8 OHM Audio Dummy Load for Amplifier testing Design and Build

A welcome addition to the workbench, this Audio Dummy load was built using two 8 Ohm 300 Watt resistors I found on eBay for a good price. I had a few challenges designing the oscilloscope output because I didn't draw out the circuit before I built it (rookie error!). In the end, I think this project turned out pretty good. Maybe you'll want to build one for yourself. Enjoy!

[@prettygoodradio]

Dude - Thank you again for sharing your knowledge and wisdom. You are on regular rotation here in my shop. SO HELPFUL! Keep up the great work!

[@Justsquareenough]

just found your channel looking for reviews of the feeltech function generator which lead to me watching two or three more of your videos. i like your presentation style. thanks for sharing this stuff.

[@michaeljohnlavin4283]

Yet another great video! Thanks Tony:)

[@robertcrawford7052]

I like the voltage divider so you can listen to a speaker at low level as you load test the amp. Also, I would frequency-compensate the scope voltage divider as I find square wave testing useful for evaluating frequency and phase response.

[@allthegearnoidea6752]

Interesting video I’m just starting to get interesting in amplifier and built a similar but much lower power audio dummy load. Thanks for sharing best wishes to you for a happy Christmas

[@navpers4764]

Awesome job ! thank you for your time :)

[@BillnBen1]

Hi, don't worry about the diagram, I followed the link below to the other video. Many thanks the vids have been really helpful.

[@dicko195]

In 1979 I had the old KRACO 40w model 131b car amp, it expressly warns to keep speaker leads separate from chassis ground. I was 19 and didn't do that, so the power cable drew a ton of amps and turned cherry red and burnt my carpet in my 71 Firebird. Lesson learned.

[@ferfromla]

Yes it would be nice to have a schematic. Like you said it is better to draw things out first.

[@patrickoconnell3896]

Nice video, I have been thinking of building something like this.

[@kenfoland]

I also DIYed my dummy load. I used an old 1500 Watt Electric Heater. I removed the nichrome wire heating element from the high voltage circuit. I shunted part of the element to match 8Ω at 1kHz, added the amp inputs, and added a switchable 1X/10X BNC output to my oscilloscope. I left the rest the HV circuit to operate the thermostatically controlled fan. But, the fan is only needed with 1000+ watt amplifiers. So, it rarely is needed and is left unplugged. Whenever there is a mains powered DUT on the bench, I power it through a variac that gets its power from a isolation transformer to remove the mains earth reference and the inherent dangers associated with it. The dummy load works great, is small enough to store away easily, and I have yet to meet an amp that it couldn't handle. And, best of all, it cost me nothing but time.

[@1959Berre]

Two 300 watt resistors means a lot of heat. Together, that comes close to a bread toaster. I have my 100 watt (!) resistors mounted on a heavy oversized heat sink (210 x 120 x 60 mm) with 20 panels. I would recommend installing a fan. Nice equipment you got, I wish... :)

[@xraytonyb]

Already had it on a 120WPC receiver at full volume. The resistors only got lukewarm. They are huge, and so is the aluminum extrusion they are attached to!

[@XavierAncarno]

I like your videos... So handy

[@BessetteMusicRepair]

Nice! You've got just a tad more eico gear than me :)

[@DoRC]

it's be cool to find a couple low profile processor heatsinks and thermal epoxy them to the inside of the tube.

[@markgilbert9930]

Nice!

[@lextr3110]

question: can you use only one dummy to test/fix each channel separately one after the other or do I need 2 plug at the same time???

[@markferrari9734]

1:52. Nice catch on the f bomb.

[@johnny7254]

he was about to say "fine"

[@JohnUsp]

Thanks for the video. I have 2 dummy loads of 4.2 and 8.4 ohms, can I use for load testing of 4 and 8 Ohms? What's the maximum tolerance? Thanks.

