NYC Big Muff Feedback mod?

[xrleroyx]

Anyone done the feedback mod? I've got a small 3-way switch that I used to have for coil tapping, and I'd like to wire that in to have feedback loop 1, feedback loop 2 and a bypass. Anyone done something like this before?

I'm also considering removing one or two of the 4 diodes. Which ones did you guys like? My muff is sounding pretty harsh and lifeless with my new amp and I'd like to change the tone of it a bit. I'm not too sure what I'm looking for, but any advice would be greatly appreciated. Thank you!

[eatyourguitar]

My muff is sounding pretty harsh and lifeless with my new amp

can you please tell me if it is closer to one of these?

"my new amp is sounding pretty harsh and lifeless"

or

"on my new amp I noticed that my NYC muff is harsh and lifeless"

cause the plan of action is completely different. what amp are we talkin about anyway?

[xrleroyx]

Nah, amp sounds great. It's a plexi combo. I'm selling it, but not for how it sounds.

I just took out the emitters on the thing, and it sounds pretty killer. I'd like to add the feedback mod in still.

[controlFreak]

Feedback/Freakout:
Replace the emitter resistors on Q2 and Q3 to something small, and take the signal off the Emitter of Q2 and switch that between the Emitter of Q3 and input. That gives you two feedback "freakout" modes on one On-off-on switch. You could wire a pot as a variable resistor between pin 2 of the switch and the E of Q3, to tune the feedback from both signal sources.

Might be worth adding another on-off-on to have different compression options at Q2s clipping diodes. The clipping Capacitor is also a good place to tweak- smaller values move the frequency knee up and give more volume and boom in the bass while the highs get progressively more squared, larger values move the freq knee down and clip/compress the bass which sounds gainier. Diode lift gives great dynamics and hugeness at the expense of sustain. I like asymmetrical diode arrangements too. You can get a lot of variety with one clipping switch in a muff that goes well beyond the typical "diode clipper" mods in Blah overdrives. Take time to experiment and decide on the three sounds you want to switch...you'll find there are a lot of options- so solder in some sockets, plug and play!

If you want more gain out of any of the stages you can up the collector resistor...

[eatyourguitar]

If you want more gain out of any of the stages you can up the collector resistor...

this part I don't follow. what if you overshoot the perfect value and keep going? value of the collector resistor that gives you the maximum gain for each transistor will depend on the Hfe of the transistor and the amount of base current. you can't just make everything bigger by some unknown value and expect all your collectors to be perfectly biased. collector biased at half the supply still applies IMO.

the exception being that the second edition triangle has a resistor missing from the base to ground Q2. it is biased hot with the CB 390k. increasing the collector there will most definitely get you closer to the maximum gain cause there is no voltage divider on the base. so 390k + bigger collector resistor will bring your base current down closer to the middle of the transistor's operating range. there is a point where you can go too far though. anything over 30k is probably going too far. your probably better off in that unique situation to increase the 390k to 680k just for Q2 as long as you have 2N5088 or some transistor Hfe ~450.

[xrleroyx]

Thanks guys,

my main question though is how would I wire this with the 3-way mini switch I used to use for phase and tapping?

[controlFreak]

You want to use that three way switch for feedback right? I'm assuming its SPST on-off-on? Three connections? You want two different kinds of feedback/freakout right?

1. replace the emitter resistors on Q2 and Q3... you can increase the collector resistors to get you back where you were gain-wise.
2. Wire pin 2 of that switch to a variable resistor (Pot) going to the emitter of Q2... 1M is probably a good value pot.
3. Pin 1 of the switch goes to your input signal, you can wire this from the tip of the jack.
4. Pin 3 of the switch goes to the emitter of Q3.

Two different modes of feedback/freak out, one SPDT on-off-on switch. I don't think I can spell it out any easier, though admittedly I'm a picture person myself so my words may be confusing to others.

Hahaha...perfectly biasing a bigmuff..

Ignore EATyourguitar's nerdy bias panic attack... There is no "perfect value." This isn't a pretty little delicate circuit where everything needs to be biased just right. Upping the collector by a few K ohms will give you more gain in this circuit, just go use a few 50-100k pots and find what sounds good to you. I fucking hate trying to calculate shit out on paper, just go do it and don't be such a math nerd. Use your godamn ears. Find what sounds good first, then scope it out and try to figure out why if you want to be a tone philosopher.

I was suggesting higher collectors because you've already jumpered the emitter resistors and if you want to use my suggested feedback mod, you'll need to replace a couple of the emitter resistors (on Q2 and Q3) and you may miss the gain from the mod you've already done.

