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Fender Champion 40 as a modding platform

Started by dimkasta, April 23, 2021, 11:05:45 AM

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dimkasta

I just bought this little amp for a very good price. The first impressions are very positive. It looks nice, it has a 12" speaker, the sound is decent and it's far louder than what is acceptable at home.

However...

It has a very audible hiss. It's irrelevant while playing, but if you leave it on while you do other things it gets annoying.

The guts are a bit disappointing. SMPS and a TDA7294 circuit on the same board. And both channels go through the DSP on the front side.
I am afraid it's one of those devices that are destined to end up in a landfill after 5-6 years (unless you repurpose it or something).

The nice thing is that the chassis is roomy with lots of space for custom stuff.

Careful how you handle the board. SMPS parts could carry lethal voltages even when disconnected from mains.

So the DIY/mod plans so far are

[] Add a 1/4 jack/plug so that I can reuse it as a cab
[] (Abandoned) Replace R409 with a voltage divider or a pot as a master volume to push the hiss down and make the volume range more home-friendly. Update: R409 is an smd part at the bottom side. Again, careful how you handle the board. I will not bother with this. The hiss and lower volume will be handled by the FX loop where I like having a permanent EQ pedal. If you do not care for the FX loop, a buffered 47K pot right at the cable coming from the front-end will do the trick.
[  ] Eventually change the speaker into something nicer
[  ] Replace the SMPS with a nice lower V chunky transformer (and glass fuses), a normal and properly filtered bridge rectifier, and nice big CRC filters.
[  ] Replace the 7x15 regulators with nicer ones with lower impedance and less noise.
[  ] Replace the TDA7294 circuit with an LM1875 at 20W (or even less, I will have to coordinate the PSU first) and proper beefy capacitance
[  ] Change the crappy plastic switches/buttons with nicer metal ones
[  ] Add a buffered FX loop
[  ] Prepare for replacement preamps or something for when the DSP goes to the big gig in the sky

Any other suggestions?


edvard

I think all the mods you're thinking of nails it pretty good.  I'd say crank it until it fries, then get busy re-making it, though it might last until next week or it might last 20 more years, who knows?  I also think what you really got was not a modding platform, but a cab, speaker, and chassis to build your own in.   8)

Not sure if the SMPS or the DSP or the regulators are the source of the hiss, but I agree that any of them could be the culprit, but it also could be your speaker is an inexpensive variety better suited for hi-fi applications.  The next best thing I can think of at the moment to do for that is suss out a good place to wedge in a 2nd-order low-pass filter around 5-6kHz; most guitar speakers won't go much over that, so you wouldn't be missing much high end. 

If none of those work, gut it and heat up your soldering iron...   :trouble

dimkasta

#2
Quote from: edvard on April 24, 2021, 03:59:08 PM
I also think what you really got was not a modding platform, but a cab, speaker, and chassis to build your own in.   8)

Yeah more or less :)
Although the DSP does have some very usable voices. I hope it does last a bit.

Quote from: edvard on April 24, 2021, 03:59:08 PM
Not sure if the SMPS or the DSP or the regulators are the source of the hiss, but I agree that any of them could be the culprit, but it also could be your speaker is an inexpensive variety better suited for hi-fi applications.  The next best thing I can think of at the moment to do for that is suss out a good place to wedge in a 2nd-order low-pass filter around 5-6kHz; most guitar speakers won't go much over that, so you wouldn't be missing much high end. 

I have already added the 1/4 socket and plug and there is no hiss from the speaker while playing through it with the other amps. There is hiss coming from the other cabs though when I plug the champion 40 into them.

What might be the problem causing the hiss is that the amp has no master volume. This means that the output of the DSP (including noise) goes straight into the TDA amp and amplified at full DB all the time. The good thing is that the hiss is not altered as you change the channel volumes.

