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As a study problem I had the idea to make an all transistor Rat. Here is what I came up with. Looks like it's in the ballpark but has a few questions.

- I need to get about x2000 gain from Q1 and Q2. My idea was to get x10 from Q1 and x1 to x200 from Q2.
- The filter low pass filters R8/C4 and R9/C5 seem like they provide a lot of Rat's sonic flavor. I'm sure if what I have is the same. Seem like the GAIN pot will affect the filter in my design.

(https://i.postimg.cc/MnpJXTs8/The-Prince.png) (https://postimg.cc/MnpJXTs8)

> ....get about x2000 gain from Q1 and Q2.

Then that is ALL your distortion. Q2 can't make even 3V, 3V/2000= 1.5mV, a VERY tiny guitar signal, about 40dB down on an average good pluck.

What does the rest of it do?

> Seem like the GAIN pot will affect the filter in my design.

Probably. But is that a flaw or a feature?

I wonder if Q1 will bias happy with its Gate near +4V.

You could also make a simple discreet opamp out of 3x trans (Boss style) and put that in place of the lm308.

Gain pot affecting tone is a desirable feature of rat or mxr distortion + or.... High gain means lots of clipping, sharp edges, but there is a hump as you push gain: less bass woof, less treble screech.

Many would consider this interactivity handy, even essential. Electrosmash website has a nice collection of freq response vs. gain graphs.

Quote from: imJonWain on July 09, 2022, 01:27:55 PM
You could also make a simple discreet opamp out of 3x trans (Boss style) and put that in place of the lm308.

Did this many years back, not quite how I'd have gone about it now, but perhaps it'll inspire.

(https://i.postimg.cc/PLrGML42/TAR-Transistor-RAT.png) (https://postimg.cc/PLrGML42)

Ahhh... Mitchell again..
(always 2 or 3 projects going on simultaneously..) :icon_wink:

1. I presume you are familiar with single BJT CE amp (like Q2 stage) maximum gain in conjunction with power supply margins..
For your info, a directly grounded Emitter (in the mean of no R8/R9) can't exhibit gain more than 20 X PS voltage (ideally..!!)
(actually much less due to next stage impedance Collector loading..)

For particular Q2 stage, R10 severely dominates R11 so gain can't be higher than X10(*)..!!
(far away from X200)

2. As Paul pointed above, Q1 bias point should be OK for a Mosfet but not so good for a JFET..
About 4V on Gate call for 5.5V (say) on Source, hence 5.5mA channel current..
I'm pretty sure that Drain can't handle 55V drop.. :icon_biggrin:
(minor note is R5/R6 high values)

(*) brute analysis:
For Q2 Collector sitting at 4.5V, quiescent current should be  450μΑ, hence intrinsic Emitter resistor about 55R..
The above resistor is added in series with R8 & R9 equivalent parallel resistance (43R)..
Q2 stage gain is [R11 // (R10+diode dynamic resistance) / (55 + 43)]..
(less than 1000 divided by less than 100..) :icon_wink:

Thanks Paul, you always cut out the heart of the matter in the fewest words. My takeaway is that I don't need to match the gain the of the original circuit I just need enough gain to get those diodes to clip, and Q1 is not biased correctly.

Thanks Scruffy, looks like you did exactly what imJonWain was describing, nice work! If I understand this correctly, it's a discreet op-amp Rat with 18v option. Have you tested this?

Thanks Atonis your insights are always illuminating! I'll read this a few more times. Seems like I need a little work on Q1 and Q2. I have a couple more projects in the works! I'm trying to discussing one per week!

Quote from: soggybag on July 09, 2022, 07:09:20 PM
Thanks Scruffy, looks like you did exactly what imJonWain was describing, nice work! If I understand this correctly, it's a discreet op-amp Rat with 18v option. Have you tested this?

Almost, it's Joe... Davidson's? Diode compression discrete op amp.

Yeah, I tested it, might even have an old PCB you can have if I didn't toss them in my previous move, nudge me with a PM on Monday to search if you're interested, has 3-way diode switching and I think I even put DIP switching for the diode in the op amp and it fit in a 1590B.

