Introduction: THR10 Effect Loop Modification

The THR10 YAMAHA is a great little amp (for me, so far, The Greatest) but doesn't have the Send/Return (Loop FX) connection.

This connection basically takes the output of the pre-amp, send it to an external Effect Chain (for example a Loop Station) and receive the signal back to feed the power amp.

First thing first.

I followed the logic that the THR10 is a Guitar Amplifier! It seems stupid statement but, it is what I used to realize my modification and… it worked.

As any Guitar Amplifier there is a preamp and a power amp. I started to found the latest.

The power amplifier for the THR10 is the tiny little IC (Integrated Circuit) called YDA147 capable of 2 x 15W and other amazing performance: I used to be an Electronic Engineer, very old fashion guy, I can be good in building some tube amp - as I did - and understand discrete components amp. Reading the features of this small beast I realized how technology has developed! Have a look at the full data sheet:

Through the YDA147 data sheet I found that the input of this power amp is on pins 5 and 9 and these inputs need to be connected through a DC-cut capacitor of 1uF.

Step 1: Discover Your THR10

On the other side I have to find a preamp or, actually, an audio Digital-to-Analog Converter (DAC). So I found another little jewell on the circuit board: the 24-Bit, 192-kHzSampling, Enhanced Multilevel, Delta-Sigma, Audio Digital-to-Analog Converter PCM1781. Amazing performance again:

The output of this DAC needs a Low-Pass Filter to eliminate the unwanted noise generated by the Delta-Sigma conversion (don’t worry if this sounds too complicated, I studied Digital Electronic 30 years ago and, at that time, the concepts were only complicated theory for us) and provide sufficient out-of-band noise rejection: in simple words, to have a clean signal output to feed properly the power amp!

If you are interested to some theory (I have enjoyed the reading) have a look at:

It looks like Yamaha opted for simple and effective Low-Pass RC First-Order Passive Filters.

Step 2: The Circuit Diagram...

After this research and hours of patience to follow the circuit diagram with my old Fluke Multimeter, I have spotted the parts I need to modify: the following diagram looks simple (and it is extremely simple indeed!) but it has required to understand first what you need, follow some logic and have the approach to simplify what appears complex!

This is it, could be not accurate but is sufficient to understand and made the modification:

Step 3: PCB View...

and this is how it looks on the Printed Circuit Board (PCB):

Step 4: The Modification...

The modification I made is a little bit dangerous because required that you have basic electronic circuits knowledge and that you are skilled in managing welding SMD (Surface Mount Devices), small electronic components…

When you open the amp and have a look at the size of these components on the PCB you’ll realize how tricky could be!

You can have a look at this for some hints:

The modification definitively void your warranty!

In other words: you need to be brave enough to try and, like me, hard wishing to have a Loop to play when you feel alone!!!

I cannot take any responsibility if you succeed or not… but I hope you do!

The modification, in theory, is quite simple: you have to remove the little capacitors C140 and C143 (don’t forget your amp is stereo - the two capacitors are circled in the previous picture) and connect a shielded cable from the side of R174/R176/C138 to a DIN 5pins connector (5) as Output R and the side of R175/R177/C139 to the (4) Output L. (strange enough the R and L are reverted compared to the data sheet you find on internet). I connected the shield (2) on the ground screw marked “B” visible on the next picture.

Step 5: PCB Modification...

On the same DIN connector you have to solder terminal (3) to a small capacitor (1uF - ceramic marked 105). The other terminal of the capacitor has to be connected to the original terminal on the printed circuit board left free from the capacitor C140 you have removed. Same operation on the pin (1) for the capacitor C143.

That’s it!

I have chosen a 5 Pin DIN Female Panel Mount Connector for 2 main reasons: first it was what I had available in my drawers, second it’s small enough to fit inside the THR10!

Step 6: Cable...

You have to prepare a cable with 4 shielded conductor to go on the stereo inputs and outputs of a stereo Loop Station: I have the BOSS RC-1 and works very well.

As you can see i have adapted an old Playstation cable!!!

It’s working surprising fine but I recommend a good shielded cable: don’t forget your signal is going all around this cable and it could be noisy!

Last, but not least, I made a fake male connector DIN 5 pins where I short-circuited IN and OUT for Left (1-4) and the same for Right (3-5) pins: this dummy connector could be installed instead of the cable if you don't need the Loop.

If you remove the cable you won’t hear any sound because the pre will be disconnected from the power amp.

Strong recommendation:

NEVER plug IN or OUT the cable or the dummy connector when the THR10 is ON.

ALWAYS perform these operation with your amp OFF.

Of course you can make more elegant solutions with a small switch or using connectors with internal switches but I found this solution simpler and, again, the DIN connectors were the only ones I had in my hand! Anyway be alerted on my recommendation to avoid damage on the THR10.

Step 7: Final Notes

I found the THR10 a magnificent amp and, going though this modification, I realized that Yamaha has used state of art of available technology. This explain the Quality of the technical performance.

I believe Yamaha has used Musicians to develop this and the other THR amps: this explain the musical performances.

Well done!

I think the Loop feature is essential to complete the THR amps (…Yamaha did it on the 100!) and, as you have seen, is so simple, in a way, that Yamaha should think to integrate it in the next generation!

Even better: considering that all the the sound elaboration is made in the digital domain (signal from Guitar is transformed immediately in digital though ADC PCM1803 and elaborated though some Yamaha excellent circuits…) why don’t install some memory chips to have the Loop Station on board, integrated in the THR10 itself! Won’t cost a fortune, if done in the original design.

Or, maybe, some young Engineer familiar with digital, could make the modification I have done on the analog part, on the digital side extracting the 1010101010 before the pin 6 of the PCM1781 and send it to a modified BOSS RC-1…

Food for thoughts.

Finally: I made the modification for the only scope to enjoy it at personal level. I hope that sharing this with other don't infringe any Copyright or any other kind of Right.

All the information that I needed are freely available on internet.

If you don’t feel confident to do the modification by yourself don’t do it: you could seriously damage your THR10.

You can provide the simplified electrical diagram of the modification to some trusted technician that will laugh at me for the approximation of my terminology but, for sure, can make a much better work than me with soldering!

To test that the modification was working, the first number I have played has been “Oye Como Va” in the Carlos Santana version. Just Am7 and D7 in loop forever…


ähöüokul (author)2016-05-09

Nicely written and extremely helpful! You've got my vote!

DIY Hacks and How Tos (author)2016-05-08

Welcome to Instructables. Thanks for sharing the cool project.

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