[@sparkey1958]

hi, i am liking this, but as there is no alternative to hearing a amp, i have been trying to come up with a dummy load that also has a speaker connection, so i can pump say 50 watts in, but actually monitor the sound with say 2 to 3 watts out to a bench speaker...i cant always crank up an amp at night got to get on with the neighbours!

[@JasonLeaman]

Radio Shack ? where is that place :) Good video sir, i bought a 8ohm 1kw unit. Soon Id like to buy another 8ohm so i can do measurements with 8 & 4 ohm. Only bought the 1kw ones because they were same prices as the 300watt ones, and allows me to run them longer when testing amplifiers.

[@pmartinlab]

Hi! great project! I´m testing an 8 Ohm prototype made of 2 pairs of 8[Ohm] 100[W] resistors, and I´m making initial tests with 3 [A], 24 [V] and making augments of 1[V] I have noticed two things, I will appreciate if we can share some info about it. Here is what I found: 1.- Do you have any specific test to confirm it´s working? I made this calculation: To get the maximum input values: P= 124 [W] @ 1k [Hz] (info from manufacturer, hand made amplifier) Z= 8 [Ohms] => I = sqrt(P/Z) which is a bit less than 4 [Amp] => Vout max from amp = 4*8 = 32 [V] So with this I decided to make the system stable for a DC signal of 32*sqrt(2) = 45.3 [V], and an AC signal, 1k [Hz], V peak = 45.3[V] 2. - The resistors gets VERY hot : I ´m making measurements, and just got a thermometer to confirm but I could estimate about 80°Celsius by touching it. Do you know if there is a wat of estimating or calculating that? I´m concerned with loosgin resistance stability due to heat in the long run. I´m building an enclosure with a 1590 C Hammond box, a fan underneath the resistors, and CPU dissipation and vents on the outside of the box. Have a great day!

[@MicskiDK]

Thanks for this very good video about measuring powerful amps. What is the final perfect diagram and resistor sizing for that voltage divider?

[@xraytonyb]

I did a Viewer question follow-up video in which I show the proper values for the divider. Check it out. kzbin.info/www/bejne/oXWTnYqvecSgnbs

[@BillnBen1]

I know this is years from when you posted the video, but is there a chance you could put a quick and nasty circuit diagram up. Cheers.

[@dougthetechguy5005]

It would have been cool if you could select 4 ohm 8 ohm and 16 ohm loads

[@Wil_Bloodworth]

Hey Tony! Can you please elaborate on why you need the ON/OFF switches? What is the use case for turning each channel on or off? Seems to me that if you're going to use this device, you're going to be testing something so I cannot think of when you would plug something into this device and NOT use it. Thank you sir!

[@BarryBirther]

Schematic, please!

[@chadhudson2046]

Would love a schematic for this design.

[@TMG363]

Can you please share a final diagram for this set up? Also how much wattage at 8 ohms would you safely measure with this setup? I also want to measure 4 ohms eventually at about 550 watts RMS. Thanks!

[@gregdesch305]

stizzytech f

[@donniefisk2252]

Help from anyone on this would be helpful. I want to build a dummy load just for testing my own personal tube amplifiers . My amps are 45 watts , 2ohms . What size resistors would be ideal? Thanks for the help

[@supremeinnovationsllc]

100W 2ohm resistors would work for max loading (give yourself some headroom). You can also test at 4 or 8 ohms with 50W resistors. Most speakers will be in this range. It is pointless to run the amp harder than needed unless you are doing a burn in.

[@Quetzalcoatl0]

Do i really need a dummy load for testing a ~150watts rated amp or i can use a speaker as a dummy load ?

[@rgones]

The point of building a dummy load is simply to be able to test & measure the amp @ near full power without blowing your ears and preventing burning a real speaker in case of failure (DC gets into the SPK output, clipping, oscillation, etc).

[@MD0MDI]

Plus it’s nice and quite, speakers aren’t, a lot easier to concentrate with no loud music belting out.