Quote from: Scruffie on July 09, 2022, 07:15:25 PM

Quote from: soggybag on July 09, 2022, 07:09:20 PM
Thanks Scruffy, looks like you did exactly what imJonWain was describing, nice work! If I understand this correctly, it's a discreet op-amp Rat with 18v option. Have you tested this?

Almost, it's Joe... Davidson's? Diode compression discrete op amp.

Yeah, I tested it, might even have an old PCB you can have if I didn't toss them in my previous move, nudge me with a PM on Monday to search if you're interested, has 3-way diode switching and I think I even put DIP switching for the diode in the op amp and it fit in a 1590B.

Awesome! I remember when the diode compression op-amp was a big discussion here! I never tried it. I have enough trouble getting regular op-amps to cooperate!

I really liked the diode compression op amp! I tried it in several pedals.

Wasn`t the virtue of the RAT the limited slew-rate of the opamp it used?

Quote from: soggybag on July 09, 2022, 07:09:20 PM
Seems like I need a little work on Q1 and Q2.

If you are fine with about 40dB gain, just implement Distort-o-Matic XI input stage configuration.. :icon_wink:
If you replace Q1 JFET with a 2N5088/89 BJT (and lower R1 value) overal gain is raised at about 52dB with corresponding input impedance issue, of course..

(https://i.imgur.com/SJnyG53.png)

For more gain, you can increase R5 value (or even delete it - which isn't recomended for stability issues..)

Quote from: puretube on July 10, 2022, 04:01:07 AM
Wasn`t the virtue of the RAT the limited slew-rate of the opamp it used?

I feel it goes this way

Rat has a peculiar sound

It's Opamp has low slew rate

Some guy who wanted to publish an analysis of the Rat chanced upon this quirk

And attributed more to this quirk than it rightfully deserved

What if I say almost every distortion pedal is slew rate limited to some extent

TL072 has slew rate of 13-V/µs

Suppose we have a distortion stage with gain 800, and feed it with 1vp guitar signal of 3khz, that demands a slew rate of 15.1V/μS. Any higher gain or frequency will demand even more slew rate. If the Opamp is not capable of slewing at that rate, the output will be slew rate limited to some extent.

Why not a single MPSA13?

Intersting project. If more gain is needed from transistors, bootstrapping comes to mind. I tried something similar, albeit geared towards medium gains here:
https://www.diystompboxes.com/smfforum/index.php?topic=123610.0#msg1169798
By using normal antiparallel Si diode clippers instead of the Zeners, tacking on a rat-style tone control, and removing some bass from the gain stage, this concept may be worth a try for a "discreet Rat".

Andy

From the datasheet of the LM301A "With proper compensation, summing amplifier slew rates to 10 V/µs can be obtained.", but later on the same data sheet it says SR typical 0.25 V/µs. The datasheet of the TL071 says "16 V/µs typical". For comparison, a 741 has 0.5 typical, 0.25 is really low.

The compensation cap is external for the LM301A, not built-in like in TL07x. Also the inputs are bipolar rather than FET.

I don't if and how much all this affects the sound of the RAT.

I've found this graph with a rule of thumb about distortion in opamps:
(https://eng.libretexts.org/@api/deki/files/24345/5.4.11.png?revision=1)
https://eng.libretexts.org/Bookshelves/Electrical_Engineering/Electronics/Operational_Amplifiers_and_Linear_Integrated_Circuits_-_Theory_and_Application_(Fiore)/05%3A_Practical_Limitations_of_Op_Amp_Circuits/5.04%3A_Slew_Rate_and_Power_Bandwidth

I remember when I first tried a RAT in the 80's, when using a power supply instead of a battery, it was much quieter (in terms of rejecting hum from the power supply) than Boss pedals with much less gain, that was a great feature.

