# Special Overdrive transistor question/mistake



## soothsayer86 (Aug 12, 2020)

I built a couple Special Overdrives and both are having the same issue. The overdrive itself is extremely sputtery and fizzy sounding and not smooth at all. I have realized I got BC548B versions of the transistors and the circuit calls for BC548C transistors... ugh. I am wondering if this mistake could be causing the issues with the pedals. I have everything socketed and have tried swapping transistors and IC's and it stays the same. 

Also, if this is indeed the issue, if someone could point me to the correct transistors at mouser that would be awesome because there are a few different varieties and I am still learning which parts can be swapped and which can't. Thanks in advance.


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## moonlightpedalbuilds (Aug 13, 2020)

Try bc549c. They are pretty easy to get anywhere.


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## soothsayer86 (Aug 13, 2020)

Anyone know if there is a difference between the BC548C and the ones mouser has that have different letters at the end like bc548CTA?


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## Chas Grant (Aug 15, 2020)

BC548CTA  is the manufacturer's part number.  BC548 is the transistor type. C is the gain bucket the transistor falls into during post manufacturing testing, in this case the C variant has a hfe of 420 to 800,  Where the B's have a hfe of 200 to 450. This may be why the B's didn't work so well in the circuit. The TA is just manufacturer's data and will tell you how its packaged, lead style or anything else specific to the component. All this information is located in the data sheets for the components. So in this case the TA means this is Ammo packaged by the manufacturer and has no bearing on the component its self.


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## soothsayer86 (Aug 15, 2020)

Chas Grant said:


> BC548CTA  is the manufacturer's part number.  BC548 is the transistor type. C is the gain bucket the transistor falls into during post manufacturing testing, in this case the C variant has a hfe of 420 to 800,  Where the B's have a hfe of 200 to 450. This may be why the B's didn't work so well in the circuit. The TA is just manufacturer's data and will tell you how its packaged, lead style or anything else specific to the component. All this information is located in the data sheets for the components. So in this case the TA means this is Ammo packaged by the manufacturer and has no bearing on the component its self.




Thank you! Just went back to look at the datasheet again, and I don't know how I missed that table with the hfe values. Thanks for pointing that out.


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## Chas Grant (Aug 15, 2020)

soothsayer86 said:


> Thank you! Just went back to look at the datasheet again, and I don't know how I missed that table with the hfe values. Thanks for pointing that out.


No Problem at All!! I haven't built this particular overdrive but I did look at the build doc for it. The DOC calls for BC548B's for Q1 and Q2. So if that's what you used then they are correct. Also they are just input and output buffers, with a unity gain, the hfe should not have any effect on the circuit, so you may have something else going on.


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## soothsayer86 (Aug 15, 2020)

Chas Grant said:


> No Problem at All!! I haven't built this particular overdrive but I did look at the build doc for it. The DOC calls for BC548B's for Q1 and Q2. So if that's what you used then they are correct. Also they are just input and output buffers, with a unity gain, the hfe should not have any effect on the circuit, so you may have something else going on.



That's so weird, because I ordered the parts based on the build doc, but then looking at the image of the silk screen on the listing for the pcb it says bc548c, here: https://www.pedalpcb.com/product/specialod/

Also in the build doc it calls for 100 and 220 resistors and I assumed those would be 100 Ohm and 220 Ohm, though in other build docs those would be referred to as 100R and 220R. I build both pedals the exact same way with the same parts and both are doing the exact same thing. 

I tried ordering fresh IC's and transistors from mouser because I thought the ones I got from Tayda could be bad but nothing changed. I am not really sure what else could be going on. The overdriven tone just sounds awful, like sputtery and fizzy and doesnt fade nicely as the note decays. It kind of sputters and then stops even though the note itself is still there, almost like theres two signals one clean and one dirty. I have built a bunch of pedals and never really run into an issue like this where everything is "working" but it just doesn't sound right.


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## soothsayer86 (Aug 15, 2020)

So I guess that means the overdrive is coming from the opamp? Which means I must have used the wrong components somewhere and it is affecting the voltages to the IC. Damn.


