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I finally got around to designing my DIY version of the Suhr Reactive Load. I was able to simplify the circuit with off-the-shelf parts and retain nearly identical frequency characteristics. I haven't formally measured the AC impedance yet, but spice said it should be nearly the same. Sine wave sweep of the circuit produced a frequency response almost identical to the Suhr, with the major peaks and dips at the same frequencies. One thing I noted was my design had a steeper rolloff of the extreme low frequencies starting around 30Hz. And mine seemed to have a little bit more high frequency energy at the extremes. I did all these tests driving them with my Traynor YVM-1 set to be as clean as possible with the tone controls at 50% and the Presence at 0. Both load boxes were set to 8 Ohms and I set the amp to 4 Ohms driving both simultaneously. I got a perfect sine wave throughout the recording (guess the bias is perfect), with the exception of the Ammo Can having a weird little kink in part of the wave. It started from the lowest frequencies and seemed to peak around 150Hz, and then quickly taper off and smooth out around 250Hz. Higher frequencies were perfectly sine. This only happened with the Ammo Can, the Suhr didn't have any kinks. I'm guessing it may have something to do with the inductors I used. One difference is the Suhr has one with a laminated core, and mine has a ferrite core. Maybe something to do with that. The ammo can is a fun box to use, but drilling that steel is not the fun part. I mounted the bank of resistors directly to the box, and it gets very hot in use, so keep that in mind. If the heat is a concern, you could mount them to a heatsink inside the box, or maybe even bolt the heatsink to the side of the box. Ventilation holes are mandatory, unless you want to run it with the top off.
I removed the fan from the Suhr, because I didn't like the idea of it adding a weird load into the mix. If you want to add a fan to this project, I recommend including a small power supply to run it. I added the shield to the transformer with copper tape. The High/Low level switch gives a boost to the level for when you're using low powered amps. Parts used in my build:

• 6.5mH ferrite core inductor: https://www.parts-express.com/6.5mH-20- ... quantity=1
• 650uH air core inductor: https://www.parts-express.com/Jantzen-1 ... quantity=1
• 250uF 100v Non-Polar electrolytic capacitor: https://www.parts-express.com/250uF-100 ... quantity=1
• 62 Ohm 10w resistor - I used an Ohmite from Mouser, couldn't find this value anywhere else I was buying from.
• 4 Ohm 50w resistor x4 - I used Arcol style aluminum body, found cheaply at All Electronics. They're sold out there now. Mouser is easiest choice, but can get a bit pricey there. But various power resistors are usually easy to find cheap at surplus outlets.
• 10K:10K 1/4w audio transformer Edcor PC10K/10K I modified this by adding a shield with copper foil tape.
• 10K audio taper potentiometer - I used a Tocos RV24 type from Amplified Parts, they're really nice.
• 22nF 400v film capacitor x2 - I used CDE 716P "orange drop" style from Amplified Parts
• 10K 1/2w resistor - I used an RN65 from Mouser
• 47K 1/2w resistor - I used an RN60
• 33K 1/2w resistor - I used an old carbon comp I had laying around. You can play with this value; make it smaller if you need more level boost.
• SPST switch for impedance select - you can omit this if you only need one impedance. Make sure the switch is high current. Look for 10A minimum just to be safe.
• SPST switch for High/Low level select - I actually used a SPDT in my bulid, doesn't really matter. This switch doesn't need to be high current.
• XLR Male Jack - I like to use Neutrik, though the mounting hole is a pain to cut, because you have to file out an odd shape to fit the keying. I used one I had on-hand. I don't remember where I had gotten it, but most likely Mouser.
• 1/4" Mono jack for input - Must use an insulated jack. I used a Cliff brand jack from Amplified Parts.
• 1/4" Stereo jack for line out - Should also be an insulated jack. I used Cliff as above.
• 30cal M19A1 steel Ammo Can - From Harbor Freight https://www.harborfreight.com/metal-030 ... q=ammo+box . I like that the lid comes off easily on this one - makes it much easier to work in. Don't use a plastic ammo can, it will melt!
• Knob for volume control - I used a "Fester" knob from Amplified Parts.
• Solder lug for chassis ground. This is attached to an XLR jack mounting bolt.
• Enough 4-40 bolts and nuts to mount all hardware that needs it. I also used nylon washers on the two transformer bolts.
• 4-40 standoffs x4
• Nylon zip ties to secure components to the circuit board.
• Large hole perf board, 3.5" x 5", for mounting reactive components: https://www.parts-express.com/White-Per ... quantity=1 (sold in pairs)
• Various Wire - I recommend teflon insulated wire so they don't melt from the heat. I used 16AWG for the main speaker level wiring to the load, and 22AWG for the line level. I used some fancy Cardas wire for wiring up the transformer and output jacks, because I had a some on-hand that I had never gotten around to trying. I also used some 20AWG uninsulated bus wire in a few spots, like on the power resistors.
• Piece of cork - I used this between the transformer and the case to give it some cushion

