In my quest to find out what gains and equalisations do pedal designers use when they claim to target the Brown sound,
I decided to first analyse the BSIAB 2 using SPICE (and make a new thread for it, so its less cluttered, and more easily searchable)
I found, like most electronic endeavors, someone already did it 10 years or more ago!!
Drschwartz' Analysis of the BSIAB 2 in October 2008
https://www.diystompboxes.com/smfforum/index.php?topic=71466.0
I know I'm very late to the party, but I will try to add more data and graphics to that initial effort.
Given that Through Hole FET are getting harder to find, and their huge vagaries, I might try and see if it is possible to get somewhat similar response with Opamps and diodes since they are more easily available, more predictable and more repeatable. I would need assistance since my knowledge and experience is very limited.
That would then be a emulation of an emulation, but still might be worth it, if it simplifies, standardises, makes more stable final pedals.
The original schematic is here http://www.generalguitargadgets.com/pdf/ggg_bsiab2_sc.pdf
Thanks to the great experimenters who developed BSIAB 2
I will need help from all of you, so please let's treat this as a community project !!
PS: It is not my intention to "teach Grandma how to eat eggs". Please pardon me if my analysis and comments appears too basic and unnecessary.
STAGE 1(https://i.postimg.cc/zfyCVBcL/BSIAB-2-first-stage-schem.png) (https://postimg.cc/8FVJY1DD)
It's a mu-amp stage
QuoteThe mu-amp is a very old circuit. It was reasonably common during the vacuum tube era, where it offered the opportunity to get a lot of gain and signal output out of common, garden variety triodes. Later, National Semiconductor published a JFET adaptation of the mu-amp in their JFET applications notes in a collection of JFET cookbook circuits. Later, effects experimenter Jack Orman used the mu-amp for a guitar gain circuit, renaming the circuit the "minibooster". The use of the mu-amp has been adapted into several DIY effects circuits from there. You can see the original circuit on National Semi's web page at http://www.national.com/an/AN/AN-32.pdf
- http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm
.
Now that same Application note can be found at https://www.ti.com/lit/pdf/snoa620
Graph of output waveshape from the first stage : (https://i.postimg.cc/Vvf50Zk7/BSIAB-2-first-stage-Output-Ampl.png) (https://postimg.cc/gX7GfDRR)
Here I ran signals of 10mvp, 50mvp, 100mvp, 200mvp, 500mvp, and 1000mv peak sine waves at 1Khz into the input and plotted the output of the first stage
An input of 50mvp leads to an output of 976mvp, ie a gain of 19.5 times. Visually, it appears that there is very low distortion for this signal level.
We also see very little distortion also in the 100mvp curve
With inputs of 1Vp, we see that the output swing is getting rail saturated at ~ +3.6Vp and =4.1Vp. This is asymmetrical.
The tops are rounded for input 1vp
Let's see what happens when we drive the first stage with 2Vp input signals :
(https://i.postimg.cc/mD8VHqSk/BSIAB-2-first-stage-Output-Ampl-2vp-in.png) (https://postimg.cc/Kk3nX0wh)
We see rail saturation +3.72vP and we see asymmetry of output waveshape.
Now looking at the frequency response of the first stage : (https://i.postimg.cc/9Fp85D9Z/BSIAB-2-first-stage-Output-Freq1.png) (https://postimg.cc/dLhmGt4V)
and with some annotation and measurements (Extended X axis):
(https://i.postimg.cc/3rC5NJpJ/BSIAB-2-first-stage-Output-Freq2.png) (https://postimg.cc/NLMPPtpW)
It is expected that there will be a serious bass cut before a distortion stage. This is to prevent harmonics from the thicker strings from overpowering the fundamental and harmonics of the thinner strings. Indeed, pre-distortion bass cut exists in almost every high gain Amp and every distortion pedal. What makes each pedal sound different is the slope of the cut and the corner frequencies.
In the BSIAB 2, the output at 5Khz is 48 db more than output at 10 Hz, while output at 10 Khz is 53 dB more than the output at 10 Hz.
