[DIY/tweaking] A session of upgrades to the Fosi Audio V3 Stereo amplifier

Spring Upgrade: upgrades for a Fosi Audio V3 amp

In other words...can we, with a small expense, turn the little Fosi V3 Stereo into a Jeff Rowland amplifier?

Product: Do-It-Yourself upgrade session for the Fosi V3 amplifier
Producer: not for sale, TNT-Audio DIY design
Author: Matteo Bruni - TNT-Audio Italy
Published: June, 2025

No! The answer is obviously no. You didn't really think we were like those YouTube stars who, just to grab some views or sponsorships, promise to turn water into wine, did you? Sorry, but that's not how it works: the upgrades I'm about to suggest won't turn your Fosi V3 Stereo into the flagship model of the famous American brand. Besides... have you ever actually heard an €80,000 Jeff Rowland power amp? I haven't.

Foreword

Before we dive into a bath of solder, let's take a step back: where did the idea for this article come from? Well, it's very simple. The Fosi V3 Stereo is an amplifier that, for what it costs, sounds very good. Back in the day, the Sure TA2024 impressed me, but it had obvious limitations: the bass required an extremely easy load to drive and the highs weren't quite up to the level of the midrange. The Fosi V3, however, is a whole different story: it's like a TA2024 on steroids. The bass is excellent, the midrange very good, and with a 32V power supply it achieves at least five times the power of the TA2024, letting you match it with practically any speaker.

Starting from these premises, I became curious to see just how far we could push this little gem (still costume jewellery though). From the beginning, I decided to set some limits; my wife always says limits aren't exactly my strong suit... or rather, when she says it, she doesn't exactly use those words... but let's move on and check them together:

Limit number 1: I set a strict budget so none of you dear readers would have the excuse not to try it. The budget was about €20.

Limit number 2: Keep the same chassis, for both aesthetic and budget reasons.

The Circuit

So let's take a look at the signal path, from the connectors to the output of the TPA3255 amplifier, which is the beating heart of the little Fosi.

As you can see, the signal, right after entering the source connection, passes through the series resistors R21 and R25 (for the left and right channels, respectively), and then meets R23/R24 and C64/C65, connected in parallel. This network acts as a simple low-pass filter, designed to attenuate any unwanted frequencies that could sneak into the signal path. Now fire up the soldering iron and, while waiting for it to reach temperature, keep reading.

1. Bypass the Volume Potentiometer

Following the signal path, we reach the potentiometer (marked in the schematic as POT 50K). Our goal is to make sure our precious signal does not pass through it: it's noisy and introduces an annoying channel imbalance.

We have to de-solder the potentiometer and short-circuit the inputs and outputs, as shown in the image below:

Don't forget to short-circuit the pins at the top in the photo: they are just ground connections necessary for the amp to power on. This way, the Fosi will always turn on when connected to the power supply. To turn it off, just: unplug the power adapter, switch off the power strip, or add a switch before the power supply. If you choose this last option, make sure the switch can handle the current. You can also bypass the low-pass filter described earlier: just solder a well-shielded coaxial cable directly from the RCA output to the amplifier input pins (IN). I did this, and had no issues.

Thanks to this modification, you'll have a power amplifier ready to be paired with your preamplifiers, active crossovers or DSP!

If, on the other hand, you use the little Fosi exclusively with a phono preamp or a DAC without volume control, and you don't have a preamplifier (even a passive one), or you simply want to keep its integrated amplifier setup, you can replace the stock potentiometer. In that case, you might slightly exceed your budget. During my tests, I used a Vishay P11 for printed circuit boards, which worked very well with some care in bending and inserting the pins. This excellent potentiometer costs around €25, but there are also good alternatives in the €10–15 range, such as the Bourns 91A. As with the Vishay, you'll need to carefully bend the pins to fit the Fosi's holes, using two pliers: one to hold the potentiometer lead steady, the other to make the bend. If the leads end up too close together, insulate them with heat-shrink tubing. Replacing the stock potentiometer with a larger one or one with a different shaft diameter, height or length could make it harder to close the chassis, and you may need to replace the knob as well.

This first step has a noticeable impact, especially on the transparency, channel balance, and dynamics: the signal is finally free from the noise introduced by the stock potentiometer. Not only do the details of certain recordings emerge with greater clarity but the the separation of instruments more precise, offering the listener a less congested and more articulate reproduction.

