Thank you for going into the details for designing a 10/1 output , you’ve got the fanciest dummy load on the web😆👍I really enjoy your videos on setup and use of audio test equipment, thanks 😃
I wanted to respond to Charlie’s post about a monitor speaker. I have been repairing high power amplifiers for many years and have used a trick for monitoring them on the bench, especially when I have to run them into dummy loads for a while. I use a 70V speaker transformer between the amp under test and my monitor speaker(s), along with the dummy loads.The transformer allows you to drop the level to the speaker(s) by a great margin, even when the amp is putting out hundreds of watts into the dummy load(s). The idea behind it is that if you were to use the 1 watt tap on the transformer, for instance, with an 8 ohm speaker connected to the secondary of the transformer, the primary impedance of the transformer would reflect 5000 ohms and as such would not change the load on the amplifier significantly. Also, at that tap, if the output voltage of the amp across the dummy loads was 70V, there would only be 1 watt of audio power presented to the monitor speaker. If you used the 1/2 watt tap, you would have 1/2 watt at the speaker, and the primary impedance of the transformer would be 10,000 ohms. BTW, 70V RMS at the output of the amplifier would represent 612.5 watts into 8 ohms. At lower output powers, you could use higher wattage taps on the transformer to obtain a comfortable listening level. Maybe Tony could do a video outlining the principles involved. Incidentally, 10 watt 70V transformers are really cheap. You can also get 5 watt transformers, and of course, higher power rated transformers. The only thing to watch out for is that the cheap transformers are quite reactive, especially at low frequencies, and will reflect a much lower impedance to the amp than expected. There are 70 V transformers that are rated for the full audio bandwidth, but they are, of course, more expensive. Also, if desired, you could still use a volume control between the transformer secondary and the speaker for fine volume adjustment. Hope this helps.
I'm interested in trying this. To clarify, the transformer primary is connected in parallel with the dummy load (this is a dummy check for myself, as I believe that is the only way it would work)? Also, if one wanted to include a volume control, I'm assuming it would need to be an L-pad configuration to ensure 8 ohm load is maintained on the secondary? ... And finally, do you think this the right type of transformer for the job? www.parts-express.com/70v-10w-speaker-line-matching-transformer--300-040?gclid=CjwKCAjw7LX0BRBiEiwA__gNw2efxZXLYZ8VzEHB837LwMZb9DYT09Go8csn4g_pFdejeiLnJ_b0gxoCq24QAvD_BwE#lblSimilarProduct
I would like to take this opportunity to wish you and your family a peaceful and prosperous new year. I have learnt enormously from your channel, and want you to know that your work is truly appreciated. I often cringe when I notice that someone is a subscriber to both your channel and mine - I cannot imagine how they could compare the level of knowledge that you impart, with my humble attempts. However, you and a few others have inspired me and continue to do so, so thank you.
Nice build. I have a suggestion for the 3.1 version. Instead of using a 2 resistor voltage divider, with minor modifications in the existing circuit, you can use 3 resistors so that the ground lead of the scope will be somewhat "isolated" from the AMP's "ground" and still make a 10 to 1 divider. This would be handy for those rare beasts that do not share the earth connection between channels. So, in the circuit you remove the Fuse and put a 100k res in its place. Raise R3 to 100k also. Tap to the voltage that is ACROSS P1 (bnc's earth should connect to point "3"). With these values , if P1 is set arround 22,5K you get 1V for 10volt input. Downside is that you loose the 1:1 option if you only do these small "invisible" changes and nothing else. Wish you a Happy new year!
Thank you for the comment! I actually tried this a while back. It ended up not working very well. By using three resistors in the divider, you are floating the signal (and the scope probe) above ground. This caused the scope to be very susceptible to noise and ground loops. This really played havoc on things like the distortion meter and some of my software-based tests. If the unit uses out-of-phase grounds (like with Carver amps), I just test one channel at a time. By only connecting the test equipment to one channel at a time, the two channels of the dummy load remain isolated from one another. Thanks for watching!
