Impedance plot of speaker cabinets for design of reactive load.

[Humbug]

Hello. First, I would like to introduce my nerdy, slightly obsessive self: My name is Thomas, I like playing guitar, figuring out How Stuff Works, drinking whisky and not working, ideally at the same time. I realise the title of this post is probably a little Borat-ish, but it's the best I could do. Jagshemash.

I'm currently working on an unnecessarily complex reactive load, much thanks to the work of "James Freeman" et al on The gear page forum (who I've been told is active on this forum), to see if it is possible to emulate the response of a speaker cabinet completely by replicating the impedance curve as accurately as possible and taking a line signal to either a linear solid state amp and the actual cab as an attenuator (like Mr. Freeman's original box) - or to the computer and an IR of the same cab.

I have attempted to replicate a fairly obscure old Marshall cab with T652 Alnico speakers, chosen for it's characteristic sound (if it works with this cab, it will probably work with any cab). It will be refined further, the 400Hz - 1500Hz area is a little high, the 1500 - 5000 area is a little low, and the rest of the rise is a hair high too, but it's pretty close:

I will perform frequency response testing in, hopefully, a few weeks. If that test is successful I will build a "production" model and share the component values, gerbers, plans, etc for others to use. However, I realise most people probably won't have much use for a load that's based on a cab as rare as hen's teeth that there are very few suitable IR's for, so I would like to ask the wonderful people who read this:

Anyone got an impedance plot of a modern 412 with Greenbacks or V30's they would be willing to share? Ideally an REW .mdat file or a .csv.

Thanks!

 

[PLX]

 

[Humbug]

That's a lovely build, but I was looking for impedance plots of actual speaker cabs :)

 

[PLX]

That's a lovely build, but I was looking for impedance plots of actual speaker cabs :)

This device produces a curve identical to a Marshall 1960A cabinet.

 

[Humbug]

This device produces a curve identical to a Marshall 1960A cabinet.

Have you measured it? Using the values in the schematic and guesstimating the R of the inductors and capacitors, the resulting curve is nowhere close to a 412.

The red curve is the calculated curve of your load (assuming the values in the schematic are accurate), yellow is a 412 with T1511, blue is T1281 and green is G12K-85s. I'm not saying your load doesn't sound good, though :)

 

[PLX]

Have you measured it?

Yep.

 

[Humbug]

Today, I had the opportunity to do a frequency response test of the cab and the load and, while test conditions were not perfect (something going on with the resonant peak of the cab, possibly a loose baffle, back panel or something in the room), the results are not discouraging. It's off in the mids, which was expected and correlates with differences in the impedance curves, but I would expect there to be more of the highs - not less. It is entirely possible that the HF inductor had moved during transportation, and I should have done an impedance test beforehand to verify the condition of the load, but I was too excited to get the SPL results

Anyway... Here's a graph!
Edit: Just to be clear - the green curve is the load box and the red is the speaker cab :-)

 

[James Freeman]

Anyone got an impedance plot of a modern 412 with Greenbacks or V30's they would be willing to share? Ideally an REW .mdat file or a .csv.

My Suhr RL measures 107Hz at resonance peak and the modified Fractal X-Load measures 115Hz (as I intended), these are 8ohm load boxes so their nominal DC resistance is slightly below 8ohm.
The Marshall 4x12 loaded with GB+V30 is a 16ohm cab.

When I will have some time next weekend I will measure the response of these loads through the JCM800 with a flat sine into the FX Return taking the signal in parallel with the cab.

 

[Humbug]

My Suhr RL measures 107Hz at resonance peak and the modified Fractal X-Load measures 115Hz (as I intended), these are 8ohm load boxes so their nominal DC resistance is slightly below 8ohm.
The Marshall 4x12 loaded with GB+V30 is a 16ohm cab.

When I will have some time next weekend I will measure the response of these loads through the JCM800 with a flat sine into the FX Return taking the signal in parallel with the cab.

That is very helpful. Thank you!

 

[James Freeman]

Frequency response of various loads through a JCM800, output transformer ohm tap was matched with the load.
The actual measured amp frequency response tells a different story than the measured impedance in REW.
If you look closely the difference is within 1dB.

 

[PLX]

 

[Humbug]

Frequency response of various loads through a JCM800, output transformer ohm tap was matched with the load.
The actual measured amp frequency response tells a different story than the measured impedance in REW.
If you look closely the difference is within 1dB.

View attachment 28090

This is true, and my comparison test from yesterday also showed less than a 1dB difference (attached with dBV-scale for illustration) except in the funny resonant peak. However, 1dB in this context is not insignificant, especially when when there is a shift along the frequency axis - ie 1dB up in the mids and 1dB down in the highs.

The load sounds fine, as do the Suhr, Fractal, or even Captor, and, I'm sure, all the other DIY versions, but if that was the goal I would just steal your (et al) original design and leave it there as so many others have done *cough* Suhr *cough*. I want to know if _identical_ is possible.

 

[James Freeman]

I would just steal your (et al) original design and leave it there as so many others have done *cough* Suhr *cough*.

All credit goes to Mr. Aiken, I just implemented the idea and used high power speaker crossover components.
Suhr, as any good businesman would, "borrowed" the implementation and mass produced it.

 

[PLX]

All credit goes to Mr. Aiken, I just implemented the idea and used high power speaker crossover components.
Suhr, as any good businesman would, "borrowed" the implementation and mass produced it.

Still owe you a Starbucks® gift card, or something, James.. because when I stumbled across your thread on teh TGP all those years ago I was just a kid with a dream..

I had been building resistive loads and cranking tube amps thru them for years.

I didn't get good tone out of these rigs until you showcased Aiken's reactive load design, and put it all together with high quality components.

You made me the man I am today !

 

[Humbug]

Slight (planned) tweak to the circuit to correct the midrange response and lift the highs a bit. The lower than expected HF response is still annoying, but I'd say this is pretty close...

 

[Humbug]

Ok, I'm calling it... Now to figure out what causes the SPL to deviate from the impedance curve in the highs.

 

[James Freeman]

Nice!
Now share.

 

[Humbug]

Nice!
Now share.

Of course!

Load resistor: 13.5r (adjust to the resistance of the rest of the circuit to get the desired nom imp)
L1 network: 0.9mH, 22nF, 138r
L2 network: 0.7mH, 1uF, 8r
L3 network: 10mH (0.4r series), 180uF.

Edit: I'm sure I don't have to tell you this, but the box (and component placement in the box) is a big variable and _will_ affect the circuit. These values work in a steel 3U 19" rack chassis that's full of holes and says "Line 666" on the front, your results may vary.

Edit2: I think I've mentioned it before, but this is replicating a pretty obscure cab and will probably not sound (or feel) right with all IR's. It's pretty sweet with closed back Alnico Blue or Celestion 100 412s, though.

 

[James Freeman]

I would love to build a 16ohm load.
The 16ohm tap of the output transformer uses all the copper winding, some some say it affects the sound, electrically it shouldn't matter though.

 

[Humbug]

I would love to build a 16ohm load.
The 16ohm tap of the output transformer uses all the copper winding, some some say it affects the sound, electrically it shouldn't matter though.

Yeah, I tend to prefer 16 ohm cabs but I think that has more to do with the cabs than the copper... However, _if_ the winding of the OT is a little off, or there is an intentional mismatch (Trainwrecks, for instance) it can make a difference, and the difference would probably be more noticeable on the highest secondary tap. Maybe :-)