[@swinde]

I used to do contract work on amplifiers for a sound company that installed and maintained public address systems for businesses. Since these amplifiers were designed to drive speakers in the ceiling or in warehouses that covered huge areas, they normally operated on a 70.7 volt distribution line. I had the appropriate load resisters with taps to adjust the load resistance. With these amps the resistance is selected depending on the amplifiers max power output. The point is that I rarely EVER listened to these amps after servicing, because any problems would show up on the oscilloscope, combined with a function generator that supplied, Sine, Triangle and Square waves including sweep and tone burst capability. There was no need to check them on speakers.

[@abfig78]

No you cannot. Speakers have all kinds of different inductive differences and as told already, do you really want to listen to a 1khz sine wave that loud while testing.

[@tubeDude48]

Where's the schematic?

[@ianshepherd917]

Very confusing - too much chat about the wrong version then no detail of the final version!

[@jordanch68]

Hello is there a schematic of the final version? Thanks!

[@xraytonyb]

Never really did a schematic on this one. Main thing to remember is on the x1/x10 switch, I used high value resistors for the voltage divider, something like 100K & 10K or 10K & 1K to get the x10 output. I used precision resistors for greater accuracy. The big 8ohm 350Watt non-inductive resistors came from eBay. Cost was about $50 for the pair.

[@timka880057]

Tony, I blew out two output transistors using my 8 ohm dummy load. I'm not sure why. Is there amplifiers that are not supposed to be used with a dummy load?? My wattage of the amp was around 60 the dummy is rated 500. It's a solid state guitar amplifier. I used an audio generator sine wave on a low setting at first then raised it slowly. My main fuse blew about less than 2 minutes. I found the transistors shorted internally.

[@nathanhatch7382]

Guitar amplifiers expect a smaller voltage as a source than audio power amplifiers, so I guess the question is how hot were you running the input? Guitar pickups output around 300mVpp, and power amplifier inputs generally run around 1-2Vrms. Transistors (and vacuum tubes, for that matter) are biased with DC to expect a certain operating range of signals - you'll have issues if you go too far outside that range.

[@timka880057]

@@nathanhatch7382 it works now, it was cheap transistors. They don't take the amps like they used to. Chinese made pieces of crap. I found old made RCA ones made it right and will blow your ears off.

[@nathanhatch7382]

@@timka880057 Good find! Glad to hear you got it sorted out.

[@timka880057]

@@nathanhatch7382 in some older solid state devices, amplifiers, tvs etc it was the 2n3055's. they were pretty common as you probably already know. :)

[@treadmillrepair754]

@@timka880057 I buy the spare parts in trusted sites like Digikey, Mouser, Newark only. Counterfeit market is a big problem in these days. Cheers.

[@amahrielove4853]

Browsing youtube for dummy load and it's seems common 100Kohm and 1Kohm precision seems to be common builds. I'm a total newbie so if someone could explain to me the concept. What I don't get is that it's suppose to be a 10:1 divider but with the resistors given in the video if 100 volts in that would be 0.99 volts out? Shouldn't it be 99Kohm and 1Kohm then 100 volts in would be 1 volt out? I'm trying to build a dummy load as well and so I want to understand this concept better if someone could please explain it to me better. Also if I order precision resistors (for voltage dividers) at what wattage should the precision resistors be? I have the same dummy loads (8ohm 300w). Thanks in advance -Mary

[@xraytonyb]

I'd have to go back and check, but I believe I used 100K and 10K in series with a 1K (for a total of 11K) for the two divider resistors. The scope output jacks are connected across the 10K/1K. This will read a ratio of 10:1. The voltage divider formula for this is: V_out = V_in X (R2 / (R1+R2)). Using 100K and 1K gives you very close (.99:1) 100:1 ratio, which is why a lot of people use these values. To get a true 10:1, you need 100K and 11K. All three of those values (100K, 10K and 1K) are readily available. The main thing you want to remember is that your measurement circuit needs to be high enough in impedance to not affect the value of the 8 ohm dummy load resistor. Invariably, some of the current from the amp under test will be diverted from the 8 Ohm dummy and into the measurement circuit, but at 111,000 ohms, it will be negligible and won't greatly affect the overall load to the amp. I think in this video (I'd have to watch it again) I had to play with the resistor values to get the ratio correct, as I didn't use 100000:11000 ratio at first. Hope this explains it.

