (crossovers), variation of inductance is very bad. Iron-core speaker chokes are made with much larger air-gaps to swamp-out the variation in the iron. Therefore they cost more per H and conductivity than DC chokes.
Convection is a function of height and temperature; and IIRC height squared.
1,000 degree smoke up 30 feet sure is big convection.
50 degree heat up a box-size height makes much less difference.
However 50d in an 8-foot rack of identical telecom boxes starts to make a difference. A box that runs hot on its own has been observed to run *cooler* when stacked by the dozens.
Hot Air Rises and Heat Sinks
1,000 degree smoke up 30 feet sure is big convection.
50 degree heat up a box-size height makes much less difference.
However 50d in an 8-foot rack of identical telecom boxes starts to make a difference. A box that runs hot on its own has been observed to run *cooler* when stacked by the dozens.
Hot Air Rises and Heat Sinks
Don't know about a toroid but I used the 8 ohm secondary of a high quality 30 watt 70 volt loudspeaker line matching transformer quite successfully. Inductance and DC resistance measured about the same as the smaller recommended Hammond choke but I'm running less voltage drop across the inductor, about 0.8 VDC, because it sounded better to me than a full 1 volt. The higher current was probably saturating the core. My power supply is 18VDC.
In general, toroids saturate "too easy" on DC. (Your 3.75A DC rating is probably for *after* a rectifier.)
E-I type cores are harder to saturate because the iron has air-gaps. Optimum is an actual gap. But even over-laps as usually found on "AC only" E-I cores delay DC saturation. A truly gapped core could be much smaller. But an OVER-size E-I core can be found in trash.
When the core saturates the inductance falls-off almost 1000:1 -- it becomes more like an air-core coil the same size. We yakked about an air-core in this thread, but I don't think anybody can afford that much copper.