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Messages - dimkasta

#16
This post is updated with my progress

That's my second ongoing small guitar amp project after the Fender Champion 40.

I just love the small pathfinder. With a few mods and a cab it sounds very nice. Plus it's a DIYer's dream, being a full old-school circuit with a transformer PSU and all through-hole parts. Keep in mind that the pcb is crappy though. It's easy to lift pads if you are not careful.

The schematic has been around for almost a decade, but there still might be things that need to be fixed.
https://imgur.com/gallery/IneD17t

My goal is to have a nice super-clean platform to use with my pedals.

So here is the list of my TODO stuff, gathering info from around the internet, plus results so far. Ssguitar and tdpri forums have much useful info.

[] Remove the hard clipping LEDs. This is the first mod that anyone should do. Just snip the LEDs and you immediately get rid of that "bees in a can" sound when the boost is on. Highly recommended mod. The only issue with it is that when the boost is on and the gain is set high, the amp is VERY loud. The volume goes from 0 to OMG very fast.

[] Change the tonestack to match the voicing of classic VOX circuits. Post 8 describes the effect and changed parts/values https://www.ssguitar.com/index.php?topic=5034.msg40409#msg40409

[] Change the last stage into a buffer.  I removed R15, R16, R17, C12 and C13, and then bridged R15 and R17 so that pins 1 and 2 of the opamp are connected. This results in a much more manageable volume. It can still go loud if you want. You might feel like you are not getting enough speed for how much you punch the gas pedal, but the top volume is still there and with a powerful pickup you can still get full volume if you need it. Removing C12 also removes a lowpass filter that kept the amp muffled (as described below).

[?] This change is rather obsolete now that we know that the last stage can be changed into a buffer without affecting the tone negatively. The original mod was to change the linear volume pot P4 to one with an audio taper. The difference was more evident without the boost on. When you engage the boost, the amp still went loud fast, but you got some more volume resolution than before.

[] Add a 1/4 jack to allow connection to a CAB. This is super easy and very straightforward.

[]  Removing C12. This cap acts as a low-pass filter on the final stage. It rounds the sound a lot, and it kills lots of clarity. Without it, when the boost is on, the sound gets a bit harsh. But the clean sound is just amazing. Something especially evident in harmonics and pick attack. Perhaps it makes sense to experiment with different values and types here to suit your taste, but without it, you get a nice percussive and super-rich sound that is almost JazzChorus-like.

[]  Replace the plastic boost switch with a DPDT toggle switch. My switch was getting scratchy. The panel now looks much better :) The switch fits only if it's mounted on its side so that it flips horizontally. You do not have to solder all 6 pads. The top right is not connected to anything, and the bottom left is no longer used since we removed the LEDs.

[] Eliminated the boost. I will repurpose this at some point, but so far my goal is to make everything work through the gain knob

[] Removing R8. I have tried this but I am not sold on it. It gives another boost in gain on the cleans that is immediately audible as extra noise. And it seems to also affect the voicing a bit since it includes C8 and R9 in the feedback loop (probably to compensate for adding R5 on the first stage).
I restored this since I do not need any more gain on the clean channel and I am not sure I like the change in tone.
This is going to be part of a bigger change to rearrange the gain structure of the amp

[ ? ] Change C28 to a bigger cap. I was wondering how starved the chipamp could be without some solid power capacitance close to it. So I changed C28 to a 1000uF one. I have not used a cap this way before so I was not sure about the effect. The truth is that I did not hear any noticeable difference. I will revert it in a few days to check again.

[] Add a simple series FX loop right before the power amp. This should be easy with two switching jacks. And we probably do not need a buffer either.

[ - ] TODO: Experiment with R27 and R28 to see how lower or higher rails on the preamp could make the amp's volume range more usable.

[ - ] TODO: Add some soft clipping diodes to get some mild clipping when digging in. I will try some stuff, but I am not 100% sold on this. A good pedal in the front or in the FX loop makes more sense.
This again will be part of a bigger rearrangement of the gain structure

[ - ] TODO: 100nF ceramic bypass capacitors between the power rails on all opamps. This might not be critical, but I have seen a big difference in other applications, so I plan to try it. It's cheap and very easy to do on the underside of the pcb. It could make a bigger difference if you plan on using the amp on higher gains, especially on the first stages.

[ - ] TODO: In general, experiment with PSU optimizations, like CRC, better rectifier diodes, a quasimodo snubber, perhaps a bigger transformer with lower V to reduce the Wattage and make it more home-friendly

[ - ] TODO: General experimentation with different caps and resistors on various positions. I am not going fancy, just going to try different parts that I have in my stash. Changing the gain might not be easy without changing the voicing though.
#17
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.
#18
Aaaaah
:D I didn't even notice it
#19
No pictures? How come?

Anyway, an easy win should be right at the cable coming from the front-end to the amp.
#20
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
#21
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?