Analyzing the Lab Series L5

[blandman74]

Hi new poster here.  I have been looking at the (Moog/Gibson) Lab L5 amplifier schematic (here it is) and thinking about why it gets the rave reviews that it does from guitarists.  As it happens, I bought one in high school but had to sell once I went off to college.  Sad.  Now I would like a similar sound but at apartment levels, not the brutal 100W volume levels the L5 could achieve.

My guess so far about the reasons for its excellent sound (and why even tube amp purists acknowledged the tonal quality of this amp) are thus:

(1) the "multifilter" feature, which had grouping of fixed hi-end boosts mixed in to the signal; probably an attempt to accommodate keyboardists, but almost certainly the thing that made this the favorite amp of Gang of Four and BB King (!).

(2) the power amp, which looks like a Class A, John Linsley Hood type of amp, on steriods.  Right now I am thinking of using a much quieter (5-10W, still very loud) power amp like this one.  As a substitute in my homebrew. Not a fan of chip amps, me.  No offense.... 

EDIT: Sheesh, looking at it again, and reading teemuk's comments on the L5, seems I missed that it is complementary and not the traditional "class A."  Nevermind on that one....

(3) the "parametric" filter in channel two.  That thing is purdy...

(4) that downright bizarre transconductance amplifier after the mixer stage, but before the compressor (the now-obsolete CA3094).  What the hell is that thing for?  Distortion, maybe? 

(5) Reverb driver.  The tank appears to be driven hard.  So much so that the make-up gain for the reverb output is only 20 dB.

Would love to know people's thinking on that, as I get set to emulate this in smaller scale.

[teemuk]

I would also add...

(1) Two limiter compressors. Yes, that "downright bizarre transconductance amplifier" was called a "Distortion trim" circuit and it was supposed to simulate "psedo tuve [sic] distortion which flat tops the waveform". I'm pretty sure that it's a soft clipping circuit of some kind.

(2) A very nicely designed power amplifier that has boatload of output devices. You just don't see as many output transistors in equally rated amps nowadays. Also, check out the way they added the "Baxandall diode" to the PNP side of the quasi-complementary circuit. Concerning stability, it's the best method to do this I've seen so far. It looks like care is put to designing the current limiter as well.

(3) Individual supplies for both preamp and power amp.


I've been simulating parts of this design few times myself but never focused on the limiter stages. However, today I just created an OTA model for SPICE (based on LM13600) so if you're interested just let me know.

[blandman74]

Thanks!  I posted that edit in my first post, just before you made your comment.... 

What's your reasoning as to how separate power supplies (pre and pwr) affects the sound?  Not surprised that it does, but can you explain how?

And I am definitely interested in the OTA model if I can use it for SwitcherCAD/LTSpice.  I will probably try to use the 13600 to emulate that little distortion circuit.  I am very curious about it.  Will PM you...

[teemuk]

Check your email.

Well, I guess I gave a bit wrong impression in my post. I believe that individual supplies really do not have any notable effect to the sound (preamp voltages tend to be regulated anyway). However, when compared to circuit that derives the preamp voltages from the same higher rail voltages that power the power amp this approach is definitely better due to total isolation of the supplies. I also like the detail that - regardless of isolation of the supplies - the preamp rails are still regulated with active regulators. Compare this power supply to those what you typically see in amps and you get the point.

[J M Fahey]

Dear Teemuk. I *think* I understand what the CA3094 does here. It´s a buffered OTA, very similar to the LM13600, great for compressors or VCFs. My doubt was: if that´s so, where´s the diode ? (or rectifier). I think I´ve found it (or at least something that functionally imitates it). The guitar signal is fed into pin#2 (suitably attenuated 15k/1k); it develops across the OTA's load (10k+20k trimmer), and is buffered by the right half Darlington pair, leaving through pin #6. The signal (480mV) is fed into pin#5 of the 3094. The audio *voltage* is symmetrical, but I think that the *current* is not, the pin 5 surely does not source or sink the same amount of current. In fact the datasheet says it sinks a few mA (the gain, or more precisely transconductance-controlling current), fed by R169 (33k) , but says nothing about sourcing current, in fact it isn´t necessary at all. I think that the non-symmetric current handling capability creates a D.C. component which modulates gain with the audio signal. Not the best "diode" in the world, but it has no threshold, and makes the output signal non symmetrical to boot. What do you think ? J M Fahey

[teemuk]

I think I understand what you mean. The signal at Iabc input does indeed experience a shift in its DC offset when signal amplitude increases.