Quote from: GFR on July 10, 2022, 09:17:39 AM

I've found this graph with a rule of thumb about distortion in opamps:
(https://eng.libretexts.org/@api/deki/files/24345/5.4.11.png?revision=1)
https://eng.libretexts.org/Bookshelves/Electrical_Engineering/Electronics/Operational_Amplifiers_and_Linear_Integrated_Circuits_-_Theory_and_Application_(Fiore)/05%3A_Practical_Limitations_of_Op_Amp_Circuits/5.04%3A_Slew_Rate_and_Power_Bandwidth

The graph above makes some assumptions that are mentioned elsewhere in the article

A) The power supply is +/- 15V

B) Therefore the maximum unclipped Vp that we can expect is 13V (assuming 2V dropout from rail)

C) The slew rate is 0.5V/uS  and expected output is 12Vp <<<<<<<<< These are the important specs that give the 12v/6631Hz data point


If any of above change, the graph will change.


Hence that graph is not a rule of thumb for all Opamps and all circuit configurations.

Quote from: Fancy Lime on July 10, 2022, 07:52:55 AM
If more gain is needed from transistors, bootstrapping comes to mind.

With all the respect Andy, bootstrapping has nothing to do with any stage gain.. :icon_wink:
(at least, in the way gain is determined..)

Quote from: antonis on July 10, 2022, 12:55:28 PM

Quote from: Fancy Lime on July 10, 2022, 07:52:55 AM
If more gain is needed from transistors, bootstrapping comes to mind.

With all the respect Andy, bootstrapping has nothing to do with any stage gain.. :icon_wink:
(at least, in the way gain is determined..)

Why not?

Quote from: Fancy Lime on July 10, 2022, 01:04:24 PM
Why not?

'Cause bootstrapping deals with input impedance (voltage dividing BEFORE gain stage) where gain deals with signal amplification AFTER any previous attenuation.. :icon_wink:

Do you presume that Q1 stage in 250 biturbo should have lower voltage gain without R3 bootstrapped..??

P.S.
Are we talking seriously or what..?? :icon_eek:

As long as we are talking about impedance bootstrapping, sure. But there is also gain bootstrapping. The reason why I called the 250 biturbo "biturbo" is, that it uses both: C4 impedance bootstraps Q1 and C6 gain bootstraps Q2 (numbers according to the first schematic in the thread). Unless you object to calling that bootstrapping, but if the nomenclature is good enough for Tim Escobedo, it's good enough for me. The technique is not exactly new, though. In fact, Q2+Q3 in the biturbo are essentially a Jordan Bosstone with DC decoupling in the bootstrapping path, so that the gain control (which actually controls the amount of bootstrapping) does not mess with the bias of Q2.

Andy

https://www.diystompboxes.com/smfforum/index.php?topic=95356.0

Bootstrapping for gain

Maybe we can call it BOOSTrapping

https://web.archive.org/web/20100330091933/http://folkurban.com/Site/BootstrappingforGain-692.html


(https://web.archive.org/web/20120419085057im_/http://folkurban.com/Site/692_25343.gif?1173243628329)

Quote from: antonis on July 10, 2022, 05:57:07 AM
For more gain, you can increase R5 value (or even delete it - which isn't recomended for stability issues..)

If C15 is "large", R5 has very little effect on audio gain. As you say, R15 is important for DC stability. Even then, this is not a plan for beginners. JFET variability makes trouble.

Any of the externally-compensated chips (now rare) can be over-compensated to "slow" or "dull". '301 can beat 70V/uS one way, struggle for 0.1V/uS another way. It is hard to know if this matters for e-guitar. If you decide to experiment, keep compensation cap leads very short (no switching).

Bootstrapping for gain is an old-old sin, distantly related to the original radio sin of regeneration.

Quote from: Fancy Lime on July 10, 2022, 02:53:06 PM
C6 gain bootstraps Q2 (numbers according to the first schematic in the thread). Unless you object to calling that bootstrapping

Not at all.. :icon_wink:
As you said, "boostrapping" is a designation for raising apparent value..

