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What's Up with the Mesa Walkabout Mid Knob?

#41
Hello Everyone,

Sorry I am late on this but I figured that I have to clarify a few things. First I own an M-Pulse 360 and love it's sound through my Boogie 15" cabinet (TL606 design I believe with EVM15). I have played bass off and on for the last 10 years and found this rig to be the best sounding that I have had (with the exception of the old Allied Radio Corp all tube PA head that I used for bass, I sounded so good).

Now, on to the fun stuff. After I purchased the amp I contacted Boogie for a schematic so I could get a feel for how things worked in this amp, a natural tendancy for the EE geek that I am. I did read how the mid control was different from the bass and treble controls and wanted to know how different, hence my request for the schematics. After reviewing the schematics, I can say with 100% assurance that the mid control is passive. The Bass and Treble controls are active and use Op amps for the boosting/cutting. I have since gone on and modeled the tube section in LTSpice (free simulation software and free tube models on the web) and ran an AC analysis to see what the frequency response of the preamp is with different settings on the mid control. The attached results speak for themselves.

I believe that the Walkabout is a trimmed-down version of the M-Pulse series (I believe that Mesa even stated so) and thus most likely uses the same preamp setup as used in the M-Pulse 360/600.

Maybe someone has a schematic for the Walkabout and would be willing to share it?

Regards,
Sean
 

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#43
KJung said:
One more comment! Just like the mid control being somewhat unusual versus other amps, the treble control is voiced uniquely also. I believe the shelving start point is around 2K... very, very low. This results in the treble control really acting more like an upper mid control. This makes sense, since the MPulse line seems targeted more toward rockers than funk/pop guys.

I find that cutting the treble control just a touch (11 o'clock), and then using the upper band of the semi-parametrice mids set at around 4K (the more typical treble center or start point) results in the amp really 'cleaning' up in the upper mids and sparkling a bit in the treble region.

Just like there is a lot of sound in that mid control, the treble control also can really 'revoice' the amp quite a bit with subtle changes.

This EQ power is a blessing and a curse with these amps. They are definitely not 'plug and play', but there is a very wide range of clean to moderately grindy tube tone in them when the mid and treble controls are varied IMO.
Click to expand...


This is a cool discussion about a very cool bunch of amps. I'm surprised that I haven't noticed this thread up until now.

I agree that the sweet spot for the passive mid is between 9 and 12 o'clock. I also keep my treble between 11 and 1 o'clock.

The one thing that I'd like to add to this discussion is that cutting slightly at 2K on the parametric eq works well for removing any harshness in the active treble control. My parametric is almost always set this way.

I always start with everything flat, with the exception of the passive mid at 12 o'clock and a slight cut at 2K. All I have to do is make subtle adjustments to get my sound from there.
 
#44
sjb64 said:
This
The one thing that I'd like to add to this discussion is that cutting slightly at 2K on the parametric eq works well for removing any harshness in the active treble control. My parametric is almost always set this way.

I always start with everything flat, with the exception of the passive mid at 12 o'clock and a slight cut at 2K. All I have to do is make subtle adjustments to get my sound from there.
Click to expand...


+1... depending on the cab, I usually have a slight cut between 1K and 2K... mostly closer to 2K to just smooth the upper mids a touch given the voicing of the treble control.
 
#45
cmhgreg,

I don't know what "one of those" is, but if you set the pre/post switch to pre then you completely bypass the preamp. This output is taken directly from the grid of the first gain stage , it is buffered with an opamp.

Another thing to point out is if you feed directly into the the Effects Return with the mix pot set to 100%, you can drive directly into the power amp section. Remember that you will still be driving the tube-driver of the power amp so you should still be getting a tube-like sound. You must have the footswitch to do this as you need to activate the Effects Loop switching. You can even use the effects loop switching to enable a second preamp if you like.

It looks like I'm going to be busy with some more experiments.:hyper:

Regards,
Sean
 
#46
TucsonSean said:
I don't know what "one of those" is, but if you set the pre/post switch to pre then you completely bypass the preamp.
Click to expand...

Sorry. I meant, could you post another one of those graphs showing the signal when it is bypassing the tone controls of the preamp completely (which can be accompished by recording the signal thru the DI with the switch set to "pre" as I understand it)?

I'm just curious to see what the signal looks like compared to the graph you posted previously. If you can. Thanks.
 
#49
TucsonSean said:
cmhgreg,

The Pre signal is taken directly from the input to the first tube (essentially right at the input jack) and buffered by a unity gain op amp (gain of 1 or a buffer). What ever you put in is what you will get out.

Regards,
Sean
Click to expand...

I understand that (I think). What I am trying to understand is why the signal I recorded that way (the "Pre" signal) matched up on the graphs I posted almost exactly to the signal I recorded with the Mid knob fully counter-clockwise, while the graph you posted (if I'm interpreting it correctly) suggests that "flat" is achieved with the knob fully clock-wise.
 
#50
KJung said:
+1... depending on the cab, I usually have a slight cut between 1K and 2K... mostly closer to 2K to just smooth the upper mids a touch given the voicing of the treble control.
Click to expand...

Ken, I always get a chuckle from your post, as your opinions are almost always exactly opposite of mine. Too funny. You must be my secret evil twin or something. Me, I almost always boost a little at 1.5K. Helps give me a good biting rock tone. Although I must admit, for a fairly simple EQ, this amp took quite a bit of experimenting to get it right for me. I really do love this amp. I crank the gain up to about 2:00 and the dynamics coming from the tube are simply wonderful and inspiring to me.
 