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## Chas Grant (Aug 16, 2020)

soothsayer86 said:


> That's so weird, because I ordered the parts based on the build doc, but then looking at the image of the silk screen on the listing for the pcb it says bc548c, here: https://www.pedalpcb.com/product/specialod/
> 
> Also in the build doc it calls for 100 and 220 resistors and I assumed those would be 100 Ohm and 220 Ohm, though in other build docs those would be referred to as 100R and 220R. I build both pedals the exact same way with the same parts and both are doing the exact same thing.
> 
> I tried ordering fresh IC's and transistors from mouser because I thought the ones I got from Tayda could be bad but nothing changed. I am not really sure what else could be going on. The overdriven tone just sounds awful, like sputtery and fizzy and doesnt fade nicely as the note decays. It kind of sputters and then stops even though the note itself is still there, almost like theres two signals one clean and one dirty. I have built a bunch of pedals and never really run into an issue like this where everything is "working" but it just doesn't sound right.



You're correct with 100R being 100 Ohms, its just another way of writing it, R is easier than spelling out ohms all the time. If you can, post pictures of the front and back of the PCB. A second set of eyes on it is always a good thing.


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## Ratimus (Aug 17, 2020)

soothsayer86 said:


> That's so weird, because I ordered the parts based on the build doc, but then looking at the image of the silk screen on the listing for the pcb it says bc548c, here: https://www.pedalpcb.com/product/specialod/
> 
> Also in the build doc it calls for 100 and 220 resistors and I assumed those would be 100 Ohm and 220 Ohm, though in other build docs those would be referred to as 100R and 220R. I build both pedals the exact same way with the same parts and both are doing the exact same thing.
> 
> I tried ordering fresh IC's and transistors from mouser because I thought the ones I got from Tayda could be bad but nothing changed. I am not really sure what else could be going on. The overdriven tone just sounds awful, like sputtery and fizzy and doesnt fade nicely as the note decays. It kind of sputters and then stops even though the note itself is still there, almost like theres two signals one clean and one dirty. I have built a bunch of pedals and never really run into an issue like this where everything is "working" but it just doesn't sound right.


I know exactly what you're talking about with the "two signals" thing. I hear it all the time when I'm screwing around with cruel and unusual ways to mis-bias transistors if I'm breadboarding some fuzz circuit. Usually this happens because there's some part of the circuit that isn't set at a high enough voltage to forward bias a P-N junction in a transistor, so you only get sound when the signal itself is greater than the diode drop of that junction. The attack portion of a note is way louder than the rest of it, so that's the part that pokes above the threshold. All this to say... check,  double check, and triple check that you've got the correct value resistors around all the transistors, that you don't have solder bridges anywhere, no backwards diodes or electrolytic capacitors, etc. I wouldn't blame component selection. Best of luck!


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## soothsayer86 (Aug 18, 2020)

Ratimus said:


> I know exactly what you're talking about with the "two signals" thing. I hear it all the time when I'm screwing around with cruel and unusual ways to mis-bias transistors if I'm breadboarding some fuzz circuit. Usually this happens because there's some part of the circuit that isn't set at a high enough voltage to forward bias a P-N junction in a transistor, so you only get sound when the signal itself is greater than the diode drop of that junction. The attack portion of a note is way louder than the rest of it, so that's the part that pokes above the threshold. All this to say... check,  double check, and triple check that you've got the correct value resistors around all the transistors, that you don't have solder bridges anywhere, no backwards diodes or electrolytic capacitors, etc. I wouldn't blame component selection. Best of luck!


Thanks for the info, I have taken a break from those two pedals for a little while, when I go back to them I will triple check all the components and make sure everything is correct.


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## soothsayer86 (Aug 18, 2020)

Chas Grant said:


> You're correct with 100R being 100 Ohms, its just another way of writing it, R is easier than spelling out ohms all the time. If you can, post pictures of the front and back of the PCB. A second set of eyes on it is always a good thing.


Ill post some pictures later on. Thanks for being willing to help out.