If I recall it right, Collector resistor bootstrapping (at least for BJTs) was firstly mentioned (albeit erronously*) in T.D.Towers https://www.keith-snook.info/wireless-world-magazine/Wireless-World-1968/High%20Input-Impedance%20Amplifier%20Circuits.pdf (https://www.keith-snook.info/wireless-world-magazine/Wireless-World-1968/High%20Input-Impedance%20Amplifier%20Circuits.pdf), figure 3 b. & d. , figure 4 a. etc

(*) Can't be obtained via a single trasistor..
P.A. Johnson https://worldradiohistory.com/hd2/IDX-UK/Technology/Technology-All-Eras/Archive-Wireless-World-IDX/60s/Wireless-World-1968-09-OCR-Page-0019.pdf (https://worldradiohistory.com/hd2/IDX-UK/Technology/Technology-All-Eras/Archive-Wireless-World-IDX/60s/Wireless-World-1968-09-OCR-Page-0019.pdf)

P.S.
Didn't realise you refer on Q2/Q3 CFP.. :icon_wink:

Quote from: PRR on July 10, 2022, 03:25:21 PM
Even then, this is not a plan for beginners. JFET variability makes trouble.

That's why I recommened to replace Q1 with a BJT (n-p-n)..

I've the feeling that Mitchell is prone to mixing/modifying building blocks troubles.. :icon_wink:

Quote from: Vivek on July 10, 2022, 10:30:44 AM

Quote from: GFR on July 10, 2022, 09:17:39 AM

I've found this graph with a rule of thumb about distortion in opamps:
(https://eng.libretexts.org/@api/deki/files/24345/5.4.11.png?revision=1)
https://eng.libretexts.org/Bookshelves/Electrical_Engineering/Electronics/Operational_Amplifiers_and_Linear_Integrated_Circuits_-_Theory_and_Application_(Fiore)/05%3A_Practical_Limitations_of_Op_Amp_Circuits/5.04%3A_Slew_Rate_and_Power_Bandwidth

The graph above makes some assumptions that are mentioned elsewhere in the article

A) The power supply is +/- 15V

B) Therefore the maximum unclipped Vp that we can expect is 13V (assuming 2V dropout from rail)

C) The slew rate is 0.5V/uS  and expected output is 12Vp <<<<<<<<< These are the important specs that give the 12v/6631Hz data point


If any of above change, the graph will change.


Hence that graph is not a rule of thumb for all Opamps and all circuit configurations.

Hence that graph is not a rule of thumb for all Opamps and all circuit configurations. => Of course not. Given a specific gain / frequency response / slew rate / supply voltage etc. you can sketch a similar graph and estimate when that circuit would be clipping or distorting due to SR limiting.

Here are a couple evolutions of the original idea. The first uses a MOSFET. I figure this has plenty of gain by itself to drive the rest of the circuit. Not sue if I got the R8/C4 and R9/C5 filter gain control correct looks close.

(https://i.postimg.cc/VSZ5kXNt/The-Prince-3.png) (https://postimg.cc/VSZ5kXNt)


I looked up the diode compression op-amp and shoehorned it into the circuit here.

(https://i.postimg.cc/Q9k8W58S/The-Prince-2.png) (https://postimg.cc/Q9k8W58S)

Yet another option: Joe Davissons CCS Drive:
(https://www.diystompboxes.com/analogalchemy/old/ccsdrive.gif)
I like this one a lot. Works well with the diodes to ground instead of in the feedback loop. Capable of impressive gains if the lower transistir is high gain or a MOSFET. Put a buffer right after the gain stage for even more gain.

Andy

The gain resistors on the RAT are set for high gain.   In order to actually get that gain the amplifier part is going to need a lot of open-loop gain.

iainpunk, recently posted this one.
https://www.diystompboxes.com/smfforum/index.php?topic=129311.0

Just after that I was looking for stuff on the Danelectro FAB D-1 (see Lounge) and I found this mindjogger.
Ibanez used all transistor discrete opamps in a few pedals; from C6 to C10
(a cheaper version of Boss's JFET + BJT hybrid):

(https://i.postimg.cc/zysx2Y7T/ibanez-cr5.png) (https://postimg.cc/zysx2Y7T)

All these circuits can be 'slowed down' by putting a cap across the C and B of the final transistor.
That might help emulate the slow LM308A.   The Ibanez circuit has C7 but it might need to be
increased.   The gain control and feedback resistors on the Ibanez circuit and be re-jigged to be
like the RAT.

