Bazz Fuss Low Output

[bassmannate]

Hoping to get a bit of assistance on this. I built a Bazz Fuss and it sounds great except for the volume! I have to really crank my amp to hear it and then it's still really quiet! I'm using the "Deluxe" schematic found here: http://home-wrecker.com/bazz.html Here are the voltages I'm reading on the MPSA13:

C - 1.26-2.18
B - 1.14-2.08
E - 0-.92

The first number is with the "dirt" control turned all the way up (shorted to ground) and the second is with the control turned all the way down. The only modifications I've done are remove the pickup simulator since I'm going straight in from the bass and I've changed the output cap to 1uf since I'm playing bass through it.

[Kipper4]

Have you checked the jellybean orientation. No idea if it's that but hey. I'm trying.

[bassmannate]

Have you checked the jellybean orientation. No idea if it's that but hey. I'm trying.

Yeah, I've checked the orientation on everything but I'll check again later to see if I missed something.

[peterg]

The collector voltage looks low. Double check your component values. Have you used an audio probe on it?

[GibsonGM]

The collector voltage looks low. Double check your component values. Have you used an audio probe on it?

+1, seems off...look at the collector resistor. And be sure of how the diode and transistor are oriented...

[bassmannate]

The collector voltage looks low. Double check your component values. Have you used an audio probe on it?

Wish I had an audio probe. I'll check the bias resistor on the collector. I don't THINK the power supply is to blame. It's a crappy 3-12v wart rated at 300mA but with the circuit hooked up, it reads right around 9v. Although, with no load, it usually reads high around 13v.

[bassmannate]

Well, collector resistor is measuring at just under 10k so that's not an issue. The other weird thing that happens with this circuit is that if I adjust the dirt control while playing, signal drops out completely and then fades back in.

Edit: The transistor and diode are in correctly. However, I'm starting to suspect my socket connection as the signal was cutting in and out when I wiggled the transistor in the socket. I'll have to have a look at that once the kid is in bed.

Another edit: I just tried bypassing the volume pot and I get a whole lot more volume but I also have hum. I guess this pot is too low a value? Not sure what's causing the hum either.

[PRR]

If using a Darlington, the Collector-Emitter voltage must be very near 1.6V.

You are showing 1.26V (any Dirt setting).

If you are sure this is a Darlington, check if the diode may be shorted.

[bassmannate]

If using a Darlington, the Collector-Emitter voltage must be very near 1.6V.

You are showing 1.26V (any Dirt setting).

If you are sure this is a Darlington, check if the diode may be shorted.

Well, the DMM shows 0.61 when connected to the diode one way (removed the MPSA13 to test) and I get no reading when connected the other way. I feel dumb asking this but the end of the diode with the line goes to the base, right? Just making sure I'm reading things right.

Just verified C-E voltage and, yup, it's at 1.26V. Do I just need to decrease the resistance between +9V and the collector increase C-E voltage to 1.6? I'm pretty sure it's a darlington. It's an MPSA13 I purchased from Tayda. Starting to wish I had ordered more than 1 though...

Gee, I thought this would be a quick one to throw together with as simple as the circuit is! lol!

[bassmannate]

If using a Darlington, the Collector-Emitter voltage must be very near 1.6V.

You are showing 1.26V (any Dirt setting).

If you are sure this is a Darlington, check if the diode may be shorted.

By the way, how do you know that it needs to be near 1.6v? I couldn't figure that out from looking at the datasheet. Is it something that you just know from experience?

Edit: nevermind. just found it. Collector-emitter saturation voltage - 1.5

[lethargytartare]

You say you get a lot more volume when you bypass the volume pot.  That sounds like you might have a bad pot.  If you can, try a different one in there.  Without the pot, are you sure the hum isn't just excessive noise (since the pedal is effectively at max volume)?  And the other pot sounds like it could be bad if you are losing signal when adjusting it. Do you have extras you can try swapping in?

And what layout did you follow -- perf and lead to lead (I know that's how it is on home-wrecker, but I didn't want to assume)?

You could also post pics and maybe we can see if there's any mismatch between the board and the schematic.

[PRR]

> how do you know that it needs to be near 1.6v?

The drop in a Silicon junction is 0.5V to 0.7V, less at lower voltage.

The diode is one junction.

The Darlington is two B-E diode junctions.

The diode and first transistor run at very low current, pencil 0.5V.

The main transistor of the Darlington runs near "normal" current, pencil 0.6V.

Darlington total B-E drop 0.5V+0.6V= 1.1V.

0.5V+1.1V is 1.6V.