[eatyourguitar]

There is no "perfect value."

there is a perfect value if you want the maximum gain from each transistor. I never said that this circuit needs to be perfectly biased. none of the production muffs are. I never ever performed the bias procedure I described here on any of the muffs I built because I honestly think it is stupid and counter productive. your giving him good information about estimating what value will get him close to what he wants with a little luck. there is nothing wrong with that. I %100 agree using your ears is a great way to build guitar pedals. I do it all the time. the question presented is how do you increase the gain. my method will work %100 of the time because you arrive at the best possible answer. anyone can follow it and they do not need to have all these resistor values memorized like it seems you have.

so I ask you this, is it worse to teach someone electronics or is it worse to tell someone to randomly make some resistor value bigger by some unknown amount?

[controlFreak]

Sorry I sounded angry in my last post... that was the tequilla talking.

I'm teaching electronics too, just differently! Giving general directions and leaving the details for discovery through experimentation can do more for someone than overloading them with all the techy math details first.

A hint is better than no advice at all, right?

Eatyourguitar, would you be so kind as to post up your maths for biasing these types of gain stages? Maybe a couple examples of the same transistor circuit with the same gains running at different voltages?

[eatyourguitar]

I sometimes make angry drunk posts here too. I totally understand.

you have lots of great ideas man. using a 50k pot on the collector to get the most volume is a great idea. you can do this with your ears or you can do it with a scope. you can use a function generator or a guitar. its all good.

before we start looking at math formulas, there are some words that need to be defined. one thing that I kinda jumped on you about was the word gain. that is a special word. it means amplification. having gain greater than 1 does not mean increase perceived loudness or RMS. it means amplification. the ideal amplifier has %0 distortion. so usually we take sine wave generators and tweak all the values with a pot till we arrive at the maximum amplification with a reasonably low distortion. basically we keep going till we start turning the transistor fully on or fully off where it clips the wave on one side or the other. if we just wanted it to be loud and distorted we would use the same sine wave generator and test the output for Volts RMS. in that case the maximum Vrms would be an ideal square wave of %50 duty cycle. a square and a sine at the same Vpp (peak to peak) do not have the same Vrms. the square has more RMS power since it contains that sine with the same amplitude but it also contains harmonics that carry energy on their own.

with an NPN amplifier, once you remove the emitter resistor, you are taking the top speed safety device off the golf cart if you know what I mean lol. the only reason anyone wants an emitter resistor is because a well behaved amplifier does not need Hfe 450. another reason is that having an emitter resistor will allow you to substitute any Hfe with very little difference in the results. this is big deal for mass produced electronics. everything that goes in or out of a transistor does so through the emitter and the emitter resistor. so basically what happens in the muff is this. you remove the emitter resistors and your actually increasing the Hfe of the transistor to whatever that transistor Hfe is. the base is already directly connected to the previous stage through a cap. there is no resistor limiting AC current from the cap to the base. there is only one resistor value left to tweak. the collector. now if you want to increase the gain, you gotta get it in that sweet spot which is increasingly difficult when you have no added components for stability like for example an emitter resistor. a complete voltage divider on the base would be great for stability but then it would not be a muff. you can tweak the base pull up resistor but every time you do that you need to redo the collector resistor again. I do not like to change the base pull up because it would not be a muff if it had zero distortion. so really, in the end, you already posted the best method for increasing the volume and distortion with that 50k pot trick. its just not the best method for increasing gain.

the basic concept of these formulas you can use is that the Hfe is some number that you can multiply the base current by to get your collector current. Hfe 100 means Ib 1ma = Ic 100ma. Ie will always be Ib + Ic. in this case Ie = 101ma. so we usually just say that Ic = Ie even though we know its not %100 true. the real world gain of any transistor is never perfectly linear. you can read any datasheet to see that there is a curve with a linear operating range in the middle. when the base current exceeds some magic number, your transistor will not amplify Ib*Hfe. it will just stay at the maximum Ic for the transistor and the collector resistor + emitter resistor, whatever it may be. this is called saturation. this is why the muff always outputs a square wave. you can always expect each stage of a muff to make things into square waves. it can happen on the saturation side but it can also happen on the negative swing of the incoming signal where it is greater than or equal to the bias current through the base pull up. that is how you get your clipping top and bottom. the different sounds we get from these muffs are the combinations of filters and transistor clippers.

[xrleroyx]

I had absolutely no idea what any of you guys said.

I think I broke my 'muff!

I'll post pics if my friend can't fix it up. I should stick to just writing songs instead of messing with electronics... D':