These types of circuit might just have enough gain.  (That's not to say the bootstrap circuits won't.   Someone needs
to check if the gain can be met.)

The bottom line is work is required to get it to sound the same as a RAT.

Quote from: soggybag on July 11, 2022, 12:20:01 AM
The first uses a MOSFET. I figure this has plenty of gain by itself to drive the rest of the circuit.
(https://i.postimg.cc/VSZ5kXNt/The-Prince-3.png) (https://postimg.cc/VSZ5kXNt)

Don't know GAIN pot value but make R5 = 1M & R4 = 1M8 - 2M2 as a start..
(max gain should be slightly under X30)

Thanks again for the feedback Atonis. I wasn't sure about the gain pot value but figure it should be C5k.

The gain pot makes sense to me. My understanding of the R8, C5 and R9, C5 network is a little vague. Let me try and explain what I think is going on here.

R8 and C4 attenuates frequencies below 1.5KHz. R9 and C5 attenuates frequencies below 60Hz. This would be affected by the gain pot setting but I'm ignoring that. The affect of R8 and C4 is to decrease gain for signals below 1.5kHz. R9 and C5 cut the gain again for signals below 60Hz.

Can I ask about the values you chose?

I had planned to use 10M for both R4 and R5. This should put the gate at 4.5v. You are suggesting 1M for R5 and about 2M for R4. This would put the gate around 6v. What's the difference here? I commonly see this circuit block biassed at half the power supply.

Second question, the change in these two resistors must affect impedance! Creating a voltage divider here to set bias voltage could generate the same voltage with larger or smaller resistors, what's the difference?

In the end,

would an all transistor Rat sound like an all opamp Rat sound like an all FET rat ?

Because sound depends upon transfer characteristics and interstage frequency response

and if each above has same transfer characteristics and interstage frequency response, they should all sound same, no ?

Quote from: soggybag on July 11, 2022, 06:26:11 PM
I wasn't sure about the gain pot value but figure it should be C5k.

No problem but you'll result into about 10% (or more) useless rotation..
Q1 gain is roughly set by R3/GAIN pot ratio so for pot set FCCW gain should be less than unity.. :icon_wink:

Quote from: soggybag on July 11, 2022, 06:26:11 PM
The gain pot makes sense to me. My understanding of the R8, C5 and R9, C5 network is a little vague. Let me try and explain what I think is going on here.
R8 and C4 attenuates frequencies below 1.5KHz. R9 and C5 attenuates frequencies below 60Hz. This would be affected by the gain pot setting but I'm ignoring that. The affect of R8 and C4 is to decrease gain for signals below 1.5kHz. R9 and C5 cut the gain again for signals below 60Hz.
Can I ask about the values you chose?

For R8 & R9 values 1% & 10% of GAIN pot respectively, you realize that there isn't "correct" R/C values combination..
(they are strongly dependent on GAIN pot setting..)

Quote from: soggybag on July 11, 2022, 06:26:11 PM
I had planned to use 10M for both R4 and R5. This should put the gate at 4.5v. You are suggesting 1M for R5 and about 2M for R4. This would put the gate around 6v. What's the difference here? I commonly see this circuit block biassed at half the power supply.

It depends on Q1 Source resistor value (GAIN pot)..
BS170 has a threashold voltage between 0.8 and 3V (for I_D=1mA)

Worst case scenario is for V_TH = 800mV so we have to add max signal amplitude (like we do for BJTs) or we'll result into negative waveform clipping.. :icon_wink:
For a grounded Source Mosfet, 1.5V to 2V V_GS should be fine but here we have to add Source voltage due to channel current..
...................(deleted)
To make long story short, Mosfets are biased likewise BJTs but with their particular V_TH instead of V_BE..

