BSIAB 2 Analysis in SPICE

[Vivek]

Please check the DC level over time for caps C4, C5, C8

Please see if there is a DC shift in the first few ms

[niektb]

[...]

I think I discovered a mistake here! Aren't C2 and C6 10x too small?

[Vivek]

You are right about C2

I will rerun SPICE and see what effect it has

[niektb]

Whoops I meant C4 not C6 

[iainpunk]

that output capacitor, which in this schematic is not loaded down, should have no effect on frequency response. only when there is loading on a capacitor, it starts to changes the frequency response.

that DC shift is a result of asymmetric amplification/clipping.
the function of capacitors to put in there to remove DC offset, makes the area under the waveequal on both sides, this shifts the 0-crossing towards the more narrow side.

hope it makes sense.

cheers

But thats the thing! I dó have an output capacitor?

which of the schematics did you use for the simulation?

cheers

[niektb]

This is the current schematic I have. I've been toying with different clipping stages but as soon as I introduce asymmetric clipping, the waveform shifts...  setting the built-in 'oscilloscope to AC coupling also makes no difference...

[PRR]

... as soon as I introduce asymmetric clipping, the waveform shifts...

That's what happens in real life also.

The time-constants around that shift make an "equivalent thump pitch" which may be part of "the sound".

[POTL]

Hello everyone, I see that the topic has died out, but I still want to leave my 2 cents. Today I decided to look at different emulation methods. I ran simulators
Rog style j201
Mu amp
Mosfet 2n7000
TL072
In parallel with each emulation method, I ran a triode simulator. I was able to reproduce the frequency response in each way and I decided to compare the readings on the oscilloscope and despite the same frequencies, the change in the waveform was different in each way.

Triodes when it distorts the signal, it mainly compresses the upper half-cycle, the distortion is asymmetric, the cathode follower at the end of the preamp compresses the lower half-cycle.
 Jfet, Compresses the LOWER half-cycle (in fact, mirror compression), with an increase in the input signal, the compression becomes symmetrical.
 Mu amp (srpp), immediately compresses the signal symmetrically.
 Op amp, square wave, diodes help, but it looks too square against the background of transistors.
 Mosfet squeezes the upper half of the half cycle (like a triode).

 Mosfets are the closest of all simulations, although I've always heard that jfets behave more like triodes.

[POTL]

upd. Just tried the Zvex circuit, in a multisim simulator for smartphones, with one change, I didn't use 2 10M resistors, I took 1 resistor and attached it to virtual ground, then changed the voltage on virtual ground and that allowed me to limit like the top half-period and lower. Jfets don't know how, don't believe about Mu amps. someone tried to run 2n7000 / bs170 from 18 volts (like I saw it in the Bogner la grange circuit).

[niektb]

well, not died out but to busy with my day-to-day job

Have you by chance also simulated a depletion-mode mosfet (f.e. de LND150)? it seems to be the new holy grail around here so I'm curious to see how it stacks up 

[POTL]

well, not died out but to busy with my day-to-day job

Have you by chance also simulated a depletion-mode mosfet (f.e. de LND150)? it seems to be the new holy grail around here so I'm curious to see how it stacks up 

Unfortunately no I do not have a model and I am not trying to figure out how to do them correctly in multisim. I tried several times and they did not work, and the model from the microchip site is not suitable for multisim, I am waiting for this model, to use the capabilities of the transistor before buying a batch for tests.

[iainpunk]

Triodes when it distorts the signal, it mainly compresses the upper half-cycle, the distortion is asymmetric, the cathode follower at the end of the preamp compresses the lower half-cycle.
 Jfet, Compresses the LOWER half-cycle (in fact, mirror compression), with an increase in the input signal, the compression becomes symmetrical.
 Mu amp (srpp), immediately compresses the signal symmetrically.
 Op amp, square wave, diodes help, but it looks too square against the background of transistors.
 Mosfet squeezes the upper half of the half cycle (like a triode).