This means that the gain and saturation will depend on frequency. Earlier we saw graph of outputs with different inputs of 1Khz. Now lets see same graph but with signals of 5 Khz
(https://i.postimg.cc/T1CW3vf2/BSIAB-2-first-stage-Output-Ampl-5-Khz.png) (https://postimg.cc/n99L35XN)
Now we see that a 100mvp signal at 5Khz is distorted while a 100mvp signal at 1Khz was not distorted. This is because the gain depends on the frequency. Gain was 19.5 at 1Khz but is 58 at 5Khz.
Signals of higher frequency will saturate faster than signals of lower frequency.
Now lets use FFT to have a look at the harmonic content with 200mvp input at 1Khz(https://i.postimg.cc/8znSRp1Q/FFT-1-Khz-200mvp.png) (https://postimg.cc/rDW3cL0Q)
We see a mix of odd and even harmonics at output of the first stage.
Now we look at the filter and drive controls in between the First Mu-Amp stage and second Mu-Amp stage
(https://i.postimg.cc/3RWWj84P/BSIAB-Drive-Pot.png) (https://postimg.cc/KkSZxy5N)
It's frequency response looks like this (for different settings of the Drive knob):
(https://i.postimg.cc/7h8JnCvf/BSIAB-Drive-Pot-Freq.png) (https://i.postimg.cc/7h8JnCvf/BSIAB-Drive-Pot-Freq.png)
For low drive settings, there is considerable treble boost due to the treble bleed capacitor. The amount of treble boost reduces as gain increases.
This is a common strategy since high gain has more shrill harmonics, which need to be removed to sound good.
( However, seeing that the first stage already did a bit of clipping, an alternate strategy could have been to have a flatter response for low drive settings and a treble cut on the higher gain settings. I guess listening tests would have determined the better approach)
Now we analyse the interstage filter/Drive knob + Second Mu-Amp + Buffer
First we biased the buffer stage trimmer for roughly 4.5V DC
(https://i.postimg.cc/3xgJ5LDf/BSIAB-second-stage.png) (https://i.postimg.cc/3xgJ5LDf/BSIAB-second-stage.png)
For this analysis, we run sine waves into the input of the 2nd stage (not the output of the fist stage)
(https://i.postimg.cc/13thMJrN/BSIAB-second-stage-Output.png) (https://i.postimg.cc/13thMJrN/BSIAB-second-stage-Output.png)
We see that 100mvp at 1Khz gave an output of 550mvp hence a gain of 5.5x
Hence input signals of more than 500mvp will be clipped due to saturation.
Combined with the roughly 19x gain of the first stage, we now have 104.5x gain till the end of stage 2
Here is the output of the second and third stage combined (drive knob at 25%) :
(https://i.postimg.cc/DfXrDXcj/BSIAB-third-stage-Output.png) (https://i.postimg.cc/DfXrDXcj/BSIAB-third-stage-Output.png)
We see that 10mvp input lead to a 151mvp output from the third stage ie total gain of second and third stage is 15.1
But second stage had gain of 5.5, hence third stage has gain of 2.75x
The total gain from input to end of 3rd stage is (Drive knob at 25%)
19.5 x 5.5 x 2.75 = 295
Which is in the normal range for distortion pedals
At max drive setting, the situation is different !!!
19.5 x 60.47 x 2.77 = total gain of 3333 from input till output of 3rd stage. This is an extreme amount of gain !!!!
Here is the frequency response from gate of stage 2 till output of stage 3 (not including intermediate filter and drive)
(https://i.postimg.cc/c1bz7wLF/BSIAB-third-stage-Freq.png) (https://i.postimg.cc/c1bz7wLF/BSIAB-third-stage-Freq.png)
TRANSFER FUNCTION OF FIRST MU-AMP STAGE IN BSIAB 2
(https://i.postimg.cc/T1y6Yzrr/Transfer-Function-muamp1-B.png) (https://i.postimg.cc/T1y6Yzrr/Transfer-Function-muamp1-B.png)
(https://i.postimg.cc/QCNwzpyH/Transfer-Function-muamp1.png) (https://i.postimg.cc/QCNwzpyH/Transfer-Function-muamp1.png)
X axis is input voltage in mv Peak, from -1200mvp to +1200mvp
Y axis is output of 1st mu-amp in volts peak
This looks very tube like !!!