Quote from: soggybag on July 11, 2022, 06:26:11 PM
Second question, the change in these two resistors must affect impedance! Creating a voltage divider here to set bias voltage could generate the same voltage with larger or smaller resistors, what's the difference?

In case you're talking about voltage divider (trimpot) with Gate big biasing resistor, it's OK..
In case of voltage divider like R4 & R5, equivalent impedance is the parallel combination of R4 and R5/(stage gain +1)..
(for a gain of 30 say, R5 is seen at the Gate as about 323k..)

P.S.
An antutored person could hold that Q1 stage gain is set by R5/R1 (80dB!!) which clearly doesn't stand 'cause open-loop gain is lower than closed-loop one..  :icon_wink:

edit: I'd start with below Q1 configuration:

(https://i.imgur.com/6PCbbba.png)

i know im late to the party, but i think the ''discrete opamp'' described in this thread would be a nice option to replace the LM308 in a RAT. the gain stated in the first post isn't accurate, as i originally made a mistake while testing it, had both PNP in reverse.
https://www.diystompboxes.com/smfforum/index.php?topic=129311.0

Quoteslewrate

at the voltages were working with, the slew rate of the 308 only affects frequency's a guitar amp can't meaningfully reproduce. frequencies above 16kHz if the clipping diodes are discarded, and over 100kHz if they are taken in to consideration. that a wave is slewed doesn't mean we hear a difference. ive been experimenting with slew rate limiters a lot lately, and it starts to be noticeable in the top end around 0,1v/us. asymmetric slew rate also has a really cool sound to it!

cheers

Thanks again Atonis! I spent yesterday considering your suggestions. I'm trying to wrap my head around the math!

Looks like the gain pot, not considering R8/9, C4/5, is R3/Gain or 4k7/1K-0. At minimum gain that would x4.7. At max gain we run into 9v limit. I'm sure how to call that, let's just ballpark it at x50.

With R8/9, C4/5 these are in series with the gain pot. Since R8/9 are parallel I can look at these as something less than the smallest value, call it 43r.

I'm trying to come up with the numbers you have. With the gain pot at min I get a gain of 4700/1043 or x4.5 at max gain I get 4700/43 or x109.

That doesn't consider C4/5.

I'm guessing your max gain calculation is plotting the gain by frequency for the filter created by C4/5.

QuoteAlmost, it's Joe... Davidson's? Diode compression discrete op amp.

My notes on Joe's DCDO

https://www.diystompboxes.com/smfforum/index.php?topic=44255

mac

Quote from: soggybag on July 13, 2022, 02:12:27 PM
I'm trying to come up with the numbers you have. With the gain pot at min I get a gain of 4700/1043 or x4.5 at max gain I get 4700/43 or x109.
That doesn't consider C4/5.

But you should..!! :icon_wink:

Just replace C4 & C5 with their capactive reactance (X_C) at specified frequencies, calculate  1K // (R8+X_C4) // (R9+X_C5) equivalent parallel resistance and add in series 1/g_m (about 5mS at I_D = 1mA but don't entrust it)..
That's by what you have to divide R3 to find the Gain.. :icon_wink:

Thanks again! That's a new concept for me to study: capacitive reactance!

JUst another mode for restless Mitchell.. :icon_wink:

(https://i.imgur.com/h2JRuec.png)

A band-pass zone variable from 340Hz - 15kHz..(you can make it as narrow as you like..)
A 22k stopper resistor in series with 1M pot should make its full rotation more usefull.. :icon_wink:

(a couple of μV DC across 1M pot shouldn't make it crackle..)

Can we put big cap on wiper so no DC on wiper ?

Quote from: Vivek on July 15, 2022, 09:23:47 AM
Can we put big cap on wiper so no DC on wiper ?

Of course but IMHO it shouldn't be essential..
(after all, 1M pot isn't placed inside signal axis..) :icon_wink:

Quote from: antonis on July 15, 2022, 08:48:51 AM
JUst another mode for restless Mitchell.. :icon_wink:

(https://i.imgur.com/h2JRuec.png)

A band-pass zone variable from 340Hz - 15kHz..(you can make it as narrow as you like..)
A 22k stopper resistor in series with 1M pot should make its full rotation more usefull.. :icon_wink:

(a couple of μV DC across 1M pot shouldn't make it crackle..)

