Marshall Power Brake Attenuator PB100 First Impression

[mpv1504]

I wonder if the "one wire" mod (bypassing L2 and C1 in the schematic) really improves the tone.

 

[playloud]

I wonder if the "one wire" mod (bypassing L2 and C1 in the schematic) really improves the tone.

It will certainly make the load act less like a speaker. Those two components create the low-frequency resonant peak in the impedance curve, which is a key element of how speaker's respond.

Of course, "there's no accounting for taste" and all that.

I calculated some spectograms and plotted the mean frequency response of the recordings above, relative to the control (which you can see corresponds to the x-axis):



As you'd expect, the divergence from this control increases with the level of attenuation.

It all looks pretty good, except for the unchecked divergence above ~15k! This is precisely what I would hope to tame with that resistor in parallel with L1.

Now it probably bears pointing out that most of us probably can't hear much above 15k (take a test online if you feel like it), so the difference won't be as dramatic as it might appear. I plotted all the way up to the Nyquist frequency (48k) because I could, but much of that data will only be interesting to bats. I used a log scale to compromise (and labelled the approx. turning points on the x-axis).

 

[marshallmellowed]

I

I wonder if the "one wire" mod (bypassing L2 and C1 in the schematic) really improves the tone.

I've done the one-wire mod on both of my PB-100's, and all technical reasoning aside, I like the results. I was originally going to make it switchable, but after trying it, didn't see a need.

 

[paul-e-mann]

I wonder if the "one wire" mod (bypassing L2 and C1 in the schematic) really improves the tone.

I'm not seeing that the tone needs improving, sounds fine as is.

 

[JohnH]

Hi @playloud I listened this morning on monitors, and I could just about hear a slight difference from control to max attenuation, but its not much, mainly in the bass as you noted. The plots are great, and it shows that through the main guitar range up to say 5kHz, response is fairly consistent within a couple of dB. Interesting how the difference plots vary up and down across the range. Some subtle interactions going on with the transformer the load and the speaker! The dB differences noted at very high frequency are probably based on signals that in that range, are at very low volume, since guitar speakers don't have much to say up there. When I do such tests, I mic a cab, but I see that yours were using a TAE after the output? I wonder how accurate the TAE is up at those frequencies?

I agree about the L1 bypass resistor idea, and itll be interesting to see what happens. On my design, the equivalent part made a big difference to my DSL401 which is sensitive to load since it has no NFB.

Bypassing the bass resonance circuit may or may not change the bass dB response, but I reckon it would give probably give a cleaner bass from a driven signal.

BTW, if I got my hands on one of these, I'd first be checking the state of the cap, since it's maybe the one thing that could degrade over time, and could be replaced.

Nice work!

 

[playloud]

Hi @playloud Some subtle interactions going on with the transformer the load and the speaker! The dB differences noted at very high frequency are probably based on signals that in that range, are at very low volume, since guitar speakers don't have much to say up there. When I do such tests, I mic a cab, but I see that yours were using a TAE after the output? I wonder how accurate the TAE is up at those frequencies?

Yes, it's definitely this. The IR was recorded at 96khz too, so it does have content above 5k (room reflections etc.), but nothing which is integral to the sound.

Good point re 160uf cap.

Edit: here's the frequency response of the control (normalized so 0dB = max) for reference:

 

[Gene Ballzz]

It would be interesting to see, hear or otherwise analyze what affect that fan power circuit might have on sound and dynamic response. Also interesting to determine if such a circuit could be added to the @JohnH design, as well as where to tap it from and any other circuit modifications that might be necessary? It would certainly be wonderful to have a JohnH M2 as a total "stand alone" unit, with no need for a wall wart to power the fan!

And given these reviews/reports/testings, I guess I need to revise my assertations from "The JohnH being the ONLY reasonably priced unit that retains tone and response" to being "one of the FEW units that exhibit that retention!"

Thanks Folks,
Gene

 

[matttornado]

Yes, I have a couple of them, and they both always stay fairly cool, barely warm when cranking a 100w amp. They use a unique design, where the cooling fan is powered by the signal flowing through the unit (fan spins as you play). There's also a one-wire mod that makes the unit sound even better (IMO), It basically shunts the electrolytic cap.

I tried that mod and made my sound not too good. Sounded way better with that cap.

 

[marshallmellowed]

I tried that mod and made my sound not too good. Sounded way better with that cap.

Like I always say, whatever works.