Another awesome video! I know it’s an old one but I haven’t seen it in a while. I and I’m sure plenty others would love somebody like you to do an oscilloscope video. I have that Siglent you’re talking about and I think it’s great but I hardly know how to use it lol. I’m trying to learn in my spare time. I have had an interest in electronics since I was a kid. It started when I took the back off my black and white tv and took the speaker output and put it into the input of my stereo. I was probably 11 or 13 years old when I thought I invented surround sound and nothing blew up! But anyway it would really be great if you could do that because you explain stuff very good and speak right at my level. Thanks Tony!! I actually asked you this on a different video and forgot to check my notifications before I asked this. If you answered I apologize for asking again.
Wow, I was thinking to build a load from 2x200w per resistor, but this is a big step up, mostly I repair normal receivers with max power output from 25 to 30w RMS bigger amps, that's for higher classified technicians. Because my budget is not that big. I'm glad I can build stuff for myself for monitoring gear.
Grate video! Just a simple question:- did the 100W dummy load distorted the sin wave of 150W or 200W and leads to wrong early clipping on the oscilloscope ?
Been following the dummy load iterations.....I would like to see a volume pot and a monitor speaker so you could hear a little audio while hooked to the dummy load....keep up the interesting videos...
Hi I enjoyed your video and it was quite timely for me as I am also planning on building a load circuit. I noticed that you placed the fuses in the probe line, wouldn't it be better to actually place them in the ground line of the scope probes as I encountered several occasions where the grounds of the right and left channels where not the same. In that case the fuse would protect from any shorts. Have you had cases where the channels couldn't be measured simultaneously due to different grounds (and since the scope will connect them internally) ? Could maybe a large resistor in the ground loop of the scope reduce the risk of amplifier damage ?
I agree, it's very common for modern amplifiers to be differential drive. Also, shouldn't the coax be driven with a 50 ohm impedance to prevent reflections? (That can be important when looking at "filter free" class D amplifiers, but then you'll also need to add some series inductance.)
ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-OZDijMDHmtI.html ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-RDLWQFgJ1uU.html Differential probes. They are expensive though.
Thanks for your video! I´ve also built a similar dummy load; but with a different scope connection: Some amplifiers have a bridged potential free output. So the plus AND minus have BNC-connectors and i can connect a scope without grounding the minus - but i need a scope with two channels in differential mode. For a stereo application i need therefore a 4 channel scope; two of them for the left and two of of them for the right channel.
Since the oscilloscopes became relatively cheap it started to be fun with measurements for everyone. But few have knowledge weather picture and circuitry show something useful. that they were looking for. .
Very well done. I never thought of incorporating the x10 (10:1) part in the load itself. I have actually built your voltage divider network and tested it on my crude 8 ohm load, and, so far, works great. Can I ask what was your reason for the choice of values (mainly the 8k2 resistor as this seems an odd value)? Is it for keeping a fairly high input impedance to the scope?
Variable resistor of 100K is determinant in this case, not the 8.2K and the 8.2 K resistor suits well, so you can adjust 100K to 73 K pot point to have 10:1 voltage divider.
Thanks for the detailed video, i was looking exactly that because I was looking to safely measure the outputs of my amps and to troubleshoot a couple of issues they have Do you sell the boards for the dummy loads? Thanks a lot again and Happy holidays
Great question I was thinking the same thing when I watch the video yesterday. If you don't have boards to sell can you release the PCB design so I could have one made or five.
Thanks for keeping the explanation for RootMeanSquared (RMS) simplistic.... I recall in college having to do "proofs" of the complicated mathematics (for more than a simple sine-wave). Suggestion for you to talk about sometime.... "Can a true-RMS voltmeter be used to measure amp. power?... Does the meter 'know' the resistive load you are measuring across?"
Tony peace Joy and Happy New Year. I would leave the amp alone till you use it it may need a good break in on the usage. If something comes up then it would make sense to look into the issues which I doubt you may have. But, Daily usage is what is needs.
Nice piece of work. One concern I have is your use of the pot and the series resistor. Both would have a different thermal characteristic which could through your alignment out of wack as the resistor and pot change temperature. I would have scraped the series resistor in favor of using the pot alone to set up the 10:1 ratio, using the center tap to go to the scope probe. As the thermal resistance changes with temperature, the ratio would remain the same across both sides of the pot.