This will change very little as the external resistance changes from 10K to 11K. (It would change about 0.020V if the external resistance doubled, so a 10% change is around 0.002V change.)

> Collector-emitter saturation voltage - 1.5

Irrelevant. That is a maximum value for very large current (over 100 times the current used here). Chart on page 2 of the Fairchild sheet shows 0.6V at the ~~1mA we use here. However we don't want it "saturated"; that's why it is wired with diode base-collector to bias it somewhere up from saturation.

While there: "ON voltage" is ~~1.1V at 1mA. This plus an assumed 0.5V in diode is my 1.6V.

I still think the fault is in or around the diode.

[bassmannate]

> how do you know that it needs to be near 1.6v?

The drop in a Silicon junction is 0.5V to 0.7V, less at lower voltage.

The diode is one junction.

The Darlington is two B-E diode junctions.

The diode and first transistor run at very low current, pencil 0.5V.

The main transistor of the Darlington runs near "normal" current, pencil 0.6V.

Darlington total B-E drop 0.5V+0.6V= 1.1V.

0.5V+1.1V is 1.6V.

This will change very little as the external resistance changes from 10K to 11K. (It would change about 0.020V if the external resistance doubled, so a 10% change is around 0.002V change.)

> Collector-emitter saturation voltage - 1.5

Irrelevant. That is a maximum value for very large current (over 100 times the current used here). Chart on page 2 of the Fairchild sheet shows 0.6V at the ~~1mA we use here. However we don't want it "saturated"; that's why it is wired with diode base-collector to bias it somewhere up from saturation.

While there: "ON voltage" is ~~1.1V at 1mA. This plus an assumed 0.5V in diode is my 1.6V.

I still think the fault is in or around the diode.

Thanks for the info! That makes much more sense.

Well, I thought I had more of the 1n914 but I can't seem to find them. The one in there now shows a voltage drop of .605 in diode test mode. I do have quite a few 1n4007 if I can just swap one silicon diode for another.

The 1k pot used on the emitter looks to be good. The one used for volume seems to be bad. I can't get a reading from wiper to one of the other lugs. The other one goes down to 3.5k but that's it.

I did a little experimenting with the collector resistor and managed to get a c-e voltage of 1.5v. I had to go all the way down to 470r to do it but it sounds better with the emitter shorted to ground. However, I don't get much when I start turning the gain down.

I made my own vero layout since I didn't have any perf on hand. I'll try to get that up for you guys to look at.

[bassmannate]

Here's my layout. Let me know if anything is unclear since I have my resistor and diode up on end. Also, if anyone can give me tips on rendering more clearly, that would be great. I couldn't find a better way to make my cuts visible while still having the components visible. Still a bit new at DIYLC. I really appreciate the help you guys are giving me!

[bassmannate]

Well, Paul, it looks like you were right. I swapped out the 1N914 for a 1N4007 and it works! I still need to replace the volume pot (A100K) but I have good volume at all gain settings.

Man, this thing is nice! I don't usually like distortion of any kind on bass but this has a really nice overdrive sound at lower gain!

The only other issue I'm running into is that I get a nasty hum when I turn the indicator LED on. If I keep it off, it sounds fine and actually quite hum free.

[lethargytartare]

when I turn the indicator LED on. If I keep it off, it sounds fine and actually quite hum free.

What does that mean?  Do you mean when you have a LED wired into your switch?

[bassmannate]

when I turn the indicator LED on. If I keep it off, it sounds fine and actually quite hum free.

What does that mean?  Do you mean when you have a LED wired into your switch?

Exactly. right now I have not connected the LED to the switch. When I do, it causes hum. I'm guessing it has to do with the crappy power supply. Can't think of why the LED would cause hum all by itself. Next project is likely going to be something like a geofex spyder.

[lethargytartare]

Ok, can you post pics of the unit with the led wired in?  If it's causing hum, it must be connecting something that shouldn't be connected.  Or can you describe how you have it wired in?  It's not even in the circuit, really -- 9v to led to ground, so any impact on the circuit has to be some wiring mistake, or a solder bridge?

[PRR]

> crappy power supply.

Life is too short for crappy power supplies.

The draw of the audio-part alone is much less than the draw of a reasonable LED. Crap-supply crap increases with current. So the idea of more buzz with LED is not very suspicious.

[bassmannate]

> crappy power supply.

Life is too short for crappy power supplies.

The draw of the audio-part alone is much less than the draw of a reasonable LED. Crap-supply crap increases with current. So the idea of more buzz with LED is not very suspicious.

Yeah, I think the whole circuit draws something like 10-15ma while the LED alone draws something like 20 so I can see how that would amplify any power issues.