High power audio amplifier load - Page 1
Forum section: electronics / projects

author: electricspark
author_rank: Contributor
timestamp: February 19, 2016, 06:03:40 pm
content: I am going to build a high power audio amplifier load. It needs to be switchable between 2, 4, 8 & 16 ohms. I plan to use 100W, 4 Ohm wirewound resistors giving me 2 Ohms 800W, 4 ohms 900W, 8 ohms 800W and 16 ohms 400W. I thought a few relays and the job would be done.The problem is, in addition to the nine resistors per channel, I need 17 relays per channel! Am I being dense, is there a simpler way of doing this?I have attached my schematic in TinyCad format.Cost is of the essence!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2016, 07:43:01 pm
content: If cost is of the essence, why not strap the resistors to a board and wrap wire around the ones you need?If automation is of the essence, expect to pay for it!I can't tell what the hell is going on in your schematic. Could you draw the networks in each state?I don't see there's any way to arrange 8 resistors to achieve R/2 total resistance at 8*Pmax ratings. I think I see R + (R || R) which is 3R/2, three times in parallel to get R/2, but the series resistor dissipates twice the power of the parallel pair so the maximum power rating will be 2/3 of 8*Pmax. I suspect you'll have similar problems with the other configurations?Tim

author: electricspark
author_rank: Contributor
timestamp: February 19, 2016, 09:30:09 pm
content: At the end of this I want a piece of test equipment that will easily and quickly give me a 2, 4, 8 or 16 Ohm load. Wrapping wire around the required resistors is not what I want to do. I listed the relays that need to be closed on the schematic to give the required resistances and powers. Can anyone figure out a way to keep the power and resistances but use fewer relays?I have to agree, the schematic shows and causes me massive confusion!What it shows is a total of nine x 4? resistors, each rated at 100W. The resistors can be switched using the relays to give;Four x 4? resistors in parallel to give 1?, in series with another 4 x 4? resistors in parallel to give 2? at 800W.Three x 4? resistors in series, in parallel with another three in series, in parallel with a third set of three in series give 4? at 900W.Four x 4? resistors in series in parallel with another four x 4? in series to give 8? at 800W.Four x 4? resistors in series to give 16? at 400W.Getting the required values of resistance is easy, but the switching makes things very confusing. I was wondering if there was an easier way to achieve this without the seventeen relays per channel.

author: dannyf
author_rank: Super Contributor
timestamp: February 19, 2016, 09:35:52 pm
content: Think light bulbs and heating elements.

author: Paul Price
author_rank: Super Contributor
timestamp: February 20, 2016, 12:01:53 am
content: ..and a big bucket of water.

author: KE5FX
author_rank: Super Contributor
timestamp: February 20, 2016, 12:23:01 am
content: I wonder if, given that you can use PWM to build a class-D audio amplifier with a lowpass filter (or by treating the speaker as a reactive load that does its own lowpass filtering), you could build a minimally-dissipative active load the same way.The amp would think it's driving a perfect resistive load, in the sense that a given level of output voltage would cause a proportional amount of current to flow in the PA stage, when in reality it's driving an LC circuit that's made to look like a broadband load through appropriate switching hackery.

author: Paul Price
author_rank: Super Contributor
timestamp: February 20, 2016, 12:36:25 am
content: The easiest and simplest and cheapest way to do whatcha wanna do is to make two sets of discrete 800-watt 2, 4, 8, and 16 ohm modules (each sets of appropriate-valued resistors on an aluminum chassis with a small snap on fan) that can be strapped together with Pomona banana jacks to fit the menu of the day.In this way you can forget about so many relays that can handle the current you will need that will cost money too, not to mention the difficulty in packaging all these relays and resistors in one hot box!Obviously with this approach, you can gang some combos of these modules together so you don't need 2 sets of 4 different 800W modules.For instance, four max. (stereo) pairs of 800W 4-ohms modules can be config'd to serve as 8, 4 or 2 ohm loads, that only leads to having two more 200W 8-ohm modules to give you 400W 16-ohm loads.

author: Smokey
author_rank: Super Contributor
timestamp: February 20, 2016, 12:46:04 am
content: Here are a few sources of high wattage resistor elements:http://www.costco.com/Presto%C2%AE-HeatDish%C2%AE-Plus-Parabolic-Heater.product.100142330.htmlhttp://www.homedepot.com/p/Everbilt-Electric-Water-Heater-Tune-Up-Kit-EB20018/204834414

author: Paul Price
author_rank: Super Contributor
timestamp: February 20, 2016, 12:57:02 am
content: KE5FX: Your plan violates the principle of conservation of energy.Smokey and dannf: Heating elements and light bulbs have very high positive Temp. Coeff. and so are not so cool to use where a known fixed value load is required for a test.