#51
Rhythm5 said:
Ken, I always get a chuckle from your post, as your opinions are almost always exactly opposite of mine. Too funny. You must be my secret evil twin or something. Me, I almost always boost a little at 1.5K. Helps give me a good biting rock tone. Although I must admit, for a fairly simple EQ, this amp took quite a bit of experimenting to get it right for me. I really do love this amp. I crank the gain up to about 2:00 and the dynamics coming from the tube are simply wonderful and inspiring to me.
Click to expand...

+1.. :) We definitely have different tone goals. I do like it clean and crisp for the most part. I too dig that mid growl of the MPulse line, and it's there when you need it. One of the impressive things to me about the Mpulse600 is that it's one of the few amps I've played or owned that does a pretty good 'imitation' of a fat all tube type tone with that gritty mid growl and big, bloomy low end, but can also stay quite clean and get reasonably 'modern' sounding with the powerful EQ (although that tubiness is always there).

It's a great amp!!!! +1 also on your comment about the EQ taking a bit of time. The EQ has a huge range, and the interaction between the bass, mid and treble can almost totally revoice the amp. The MPulse line is one of those that can go from sounding great to sounding really bad (scooped, nasal and everything in between).
 
#52
how bad is it for your speakers to really crank the gain on the m pulse? In other words substantially clipping the pre amp section. I know on bass tasters they do a walkabout with the preamp in full distortion. Your still talking about a solid state power section on this amp. Is it much different than having a distortion pedal or is it different because with a disotrtion pedal you might have head room with both the pre and power amp.

Maybe it's a case that it's fine so long as your not running the power section too hot and leave a bit of head room.

Any thoughts by anybody?
 
#53
theshadow2001 said:
how bad is it for your speakers to really crank the gain on the m pulse? In other words substantially clipping the pre amp section. I know on bass tasters they do a walkabout with the preamp in full distortion. Your still talking about a solid state power section on this amp. Is it much different than having a distortion pedal or is it different because with a disotrtion pedal you might have head room with both the pre and power amp.

Maybe it's a case that it's fine so long as your not running the power section too hot and leave a bit of head room.

Any thoughts by anybody?
Click to expand...

Zero issue as long as you control the absolute volume, as you mention above.... no different from a distortion pedal as far as I know.
 
#54
cmhgreg,

Even when you play your bass into the amplifier and set the tone controls to flat, your pickups will not be providing a flat response and there will be harmonics produced by your strings (they are not producing pure sine waves). You need use a frequency generator and sweep the frequency from 20Hz to 20kHz to get a more accurate representation of the tonal characteristics of the amplifier. Most speaker simulation software can do this. Check out Speaker Workshop from Auda. There are others but that is the first that comes to mind. I am guessing that you would have much more similar results to mine than your previous attempt.
 
#55
theshadow2001,

If Mesa did not change the design then you will not be able to overdrive the output stage. There is a limiter that does not allow a large enough signal to be sent to the output stage. This makes sure the the output does not clip. If you do happen to send a large enough signal to the output and it sounds like it is clipping, that is the limiter kicking in.
 
#56
TucsonSean said:
theshadow2001,

If Mesa did not change the design then you will not be able to overdrive the output stage. There is a limiter that does not allow a large enough signal to be sent to the output stage. This makes sure the the output does not clip. If you do happen to send a large enough signal to the output and it sounds like it is clipping, that is the limiter kicking in.
Click to expand...

Ok so when I start this guy distorting it's not the tubes breaking up it's a limiter after the preamp thats causing the signal to clip is that correct?

Even though there is a limiter in place. Is it still possible to really crank the power amp and do damage. Or is it a case of you'd really want to be an idiot to damage your speakers with this amp. (Like everything below 100Hz turned to 11 and the power amp cranked on full etc etc.)

I was always wary of really cranking this amp up. Since it's kind of grindy anyway (especially with my sterling) and there didn't seem to be any sort of clip indicator or anything. It's things like this you'd think that mesa would tell you. Why is there always such smoke and mirrors when it comes to music equipment.
 
#58
TucsonSean said:
cmhgreg,

Even when you play your bass into the amplifier and set the tone controls to flat, your pickups will not be providing a flat response and there will be harmonics produced by your strings (they are not producing pure sine waves). You need use a frequency generator and sweep the frequency from 20Hz to 20kHz to get a more accurate representation of the tonal characteristics of the amplifier. Most speaker simulation software can do this. Check out Speaker Workshop from Auda. There are others but that is the first that comes to mind. I am guessing that you would have much more similar results to mine than your previous attempt.
Click to expand...

I understand what you are saying, but I disagree that it matters much whether I used the pickups on my bass or a speaker simulation software in doing the test. I believe my test was still valid and it still shows that the bypassed signal is identical to the signal generated when the MID knob is fully counter-clockwise. In other words, if one wants the tone from the amp to match the tone of a bypassed signal, center the BASS and TREBLE controls, turn the MID control fully counter-clockwise and deactivate the parametric EQ.

If there is something I did wrong that is a flaw in my test, I certainly don't mind if someone points it out. Likewise, I posted the steps I took in doing my test so that the test could be duplicated. Finally, if anyone were to repeat my test I'm sure their ears would be the ultimate judge and they would hear what I heard -- that the MID knob fully counterclockwise (and everything else "flat") produces a tone that is indistinguishable from that produced when the preamp is bypassed, and that when the MID knob is cranked fully clockwise, the tone is noticeably more mid-oriented than when the signal is being bypassed.
 
#60
I just took the manual as indicating the passive mid to be a cut only control that's flat when "dimed", but the freq response chart posted early in this thread is quite conclusive that it is boost only (to my untrained eye)-can an EE comment on that? I cannot make out the x axis scale-maybe it's lows and highs that are relatively boosted??. In any event, I get my magic dust with it set at 10 o'clock on both my WA and M9.
 

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