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lukeamdman

Luke's basic amplifier tests

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SME    211

This setup looks good.  I think you want a resistor rated for more than 50W or a lower resistance.  Didn't you say you had a load that dropped down to nearly 1 ohm?  Supposing your amp is pushing 80 V rms to achieve 6.4kW into 1 ohm, your 0.1 ohm resistor will see about 8V and will be dissipating something like 640 W.  Yikes!

 

I don't think you have to worry about inductance in the resistor for testing subs unless it's unusually high, like 1 mH or more.

 

Edit: I'm fairly certain the power dissipation problem is the reason the maximum current measurement in the Fluke and other meters is limited.

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MemX    97

I wish I could get my head round electricity :(  I just have to file it under the section marked 'things that happen by magic' in my head :lol:

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lukeamdman    154

This setup looks good.  I think you want a resistor rated for more than 50W or a lower resistance.  Didn't you say you had a load that dropped down to nearly 1 ohm?  Supposing your amp is pushing 80 V rms to achieve 6.4kW into 1 ohm, your 0.1 ohm resistor will see about 8V and will be dissipating something like 640 W.  Yikes!

 

I don't think you have to worry about inductance in the resistor for testing subs unless it's unusually high, like 1 mH or more.

 

Edit: I'm fairly certain the power dissipation problem is the reason the maximum current measurement in the Fluke and other meters is limited.

 

Yeah I think you're right.  From what my buddy was telling me, most of the time these low impedance wirewound resistors are wound in a way that cancels out inductance anyway even if they don't say it.

 

I'm going to drop down to a 0.01ohm 50w resistor.  Even in the worse case scenario of 6kw with a 1ohm load that should be no more than ~40w seen be the resistor.

 

http://www.mouser.com/ProductDetail/Vishay-Dale/RH050R0100FE02/?qs=sGAEpiMZZMtbXrIkmrvidDNaDpN5VXc5fz7udgZqacQ%3d

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lukeamdman    154

You can also wire your resistors in parallel, you'll get a lower resistance and higher power handling.

 

I was thinking of that as well, but I need/want to keep the resistance at 0.1, 0.01, 0.001, etc so I basically won't have to do any math.  0.08v will equate to 8a, and 0.10v will equate to 10a and so on.  I'll know for sure once I test the rig simultaneously with another meter wired in series to compare the results to confirm it's accurate. 

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SME    211

FWIW, I estimate that the 0.01 ohm resistor will still see 60 W, not 40 W with the amp pushing 6 kW.  At least that's in the ball park, but I'd probably opt for more power handling.  As lilmike suggested, you can always combine resistors to spread out the heat.  Four resistors in a series-parallel or parallel-series configuration will get you the same resistance but 4X the power handling.

 

Sorry in advance if you don't like the math.  For the amp, P_amp = I^2 * (R_load + R) with P_amp being power in watts, I the current in amps, R_load being the resistance (DC impedance) of the driver in ohms, and R is your resistor value in ohms.  Your resistor will have to dissipate P = I^2 * R power.  These equations combine to yield:

 

   P = P_amp*R/(R_load + R)

      = (6000 W) * (0.01 ohm) / ( 1 ohm + 0.01 ohm) = 59.4 W

 

With R being so small we could "assume" it is zero (negligible), giving a very close (1% error) estimate:

 

   P = P_amp*R/R_load = 60 W

 

Or for any particular *small* value of R:

 

   P = (6000 W/ohm) * R

 

Note that because you are trying to measure the amp power output rather than the power input to the driver (right?), you will probably want to put the volt meter across the amp terminals instead of the speaker terminals.  Otherwise, your math must consider the effect of R on the total power delivered by the amp.  In fact, your error will be equal to the amount of power dissipated in your resistor.  For your 0.01 ohm resistor, this will be 59 W, or about 1%.  This is of borderline importance if you are using a resistor with 1% tolerance (i.e., about the same amount of error), but at least you can avoid this error.

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lukeamdman    154

FWIW, I estimate that the 0.01 ohm resistor will still see 60 W, not 40 W with the amp pushing 6 kW.  At least that's in the ball park, but I'd probably opt for more power handling.  As lilmike suggested, you can always combine resistors to spread out the heat.  Four resistors in a series-parallel or parallel-series configuration will get you the same resistance but 4X the power handling.

 

Sorry in advance if you don't like the math.  For the amp, P_amp = I^2 * (R_load + R) with P_amp being power in watts, I the current in amps, R_load being the resistance (DC impedance) of the driver in ohms, and R is your resistor value in ohms.  Your resistor will have to dissipate P = I^2 * R power.  These equations combine to yield:

 

   P = P_amp*R/(R_load + R)

      = (6000 W) * (0.01 ohm) / ( 1 ohm + 0.01 ohm) = 59.4 W

 

With R being so small we could "assume" it is zero (negligible), giving a very close (1% error) estimate:

 

   P = P_amp*R/R_load = 60 W

 

Or for any particular *small* value of R:

 

   P = (6000 W/ohm) * R

 

Note that because you are trying to measure the amp power output rather than the power input to the driver (right?), you will probably want to put the volt meter across the amp terminals instead of the speaker terminals.  Otherwise, your math must consider the effect of R on the total power delivered by the amp.  In fact, your error will be equal to the amount of power dissipated in your resistor.  For your 0.01 ohm resistor, this will be 59 W, or about 1%.  This is of borderline importance if you are using a resistor with 1% tolerance (i.e., about the same amount of error), but at least you can avoid this error.

 

Good points.

 

The drivers that I will be using for this testing are a dual 2ohm configuration, with each coil having a dcr of 1.5ohm.  That being the case, I don't ever expect the resistance to ever drop below that 1.5ohm.  I see the worst case scenario between 1.5-2ohm. 

