Trouble shooting a Power Boost circuit

[D.M.N.]

Hi folks, first post, and I could use some assistance. I'm in the process of building a somewhat modified Power Boost/Overdriver. I've used this schematic here (the top, early version):


Now I've made a few changes per ideas I've seen floating around, namely a switchable 9v/18v power supply so it can go between the Power Boost and Overdriver sound, and I've added the master volume as per many recommendations. I also upped the 22uF power filtering cap to 47uf. I've got continuity on my bypass wiring, and the voltage doubler/selector is putting out the correct 18 or 9v.

However, for the life of me I cannot get any sound out of it when engaged. Unfortunately, my multimeter doesn't appear to have a strong enough continuity test to check from the in to the out of the circuit through all the resistors and caps. Here's the schematic I made up including my changes (not the cleanest schematic, sorry):



Any suggestions on where to start tracking things down would be super appreciated. Through what testing I've done, it seems I have continuity from the output all the way back to the tone stack section, just prior to Q3. Thanks!

[MikeA]

Welcome D.M.N.!

I'd recommend cobbling together a simple audio probe, to trace the signal starting at the input until you find the fault point.  Engage the circuit, put some kind of signal in, follow the schematic with the probe and note where the signal stops.   There's some how-to info available at https://www.aronnelson.com/diywiki/index.php?title=Debugging

[aron]

Yes! Audio Probe!

[D.M.N.]

Ah, excellent, I was just looking at that article the other day, should have remembered that! Excellent starting point, I'll do that this coming week, unfortunately out of town filming a music video all weekend.

[aron]

What? You have music where you are?

[D.M.N.]

What? You have music where you are?

Unfortunately no, but we figured we'd get out of the city and shoot a video for our upcoming album since we've got very little else going on. Might as well make good use of our time!

[D.M.N.]

I'll be able to make the audio probe and check everything tomorrow, but in the mean time, does anyone see any error in the schematic I used itself that could cause any issues? I tried being as careful as possible, but it's always helpful to have another set of eyes confirm I did't just miss something silly.

[D.M.N.]

Welcome D.M.N.!

I'd recommend cobbling together a simple audio probe, to trace the signal starting at the input until you find the fault point.  Engage the circuit, put some kind of signal in, follow the schematic with the probe and note where the signal stops.   There's some how-to info available at https://www.aronnelson.com/diywiki/index.php?title=Debugging

Yes! Audio Probe!

Thank you guys for the help! Built the audio probe today and sat down to go through the circuit. After testing for about an hour I was a bit stumped, seemed there was an issue with the transistors I had socketed. Swapped in some PN2222s that I knew worked, and voila, there it was! Turns out I managed to gloss over the fact that the BC184LCs have a different pinout, and are ECB, not EBC! While it works now, though, it seems the output volume/power is a little low. So more testing is required I suppose, but at least it's passing sound!

[D.M.N.]

I should say, while passing sound, it is this rather sputtering, bubbling sound when playing, no matter what transistors I pop in there.

[PRR]

Are the voltages burbling too? Or aren't there any voltages?

[D.M.N.]

Are the voltages burbling too? Or aren't there any voltages?

I thought it might be something with the power supply, and checked that, but the voltages appear stable leaving the voltage selector. Got the audio probe out and the signal appears to be fine heading into the master volume pot, loud and high output, but getting that bubbling sound or nothing at all on occasion when probing after the volume pot. I'll be running more diagnostics on it tomorrow and will report back any findings. I also suspect some noise may be from the transistors being a little loose in their sockets, so I'll be tightening those as well.

[antonis]

Got the audio probe out and the signal appears to be fine heading into the master volume pot, loud and high output, but getting that bubbling sound or nothing at all on occasion when probing after the volume pot.

Replace Volume pot.. 

P.S.1
Any specific reason for R16 to be placed there..??
(its existence sets different DC & AC Q points..)

P.S.2
@aron: I presume, a correction is needed here https://www.aronnelson.com/diywiki/index.php?title=Debugging 

[D.M.N.]