Ok, now take a break and let your soldering iron rest before moving on to the second upgrade.

2. Replacement of Coupling Capacitors

Continuing along the signal path, we encounter the coupling capacitors, four per channel, highlighted in the schematic above (C1, C2, C4, C17, C21, C22, C23, C27). What do we need to do? Simple: replace them all with higher quality capacitors. In my opinion, good non polar electrolytics are more than adequate. I also tested polyester capacitors, which cost about six times more, but — after long listening sessions and multiple “plug and unplug” tests thanks to pin headers — I didn't notice an improvement that would justify the extra cost. Personally, I chose Nichicon MUSE series capacitors: non polar electrolytics specifically designed for audio applications. Ten Nichicon MUSE 10µF caps will cost you around €3.50. If possible, buy from trusted dealers, because the web is full of counterfeits. Warning: the fact that they are counterfeit doesn't necessarily mean they sound bad... but it's not exactly a good starting point.

In my tests, alongside the stock “Elna” capacitors and the polyester TDKs, I also tried the disreputed Jamicon (not exactly audiophile grade, but for what I spent on the Nichicons, I could have bought 70 of them!). I must say that the Jamicons didn't perform worse than the “Elna”, but they definitely don't match the Nichicons.

The improvement, once again, is noticeable: especially the reproduction becomes more natural compared to the stock Fosi configuration. Matt Elliott's voice in Drinking Songs (Live 20 Years On) sounds more realistic and moving, and the reproduction of acoustic instruments is also more convincing and refined.

3. Operational amplifiers

During my tests, I tried to implement the well-known Post Filter Feedback (the circuit circled in the schematic), but in my case, the result didn't meet expectations; in fact, adding this circuit with unshielded wires (there's no room for proper shielding) made the Fosi less quiet, and in the end I decided to remove it.

The good news about skipping the PFFB filter is that you'll save some euros from the budget. You can then indulge in replacing the operational amplifiers and find out for yourself if you can really hear a difference. Will it be true? In my case, I had fun testing several (prices shown are for a pair): LME49720 (€14), OPA1642 (€4), the historic JRC2068 (now produced by Nisshinbo €3), and the prehistoric MC1458DR (€1).

If after doing the suggested mods you still crave upgrades, you could: replace the power supply capacitors (CA1 and CA2) and the connectors, or — for the bravest (and the ones willing to spend more) — you could build a linear PSU (just the transformer alone would cost at least €100), or you might opt for the more modest addition of a filter to the switching power supply.

Personally, the more time I spent disassembling the Fosi, the more I felt the urge to replace everything (luckily, I stuck to limit no.1 and didn't want to prove my wife right), and I wondered if it might not be better to start from this schematic to create something completely different. We will see...

One thing is certain: I'm glad I first listened to the Fosi, and only afterwards disassembled, measured, and analyzed it. If I had done it the other way around, some prejudices would probably have prevented me from fully appreciating the qualities of this little budget amp.

Conclusions

If you want to start getting hands-on with a soldering iron (a great idea at any age!!!). All the suggested mods are easy. Also, you won't risk getting an electric shock, unless you decide to lick the power connector while the adapter is plugged in; if that's your fantasy... well, you might be a candidate for the next Darwin Awards.

If you're already comfortable with soldering and you like the stock Fosi! With a few euros you'll achieve noticeable improvements, assuming your system and listening environment allow it.

If you're among those convinced that capacitors, potentiometers, or opamps don't affect the sound at all!!! You'd be the ones benefiting the most from the “treatment” prescribed here, provided you approach these changes without the intent of proving “nothing changes”, because you might just confirm your theory... by forgetting to connect the speakers to the amplifier.

Yes, you, dear reader, who have always wanted to pick up a soldering iron but, out of fear of shocks or wasting money, never dared. This is the perfect opportunity to take your first steps into DIY and start this journey. And you know what? It doesn't even matter where it takes you — near or far — because it will be an extraordinary journey either way. I promise you.

The tests and listening sessions were conducted using my main audio system. The Fosi, configured as a power amplifier, was driven either directly by my DAC or through the DIY BAF2018 preamplifier designed by Wayne Colburn. The speakers are three-way: low frequencies are handled by a Faital Pro 8PR200 in a 16-liter net bass-reflex enclosure, while a 6FE200 (also by Faital Pro) serves as the midrange, and the ribbon tweeter is a Monacor RBT-35SR. The midrange and high-frequency sections are configured in open baffle.

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