Asymmetric
Harder clipping on one side, softer clipping on the other. It will have great crunch and still have dynamics !!!
No wonder people love the BSIAB 2 !
And here are actual readings from LTSPICE, for the boffins amongst us:
-1199.90 -5.22503
-1069.97 -5.19632
-912.967 -5.15237
-809.9588 -5.11507
-610.936 -5.02147
-461.914 -4.9245
-343.878 -4.7977
-252.860 -4.4698
-181.927 -3.8878
-120.955 -2.5879
-82.9600 -1.6638
-23.96500 -0.44859
0 0
7.0000 0.13846
28.547 0.5319
60.461 1.1182
86.459 1.45482
122.5 1.8787
129.64 1.9609
133.47 1.9868
141.49 1.9848
156.47 1.9799
173.52 1.9929
215.53 2.0347
299.54 2.1025
404.58 2.1828
557.72 2.2558
773.69 2.3258
831.59 2.34044
1026.00 2.3793
1199.6 2.4055
Suggested reading: Yamaha and Line6 have patents that explain their transfer curves for different Amp emulations and also propose mathematical equations to fit the curves (for easier computation in the digital domain)
(https://i.postimg.cc/tRL3Rhb6/Yamaha-Patent-Transfer-curve.png) (https://i.postimg.cc/tRL3Rhb6/Yamaha-Patent-Transfer-curve.png)
https://patents.google.com/patent/US5789689A/en
reserved 4
Hello About a month ago, I wondered what could help us make an amp in a box without the classic ROG-style circuits, with single Jfets and their cons. Alternatives found 3 1) Mu - amp, which, although it works on jfet, is devoid of a number of disadvantages 2) Op-Amp, flexible, affordable and modern, but the problem is I haven't heard any good examples of op amp sound, the closest way to simulate is using Bajaman 3) Mosfets - These are available but have many disadvantages, however I recently learned about LND150 circuits as a good way to simulate amplifiers. I haven't tried them yet and I don't have a simulation model.
Now let's talk about mu amp I simulated the frequency response of the triodes in multisim by comparing both at the same time. 1) C2 must be 22uf or more 2) To reduce the output impedance and gain loss, there must be a resistor between the transistors (1k-10k) 3) R3 gave the desired response at a large value, about 18k 4) C3 was calculated at the desired frequency, usually there is a resistor between it and ground, which attenuates the gain. I simulated this about a month ago, the calculation was fast, it is possible to recalculate the circuit bypassing the "extra" resistor in step 4.
Hi,
I really like these analysis-es-es-es post and I appreciate the work.
I bet most BSIABs try to copy the general topology of Marshall amps, with the same inter-stage eq (such as that 470k//470p between stage 1 and 2) and different amp/clipping elements.
instead of the mu-amp, you can try SPRR gain stages, they work a bit differently but sound great as well, with similar gain range, but a different sound when clipping.
this one uses diode bias instead of resistance bias, using 1n4004 diodes. swapping out the JFETs makes a big difference in sound, and they need to be somewhat close to sound good, but matching is a fun activity IMHO.
(https://i.postimg.cc/hzZjWfq4/gainstage.png) (https://postimg.cc/hzZjWfq4)
originally done with resistors, they are praised for their sound quality by the DIY HiFi community.
(https://i.postimg.cc/BLn6V2kw/unnamed.jpg) (https://postimg.cc/BLn6V2kw)
i haven't tried combining them, tubes with diode bias, might give that a try later today.
cheers, Iain
Quote from: iainpunk on May 11, 2021, 10:11:46 AM
instead of the mu-amp, you can try SPRR gain stages...
cheers, Iain
I want to try and redesign something equivalent with Opamps and diodes.
By matching the amplitude, frequency response, clipping of each stage as close as possible
^ interesting , i for one would like to see what an opamp equivalent to a mu-amp
looks and sounds like....
i like mu-amp dirts...they have a certain ampy type flavour.
as you were.... 8)
Quote from: deadastronaut on May 11, 2021, 12:11:25 PM
^ interesting , i for one would like to see what an opamp equivalent to a mu-amp
looks and sounds like....
i like mu-amp dirts...they have a certain ampy type flavour.
as you were.... 8)
Let's work on it together !