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## soothsayer86 (Aug 18, 2020)

Chas Grant said:


> You're correct with 100R being 100 Ohms, its just another way of writing it, R is easier than spelling out ohms all the time. If you can, post pictures of the front and back of the PCB. A second set of eyes on it is always a good thing.




heres a shot of the top side of the board for the first pedal. I just went through and confirmed all of the components one by one. Everything checked out. I am completely stumped. I would appreciate any help on this.


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## soothsayer86 (Aug 18, 2020)

p.s. the LED spot is jumpered because I used a LED foot switch instead.


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## Jbanks (Aug 18, 2020)

Show us the back side too please. Lots of things can go wrong on the solder side. But top side is good to double check values as well.


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## soothsayer86 (Aug 18, 2020)

Jbanks said:


> Show us the back side too please. Lots of things can go wrong on the solder side. But top side is good to double check values as well.


Yeah I will go pull it apart and get a picture of the bottom, however because more than one pedal is doing the exact same thing, it's unlikely to be a solder bridge or cold solder joint on the exact same component. But hey, you never know!


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## soothsayer86 (Aug 18, 2020)

Ratimus said:


> I know exactly what you're talking about with the "two signals" thing. I hear it all the time when I'm screwing around with cruel and unusual ways to mis-bias transistors if I'm breadboarding some fuzz circuit. Usually this happens because there's some part of the circuit that isn't set at a high enough voltage to forward bias a P-N junction in a transistor, so you only get sound when the signal itself is greater than the diode drop of that junction. The attack portion of a note is way louder than the rest of it, so that's the part that pokes above the threshold. All this to say... check,  double check, and triple check that you've got the correct value resistors around all the transistors, that you don't have solder bridges anywhere, no backwards diodes or electrolytic capacitors, etc. I wouldn't blame component selection. Best of luck!


Your description sounds exactly like im experiencing, but I was told that in this circuit the transistors aren't what is creating the overdrive. Honestly, I don't know enough about it to know for sure. Hopefully I can get this figured out!


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## soothsayer86 (Aug 18, 2020)

Jbanks said:


> Show us the back side too please. Lots of things can go wrong on the solder side. But top side is good to double check values as well.



Here are the rest of the pictures:


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## Chas Grant (Aug 18, 2020)

OK first thing is to remove the jumper for the  LED.  Not sure if this will help or not, but the jumper will alter the current flow through the circuit. If not using the LED just leave it and the CLR out of the circuit.


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## Chas Grant (Aug 19, 2020)

soothsayer86 said:


> Your description sounds exactly like im experiencing, but I was told that in this circuit the transistors aren't what is creating the overdrive. Honestly, I don't know enough about it to know for sure. Hopefully I can get this figured out!


The transistors in this circuits are buffers only, they are not clipped and have a unity gain. What’s creating the overdrive is the exuberant amount of gain on the first stage op amp. The high gain is causing the op amp to clip the signal and cause your overdrive. Normally this is done with diodes in the feed back path, but this circuit has none. Without the diodes you have less compression, more headroom but it’s a hard clip when you hit the rails.


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## soothsayer86 (Aug 12, 2020)

I built a couple Special Overdrives and both are having the same issue. The overdrive itself is extremely sputtery and fizzy sounding and not smooth at all. I have realized I got BC548B versions of the transistors and the circuit calls for BC548C transistors... ugh. I am wondering if this mistake could be causing the issues with the pedals. I have everything socketed and have tried swapping transistors and IC's and it stays the same. 

Also, if this is indeed the issue, if someone could point me to the correct transistors at mouser that would be awesome because there are a few different varieties and I am still learning which parts can be swapped and which can't. Thanks in advance.


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## Chas Grant (Aug 19, 2020)

soothsayer86 said:


> Here are the rest of the pictures:


Had an emergency at work I have to babysit for, so I looked over everything on my phone. I don’t see anything obvious. All your components look correct. Have you taken any bias voltage readings? If they are good you may need to check for audio through the circuit with a probe.


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## Chuck D. Bones (Aug 19, 2020)

Measure the voltage at IC1 pins 1 & 7, should be close to +4.5V.  Check voltages on Q1 & Q2 emitters, should be around +3.9V.  You used a blue LED for D1. The BOM does not specify LED color; the layout on Guitar FX Layouts shows red.  This will only affect the tone when you turn up ACCENT.


