LND150 mu-amp/Minibooster/SRPP

[Fancy Lime]

Hi there,

quick breadboard note: I built an SRPP stage using two LND150. Pretty much R.G.'s SRPP plus a 1k source resistor, not bypassed. Very nice low to mid gain overdrive. Can be dialled back to squeaky clean with the guitar volume. Highly recommend to try that thing. To me the gain range is much more useful (much lower) than with the JFETs I have (all fairly high transconductance).

I have half a mind of calling it Jenny Rockless

Cheers,
Andy

[Vivek]

Is the gain determined by external components ?

and can a Mu Amp stage be designed to have same / similar transfer function as the other topologies  ?

[Fancy Lime]

Hi Vivek,

unfortunately, gain of a mu amp is heavily dependent on device properties. The source resistor reduces gain a bit, though. You can also reduce gain by adding a resistor in series with the cap from the drain of the lower to the gate of the upper FET. I tried a logarithmic 500k pot wired as a variable resistor in this place and it works very well as a gain control.

I am not sure what you mean by similar transfer function compared to other topologies. That totally depends on the test case (e.g. frequency* of the incoming signal), what other topologies you have in mind and what you define as "similar". The transfer function of a mu amp is a bit asymmetrical IIRC and it is not very linear, in case that helps answer your question. The clipping is hard but the transition into clipping is nice and gradual without unpleasant fizz.

Andy

Edit: the "frequency" with the * was originally "amplitude", which makes no sense. Sorry.

[Vivek]

That was transfer function of Mu-Amp first stage of BSIAB

[Fancy Lime]

Ah, I see. The x-axis is input in mV and the y-axis is output in V, correct? Are we talking DC? That is very tricky with the mu amp because the upper FET is AC coupled to the lower. So knowing the DC transfer function is not going to provide us with much useful information about the audio behavior of this thing. Or are we talking DC at a specific frequency? In that case, what did you mean by "similar as the other topologies"? Minibooster and SRPP? Minibooster and Mu Amp are used more or less as synonyms. The SRPP will be a little different with slightly more gain and lower output impedance but otherwise very similar. IMO the SRPP is usually the better, more consistent topology if the stage is not immediately followed by a buffer.

Andy

[Vivek]

I had plotted AC transfer function of Mu Amp of BSIAB

with inputs in mVp on X axis

and outputs in Vp on Y axis

at 1Khz


As Minibooster and SRPP? Minibooster and Mu Amp are used more or less as synonyms, I suppose their transfer functions are similar.

[iainpunk]

for Jfets, i use this topology:


it sounds really cool with Jfets, although it really doesn't like having its input overloaded.
i wonder how that sounds with LND150's, it would be less vulnerable to input overload problems.

cheers

[bluebunny]

I have half a mind of calling it Jenny Rockless

Just this simple sentence deserves a sausage.   

[Fancy Lime]

I have half a mind of calling it Jenny Rockless

Just this simple sentence deserves a sausage.   

Why, thank you, kind Sir! I'll share it with Jonny for pushing me into the LND150 rabbit hole (no pun intended... well, maybe a little).

Andy