author: KE5FX
author_rank: Super Contributor
timestamp: February 20, 2016, 01:16:08 am
content: Quote from: Paul Price on February 20, 2016, 12:57:02 amPossibly, but if it's a matter of forcing the output stage to do the dissipating...It does lead to the question of what one would expect to happen if such a load were, itself, driven by a class-D amp.

author: Paul Price
author_rank: Super Contributor
timestamp: February 20, 2016, 01:36:45 am
content: If you force the amplifier being tested to dissipate the energy to be delivered to the load then you are testing the amplifier in a way that would likely cause it to emit Smokey and the amplifier would not be connected to a desired fixed value known resistance value.

author: Smokey
author_rank: Super Contributor
timestamp: February 20, 2016, 03:21:31 am
content: Quote from: Paul Price on February 20, 2016, 12:57:02 amTrue... True...Constant resistance active load. Problem solved.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2016, 03:30:51 am
content: You can arrange a switching converter with feedback, such that its input looks resistive; but the energy necessarily has to go *somewhere*. Only an infinite LC network (aka transmission line) could sustain that for a long period of time. With a converter, you can dump it into a supply rail, but obviously, that's not an infinite sink either... If you loop it back to the amplifier's supply rail (not a very general approach, sadly), you circulate the power back where it came from and save on some total dissipation.Ideally, an active load would have a very general input, and a grid-tie output (so it circulates back into the grid, where it eventually comes back to the amplifier, or whatever's generating the power anyway), but that's pretty hard to do, so it's easier just to burn it.Tim

author: Smokey
author_rank: Super Contributor
timestamp: February 20, 2016, 04:57:45 am
content: Do you actually have 800W of drive in your amplifier?

author: pmbrunelle
author_rank: Regular Contributor
timestamp: February 20, 2016, 05:50:34 am
content: If this is for audio, don't you want some inductance in series with the resistance?This way, the impedance will rise with frequency, just like a real speaker's voice coil.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 20, 2016, 06:12:20 am
content: Hi,I don't know about the ratio of cost resistors versus relays, but I propose this solution:It minimizes the switches but requires 16 resistors instead of 9.Regards,Jay_Diddy_B

author: Synthetase
author_rank: Regular Contributor
timestamp: February 20, 2016, 06:22:09 am
content: Quote from: electricspark on February 19, 2016, 06:03:40 pmSimplest solution I've found:http://sound.westhost.com/project124.htm

author: diyaudio
author_rank: Frequent Contributor
timestamp: February 20, 2016, 02:12:54 pm
content: Quote from: electricspark on February 19, 2016, 06:03:40 pmlooks like you building an audio amplifier dyno rig. (the loading part) you could probably do RMS measurement with a cheap micro and a LCD.http://www.12voltnews.com/2013/07/24/damore-engineering-intros-amp-dyno/

author: electricspark
author_rank: Contributor
timestamp: February 20, 2016, 02:29:32 pm
content: Quote from: Jay_Diddy_B on February 20, 2016, 06:12:20 amThanks Jay_Diddy_B – You have solved my problem. I think I had a block and got stuck with the minimal number of resistors and way, way overcomplicated the switching. I have a stack of assorted DJ amps from 100W to 1Kw to get fixed and need the load to test them. Thanks again.

author: djQUAN
author_rank: Frequent Contributor
timestamp: February 20, 2016, 02:45:43 pm
content: Inspired by Rod's P124 project, I built mine this way. Probably not what you wanted but might give some ideas?http://quan-diy.com/projects/dummy/load.htm

author: Smokey
author_rank: Super Contributor
timestamp: February 20, 2016, 08:14:22 pm
content: Quote from: djQUAN on February 20, 2016, 02:45:43 pmThat's a good looking fixture. You will get WAY more heat dissipation if you use chassis mount resistors on plates and blow across the plates with a fan.http://www.mouser.com/ProductDetail/Ohmite/HS100-16R-F/?qs=bvCpHSZmQjnTyGOZqaJnNg%3D%3D&gclid=CNSqpO6Th8sCFQyNaQodfwALQw