What's going on here? Is that a low pass filter R12+C9 followed by a high pass filter C8+1M pot?

R12 and C9 form a low pass of 16kHz = 1 / (3000 * 0.0000000033 * 6.28)

C8 and 1M pot form a high pass. 338Hz = 1 / (1000000 * 0.000000000470 * 6.28)

The 22k resistor limits the high pass filter to 16kHz = 1 / (22000 * 0.000000000470 * 6.28)

I'm starting to get the math! I need more practice problems to practice!

So what's happening here? Looks like two filters, one cuts lows below the threshold while the the rolls offf highs above the threshold?

Quote from: soggybag on July 15, 2022, 03:52:43 PM
R12 and C9 form a low pass of 16kHz = 1 / (3000 * 0.0000000033 * 6.28)

Yes, buy only for CUT pot set FCW..

Actually, they form a LPF of 468Hz (pot FCCW) to 16kHz (pot FCW)

Quote from: soggybag on July 15, 2022, 03:52:43 PM
C8 and 1M pot form a high pass. 338Hz = 1 / (1000000 * 0.000000000470 * 6.28)
The 22k resistor limits the high pass filter to 16kHz = 1 / (22000 * 0.000000000470 * 6.28)

15.5kHz to be more precice but yes.. :icon_wink:
(you've got the point..)

So you have a band-pass filter of variable Q (abusive use here but study link below..) :icon_wink:
https://en.wikipedia.org/wiki/Q_factor (https://en.wikipedia.org/wiki/Q_factor)

Next challenge will be to place one of hard clipping diodes into Q1 NFB loop to mix soft and hard clipping configuration..
(I let you decide for proper diodes orientation for maintaining symmetrical clipping..) :icon_wink:

If that 1M pot is set with wiper to pin 3,

Instead of being a filter

It's just a short to ground

Ie FET gets no signal


Maybe a resistor in series with the pot to set minimum resistance of that leg would be a good idea.

Quote from: Vivek on July 15, 2022, 06:52:58 PM
If that 1M pot is set with wiper to pin 3,
Instead of being a filter
It's just a short to ground
Ie FET gets no signal

As long as it's wired as variable resistor, it makes absolutely NO difference for "grounding" Q3 Gate.. :icon_wink:
(it just sets resistance change in relation with rotation direction..)

Quote from: Vivek on July 15, 2022, 06:52:58 PM
Maybe a resistor in series with the pot to set minimum resistance of that leg would be a good idea.

Already suggested a 22k "stopper" resistor above.. :icon_wink:

P.S.
What I'd expect is an objection concerning one diode placed inside Q1 NFB loop in relation with input impedance.. :icon_wink:

What's the impact of dropping to the resistor from Q3 gate to ground to 22k on Q3? Is it solely a highpass with C8 or will it have some other effect on the operation of Q3?

I ask, because it seems like it would make a fantastic variable highpass in earlier stages of other circuits. Like ye olde FAC controls on older orange amps.

Quote from: D_Ex_Patria on July 16, 2022, 11:23:08 AM
What's the impact of dropping to the resistor from Q3 gate to ground to 22k on Q3? Is it solely a highpass with C8 or will it have some other effect on the operation of Q3?

Just a HPF..

Secondary (and negligible) impact is about 45 times lower Gate to GND voltage difference, compared to the one for 1M..

I put the MOSFet version on the breadboard. It works pretty well. Here's a few thoughts.

- over all output is not too far above unity at max volume.
- the range of gain is as great as the Rat. Though I don't have a real Rat to compare with at the moment. Just my impression.
- the voice is very suited to rock guitar.

I tried the  470p and 1M + 22k bass roll off. This worked but the volume drops significantly.

Seems like I could  rearrange Q2 to get a volume boost at the output. Looks like the current configuration is a source follower which is just under unity gain.