What would you recommend to acheive same transfer function distortion curve as the first stage mu-amp but using Opamp and diode function generator ?
No idea....just along for the ride really... 8)
I have wondered how an op amp would be configured to replicate a mu-amp though.....
Lets hope some more knowledgeable, and also curious chaps chime in... 8)
Pertinent thread
https://www.diystompboxes.com/smfforum/index.php?topic=123269.0
There is a paper from 2004 by Dimitri (Daniuk) about triode emulation which is using op amps along with a single jfet. I don't have the paper to hand at the moment but will try and find the link to it asap.
Quote from: deadastronaut on May 11, 2021, 12:11:25 PM
i like mu-amp dirts...they have a certain ampy type flavour.
I posted the Transfer function of the first mu-Amp of the BSIAB2.
Yes indeed it looks quite tube like !!!
I feel that an Amp sound is basically a mix of
Correct pre-distortion EQ
Correct transfer response curve
Correct post-distortion EQ
So far into my analysis, BSIAB is great,
1. Cut bass before distortion
2. Tube like transfer response
3. some interstage filter ( Which I feel should have been a treble cut instead of a treble boost)
4. More clipping stages ( I hope they too have same tube-like transfer function)
5. Tone controls ( Some reports say that could be improved / had been improved but I dont have schematics of the Mods)
6. Sufficient output to drive / overdrive an Amp
Maybe the fortuitous transfer curve was a serendipitous discovery, since the Mu-amp was designed for high linear gain and all bets were off when the Mu-amp is overdriven. Great respects for experimenters like Jack AMZ who bravely went where no man had been before !!!
what is on the x-axis of the mu-amp transfer curve? Can you convert it to Vin? (So you have Vin plotted against Vout :))
X axis is input voltage in mv Peak, from -1200mvp to +1200mvp
Y axis is output of 1st mu-amp in Volts peak
To be able to generate these graphs with automatically increasing Vin, I use as V(in), a BEHAVIORAL VOLTAGE SOURCE which uses the formula V(sine source) * time
as time increases, the Vin increases automatically.
Hence the X axis shows as time units
But I have standardised it so that 1 second = 1000mvp linearly.
i strongly suggest a CMOS inverter gain stage to simulate ''tube style'' clipping.
have been experimenting with a Marshall / Tube amp in a Box type sound using some CMOS and opamp gain stages. currently battling the ''high gain hiss'' sound that it also has on lower gains. i base my tone off of the clean channel of my Bugera V55 when over driven using a mid boost pedal, instead of a Marshall.
cheers
All amplifier in a box projects have one limitation, they try to simulate a triode in a class A amplifier mode with hot biased and standard anode resistor value. This, of course, is interesting, but the study narrows our possibilities, guys.
Let's discuss some of the obvious things that strongly affect tonality in real preamplifiers, but we are not paying attention to them.
1) DC cathode follower. Roughly speaking, this is a triode operating in buffer mode, it is located between the last stage of amplification and the tonstack.
A) It reduces the output impedance and your circuit does not lose high frequencies in the tonstack.
B) It compresses the signal, and only 1 (!) Half of the wave. Almost asymmetrical clipping that is loved in overdrive pedals.
C) This is part of the sound of cult amps, almost all Marshall amps (except the Silver Jubilee and a couple of other models), Vox, Fender Tweed (later models), Soldano, Diezel, etc. have it and that's part of their sound.
2) The absence of a cathode follower, it is easy to simulate, you need to install a resistor between the tonstack and the last stage of amplification, the resistor will be equal to the output impedance of the triode.
3) The stages of cold clipping, this is a cathode resistor with a large value, part of the sound of JCM800, Soldano, Mesa Rectifier, Peavey 5150/6505, Framus, PRS Archon / MT15. the resistor value is 10k-39k, this also gives an asymmetric distortion, 39k, looks almost like a half-wave rectifier. some amplifiers (Friedman, Bogner) can switch the clipping mode between cold and hot.