[@amahrielove4853]

Thanks for the quick response (: Couldn't quite understand why the 100Kohm and 1Kohm resulting in 0.99 if Vin is 100 when it would have been more accurate if 99Kohm and 1Kohm is used giving a true 1Vout? Also what wattage should my precision resistors be should I build a voltage divider similar to yours? Thanks for answering since I'm so new I have so many questions. Your videos are very thorough and informative. Subscribed (:

[@xraytonyb]

Check out the Answers video I posted yesterday. It may explain a bit better.

[@InssiAjaton]

10k and 100k makes a 1 : 11 divider, not 1 : 10. You waste precision resistors if you are satisfied with the 10% difference. The formula is like 10 / (10+100). PJ

[@barski1960thomas]

Finally! Someone has the math right!!

[@jp040759]

Just make life easy for yourself. Use a 10 turn 100k pot across the 8 ohm load and finely adjust the pot to get you 1/10 division ratio. Just make sure pot sized for proper dissipation at what your highest power amp you will be testing with.

[@allansmith6140]

Ok so after 4 years lets get this right. There seems to be a lot of confusion as to how to properly construct a 10x divider setup for use with an oscilloscope, and by that I mean correct termination. Oscilloscopes (generally) have a 1 megohm input impedance. That means there is a 1 megohm resistor connected to ground at the BNC inputs. To get a 10 division a series 9 megohm resistance is required at the output of your apparatus, nothing more and that is how a 10x oscilloscope probe is wired. 9 megohm resistors are hard to find, even more so precision ones. What I normally use is a 8.2M and a 820K (any tolerance) and add a 20 turn 200K pot. I see a lot of people using oscilloscopes and demonstrating all sorts of complicated measurements, yet they lack some very basic knowledge on them and other test instruments which is needed to get correct results. Look at your instrument inputs and outputs and terminate accordingly. For example, if your audio generator says it has a 50 Ohm or 600 Ohm output, then provide the proper load for it. Not only will you not get the voltage you dial up on the generator into what you are feeding (unless its input impedance happens to match by accident) you can also get other errors and you will be none the wiser. In most cases people using audio generators into guitar amps they are actually providing the amp with at least twice the input voltage they have dialed up. It may not matter so much unless you are doing some accurate testing (or attempting to), but get into the habit of using your gear correctly and professionally, especially if you are doing youtube videos, that way you won't come off as a dummy. Nothing personal.

[@allansmith6140]

How to construct proper cables for use use, in this case, guitar amps to be tested with audio generators. Easiest way is to to snip the BNC plug off on end of a BNC to BNC coax and solder in a 1/4" plug. Solder in either a 600 Ohm or 50 Ohm resistor at the phono plug from the center conductor tab to the earth side. Choose the resistor according to your audio generator output. The voltage or dB you dial up on your generator will then be "seen" by your guitar amp. For proper termination between instruments you can use a BNC "T" piece and plug in the appropriate terminator into one of the two connections. If you can't find a 600 Ohm terminator, buy a 50 Ohm terminator that you can disassemble, unsolder the 50 Ohm resistor and replace it with a 600 Ohm one. Alternatively you can use a T piece at the output of the generator with a proper terminator on it, though I prefer to to use custom cables on amps rather than fiddling with T pieces. Be sure to mark your cables with their impedances.

[@dennislarson9560]

Another, I'm gonna! How about just do it and stop all the side commentary!

[@xraytonyb]

Perhaps a good stilton, to go along with that whine. ;)

[@fghjfghgj]

isn't 300 watt to small i mean you need at least 10000 watt lol