8 OHM Audio Dummy Load for Amplifier testing Design and Build 

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!

I did a Viewer question follow-up video in which I show the proper values for the divider. Check it out. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-kD0lWyD_mgY.html

he was about to say "fine"

stizzytech f

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.

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).

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

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.

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.

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.

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.

@@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.

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

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

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

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.

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 (:

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

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

Finally! Someone has the math right!!

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.

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.

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.

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