4) The value of the anode resistor. the sound feature of some amplifiers is the larger anode resistor value (220k-330k versus standard 100k). it affects the gain, tone, distortion and how the cathode resistor and capacitor filter changes. As examples Engl, Soldano, Mesa, Friedman. 5) Tonstek, yes yes yes, he plays a huge role in shaping the tone, which is why ancient schemes like Bsiab with a tone ala Big Muff look strange.
6) phase inverter and power amplifier (look at the PAL pedals, which were inspired by the early designs of R.O.G.), Presence, reaonanse, response controls are important. In addition, my personal experience says that all pedals on field-effect transistors, after the tonstack, MUST have an amplification stage otherwise they will sound bad.
Now let's think about what every possible way can give us.
1) Jfet class A, they cannot simulate different values of the anode resistor, the stage of cold clipping (more than 10k), without the selection of transistors, the circuit is poorly repeated and requires adjustment. Pros, sounds good, but the choice of circuits to simulate is small.
2) Mosfet (2n7000 / bs170), are noisy, have a large miller capacity and require an input buffer to prevent high frequency losses. Poyes, they are more flexible in tuning, do not require selection of transistors and bias tuning.
3) Mu Amp, about the same as a mosfet, but prone to sagging when heavily distorted.
Also I have often seen reports that the Mu Amp does not simulate a Class A amplifier, but rather a Push-Pull amplifier.
4) Op amps, the most flexible, but I am a skeptic and have never heard of a good amp in a box using op amps. Perhaps the problem is high speed and square distortion without diodes.
5) LND150, potentially the main candidate for the title of the new king of field-effect transistors (so far j201, 2n5457, 2n7000, bs170). Judging by the description, this transistor has the best of jfet and mosfet, but does not have their drawbacks, does not require selection, is used in real tube amplifiers, in cathode follower circuits (good as a buffer, bad as a wave amplifier), amplification stages, effect loops, able to replace the lamp without reworking the circuit.
In my humble opinion, it is worth paying attention to operational amplifiers and LND150, as the least used (unlike jfet, mosfet, mu amp) devices for simulating an amplifier in a box. Mu Amp, I would put it in third place in terms of potential (it has a lot of limitations).
i think i found a way to have the triode artifacts you mention, like a curve/asymmetric transfer function of the triode.
you mention the cathode and anode resistors making a difference in a triode's sound, but what is basically does is changing the bias, which is also doable with CMOS gain stages, using asymmetric loading, which reproduces triode sounds quite convincingly.
(https://i.postimg.cc/XXn1cNjj/cmos-output-vs-gain.jpg) (https://postimg.cc/XXn1cNjj)
the cathode follower's sound can be done with a capacitively loaded Jfet Buffer.
a good frequency response can be fully designed with opamps.
Quote3) Mu Amp, about the same as a mosfet, but prone to sagging when heavily distorted.
Also I have often seen reports that the Mu Amp does not simulate a Class A amplifier, but rather a Push-Pull amplifier.
you imply that 'push-pull' and 'class A' are mutually exclusive, which is false, lots of push pull amps operate in class A (just look up the Zen Amp, which is a class A push pull ''''HiFi'''' amplifier). the MuAmp and the SRPP topological family are fully class A, since all devices used are in conduction during the whole 360° of the signal, unless cilpping occurs, when it turns in to class C operation. (a triode clipping to the rails is also operating in class C)
i don't think that if you design a pedal, you have to include the tone stack, since the amp you put that pedal in front of already has a tone stack. this also counts for the phase inverter and the power stage.
i mean my, project isn't to recreate a tube circuit or my amps sound, but to come close to the overarching ''vibe'' of my amp's clean channel cranked, changing a bunch of stuff to sound and feel better to my taste, so my methods might not apply in the process of cloning a marshall's circuit, but its food for thought.
cheers
So I've been toying with this idea to roughly match a transfer function of a mu-amp:
(https://i.postimg.cc/vcwVpBjn/2021-05-13-11-49-36-Window.png) (https://postimg.cc/vcwVpBjn)
It's a combination of a single-ended soft-clipper and dampened single-ended hard clipper (the later because the mu-amp doesn't clip as hard as a diode).
I've deliberately added some high-end roll-off to keep it smooth :)
Also, probably use a low slew-rate opamp as it will likely clip :)
Quote from: niektb on May 13, 2021, 05:53:09 AM
So I've been toying with this idea to roughly match a transfer function of a mu-amp:
Wow that's great !!!