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## Chas Grant (Aug 19, 2020)

Looked aat the pics on a real monitor, everything does look correct. I think Chuck nailed it with voltage checks at those points above! Take those and we can move forward from there.


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## soothsayer86 (Aug 20, 2020)

Chas Grant said:


> OK first thing is to remove the jumper for the  LED.  Not sure if this will help or not, but the jumper will alter the current flow through the circuit. If not using the LED just leave it and the CLR out of the circuit.



Without a jumper the circuit doesn't function. The LED footswitch with the 3pdt breakout board keeps the CLR in the circuit so those pads need to be jumpered.


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## soothsayer86 (Aug 20, 2020)

Chuck D. Bones said:


> Measure the voltage at IC1 pins 1 & 7, should be close to +4.5V.  Check voltages on Q1 & Q2 emitters, should be around +3.9V.  You used a blue LED for D1. The BOM does not specify LED color; the layout on Guitar FX Layouts shows red.  This will only affect the tone when you turn up ACCENT.


Thanks Chuck, I'll check this when I get home from work. As to the blue LED, I tried various colors and the blue one just happened to be in there when I took the photos.


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## soothsayer86 (Aug 20, 2020)

Chas Grant said:


> Had an emergency at work I have to babysit for, so I looked over everything on my phone. I don’t see anything obvious. All your components look correct. Have you taken any bias voltage readings? If they are good you may need to check for audio through the circuit with a probe.


I get audio through the pedal totally fine, the issue is that it just sounds terrible ?


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## soothsayer86 (Aug 20, 2020)

So, reading through the comments on Guitar FX Layouts (https://tagboardeffects.blogspot.com/2014/03/shins-music-dumbloid-standard.html#comment-form), it seems like the issue I am describing is just standard for this design...sigh. So I am gonna try out some different op amps and see if I can find one that clips better.


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## Chas Grant (Aug 20, 2020)

soothsayer86 said:


> Without a jumper the circuit doesn't function. The LED footswitch with the 3pdt breakout board keeps the CLR in the circuit so those pads need to be jumpered.



My Bad! I didn't look at the 3PDT board.  I use a different breakout board that has the CLR mounted on the 3PDT board and, so I don't install the LED or the CLR on the circuit PCB, just on the breakout board. I just assumed yours did also, but it doesn't. So you are correct with needing them.


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## soothsayer86 (Aug 20, 2020)

Chas Grant said:


> My Bad! I didn't look at the 3PDT board.  I use a different breakout board that has the CLR mounted on the 3PDT board and, so I don't install the LED or the CLR on the circuit PCB, just on the breakout board. I just assumed yours did also, but it doesn't. So you are correct with needing them.


No worries, just wanted to make sure someone who comes along and reads this later has accurate info!


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## FloydianPulse (Feb 27, 2021)

About to build this circuit and realized that I bought BC548 instead of B or C, looking at data sheet seems the unlettered one just has a wider gain potential. Since not being used as gain am I ok to proceed with the unlettered BC548?


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## Chuck D. Bones (Feb 27, 2021)

Should be fine, that circuit is not sensitive to the transistors' hFE.  For that matter, they don't need to be BC548.  Any of the usual suspects will work.
If you're curious about what you got, you could always try measuring them.

BTW, Soothsayer, did you ever measure the voltages I asked you to measure?


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## odourboy (Jun 13, 2021)

soothsayer86 said:


> So, reading through the comments on Guitar FX Layouts (https://tagboardeffects.blogspot.com/2014/03/shins-music-dumbloid-standard.html#comment-form), it seems like the issue I am describing is just standard for this design...sigh. So I am gonna try out some different op amps and see if I can find one that clips better.


Did you ever resolve this? I built one recently and it exhibits the same characteristic. I hadn't noticed it but after reading this thread, it jumped out at me. Sadly, it's something you can't unhear once you've heard it. LOL! I love this pedal except for that weirdness.


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