Some modulation

By aphid310, 2 November, 2011

Hi, It seems if i crank up all the knobs, i get some odd modulation type sound. I also am the person who had to replace my 1uf capacitor with another ceramic layered one. its a 50v instead of a 25v though if that will affect it. Is there anything to be done or check?

Member for

12 years 10 months


12 years 5 months ago

The voltage rating won't affect the sound at all. When I made my original prototype, I used 50V caps.

When you say "modulation" do you mean oscillation? Like, is it making a sound all on its own, without strumming the strings, or even having anything plugged in? If so, that's normal. I left that oscillation in there for a number of reasons.

First off, it sounds really good if you let it oscillate just a little bit and then start playing. You can get some really crazy feedback effects. And the feedback effects you can get are very different depending on your supply voltage, what kind of pickups you have, and whether or not you are running your signal through a pedal or preamp. I didn't want to lose that.

Let me back up a bit and explain why it's doing it. It's oscillating because the incoming voltage is too high on the input of the TBA820M. Now, I could have clamped that voltage down with an appropriate voltage divider, but if I would have done that, I would have lost the wide supply voltage range at which the amp can operate and still sound good.

I love running my amp at 4.5 to 6 volts. That's 4 AA batteries, either rechargable or disposable, respectively. But if I would have designed around that supply voltage then the oscillations would show up again at 9 volts, and at the same time I would have worse distortion performance. Designing for 9-12 volt supply would make a clean tone at 4 volts almost impossible, in fact, it would sound absolutely horrible without a major redesign of the front end.

I could be wrong, maybe I'm just not clever enough about it. Perhaps if I would have went to college for Electrical Engineering I could have figured out a solution.

Instead of finding a solution to eliminate the oscillation, I put in something to tame the oscillation.

The trim pot. The trim pot is there to tame that oscillation for all different supply voltages.

With the guitar plugged in, crank everything, and then dial back the trim pot until it stops oscillating. (Depending on how much voltage you're using as a supply, you may have to dial the gain back just a smidge as well)

Once you found this setting, you can change the gain and level knobs all you like and it's still going to sound good, and you can get a pretty wide range of distortion effects. If you crank the level pot again, and turn up the trim until it just barely starts oscillating, you can get those feedback effects I mentioned earlier.

If you turn it down so it stops distorting, your clean tone will be quite quiet. For a louder, clearer clean tone, turn the gain all the way down, crank the trim pot all the way up, turn the level to about halfway, and then start turning up the gain until it's at the volume you like. I mean, the level knob doesn't have to be exactly halfway. There's a range at which it sounds good, and that range depends on the gain setting.

Got a bass? I use my Stella amp as a quiet bass practice amp. I crank the gain all the way down (and leave it there) and the trim all the way up, then I bring up the level until it's at the volume I like without distorting (it will be quite quiet, I use it at night to keep from waking up the kids.)

And of course, you can leave the guitar unplugged and play the Stella like a synthesizer by tweaking the knobs. There's a certain setting where the frequency of the oscillations are highly dependent on what is
currently being played through the amp. You can get a delayed thump, like, the thump works its way through almost like a drum beat, and slowly decays over time. And then you can control it by touching the input lead on the guitar cable to goose the oscillations a bit.

My main Stella has a Tillman preamp soldered on the front, and when I run the Tillman at such a low voltage, it really brings the signal down and helps keep those oscillations from getting out of control. At least, that's been my experience! I keep meaning to do a side by side comparison. Really this circuit is so new and I've made so many mods and iterations of it I sometimes forget what the performance of version 1.1 really is. Please correct me if your experience is different!

Anyway I hope that answered your question. :)

Member for

12 years 3 months


12 years 2 months ago

Thanks for the detailed overview. I was not sure if the oscillation was normal or not, but your description is spot on. I'm running with 4 AA batteries and it sounds great.

Thanks for a great kit, well thought out, nice board and easy to follow pictorial instructions. I've been messing with breadboards and building kits for a long time and really enjoyed this one.

Thanks for taking the time to share your passion, or at least one of them. Next Step - build the enclosure.

More to come.

Member for

12 years 10 months


12 years 2 months ago

You're very welcome. Thank you for making the time to post an update. I love updates!