After checking the schematic, a very convenient solution could be to replace the TDA's 47K input impedance resistor R409 with a 25K/25K voltage divider (or a pot). This will push the hiss close to the noise floor and significantly increase the SNR ratio. And as an extra benefit, the amp is going to have a much more home-friendly volume range (currently 4 is my max)

I'll keep updating the first post with some progress

joecool85

Quote from: dimkasta on April 24, 2021, 04:55:47 PM
After checking the schematic, a very convenient solution could be to replace the TDA's 47K input impedance resistor R409 with a 25K/25K voltage divider (or a pot). This will push the hiss close to the noise floor and significantly increase the SNR ratio. And as an extra benefit, the amp is going to have a much more home-friendly volume range (currently 4 is my max)

I'll keep updating the first post with some progress

This sounds like a very good idea!  Let us know how it turns out!
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

flester


DrGonz78

Quote from: flester on April 28, 2021, 09:21:37 AM
Where would the low ass filter go?

I can only guess...??? But please don't post any pictures of it.
"A person who never made a mistake never tried anything new." -Albert Einstein

dimkasta

No pictures? How come?

Anyway, an easy win should be right at the cable coming from the front-end to the amp.

Enzo

No pictures?   Reread your typo in your previous post.

g1


dimkasta


dimkasta

#10
I finally found some time to work on the amp a bit more.

I tracked down R409 on the main PCB. But after all, the entire thing is mostly SMD parts at the bottom side. Combined with the entire high voltage SMPS stuff, this is a bit more difficult and dangerous than what the average DIYer can or should do.

So I won't bother with this after all. The hiss and lower volume will be covered by the FX loop where I usually add an EQ pedal.

If you do not care for the FX loop, a buffered 47K pot right at the cable coming from the front-end will do the trick.

dimkasta

Sorry everyone I was super busy and did not have much time to play. But we have lots of progress this weekend.

Here is what I did

You cannot view this attachment.

I added two switching quarter inch sockets. The right one can be used to get the signal from the DSP, and the left one to feed the signal to the power stage. Of course you can use them separately as you want. And with nothing connected, the thing defaults to feeding the DSP to the amp.

So this works like an unbuffered simple FX loop. Keep in mind that both the DSP and the power amp are connected to the ground, so you might have to fix ground loops. I only tried this with a battery powered pedal.

The main culprit for the hiss unfortunately is the DSP pcb. I do not know if it comes from the PSU or from the design itself, or from the buffer stages (yet). There is still some noise on the power stage, but it's significantly lower, and for me it was lower than the noise from my guitars.

Testing the power amp with my mojomojo in the middle like a buffered volume, I was surprised how much better the speaker was behaving. It is now ringing loudly and with much authority. Which further points that the output stages of the DSP are also probably weak.
And of course, the volume is now much more manageable for home-use

I do not know how I will proceed. I will play a bit more with it using my preamp pedals and see how it goes. It was already working nicely as a plain cab (for example with my VOX pathfinder 10). But it's power amp is powerful and behaves nicely even at low volumes. At this point I think that its best bet is to work like a powered cab for pedals.

Here is the mod list updated and re-ordered

[] Add a 1/4 jack/plug so that I can reuse it as a cab
[] Add an unbuffered FX loop. This was huge with a pedal as a powered volume. It's surprising this way, and it might even make sense to use as a powered cabin.
[ ?? ] Prepare for replacement preamps or something for when the DSP goes to the big gig in the sky. Again, the loop gives nice options to use the amp as a final stage with some kind of preamp pedal. I will probably eventually end up doing just this.
[ ] Replace R409 with a voltage divider or a pot as a master volume to push the hiss down and make the volume range more home-friendly.  I abandoned this, since this can be covered by either an EQ pedal in the FX loop, or a buffered 47K pot right at the cable from the front-end).
[    ] Eventually change the speaker into something nicer. I will eventually get a V30, but with the FX loop and a buffer in between, the speaker is much better driven and it works surprisingly nicely as it is.
[    ] Change the crappy plastic switches/buttons with nicer metal ones

The rest are currently probably abandoned. It's probably not worth it.

[    ] Replace the SMPS with a nice lower V chunky transformer (and glass fuses), a normal and properly filtered bridge rectifier, and nice big CRC filters.
[    ] Check if the DSP for noise and performance in isolation. And check if its stages can be improved.
[    ] Replace the 7x15 regulators with nicer ones with lower impedance and less noise.
[    ] Replace the TDA7294 circuit with an LM1875 at 20W (or even less, I will have to coordinate the PSU first) and proper beefy capacitance