Please post the transfer function that you acheived.
Let's collaborate to build a new BSIAB with Opamps and Diodes
I too was about to start trying to use Opamps and diodes to approximate the transfer function of the Mu-amp
I was thinking of using Diode Function Generators
- Vivek
Quote from: niektb on May 13, 2021, 05:53:09 AM
It's a combination of a single-ended soft-clipper and dampened single-ended hard clipper
Are the diodes pointing the correct way ?
I want a circuit that does not depend on rail saturation of the Opamp
iampunk I see such circuits as an imitation of an amplifier or preamp. I think these pedals should be versatile, sound good in a clean channel, sound good in a power amp or regular speakers (using a cabinet simulator). Today I simulated mosfet, jfet, muamp and tube, I set them to the same aspiration, the same frequency response and looked at the oscilloscope. transistors connected in the triode simulation mode (class A amplifier) had a similar distortion shape, it was asymmetric. Mu Amp had symmetrical clipping, the signal was compressed on both sides. when there is time, I will add Cmos to the comparison.
yeah, i think you're right about the implied versatility of an ''amp in a box'' pedal, which isn't my personal goal, i'm designing a good sounding overdrive, that happens to be inspired by my tube amp's clean channel.
yes, normal transistors are quite tube like, especially with appropriate miller caps. i must say that to my ears, the bias of a transistor can make it morph between pentode and triode sounds, but i haven't checked on a scope.
CMOS is somewhat tube like, but it is just to symmetrical in self-bias mode, i like using external bias voltage to get a touch more gain and way less symmetric amplification/clipping.
must say that if you consider the mu-amp with jfets, you should also try that topology with triodes. while you are at it, take a look at SRPP topology, its simpler than - and superior to mu-amp topology.
one thing nit-picky thing tho:
Quotetransistors connected in the triode simulation mode (class A amplifier)
the class A part, while its probably correct that its operating in class A, i think you mean 'single ended', because this is also class A: (https://sound-au.com/p36-fig4.gif)
cheers, Iain
Quote from: Vivek on May 13, 2021, 08:58:38 AM
Quote from: niektb on May 13, 2021, 05:53:09 AM
It's a combination of a single-ended soft-clipper and dampened single-ended hard clipper
Are the diodes pointing the correct way ?
I want a circuit that does not depend on rail saturation of the Opamp
You were right! So I inverted one end! I also softened the soft-clipping stage slightly! I can't find the transfer function option :o
But I cán plot some graphs like this. This is with a 1kHz wave. Inputs are: 5mV, 10mV, 25mV, 50mV and 100mV. Gain is maxed out.
(https://i.postimg.cc/Ln7TgDYK/tinadiag.jpg) (https://postimg.cc/Ln7TgDYK)
I made transfer function graph using excel
By taking readings at about 20 points in between -1200mvp and +1200mvp inputs from the simulation software and typing it into XL
Does your software allow BEHAVIORAL VOLTAGE SOURCE where voltage = sine wave x time ?
I assumed that 1200mvp would represent approximately the largest guitar signal.
Even for transient analysis graphs like the one you posted, I normally use 10, 50,100, 200, 500, 1000 and 1200mvp as inputs
I appears I actually cán do X-Y plots in TINA-TI! 8)
So this is what I came up with. I lowered the gain a bit to match that of a mu-amp (27dB). The 120pF in parallel to the diode wasn't doing anything, increased that to an Fc of around 3kHz. The low-end roll-off matches that of a tubescreamer. Add a volume pot (and maybe a tone pot) and you should have a nice simple overdrive on it's own ;)
0
(https://i.postimg.cc/Wq6s4TsX/2021-05-14-14-37-09-Pedal-Orig-Proto-BSIAB2-Schematic-Editor.png) (https://postimg.cc/Wq6s4TsX)
(https://i.postimg.cc/t7hqktFW/tinadiag.jpg) (https://postimg.cc/t7hqktFW)
(https://i.postimg.cc/ZCHTtb7q/tinadiag2.jpg) (https://postimg.cc/ZCHTtb7q)
(https://i.postimg.cc/MvqWFMY4/tinadiag3.jpg) (https://postimg.cc/MvqWFMY4)
In the BSIAB2
First Mu amp had gain of 19.5
I did not understand your transfer response graph properly
Your waveshape graph does not match mine.
