The Marshall DSL "Resonance" circuit

[Helmholtz]

1) Is there a circuit analysis available?

2) If not, anyone interested in a circuit discussion?

[g1]

I noticed that one as well and was also hoping someone had some info on it. I guess we should specify that it's the newer MkII or HR version of DSL.
schematic attached, resonance circuit on pg.2

[Helmholtz]

I noticed that one as well and was also hoping someone had some info on it. I guess we should specify that it's the newer MkII or HR version of DSL.
schematic attached, resonance circuit on pg.2

Good point.
Can you advise how to change a topic/headline?

[Helmholtz]

To be more precise, this is about the newer active "resonance" circuit of MKII or HR versions.

[loudthud]

All that business with TR1, TR6 and C139 looks like a synthetic inductor. With C138, VR9 and R146 you have a low Q series resonant circuit. A series resonant circuit has a low impedance at the resonant frequency so when that's in parallel with the Presence control, it makes a variable peak at the resonant frequency.

[g1]

looks like a synthetic inductor.

I just remembered there had been some mention of 'gyrator' resonance circuit in Marshall JVM series. I guess this would be a different set up for the same idea?

https://sound-au.com/articles/gyrator-filters.htm

[Helmholtz]

Yes meanwhile I'm sure it's a gyrator acting like an inductor.
So this is a real resonance circuit giving a resonant peak in the amp's response. Other than simple RC "resonance" controls which just boost the bass.

When I first looked at the circuit I just saw an emitter follower with its output connected to the input via a 100R resistor. What??

With a Darlington pair the betas of the transistors multiply, which in turn gives very high input impedance, very low output impedance and a voltage gain (G) close to 1.
At first I thought that the low output impedance would result in a total circuit impedance close to 100R.

But I forgot to consider the bootstrap effect which largely increases the resistance between input and output (as well as ground).
With a voltage gain of say 0.999, the 100R resistor looks multiplied by a factor of 1000, so total input or load impedance of the gyrator is high.
But only as long as there's no significant voltage drop across the 47n input coupling cap.
The impedance of the cap increases with 1/f . So the gain between the ends of the 100R resistor drops accordingly
This effect is quite dramatic: When the signal at the base at some lower frequency drops by only 1%, the gyrator input impedance drops by a factor of around 10 in my example.

So I can see that the circuit impedance drops at low frequencies just like an inductor.
Of course it also needs the phase relation of an inductor, but proving that will require to set up the circuit equations and solve for complex impedance.

BTW, how effective and useful is the Resonance control?

[Helmholtz]

Of course it also needs the phase relation of an inductor, but proving that will require to set up the circuit equations and solve for complex impedance.

Ok, I did (assuming an ideal buffer).
The impedance formula clearly shows inductive phase.
So the circuit behaves like a real inductor.

[GoldtoneWorkshop]

What would be the advantage of using this type of circuit, which acts like an inductor, instead of just using an inductor? Would it be just to reduce noise?

[Helmholtz]

What would be the advantage of using this type of circuit, which acts like an inductor, instead of just using an inductor? Would it be just to reduce noise?

Hard to say.
A well shielded low tolerance and good linearity inductor might be more expensive than a gyrator circuit, though.

[Helmholtz]

Still hoping for feedback regarding the performance/usefulness of the circuit as I don't have a chance to test one of these amps.

Especially interested in the actual peak frequency.

[Greg Robinson]

I haven't had a chance to directly A/B the Marshall gyrator resonance circuit vs the more common Peavey style series capacitor resonance circuit, and certainly haven't done a direct comparison in an otherwise identical circuit with the filters tuned to match, but I have had quite a few of both types across my bench fairly regularly at different times. I haven't noticed a difference in "feel" or anything that made me think more highly of one over the other, they both increase gain at low frequencies, and accentuate the cab's bass resonance by reduced damping (giving a "looser" feel).
Maybe next amp I build with a resonance control I'll try both circuits

As for whether these controls in general are useful? I'd say yes, but that's going to be dependent on playing style and taste.

[Helmholtz]

I might add that the inductance of the gyrator can be easily tuned by varying a single resistor.
This can be a big advantage in the design phase.

[mhuss]

I wonder if you can vary the Q as well. This could be generalized to be useful in, say, Ampeg-style mid circuits (instead of the expen$ive inductor).

[Helmholtz]

Yes, Q can be varied by adjusting the feedback resistor. But that also changes L and there is an upper limit.
A way to lower Q (without changing inductance) is adding a (variable) resistor in series with the gyrator.