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Boost Effect My Designs

Micro Thunder Boost Design

I was thinking for a while to create a booster that is as small as possible. Ever since I bought an old guitar that was sitting unused on someone’s attic for 15 years I was thinking what could I do with it.

I have a thing for Ibanez guitars … I don’t know, I just couldn’t let the guitar go on a skip. In the end, buying tremolo arm for it was more expensive than the guitar ?. It is great thus for my own little experimentation.

All the boards in this article were designed by me, and I got prototypes for free from PCBWay – thanks for this fellas, great service as usual ?.

You can skip straight to the video if you wish, below are some extra details of the build.

Intro

I did a bunch of booster pedals before, so I had a sort of a template of what I was going for. What was different this time was that I wanted to make this as small as possible, battery operated and get it to use as little power as possible.

The reasons for the above are – I wanted to be able to put it into my guitar’s body – hence small … and battery operated. And since I didn’t want to change the battery for as long as possible, low power usage was essential.

Natural choice for the battery was 3V coin cell battery. I wasn’t really thinking beyond that. I might in future consider something else, but one of the design goals was using a 3V coin cell battery.

Since the size of the battery is the minimal size I can get away, I thought, that’s the size of the board I’ll aim for. Naturally, I’ll need to use SMT components, and I’m no stranger to SMT.

Finally, I thought that the simplest and the most efficient design would be with an op-amp. I was looking for something preferably rail-to-rail to give me maximum head room, and something that will be very low power when idle.

There’s a few op-amps I considered and I settled for giving a try to: OPA379 (OPA379AIDBVT) and TLV313 (TLV313IDBVT) both in tiny SOT23-5 package. For this build I used OPA379, I’ll try the other op-amp later.

Schematic

Here’s the schematic:

Schematic of a non inverting amplifier with 3V power source
Micro Thunder schematic

Pretty simple. It’s really just a simple non-inverting op-amp design. Nothing special to be honest. The biggest thing was figuring out which components to use.

If you find some of the values odd – I had leftover resistors and capacitors from earlier so I designed the circuit around them. A conscience decision was made to keep gain lower and fixed (about 10dB doh! that’s rather about 12dB and that’s higher than some commercially available boosters … might need to lower the gain for the next design).

I used a through hole version of coin cell battery holder, I’ll change that to surface mount for the next version of the design.

The op-amp I used is not really designed for audio so we’ll see what we get out of it. I’m pretty certain it will work, how good though, we’ll see. This is a proof of concept more than anything else, so another iteration is coming for sure.

Consumption

One of the goals was to use an op-amp that can last a while on a single battery. If we assume that 3V coin cell has capacity of about 500mAh, let’s see what can we get out of our battery. This is a very crude calculation and I probably got it wrong on the video.

Quiescent current per amplifier is maximum 5.5uA. Our chip has only one op-amp, so, in theory, if the op-amp is idle, we should squeeze about 90,000 hours???? Is this right? That’s over 10 years. (Typical consumption is half of this so … 20 years!???)

Reality is that I used 100Kohm resistors for biasing, so they alone use 10uA … doh! That means the battery should last around 3 years. Again, this is not entirely true, because the op-amp won’t be completely idle in reality.

One thing for certain, it beats my Vox amPlug 2 … that thing is eating batteries. Unfair comparison … I know, amPlug has a bit more going on, but still.

If anyone knows how to better calculate consumption, please chime in the comments. I wonder how far off I was with this super crude calculation.

Noise

Another thing to consider is noise. When I compare this OPA379 to TL072, it will be probably more noisy (just by looking at the parameters). All I can hope is that it will be good enough ?.

TL072OPA379
Input voltage noise density (nV/√Hz)20-3780
Input current noise density (fA/√Hz)801
Gain Bandwidth Product (GBW)5.25 MHz90Khz
Some parameter comparison

Current noise density is negligible compared to voltage noise, but voltage noise might be a deal breaker in the end.

Also, it’s worth noting that GBW is way wider for TL072. For OPA379, I chose 10dB gain in part because of GBW. Closed loop bandwidth is about 30Khz. When I plug-in noise figure for the noise bandwidth and include noise contributed by resistors … I get 0.3mVpp … for the equivalent bandwidth TL072 would have 0.2mVpp … I have no idea what to take of these numbers though :))))

Final Result

Anyway, here’s the final result:

Looks pretty much the job!!! The finished board looks the same as the 3D rendering from KiCad. Well, my soldering is suspect, but the goal – size-wise, complete success!

Action Time

Conclusion

In the end I wasn’t really happy with the result. While I did manage to make it work, the circuit is too noisy. I am not entirely sure if I messed up something, maybe it’s my soldering. TLV313 has comparable voltage noise to TL072, so maybe I’ll have more luck with it. I haven’t given up on OPA379 yet, so stay tuned for more messing around with it.

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