Quote from: Vivek on May 14, 2021, 09:52:22 AM
In the BSIAB2
First Mu amp had gain of 19.5
I did not understand your transfer response graph properly
Your waveshape graph does not match mine.
My bad, got the wrong online calculator (power instead of voltage), but the gain at 1kHz is 25.86dB which is 19.6x. :)
The transfer response plots both the upgoing (the right curve) ánd the downgoing (left curve) flank (similar to an eye diagram). As you can see, there is a bit of skew...
I know, you can play around with the diodes (should probably LEDs) and the series resistors to get the exact clipping response you want...
Dear niektb
Could you send me a table in mvp
Input, output
-1200
-1100
-1000
.
.
-100
0
100
200
300
.
.
.
1100
1200
There is an export to text option in graphs in TINA-TI so here you go :)
IN OUT
-1.19948686640175 -1.99172831120686
-1.18903511572659 -2.03126952638068
-1.15598285617605 -2.02599553702016
-1.09157628729542 -1.97192473513099
-1.00029150598102 -1.86805974649348
-0.884376228958682 -1.7164942072197
-0.746684678641574 -1.51916223416013
-0.590607282127369 -1.2716353831398
-0.51791956573537 -1.1215739668619
-0.495495115473969 -1.06622336963506
-0.475765732250903 -1.01059522403624
-0.449178032528607 -0.921253796095772
-0.421439703392049 -0.810524809331847
-0.397326650713596 -0.705874645706524
-0.387202009946283 -0.660812189527395
-0.379039781821198 -0.62452228584848
-0.368298300708477 -0.577234458472926
-0.357674616349312 -0.531143559692211
-0.346080903818751 -0.481675745796077
-0.33063025567917 -0.417110810867088
-0.305877591726118 -0.316821746555281
-0.255963445025229 -0.125548673814609
-0.178279004130879 0.14670077310334
-0.05716587475708 0.522063581972764
-0.024502881647189 0.613813337272989
0.001508367517196 0.683223979898092
0.024044418585108 0.739535804230277
0.069083066403761 0.83671101664916
0.112455125435754 0.910079472880794
0.166504512361057 0.977265078299259
0.242093971159183 1.03808670419737
0.352184709596205 1.08472056039832
0.510379219072326 1.10974625305615
0.673931730028114 1.1138029810462
0.820890018091734 1.11108066493533
0.947635372347442 1.10768129106118
1.05104681033356 1.10472410616672
1.12857797635659 1.10206997492198
1.17831979701013 1.09953933629877
1.19904746745494 1.0970455679434
1.19025060624046 1.09455923609091
Here's what I see from your data values
(https://i.postimg.cc/Y0qdpvKS/Niektb-Transfer.png) (https://i.postimg.cc/Y0qdpvKS/Niektb-Transfer.png)
I would say that the basic concept is present, but work is needed to fine tune the diodes and gain determination resistors to get closer to the intended transfer function
Its very interesting that your graph does not go through (0,0). Any reason why you designed it with that offset (It could be wonderful if that offset could follow the integrated envelope !!!)
Could you try D3 going to ground via a decoupling cap, or even going to vref via a decoupling cap ?
Is there DC buildup on the caps C4, C5, C8 ?
Also, your positive side clipping is a bit more severe ie chopping to 1 V output while BSIAB does around 2 volts
On positive swings, BSIAB shows some compliance after knee, and does allow the wave to get a bit bigger. Yours is less forgiving.
Please tweak your transfer function by changing type/number of diodes and the resistors
Thanks
Quote from: niektb on May 14, 2021, 02:39:06 AM
Quote from: Vivek on May 13, 2021, 08:58:38 AM
Quote from: niektb on May 13, 2021, 05:53:09 AM
It's a combination of a single-ended soft-clipper and dampened single-ended hard clipper
Are the diodes pointing the correct way ?
