I'm trying to modify the last transistor stage on a Big Muff type circuit to get a higher maximum output level.

Working on the basis that this last stage of the circuit is essentially an LPB/common emitter configuration, the things I read lead me to believe that I might be able to achieve this simply by reducing the value of R4 (referenced to the calculator page here: https://stompboxelectronics.com/resources/common-emitter-amplifier-calculator/ )

However, this doesn't work as, below my original value for this resistor, the output signal is just choked off entirely. I imagine this might be because I'm also inadvertently messing up the biasing of the transistor? (I'm using 2N5088)

(Full disclosure: my grasp of electronics is extremely feeble...!)

So... any advice on what I need to do to achieve my aim?
Or have I misunderstood it all sufficiently that a Big Muff type circuit is likely to already be tuned to a maximum-practical output level?

Thanks in advance for your input! 

How about bypassing the emitter resistor with a largish capacitor like 10uF? And with bypassing I mean put the cap in parallel with the resistor.

Thanks for the reply Petri. Sounds intriguing!

Could you explain to an electronics novice what this would do / how it would work?

Quote from: Elektrojänis on March 24, 2024, 06:05:52 PMHow about bypassing the emitter resistor with a largish capacitor like 10uF? And with bypassing I mean put the cap in parallel with the resistor.

I've just seen elsewhere a suggestion of swapping the last 2N5088 for a higher gain transistor such as an MPSA13.

I've ordered a few anyway as they're cheap, but thought I'd ask here: does this seem like a viable solution?

Quote from: paulbuzz on March 24, 2024, 06:20:36 PMThanks for the reply Petri. Sounds intriguing!

Could you explain to an electronics novice what this would do / how it would work?

I'm not too experienced myself but here goes. The ratio of collector and emitter resistors affects the gain of the transistor stage. They also affect the bias (DC-operating point) that can be measured from the collector. If the bias moves too far towards ground (0 volts) or power supply (9 volts), it cannot swing anywhere anymore with your AC (audio) signal. Those are the limits how far the output voltage can swing. (And actually it won't even get fully to those.)

The capacitor is open circuit for the DC, so it will not affect the biasing. However, for AC it's impedance gets lower when the frequency get's higher. Placing a capacitor in parallel to the emitter resistor leaves the bias alone, but for audio it is close to zero ohms. The lowest frequency it affects like that depends of the ratio between the value of the cap and the resistor. Make the cap smaller and it will only boost higher frequencies.

This is probably a very crude simplification but it has worked for me.

Quote from: paulbuzz on March 24, 2024, 09:42:28 PMI've just seen elsewhere a suggestion of swapping the last 2N5088 for a higher gain transistor such as an MPSA13.

I've ordered a few anyway as they're cheap, but thought I'd ask here: does this seem like a viable solution?

Please provide the actual diagram you are working on.
In short — replacing the output transistor will not provide any benefits, the circuit is already full of gain.
The circuit should work both, with R4 shunting by cap and with complete absence of R4.
Couple of points can be discussed.

Quote from: Elektrojänis on March 25, 2024, 02:33:57 AMThe lowest frequency it affects like that depends of the ratio between the value of the cap and the resistor. Make the cap smaller and it will only boost higher frequencies.
This is probably a very crude simplification but it has worked for me.

Crude but very realistic..

To make things more complicated:
There is an intrinsic resistor (r_e=0.026/Ic) in series with Emitter (the reason for BJT voltage followers gain always less than unity and grounded Emitter CE amps gain less than infinite)..
As Emitter total resistance is considered Emitter resistor in parallel with bypass capacitor impedance (capacitive reactance) equivalent resistance in series with than intrinsic resistor..
To obtain maximum gain (if desired), Emitter resistor bypass cap impedance should be much lower than that resistor (and not only than Emitter resistor, which nornally is much bigger than intrinsic one..) 

In our case now, considering a working current of about 350μA, r_e is about 70R so Stage gain is 10k / (70 + Xc), where Xc = 0.159/f*C and f = frequency of interest..
(e.g. X111 for 10μF at 800Hz..)

P.S.1
To be more close to reality, Volume pot value should be considered in parallel with R6 so voltage gain should be X100..

P.S.2
For a CE amp, biased at Vcc/2, stage gain can't be higher than 19 times Vcc..

Dormammu: As you stated, changing the output transistor was unhelpful
The circuit I'm using is the Baxandall Muff from Fuzzdog (pedalparts.co.uk)
Schematic is below, with values included for the later parts of the circuit.

--------

Elektrojänis - Thank you very much for the explanation of the parallel capacitor suggestion, pitched just about right for my novice understanding.
In short, it worked excellently for boosting the output level!
However, it also seems to change the nature of the fuzz sound significantly, even at lower output volumes. It's always hard to describe these things but I guess it makes the fuzz seem much more coarse-grained rather than the muff's characteristic fine-grained sound...?

I'm wondering now whether my best plan might be to simply tack another LPB stage onto the end of the existing circuit...

Quote from: paulbuzz on March 25, 2024, 08:02:16 AMElektrojänis - Thank you very much for the explanation of the parallel capacitor suggestion, pitched just about right for my novice understanding.

I quess it helps that I'm not too far from the novice level myself.

QuoteIn short, it worked excellently for boosting the output level!
However, it also seems to change the nature of the fuzz sound significantly, even at lower output volumes. It's always hard to describe these things but I guess it makes the fuzz seem much more coarse-grained rather than the muff's characteristic fine-grained sound...?

It is possible that the gain of that last stage is now too much and the last stage is now clipping. The volume control is after the gain, so lowering it does not help.

You could try adding a resistor in series with that added cap like this:



That will tame it down a bit but still give you a bit more than stock. I'd start with 1 kohm resistor and if it still sounds coarse grained, make it bigger. If there is not enough volume, make it smaller.

The limit on the output level will always the power supply. You can always slap another stage in the end, but that will also clip at about the same level. After that level it will just distort more.

Quote from: paulbuzz on March 25, 2024, 08:02:16 AMI'm wondering now whether my best plan might be to simply tack another LPB stage onto the end of the existing circuit..

1. Place a humble Emitter follower (BJT buffer) between R17 & R18 junction..
(unless R18/R19 values are much higher than R17 one..)
2. Lift R19/C16 junction from GND and connect it to Q4 Collector (via a cap)..

What big muff are we talking about? I con't imagine not getting enough output unless its a bubble font clone. if there isn't enough output you prob have some other problem in the circuit. any normal muffer has plenty of output.

Quote from: paulbuzz on March 24, 2024, 09:42:28 PMI've just seen elsewhere a suggestion of swapping the last 2N5088 for a higher gain transistor such as an MPSA13.

I've ordered a few anyway as they're cheap, but thought I'd ask here: does this seem like a viable solution?

That will have almost no effect on the output level.

Quote from: antonis on March 25, 2024, 12:30:42 PM1. Place a humble Emitter follower (BJT buffer) between R17 & R18 junction..

Are you sure that this thing will work, without interstage caps?

@Dormammu: You do know you're the only person in my ignore list, don't you..??

Quote from: antonis on March 26, 2024, 06:02:42 AM@Dormammu: You do know you're the only person in my ignore list, don't you..?

U 2.   
But some mistakes are — unforgivable to anyone.

Thanks everyone for all the replies. Sorry I haven't updated for a bit; I'm still tinkering with it!

For such a tiny fragment of such a well-known circuit it's certainly given my tiny ignorant brain some new understanding and things to think about. One thing I have learned is that I had failed to realise just how easy it is to throw the bias voltages waaaay off with small resistor changes.

I'll post another update when I reach an end point...

Quote from: paulbuzz on March 29, 2024, 03:29:50 PMOne thing I have learned is that I had failed to realise just how easy it is to throw the bias voltages waaaay off with small resistor changes.

This is not the case.
I would change the tonestack a lot.
And maybe I would cut off D3, D4.
Feel free to ask if you need details.

Quote from: Dormammu on March 29, 2024, 03:42:52 PM

Quote from: paulbuzz on March 29, 2024, 03:29:50 PMOne thing I have learned is that I had failed to realise just how easy it is to throw the bias voltages waaaay off with small resistor changes.

This is not the case.

Hmm, interesting! Certainly seemed that way to me when experimenting with the Common-Emitter Amplifier Calculator here:
https://stompboxelectronics.com/resources/common-emitter-amplifier-calculator/

...but it's entirely possible that I may mave wildly misunderstood something...! 

paulbuzz
Are you familiar with the term "stage\cascade load capacity"?