 

Good call on measuring the RMS voltage from the amp terminals rather than the sub terminals.  This was my plan but I goofed it on my super professional CAD-like diagram. 

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SME    211

Okay.  I wasn't sure what the actual dcr of your setup looks like.  You look to be fine for 1.5 ohm.  FWIW, the magnitude of the impedance never drops below the DCR.  I gather you are running two drivers on each amp channel then with a series-parallel or parallel-series configuration between each pair of drivers and each pair of voice coils on each driver?

 

Good luck on this.  It should be interesting to see what you find out.

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lukeamdman    154

Okay.  I wasn't sure what the actual dcr of your setup looks like.  You look to be fine for 1.5 ohm.  FWIW, the magnitude of the impedance never drops below the DCR.  I gather you are running two drivers on each amp channel then with a series-parallel or parallel-series configuration between each pair of drivers and each pair of voice coils on each driver?

 

Good luck on this.  It should be interesting to see what you find out.

 

Yes, with the two drivers in each box I'll series the coils and then parallel the drivers.  With 4 cabinets, each having a DCR of 1.5ohm, I have good ~2ohm and ~4ohm testing options.  For the ~2ohm testing I'll be able to share the load between all 8 drivers. 

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lukeamdman    154

I'm fairly certain I'm going to buy an entry level scope.  Maybe the Rigol DS1054Z. 

 

Doing this with multi-meters is clumsy and requires longer sine wave durations since I have to keep track of readings on two different displays. 

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dgage    67

I'm fairly certain I'm going to buy an entry level scope.  Maybe the Rigol DS1054Z. 

 

Doing this with multi-meters is clumsy and requires longer sine wave durations since I have to keep track of readings on two different displays. 

 

I just bought this one based on ShredHead's recommendation.

http://www.circuitspecialists.com/hantek-100mhz-digital-storage-oscilloscope-dso5102p.html?otaid=gpl&gclid=CNqi2brcrMkCFYI7aQodNTEBmg

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lukeamdman    154

 

Arggg.....

 

This Hanek and the Rigol are limited to 150v RMS?  My testing with the bridged CC5500 exceeded that dang it. 

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lukeamdman    154

It looks like the Hantek is actually rated for a little more voltage with a CAT III rating of 150Vrms.  If it can handle that then I doubt 154Vrms for a few seconds would cause it any problems.

 

Or I could always just buy 10x probe and call it a day. 

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dgage    67

It looks like the Hantek is actually rated for a little more voltage with a CAT III rating of 150Vrms.  If it can handle that then I doubt 154Vrms for a few seconds would cause it any problems.

 

Or I could always just buy 10x probe and call it a day.

 

$17 on Amazon. Ordered. Thanks.

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lukeamdman    154

$17 on Amazon. Ordered. Thanks.

 

On that link for the Hantek you posted is says it's a discontinued product? 

 

I'll probably go with the Rigol.  It's more expensive but it comes with two 10x probes and it's software upgradeable to be a DS1104Z ($850). 

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dgage    67

On that link for the Hantek you posted is says it's a discontinued product? 

 

I'll probably go with the Rigol.  It's more expensive but it comes with two 10x probes and it's software upgradeable to be a DS1104Z ($850).

 

I noticed that today as well. Supposedly mine can be upgraded since they're all basically the same core unit, at least that is my understanding. But I was leaning towards the Rigol as well until Shreds pointed me to the Hantek. Likely can't go wrong with any of them.

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lukeamdman    154

I noticed that today as well. Supposedly mine can be upgraded since they're all basically the same core unit, at least that is my understanding. But I was leaning towards the Rigol as well until Shreds pointed me to the Hantek. Likely can't go wrong with any of them.

 

I just pulled the trigger on the Rigol. 

 

It's going to make my head spin learning how to use this thing, but it'll sure be fun!

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lukeamdman    154

Sweet, the Rigol came with (4) 10x probes.  

 

Maybe tomorrow I'll have some time to assemble my resistor jig and try and measure something small.  

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dgage    67

What did you end up ordering for resistors?  Hurry up and assemble and test it so I can place my order. :)  I received my 10x probes the other day but didn't pull the trigger on the resistors until my beta tester gives me feedback. :)

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lukeamdman    154

What did you end up ordering for resistors?  Hurry up and assemble and test it so I can place my order. :)  I received my 10x probes the other day but didn't pull the trigger on the resistors until my beta tester gives me feedback. :)

 

I bought this one:

 

http://www.mouser.com/ProductDetail/Vishay-Dale/RH050R0100FE02/?qs=sGAEpiMZZMtbXrIkmrvidDNaDpN5VXc5fz7udgZqacQ%3d

 

0.01ohm 50W. 

 

Should be good for up to ~6kw at 1.5ohm loads.  If you need to measure more power than that, the cheapest way is four of these same resistors in a series/parallel configuration for 4x the power handling. 

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lukeamdman    154

Capture_zpsiibj0pur.jpg

 

This wiring diagram, where the leads to measure current are across the resistor, is not correct. 

 

When connected this way the current draw on the amp goes through the roof.  I'll call my buddy tomorrow to figure out what I'm doing wrong. 

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lukeamdman    154

Here's what I threw together today.  Amp connects on the left side. 

 

20160131_175550_zpstlq7ktek.jpg

 

 

The stubby wires on the left make it really easy to connect a 10x probe to measure Vrms. 

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lukeamdman    154

This wiring diagram, where the leads to measure current are across the resistor, is not correct. 

 

When connected this way the current draw on the amp goes through the roof.  I'll call my buddy tomorrow to figure out what I'm doing wrong. 

 

The wiring is correct, so I'm unsure if why I'm having issues.  I'm going to double-check everything and try again. 

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