 Mosfets are the closest of all simulations, although I've always heard that jfets behave more like triodes.

really interesting, i never really compared them on a scope comparing them with tubes.
there is one thing tho, the bias level. did you bias them all to the center of the operating range? some things biased differently, especially BJT's, can have a plethora of tones available this way.

Mu amp (srpp), immediately compresses the signal symmetrically.

two things here:
the SRPP is not the same as a Mu Amp. the distortion characteristics are different, the SRPP sounds 'softer' than the Mu Amp.
did you compare the solid state muamp with a single ended triode? i wonder how a tube and solid state SRPP actually differ.

cheers

[POTL]

I think of SRPP as Mu amp by default. All comparisons I made with the SRPP scheme. Later I will try to compare Mu amp with other circuits, but I see no reason to use it in real life, there are too many drawbacks.

[iainpunk]

I think of SRPP as Mu amp by default. All comparisons I made with the SRPP scheme. Later I will try to compare Mu amp with other circuits, but I see no reason to use it in real life, there are too many drawbacks.

to much drawbacks? its excellent linearity, and low output impedance make it quite popular in the DIY tube HiFi scene. a single 12AX7 and only a few passive components give you super linear and powerfull signal. one of the downsides is the relatively low signal gain, compared to the MuAmp or single ended designs.

i personally love the SRPP topology, but i generally use Jfets and diode bias.
this specific topology with the diodes, it really doesn't like having an input singal that's larger than the Jfet's internal diode and the bias diode combined forward voltage, and weird octave-down glitches can appear in the decay of notes, if no pre-limiting is applied.


Cheers, Iain

[Vivek]

i personally love the SRPP topology, but i generally use Jfets and diode bias.

Cheers, Iain

I am more interested in overdriven stages, to generate distortion

I request you to share data outside the linear zone please. Did you generate the output function ?

[iainpunk]

i personally love the SRPP topology, but i generally use Jfets and diode bias.

Cheers, Iain

I am more interested in overdriven stages, to generate distortion

I request you to share data outside the linear zone please. Did you generate the output function ?

when i get home, ill fire up the old oscilloscope and do some XY plotting of the in and output of that particular gain stage, and use my old phone to take photo's. should give a good in/out curve, since i can get away without any in/out DC coupling in srpp stages.
ill try with different resistors and diodes for the bias, to see if that makes much difference, also matched and un-matched Jfets.

cheers

[POTL]

I took the time and brought more simulations, I took the first stage of a Fender Twin blackface and a Marshall as a basis for comparison. The goal was to learn how to simulate filters (cathode resistor + capacitor pair). Compared SRPP, 2N7000 and TL072. The first thing I figured out is the maximum net gain that is available at 18 watts, just over 17dB, which is about 1/2 of what the triode can do with a standard plate resistor value of 100K. And yes, all 3 ways worked great.
1) The 2N7000 had components in the source similar to the cathode in the triode and could accept 1VPP, the gate was biased by 4v (depending on the source resistor, the voltage varied in the range of 4v-4.5v). A 1k-1.5k resistor was installed between the capacitor and the source to limit the gain.
The frequencies coincided, but the gain was equal to 1/2 of the triode. Changing the voltage at the gate made it possible to adjust the limiting, you can choose which half-cycle will be compressed more, which means you can set up symmetrical limiting or asymmetric, without using diodes. The AMZ mosfet booster circuit was taken as a basis, but with a different offset and adjustments as in tube amplifiers.
2) TL072, gave similar results, except limiting, limiting is always symmetrical, diodes can be used, but this does not look like the shape of tube limiting.
3) SRPP was only able to receive a signal of 0.6VPP, theoretically, you can put a voltage divider at the input and increase the gain, but you will also need a buffer at the input.

[Steben]

Looking at the graphs, it definitely makes the mu-amp a great input valve stage equivalent because of its non-linearity and gain. I'ld let this be followed by op-amp design for clipping. Especially if non-master volume is to be mimicked, because of the similarities to a class AB power amp and sag emulation is easier. Pre-amp distortion might still be done with FETs.