I want a circuit that does not depend on rail saturation of the Opamp
You were right! So I inverted one end! I also softened the soft-clipping stage slightly! I can't find the transfer function option :o
But I cán plot some graphs like this. This is with a 1kHz wave. Inputs are: 5mV, 10mV, 25mV, 50mV and 100mV. Gain is maxed out.
(https://i.postimg.cc/Ln7TgDYK/tinadiag.jpg) (https://postimg.cc/Ln7TgDYK)
Yeah I also stared at that offset a bit and I'm not sure yet what's causing it! But it does seem to be related to the ingoing signal amplitude, as is also shown in the image in the quoted post :) (notice how the signal spends more time in the positive half when the input signal gets larger).
Its DC offset
Maybe due to DC buildup on caps
Due to asymmetric clipping
When the signal gets larger and starts to clip, the charge starts to build up and the wave shifts up
If you check the voltage on the caps, you might find that they build up in the first few ms
That's really great, if its controlled and not too huge a DC shift
But so far, we are not close to our target
Regarding the filter and drive controls in between the First Mu-Amp stage and second Mu-Amp stage
(https://i.postimg.cc/3RWWj84P/BSIAB-Drive-Pot.png) (https://postimg.cc/KkSZxy5N)
I saw that many Tube Amp circuits have a treble bleed circuit near the volume pot
It appears that is good if the earlier stage did not create too much distortion and is quite clean
It means that the circuit is trying to correct for the Loudness perception as per Fletcher- Munson curves.
How it does that properly, I do not understand, since the volume control in the pedal of Amp does not know about the final loudness curve created by the PA system.
However if there is too much distortion and high harmonics created by the earlier stage, there is a need to cut the treble after the distortion.
Quote from: Vivek on May 15, 2021, 08:39:39 AM
Regarding the filter and drive controls in between the First Mu-Amp stage and second Mu-Amp stage[/b]
(https://i.postimg.cc/3RWWj84P/BSIAB-Drive-Pot.png) (https://postimg.cc/KkSZxy5N)
I saw that many Tube Amp circuits have a treble bleed circuit near the volume pot
It appears that is good if the earlier stage did not create too much distortion and is quite clean
It means that the circuit is trying to correct for the Loudness perception as per Fletcher- Munson curves.
How it does that properly, I do not understand, since the volume control in the pedal of Amp does not know about the final loudness curve created by the PA system.
However if there is too much distortion and high harmonics created by the earlier stage, there is a need to cut the treble after the distortion.
I 'm not sure I understand the question.
The treble bleed is there because the impedance of the volume control, combined with the impedance of the next stage (they are in parallel, so lower volume == lower impedance), may cause treble attenuation. The capacitor is there to bypass this, so the treble range sees the full impedance of the next stage.
Someone please correct me.
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
Quote from: iainpunk on May 15, 2021, 11:43:07 AM
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?
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
Quote from: Vivek on May 11, 2021, 05:59:32 AM
(https://i.postimg.cc/zfyCVBcL/BSIAB-2-first-stage-schem.png) (https://postimg.cc/8FVJY1DD)
[...]
I think I discovered a mistake here! Aren't C2 and C6 10x too small?
You are right about C2
I will rerun SPICE and see what effect it has
Whoops I meant C4 not C6 :o
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.
Quote from: niektb on May 15, 2021, 12:04:52 PM
Quote from: iainpunk on May 15, 2021, 11:43:07 AM
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
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...
(https://i.postimg.cc/S2Ks3HtT/2021-05-17-19-29-08-Pedal-Orig-Proto-BSIAB2-Schematic-Editor.png) (https://postimg.cc/S2Ks3HtT)
Quote from: niektb on May 17, 2021, 01:32:27 PM... 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".
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.
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).
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 ;)
Quote from: niektb on May 19, 2021, 03:19:40 AM
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.
QuoteTriodes 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.
QuoteMu 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
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.
QuoteI 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.
(https://i.postimg.cc/hJgSt16R/gainstage.png) (https://postimg.cc/hJgSt16R)
Cheers, Iain
Quote from: iainpunk on May 19, 2021, 07:09:42 PM
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 ?
Quote from: Vivek on May 20, 2021, 04:57:58 AM
Quote from: iainpunk on May 19, 2021, 07:09:42 PM
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
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.
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.