I think that's everything...

Nice work!

A question from a rookie,
About the frequency plot, arent those harmonics in the original preferable?

din_mentor wrote: ↑10 Apr 2023, 13:14 About the frequency plot, arent those harmonics in the original preferable?

Well, which is "preferable" is highly subjective. I will say that in use, they sound slightly different from each other, but not in any dramatic way. With a clean signal, you'd be hard pressed to hear a difference. With a distorted tone, the Suhr sounds a little bit smoother maybe, and a little bit more wooly in the upper bass region. But it's very subtle difference that seemed to be less noticeable the more I played on them.
And note that I ran the sweeps thru a guitar amp, so the frequency plot is reflecting the response of the amp as well. I plan to do more tests later with a tube hi-fi power amp that should be less colored.

Another thing to note that is also worth experimenting with: the load puts out alot of sound in use, similar to the Suhr. I had assumed the Suhr's sound was coming from the fan, but this one doesn't have a fan, and I'm still getting alot of sound from it. The inductors seem to be the main cause, as they would vibrate rather strongly from the signal (not surprising!). I'm going to play around with different ways to possibly damp that vibration to quiet it down.

Oh ok,
i guess one shouldnt judge sound based on a graph

Keep up the good work, very interesting to follow your findings

Any partlist btw?

din_mentor wrote: ↑10 Apr 2023, 15:36 Any partlist btw?

I updated the first post with a list of the parts I used.

Great DIY project thanks!

Because the two coils are oriented in the same manner, I guess the magnetic field of one inductor coil can couple into the other inductor coil. This will change the effective inductance and frequency response of the load box.

Shouldn't the two inductors be oriented 90 degrees in regard to each other to avoid that?

floris wrote: ↑12 Apr 2023, 13:07 Great DIY project thanks!

Because the two coils are oriented in the same manner, I guess the magnetic field of one inductor coil can couple into the other inductor coil. This will change the effective inductance and frequency response of the load box.

Shouldn't the two inductors be oriented 90 degrees in regard to each other to avoid that?

Yes, I hadn't thought of that. I was thinking about it in regards to the fields affecting the transformer, because I think this happens in the Suhr. I noticed even with the volume all the way down, I would still hear a faint signal. I think this was the fields from the inductors getting into the transformer, essentially bypassing the volume control via inductance. That's what prompted me to add the copper shield and mount it well away from the other components.
I think there is enough room inside the box, I will try re-orienting one of the inductors and see if anything measures differently.