That's not actually a concern for several reasons. First, is the values that I chose for the divider network. The total resistance is 82000 ohms. Let's say we apply 100 volts RMS across the dummy load (this would equal 1250 watts with the 8 ohm load selected, which is way more than I would ever need). Since the divider network is in parallel with the 8 ohm load resistors, it would see the same 100 volts. I=V/R = 100/82000 = 0.00122 amps, or 1.22 milliamps. W=VI = 0.00122x100 = 0.122 watts, or just under 1/8 of a watt dissipated across the reference resistor (which is 8200 ohms) and the pot (which is set to 73800 ohms). The pot is rated at 1/2 watt and the resistor is rated at 1 watt. 1/8 watt would not produce any appreciable amount of heat. In addition to this, I used a metal film resistor. Metal film resistors have very little change in resistance with temperature (as opposed to carbon resistors). Even if the pot changed a couple of ohms, it wouldn't be enough to make much of a difference. Now when we use a more realistic voltage of 50 volts (for a 312 watt amplifier output), the wattage dissipated in the divider becomes negligible. This is why you use such a high value of resistance for the divider. Last, I used a reference resistor along with a potentiometer because it allows more precise adjustment. If you just use the pot alone, as the wiper moves, it will increase the resistance on one side of the wiper while at the same time decrease the resistance on the other side of the wiper. In other words, you are changing both "resistors" in the divider at the same time. When you use a fixed resistor, then the pot only adjusts one side of the divider. this makes it much more accurate to adjust. Thank you for the comment! I should have explained this a bit more in the video.
@@xraytonybtony, thank you for your response. I enjoy watching your videos. I wasn't thinking of the heat produced by the power draw through the resistors. As you have stated that is negligible. I was more concerned about the environmental temperature. In your case that may not be of much concern since your PCB is not in an enclosure but for those who would want to build a box to enclose the PCB and the power resistors (hopefully with some sort of fan) as temperatures in the box change you could have a ratio problem with the pot and resistor. My point is that in using the pot alone once the ratio is set it will remain the same throughout the temperature range. So even though the resistance will change with temperature in the POT the ratio will remain the same and that is what is important here. I have seen this effect in some Audio designs that I have done in my work over the years. Especially with digital POTS that are notoriously temp unstable. I have found that using them in a ratio metric configuration eliminates the termal problems. Just my 2cents for what it is worth.
Very nice... For my own curiosity (still learning)-- why not just use a 9M resistor in series with the output to the scope and a trimmer cap across that? Wouldn't that be exactly the same as a 10:1 probe? The 9M forming a 10:1 voltage divider with the 1M scope input.
Good Question! That would work with a 1Meg input scope, but would not work with a device with any other input impedance. In addition, when you use the potentiometer, you can perfectly adjust the ratio to 10:1, eliminating the need for precision resistors. Thanks for watching!
Regarding bridged amplifier testing, even if you only test one channel at a time, the scope ground connection will still short one amp channel to ground. One method would be to use both channels of the scope with only the test probe tips connected to the speaker terminals, and the ground of the scope tied to the amp chassis. Then invert one channel of the scope and use the add function. Of course, if you have a true differential probe, that the best way.
That was a concern/question of mine. If the minus side of each channel is not referenced to ground, as soon as you connected both O-scope inputs, you would tie the minus side of each channel together. Producing smoke and sparks no doubt. An isolation xformer on the scope would not help, as the shields of both probes are tied together. Trying to think of a way around this.
In reading more of the comments... it seems that connecting only one channel at a time maybe works. Still wondering the the minus side of the amp is not referenced to earth ground...
Nice setup Tony, but you kind of glossed over the placement/heat sinking for those 1000 watt resistors. Seems like there is a chance that the shelf above could get pretty toasty under the right conditions . . ..
No heat sink used. These resistors are so large that they barely get warm, even at 150 WPC for 5 minutes. The advantage of using two 4 ohm 1000 W resistors in series is that they really dissipate the heat very well.
Instead of arranging the speaker inputs as + - + - I did mine as + - - + because that matches the way most amps have them. I would also be concerned about flexing of that board over time as you connect and disconnect banana plugs. The other thing I did was use two 8 ohm resistors per channel so I could get 4, 8, and 16 ohm loads with one switch.
I also am a little concerned about the mechanical stresses on that PCB in the long run. I would mount the sockets and the switches on a sturdy face plate in front of the PCB and connect them through with short leads.
Your resistor configuration is a better option. I just used what I had on hand, but if I were to order the parts, I would do as you did. As for the durability, these FR4 boards are quite tough. In addition, I have a mounting bolt positioned between the terminals for increased support. I'm sure I'll be on to the next design long before this one wears out! Cheers!
I've never worked with one of those, but I know they have the math function, so there is a good chance they can. You would just have to check and see if you can program a custom math expression.
XRay, I love your channel and I want to thank you for all I have learned through your videos. I just made an amplifier test panel that makes it easier to switch between speakers and the load so this was great timing. I was worried about a bad offset so I added a speaker protection circuit to protect the speakers. The video is here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-dFzoCkFdzhQ.html I used much smaller resistors! I wish I had watched this first, I would have added BNC connectors for the scope. Were you worried about capacitance on the scope probe?
I looked them up. They look interesting, but the 100 watt model has about 20nH of inductance per module. Not sure if that would be enough to affect anything. I also noticed that the power rating drops considerably as the case temperature goes up, so you would need a pretty good heatsink, which might make it as big or bigger than a standard power resistor. Still might be worth looking into, though. Thanks for sharing!
@@xraytonyb I forget who made the comment but someone said non-inductive or inductive, it made no difference. I always heard non-inductive was the way to go. But yeah, a hell of a heat sink is still needed. The planar resistors look interesting as well... Thanks for sounding--off...
Purely resistive dummy loads have little relevance to actual speakers, as we all know, real-life speakers have complex resistive+indictive+capacitive impedance characteristic that is highly dependent on frequency. Here is a good explanation of a speaker equivalent circuit or electrical model of a speaker. audiojudgement.com/speaker-equivalent-circuit/ Maybe Tony can design his next dummy load to more closely resemble an actual speaker load.
I believe that only your ear can tell you if an amp/speaker sounds good. That said, the resistive load is very important when troubleshooting the amp, checking for clipping points, crossover distortion, etc. It also helps by giving a standard, by which you can reference other measurements. I have been working on a transformer-based load that has adjustable reactance, but it's not ready for use yet. Even so, each speaker has its own reactance "fingerprint", making it impossible to reproduce every type of speaker load. The main purpose would be for testing class-d amps and for better testing of amplifiers for load-induced oscillation. Maybe someday I'll get around to finishing it! Thanks for the comment!
@@xraytonyb - Agreed, pure resistive loads are useful for troubleshooting, however NOT for testing THD, frequency response or clipping points. A cheap amp may have a flat frequency response in pure resistive load but sound terrible when driving an actual speaker enclosure. While the goal isn't to model every speaker, it would be useful to model a typical one-way speaker enclosure with known resonant frequency that would resemble more closely an actual load.
@@1959Berre - What is the point of testing a piece of equipment when the test has "little relevance" to actual load? Two 100W rated amplifiers may have a flat frequency response into a pure resistive load, but one may handle inductive (cone drivers) or capacitive (electrostatic speaker) loads much better than the other one. Pure resistive loads testing cannot tell you that. As you may know, there is no such thing as pure resistive loudspeaker.
@@NICK-uy3nl The point is ruling out all possible inductive and capacitive influences which strongly vary depending on the load (the speakers) ergo make it impossible to assess the amplifier in an 'objective", standardized way.
For us mere mortals, us non-Electronics Engineers, what is the purpose of this video? I've built a few dummy loads in the past for my tube amps. I am currently in the process of upgrading those dummy loads to reactive loads that more closely mimic the dynamic resistance of a real speaker moving real air. How do you apply this circuitry ... this device in creating resistive load boxes or in testing the output of tube amps?
Every model of speaker out there has its own specific impedance curve. If you were to build a reactive load for your amp, which speaker would you simulate? I think this is why all testing is done with a resistive load, as it is easier to measure the amplifier at different frequencies and amplitudes with a known load.