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2016, 09:33:09 pm
content: FYI, heatsink type resistorsonly achieve their ratings with a suitable heatsink.Check the datasheet for Pd vs. Tc. Failure to observe this will result in... well, consider what expanding gas does inside a trapped cylinder...They are the cheapest and poorest-performing power resistor in their class, because the resistive wire has to dissipate heat through an epoxy potting, rather than a high temperature ceramic or enamel.They are handy when you already need a relatively low temperature heatsink, i.e., alongside a lot of semiconductors. The deratings for transistors are the same way, so it works out well, as long as the dissipation ratings have enough margin.As for ceramic resistors,you literally can't use them at their theoretical limit, because the connections melt off, or something like that. (Okay, if you bolt rather than solder to the terminals, you can keep on cooking for a while...) Short-term power dissipation is limited by thermal shock (you can't instantly heat just the wire element to red hot, and expect the enamel to not spall off..), and long-term reliability is limited by contacts, oxidation and cycling stress.For any resistor type, ratings with forced cooling may not be higher (in other words, the max. power limit doesn't depend on cooling, above the minimum amount of air flow or whatever's necessary), because mechanical / strain / thermal shock limits may come into play first. That is, the resistive element will have a constant temp rise above the outer surface, at some given power level; too much difference there, and the materials break down or crack or whatever. Obviously, any resistor that has better thermal transfer, will do a better job with forced cooling -- open wire type resistors, for example, might operate at many times their in-air ratings, when submerged in circulating water or oil.For sheer dissipation/dollar (including hardware, like the lack of heatsinking!), silicone-enamel and vitreous ceramic types seem to win the most.Tim

author: GamerAndds
author_rank: Regular Contributor
timestamp: February 21, 2016, 12:35:02 am
content: I cant wait to read this tread later. Ive been looking for a tread like this!Just as an asside I used 2 water heating elements from an old water heater to test my amp (100 watts per channel into 2 ohms). They got hot to the touch! Putting them in water would have been smart but something to think about....

author: RobertBG
author_rank: Regular Contributor
timestamp: February 21, 2016, 03:17:31 am
content: Like T3sl4co1l mentioned open wire resistors do quite well in these cases when submerged.I have a load bank using them but they are in cementwhen I was testing things in open air I would get nervous approaching the limit but thus far they have survived 4 times the rated power encased in a bucket of concrete.

author: djQUAN
author_rank: Frequent Contributor
timestamp: February 21, 2016, 03:18:34 am
content: Quote from: Smokey on February 20, 2016, 08:14:22 pmI was originally thinking that way but the aluminum cased resistors available here are pricey and of dubious quality. These ceramic ones are very cheap and readily available. My dummy load only gets used once in a while so I didn't think of spending a lot on it.If I need a lot higher power dissipation, I could just immerse the whole thing in a bucket of water

author: German_EE
author_rank: Super Contributor
timestamp: February 21, 2016, 10:28:05 am
content: Way WAY WAY too complicated!Wire 2 x 2 ohm, 1 x 4 ohm and 1 x 8 ohm resistor in series and put a normally closed relay contact across each resistor (four relays)For 2 ohms open the contact across one 2 ohm resistorFor 4 ohms open the contacts across both 2 ohm resistorsFor 8 ohms open the contacts across 2 x 2 ohm and 1 x 4 ohmFor 16 ohms open all relay contacts

author: John Heath
author_rank: Frequent Contributor
timestamp: February 21, 2016, 09:41:52 pm
content: Quote from: Paul Price on February 20, 2016, 12:01:53 amI can second big bucket of water. 2 cups of water with 2 resistors 10 ohm wire wound 10 watts each. I blasted these puppies with 300 watts for 10 minutes for a power amp distortion test and bias current creaping. The resistors survived this.

author: cvanc
author_rank: Frequent Contributor
timestamp: February 21, 2016, 11:16:25 pm
content: Quote from: German_EE on February 21, 2016, 10:28:05 amSure, but didn't the OP say he wants to handle at least 800 watts regardless of resistance setting? That's gonna be some expensive resistors if built as you describe.

author: German_EE
author_rank: Super Contributor
timestamp: February 22, 2016, 04:50:59 pm
content: Ouch! I missed that bit. 800W of anything demands some serious engineering.

author: AF6LJ
author_rank: Unknown
timestamp: February 22, 2016, 05:55:01 pm
content: This is a good thread, looking forward to seeing what others have to say in regard to the 800W load.

author: German_EE
author_rank: Super Contributor
timestamp: February 22, 2016, 09:00:55 pm
content: For reference here is a 7.5 ohm 800W resistor:http://www.mouser.com/ds/2/303/res_tap800-753024.pdfMouser will sell you one for 184.10 Euro. You can also get them at 4 ohms and 16 ohms for about the same price then after that you will need a very large heatsink. I suspect that there are cheaper solutions but 800W in the wrong place if something goes wrong will lead to an interesting day.

author: dannyf
author_rank: Super Contributor
timestamp: February 22, 2016, 09:15:22 pm
content: QuoteLike light bulbs or heaters?