Well, something was going wrong last night. Was playing my modded Traynor YVM-1 into it last night and got something crazy happening. Sound started to dip out and sag - this amp doesn't normally sag. I looked around back and saw the tubes flash brightly a few times as I was playing, then I stopped playing and they continued to flash a bit. Then the fuse blew.
Today I replaced the fuse and found one of the KT66 output tubes is dead (OUCH!).
Aaaargh... don't know what failed where to cause this situation. Can't help but wonder if the inductors in the load were somehow to blame. I'm going to go over the whole thing thoroughly and try to figure it out. The amp is only around 50-ish watts, so I don't expect enough power would come out to burn up the load - I was running it at 4 Ohm with 200W worth of resistors - and nothing appears (or smells) damaged in there.
Could also have been something in the amp failing on its own, regardless of the load, since it's so old and I was blasting it. I'll give it all a check and see what I find.
I also wonder if it could have been some kind of feedback happening at an inaudible frequency. The load box was sitting on top of the amp, physically right over the input preamp section - so maybe the magnetic field from the load was strong enough to induce itself into the preamp and get some chaotic feedback situation happening. I honestly don't think I ever operated this amp with a load box sitting directly on top of it before - I always had the Suhr off to the side when using it. Lots of things to investigate.

Things to do:

• Mount resistors to an aluminum plate in the box - to get better heat sinking and keep the box from getting so hot.
• Change orientation of inductor to keep them from interacting with each other.
• Experiment with damping the inductors to keep them from vibrating so strongly.
• Investigate polarity issues. An odd thing that I noticed when comparing the Ammo Can to the Suhr is that their outputs were opposite polarity. This would be caused by the output transformer in one of them being wired with the polarity swapped. I double-checked the wiring in the Ammo Can several times to make sure the polarity should be correct, but I will verify with a scope because there is always a chance Edcor messed up and wired the pins wrong. It's also possible the Suhr is the one inverting the polarity, though I'm not sure why or if it would be intentional.

I hope that you find the problem!
Please keep us posted.

Back to the attenuator. I found the Edcor transformer is the cause of both the reversed phase and the distortion at low frequencies. That's a bummer. I checked and double-checked that my wiring on the transformer was correct - it must be a matter of them having wired the pins wrong. I don't know what is causing the distortion - if it's a design flaw or something caused by how I've mounted it (I'm thinking because of the steel chassis, somehow?). I have several more of these transformers, including one that I had bought almost 10 years ago, so I can make some comparisons to see if this an issue of just one being defective, or maybe they all are like this. I'm especially interested to see how the old one compares.
I also have a couple 1:1:1 trifilar ones that are the next size larger, so I will see how those compare distortion-wise.

Got to thinking,
A good budget approach, or at least easy if you don't want to start from scratch, could be to mod a prebuilt cheap attenuator, like a Harley Benton PA-100, with the reactive parts?
But i guess it could be hard to fit?

din_mentor wrote: ↑29 Apr 2023, 07:40 Got to thinking,
A good budget approach, or at least easy if you don't want to start from scratch, could be to mod a prebuilt cheap attenuator, like a Harley Benton PA-100, with the reactive parts?
But i guess it could be hard to fit?

Problem is there's no reactive parts in that thing. The inductors I specified here are cheap cheap. If you scrounge up cheap resistors, you should be able to to build this for well under $100. That's why I used the ammo can, because it was the cheapest suitably sized box I could find off-the-shelf that most people should be able to easily find.
I could see more like the opposite happening: adding components to the attenuator to make it reactive, but I honestly think that's more trouble than its worth.

You can use a load box like this as a really good active attenuator by sending the line out to a power amp to drive a speaker. This is how the Fryette Power Station works; reactive load box that is then reamplified to act as an attenuator (or power booster, if the attenuator's amp is more powerful than the original amp). I personally think active attenuators are better in every way than passive ones.

UPDATE:
I found why the transformer was reversing the polarity. The mistake was in the schematic drawing from Edcor. The diagram they provided showed Pin 5 being Hot and in 7 being ground, but it is actually supposed to be the opposite of this. I will update my schematic with the correct pin numbers later this evening. Edcor has now updated the diagram on their website to show the correct pinout. Glad it was something as easy as that!

It won't let me edit the original post with the new schematic, so here it is: