Mach1 vs Southland: Frequency Response

[debrad]

Hi everyone,

I've been reading a little more about the sonic differences between the Greer Lightspeed and Southland overdrives and the general consensus seems to be that the Southland is more "mid-scooped" (in addition to the differences in gain).

Looking at the PedalPCB schematics for the Mach1 (Lightspeed) and Southern Belle (Southland) and reading this post from @Markus Kersius, the only differences are:

LS:​

a100 vol, a10k tone, a500k gain, R4=4k7, and opa2134 ic​

​

SL:​

a 250k vol, a10k tone, a1meg gain, R4=22k, 2 extra Bat41's, and tl072 ic​

Given that the tone pots are the same, I'm curious to know what is creating the Southland's "mid-scoop"...the difference in the other pots, R4, or something else?

 

[manfesto]

It’s probably mostly a combination of the bat41 hard clipping and the increasing of the volume pot to 250K.

R4 just sets the gain of the second op-amp stage so shouldn’t affect the EQ, just the volume. The difference between a TL072 and an OPA2134 is probably noticeable but probably not the important change.

EDIT: so I think I misread the schematic and there’s no capacitor touching the volume control at all so it shouldn’t really be affecting the frequency response either I think?

If so, it really is mostly down to the hard-clipping BAT41s?

 

[debrad]

Thanks @manfesto. So, if I was to put those BAT41 diodes on a switch, I shouldn't hear that "mid-cut" until I engage the extra diodes?

 

[manfesto]

Thanks @manfesto. So, if I was to put those BAT41 diodes on a switch, I shouldn't hear that "mid-cut" until I engage the extra diodes?

The bigger problem would be if you put the BAT41s on a switch, when they’re out of the circuit you’ll have a sizable volume boost because you’ve removed a clipping stage

 

[swyse]

R4 is making up for the volume loss from the hard clipping, I’d put that on your switch too so when you switch you don’t get a big volume loss/gain. I can model the circuits in LTSpice later if you’re interested to see them on a graph, I don’t think you’re going to see any significant EQ difference though

 

[debrad]

Yes, I was anticipating the volume drop and thought I'd test a variety of diode pairs in addition to the BAT41s to see if I could find a happy compromise between extra gain and volume loss...never thought about putting R4 on the switch too (mostly because I didn't know that was its function!).

 

[swyse]

Here are models. First up is the gain sweeps. Note due to the 1Meg pot we do see more treble roll off on the southland with the 100pf capacitor at the highest gain settings. This is with volume at 100% and tone at 5k ohms.




Next up is tone sweeps with gain at 0 and 100%




Finally here is gain at minimum, tone at 5k, volume sweep.


I think the "mid-scooped" tone that people are hearing must either be due to the high end roll off of the larger gain pot, or how they use it with their extra clipping maybe making the preferred tone setting different?

 

[manfesto]

I think the "mid-scooped" tone that people are hearing must either be due to the high end roll off of the larger gain pot, or how they use it with their extra clipping maybe making the preferred tone setting different?

I think it could also be the hard-clipping itself, which'll generate different harmonics than soft-clipping by itself and change what we hear