FX LOOP for Katana 50 finally added!

Started by ColorMaker, August 21, 2021, 08:40:01 AM

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   I have heard more than once that the Katana 50 is very good, compact and lightweight amp. But many note the lack of an FX Loop as a major drawback.
The question "How to add a loop to a Katana 50" is quite popular. There were some considerations on this, but most of them, for some reason, were in a negative way, completely not supporting the principle of "Do it yourself".
I bought my amp in advance knowing that I would be adding FX Loop functionality to it. Although I did not know in advance if it was possible to implement it in such a way that it was practical and functional enough. The network asked questions about how the amplifier is arranged inside, but there were no answers. Only a few photos with a general view of some of the circuit boards, according to which it was impossible to draw any conclusions.
I spent some time, sketched some diagrams of the analog part and as a result completely solved my task - connecting a guitar looper pedal to a my Katana 50. I think when someone says that they would like to have an FX Loop in a combo amplifier, most often they mean it is exactly the ability to connect a looper pedal (Boss RC-1, Ditto Looper etc.).
However, almost any other effects unit can be connected to the device I made.
I will describe some features:
The Master knob does not affect the volume of the signal in the Loop.
The Power Control knob also does not affect the volume in the Loop circuit.
The Loop works with both speaker and headphones.
There is a switch for the level of the sent signal -12dB
The Loop does not affect the sound quality.
It is safe.

Some disadvantages include the following:
The Loop signal cannot be transferred to the PC via USB.
There is no way to use the Katana reverb and delay after the Loop (For Katana 100 Tone Studio allows you to change the position of the reverb in the chain relative to the effect loop. In my version, the effects loop is always after the reverb).
Depending on whether the headphones are connected, the Looper records either the sound prepared Katana for (Speaker Simulator, Air Feel) the headphone or prepared for output to the speaker. This implies that a recording made with headphones connected will sound unpredictable when played through the Katana speaker. Conversely, a recording made without using headphones will sound rather poor on headphones (also on a PC if your looper has the ability to export recordings to a PC).
The -12dB signal level switch first lowers the signal sent to the Looper pedal, and then raises it to the previous level (so as not to affect the playback volume through the speaker or headphones), which means that the recording and playback volume of the Looper depends on the position of the switch. That is, switching -12dB does not affect the volume of the guitar, but it does affect the playback volume of the Looper. If you do not understand me then do not disturb. This is just a small feature of the many FX Loop.
This is not a problems at all in my opinion.

The -12dB switch is needed because the Distortion effect at a high preamp volume will overload the input of any external effect (with the exception of some studio effects), since it was not originally intended for output outside the amplifier. On the other hand, many stompboxes make a lot of noise, so when using the Clean amp channel it is best not to lower the signal going into the looper to increase the signal-to-noise ratio. Use Overdrive / Distortion = turn on -12dB. Play Clean = keep the FX Loop high. A potentiometer is often used, but they bring crackling problems with poor contact, so I just used a high quality mini toggle switch.

I placed the circuit board in a separate shielded case (model plastic covered with copper foil from the inside) covered with something like Tolex. I could place the board inside the amplifier by drilling holes in the case, but then it will be near the power transformer, which can cause additional hum. I also think it is important that I can easily remove my device, returning the katana to its original state. In addition, it made setup and testing a lot easier for me.

The idea of ��installing the missing parts on the Katana's circuit board (50W and 100W boards are identical) is good, but will it work? Tone Studio sends commands to control the FX Loop to the Katana 50 too, but from what I understand (correct me if I'm wrong) it doesn't send anything back. So I doubt it will work. The ADC chip is difficult to obtain and expensive.

In the course of my work, I also tried to figure out how the Power Control works. But I have not researched the circuitry of this node in detail.
Since it adjusts the volume after the Master, and my device takes the signal before the Master, then you can record the signal to the Looper without changing the volume, but the sound will change (if the Katana's MCU is actually doing something with the sound).
But, I could not hear the difference in sound. I tried very hard to hear. I'm not an audiophile, but I'm not deaf either, and I have excellent hearing. Apparently, unfortunately, the Power Control knob only changes the volume. However, it is possible that by changing the gain of the power amplifier, the Katana simultaneously introduces a frequency correction into the feedback loop. It is possible.
The official BOSS website states: "Power Control for achieving cranked-amp tone and response at low volumes." I do not accuse anyone of lying, but let them prove the opposite to me, because this function is declared among the main advantages and the benefits of using it should be obvious! I don't see anything other than the convenience of using the volume knob and the MUTE mode (yes, the panel knob signed as the STANDBY, but it includes exactly the MUTE mode of the power amplifier IC, which has both the STANDBY mode and the MUTE mode. What's the difference between them? The datasheet doesn't talk about it).
I leave this question for you. It is still interesting.

A simplified diagram looks like this:

So I spend about 25$ for quality input jacks, switch and other parts and got the FX LOOP, the guitar input board schematic, the power amp board schematic, the analog part of main board schematic, and the pleasure of the work done.
It's easier than it might seem at first glance!
DIY is fun guys. Marketing is evil.

After what I wrote, I think many can easily draw all the necessary conclusions to repeat this project.


(UPDATE: Correction of a simplified diagram in AUX junction)


I've been waiting for someone to figure this out! The katana was gifted to me and I like it exactly as is. So the whole "It's easier to sell the 50 and buy the 100 with the loop built-in" doesn't apply to me. I've been wanting to add the effects loop so I can use a looper pedal and have all the distortion/effects pre-looper.

I've sent a PM asking for more details!


Hi Steven!
I understand your expectation, since I myself waited a whole year until the warranty expired and I could start working on this project :)
I answered you in a PM.


Hi I am interested in this mod, need help tho.







Thank you for providing this info.
However, the manual.pdf file that i unzipped does not contain all the diagrams.
Anyone else having the same problem?


I've just stumbled across this, it sounds fantastic! I would be interested in trying to implement this as well, would I be able to get a copy of the plans?


I too came across this post recently, and I'm as well interested in any further info - bcguitars74 if You are getting further with the project I would like to catch up and possibly share experiences :)

Andy Pro

Hi ColorMaker,

My internet search lead me to this post. I like what you have done and I love my Boss Katana MII 50 except for the lack of the effects loop feature. I really don't want to go out and replace it for the 100 when the 50 does everything I need and is more than loud enough for me.

I would love to know more about this project please.


Hi, it seems to be great ! Could you please send me the details for implémentation in PM ?
Thank you son much !


Thank you for the solution. May I ask for more detailed instrucions how to implement this? Thanks!


Hello Color Maker!
Nice job! Grats!
Could you send me more detailed blueprint and description? I'm really interested also!
Thanks in advance!