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Author Topic: Fender Champion 40 as a modding platform  (Read 1944 times)

dimkasta

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Fender Champion 40 as a modding platform
« on: April 23, 2021, 11:05:45 AM »
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?

« Last Edit: May 09, 2021, 09:42:36 AM by dimkasta »

edvard

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Re: Fender Champion 40 as a modding platform
« Reply #1 on: April 24, 2021, 03:59:08 PM »
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

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Re: Fender Champion 40 as a modding platform
« Reply #2 on: April 24, 2021, 04:55:47 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.

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
« Last Edit: April 24, 2021, 05:00:08 PM by dimkasta »

joecool85

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Re: Fender Champion 40 as a modding platform
« Reply #3 on: April 26, 2021, 01:54:34 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!
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flester

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Re: Fender Champion 40 as a modding platform
« Reply #4 on: April 28, 2021, 09:21:37 AM »
Where would the low ass filter go?

DrGonz78

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Re: Fender Champion 40 as a modding platform
« Reply #5 on: April 28, 2021, 03:09:00 PM »
Where would the low ass filter go?

I can only guess...??? But please don't post any pictures of it.
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dimkasta

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Re: Fender Champion 40 as a modding platform
« Reply #6 on: April 28, 2021, 03:30:35 PM »
No pictures? How come?

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

Enzo

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Re: Fender Champion 40 as a modding platform
« Reply #7 on: April 28, 2021, 08:39:51 PM »
No pictures?   Reread your typo in your previous post.

g1

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Re: Fender Champion 40 as a modding platform
« Reply #8 on: April 28, 2021, 09:44:21 PM »
Typo was in Flester's post.   :)

dimkasta

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Re: Fender Champion 40 as a modding platform
« Reply #9 on: April 29, 2021, 03:47:13 AM »
Aaaaah
:D I didn't even notice it

dimkasta

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Re: Fender Champion 40 as a modding platform
« Reply #10 on: May 09, 2021, 09:19:47 AM »
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.
« Last Edit: May 09, 2021, 10:11:34 AM by dimkasta »