Got the audio probe out and the signal appears to be fine heading into the master volume pot, loud and high output, but getting that bubbling sound or nothing at all on occasion when probing after the volume pot.

Replace Volume pot.. 

P.S.1
Any specific reason for R16 to be placed there..??
(its existence sets different DC & AC Q points..)

P.S.2
@aron: I presume, a correction is needed here https://www.aronnelson.com/diywiki/index.php?title=Debugging 

Yep, that'll be the first thing this morning when I get to the the bench, going to confirm with the audio probe again.

Regarding R16, it's there in the earliest iterations of the Power Boost, though disappears on the later version and Overdriver. I actually did end up leaving it out in my build.

[D.M.N.]

So, went through, sorted a couple of things. Secured the transistors in their sockets, that helped stability, and had a bit of a ground on the volume pot from a stray bit of solder. It's working for the most part now, however, when I turn the gain pot up all the way, I get this gurgling/bubbling sound, almost like it's too saturated and the signal is too hot, if that makes sense. It's not the amp being over driven, as I've tested it on two different amps. I've included a video clip below. You can hear it a little bit around the 30 second mark, and that's not actually from the gain knob but rather cranking the bass knob. You can hear it from the gain knob, and more easily, at 1:18. Apologies for the poor playing, I'm a bass player by trade.

[D.M.N.]

Here's a clip I made with bass, where the issue becomes quite readily apparent.

[D.M.N.]

So, back at it again today with the audio probe. With the gain turned down, everything tests good all the way through the circuit. However, as soon as I turn that gain up to where the noises start happening, I find that it's occurring immediately after transistor 1. Checked the power supply, voltages are correct at all entry points into the circuit. Is it some sort of biasing issue when the gain is turned up that high, or when there's that much signal going in? I'm a bit stumped.

[D.M.N.]

Any suggestions?

[bushidov]

I made a couple of PCBs of these for myself and some friends and it seemingly works well.


I build mine more as an overdriver than the power booster, but it seems to work ok. I noticed your overdriver schematic had a mistake on it,  though whoever drew it, corrected it on the power booster schematics where the drive pot's pins 1 and 2 were going to some resistors incorrectly.

As of the first stage transistor, I have my emitter pin resistor a little higher at 6.8K and my collector pin resistor a little lower at 100K, but still keeping my feedback resistor to the second stage at 150K like you have it as well as my drive control being a 10K pot. I also had a heftier miller cap for that second stage at 470pF and a much lower emitter resistor for the second transistor, going down to 470 ohms. I am not sure if these values will matter much in your design, but I know it works, so if you want to give it a crack, let me know.

[Electric Warrior]

I made a couple of PCBs of these for myself and some friends and it seemingly works well.


I build mine more as an overdriver than the power booster, but it seems to work ok. I noticed your overdriver schematic had a mistake on it,  though whoever drew it, corrected it on the power booster schematics where the drive pot's pins 1 and 2 were going to some resistors incorrectly.

I drew that and it's correct. It connects differently in the Overdriver. That's why the resistor is in a different spot on the Overdriver PCB. The reissue has a translucent board on which you can actually see how it connects: http://www.macaris.co.uk/overdriver-by-stu-castledine/p/p1800

[D.M.N.]

I made a couple of PCBs of these for myself and some friends and it seemingly works well.


I build mine more as an overdriver than the power booster, but it seems to work ok. I noticed your overdriver schematic had a mistake on it,  though whoever drew it, corrected it on the power booster schematics where the drive pot's pins 1 and 2 were going to some resistors incorrectly.

As of the first stage transistor, I have my emitter pin resistor a little higher at 6.8K and my collector pin resistor a little lower at 100K, but still keeping my feedback resistor to the second stage at 150K like you have it as well as my drive control being a 10K pot. I also had a heftier miller cap for that second stage at 470pF and a much lower emitter resistor for the second transistor, going down to 470 ohms. I am not sure if these values will matter much in your design, but I know it works, so if you want to give it a crack, let me know.

I'm out of town this week but I'll switch some components around when I get back and see if that alleviates any of the flubbing. As of right now, with my current components, I haven't been able to eliminate it, still a bit mystified.