I'm going to breadboard the transistor op-amp version also and see what that sounds like.

Quote from: soggybag on July 20, 2022, 12:51:20 AM
- over all output is not too far above unity at max volume.

You're free to put the blame on D1, D2 pair.. :icon_wink:
(and secondarily on LPF..)

Quote from: soggybag on July 20, 2022, 12:51:20 AM
I tried the  470p and 1M + 22k bass roll off. This worked but the volume drops significantly.

But of course.. :icon_wink:
(that's the way ALL passive filters work..)
Just consider CUT pot FCCW and 1M pot FCW (only 22k stopper resistor between Gate and GND)..
You have a voltage division of 22/125 = 0.176..
By "adding" Q3 voltage gain of about 0.9 we result into 0.158 X D1/D2 forward voltage drop, which hardly is 100mV on Q3 out..
(it actually is even lower due to C8 & C9 voltage dividing effect..)

Quote from: soggybag on July 20, 2022, 12:51:20 AM
Seems like I could  rearrange Q2 to get a volume boost at the output.

You can try Jack's boosters http://www.muzique.com/lab/boost.htm (http://www.muzique.com/lab/boost.htm) or some kind of BJT booster (like Big Muff Π last stage..)

I got back to this project after a break. It was sounding alright but the volume was a little low. The original J201 follower provided about unity gain on the end, if I understand correctly. I replaced this with a second MOSFET with a gain of ~x2.

I can adjust the volume of the output with R14. It also inverts the signal to get it back in phase, so that's a small improvement.

I used BS170 for Q1 but ran out of these. I had some 2N7000 types and used one of these for Q2. The internet says these are interchangeable but the pin out is reversed.

I added D5 to protect the MOSFET.

Q: what exactly is going on there? I saw this on other schematics. People were using 1n4001 9.1v zeners and LEDs? Should I add a diode like this on Q2?

(https://i.postimg.cc/3yQ7s391/The-Prince-4.png) (https://postimg.cc/3yQ7s391)

Quote from: soggybag on September 18, 2022, 01:35:05 PM
I can adjust the volume of the output with R14.

You can adjust the GAIN, to be more precise.. :icon_wink:
From signal amplitude point of view, Gain & Volume are interchangeable only for undistorted signals..
(which is the case here for D1/D2 Si, Ge or Schottky diodes..)

Quote from: soggybag on September 18, 2022, 01:35:05 PM
Q: what exactly is going on there? I saw this on other schematics. People were using 1n4001 9.1v zeners and LEDs? Should I add a diode like this on Q2?

https://electronics.stackexchange.com/questions/573662/is-a-diode-on-a-mosfets-gate-source-really-necessary (https://electronics.stackexchange.com/questions/573662/is-a-diode-on-a-mosfets-gate-source-really-necessary)

A simple diode protects one way not the other.

5V Zener is fine for small MOSFETS.

The Base-Emitter junction of any small BJT silicon transistor will work (~~7Vz) but it is awkward to figure the polarities.

Never ever experimented with Mosfets... but is the gain of q1 stage at its max? If not, have higher gain and use double the amount of diodes. There you have volume. Classic ds1 mod....

I guess my strategy was something like what is happening in the Big Muff. The signal is clipped pretty heavily at the 0.7v diode threshold. The tone section brings the signal down more but the last stage has a gain of roughly x5.

This set up gives you some room to play with the volume control. There's a spot about 75% that is unity and some room to boost or reduce the output.

I'm guessing the clipping setup in the big muff clips similarly to diodes to ground giving each clipping section an output range of roughly +0.7v to -0.7v?

In my schematic Q2 is about x2 gain. Which gives a little more range for the volume control.

I'm wondering if I'm correct about this thinking about the "Haunting Mids Fuzz." Haunting Mids is basically a Big Muff without the tone and output transistor. It's really loud.

Looking at the schematic it seems Haunting Mids uses 3mm red LEDs for the first stage which the internet says have threshold of 1.58v and the second set of clipping diodes are 1n4742 types which look like 12v zener diodes. These diodes could explain the higher output.