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.
The schematic has been around for almost a decade, but there still might be things that need to be fixed.
Vox Pathfinder 10 schematic (rev.D 4-28-2021) - Album on Imgur
My goal is to have a nice super-clean platform to use with my pedals. I do not expect it to clip or sound like a tube amp on its own, but it can sure sound awesome with 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.
This brings me to my second recommended mod.
[✔] Change the linear volume pot P4 to one with an audio taper. The difference is more evident without the boost on. When you engage the boost, the amp still goes loud fast, but you get 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.
[✘] 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 will probably restore this since I do not need any more gain on the clean channel and I am not sure I like the change in tone.
[ ? ] 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.
[ - ] 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.
[ - ] TODO: Eliminate the boost circuit and repurpose the switch to control the clipping diodes.
[ - ] 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: 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: 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.
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.
The schematic has been around for almost a decade, but there still might be things that need to be fixed.
Vox Pathfinder 10 schematic (rev.D 4-28-2021) - Album on Imgur
My goal is to have a nice super-clean platform to use with my pedals. I do not expect it to clip or sound like a tube amp on its own, but it can sure sound awesome with 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.
This brings me to my second recommended mod.
[✔] Change the linear volume pot P4 to one with an audio taper. The difference is more evident without the boost on. When you engage the boost, the amp still goes loud fast, but you get 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.
[✘] 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 will probably restore this since I do not need any more gain on the clean channel and I am not sure I like the change in tone.
[ ? ] 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.
[ - ] 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.
[ - ] TODO: Eliminate the boost circuit and repurpose the switch to control the clipping diodes.
[ - ] 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: 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: 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.
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IMHO perhaps all that is overkill.
Removing the clipping diodes and external jack a speaker is all you need. The little 6.5” speaker is horrid.
Even into a celestion 10 /30 in a blues junior style cab the amp shines a lot more.
Remember that a lot of pedals are designed to work well with amps on the edge of breakup etc and delays and reverbs don’t have to be HiFi to sound good.
Be interested to see what you think was worth the effort when done.
Removing the clipping diodes and external jack a speaker is all you need. The little 6.5” speaker is horrid.
Even into a celestion 10 /30 in a blues junior style cab the amp shines a lot more.
Remember that a lot of pedals are designed to work well with amps on the edge of breakup etc and delays and reverbs don’t have to be HiFi to sound good.
Be interested to see what you think was worth the effort when done.
Fair enough, but don't forget that most of this is playing with electronics. It does not have to give you the best sound. Even the experimentation is enough reward on its own.
I would add experimentation with C12 to this. The difference is so profound. Even with the crappy 6" speaker. Especially if you like very dynamic fingerstyle playing, removing the cap is just bliss. But it does sound a bit hifi, and you need a pedal that can work on its own to give you overdrive that is nice and round.
Fair enough
With regards to the can of bees clipping, I would suggest looking at removing the LEDs and make a little perfboard circuit to wire into that section. As it is in the stock circuit R9 220 Ohm and then the LED's shunt directly to ground. Change R9 to something like 2K to 10 K. Then on your perfboard wire opposing pairs of 1N4148 diodes (or put two in series on each leg for higher voltage threshold) and another variable resistor (50 K or so) in series with the opposing pairs clipper. This should soften up the distortion making it more gradual then direct shunt clipping, you could add a low value capacitor across the clipper to dampen out some of the higher frequencies.
Shanx I am currently leaning more towards adding something like the variable clipper used in VOX beattle.
In the meantime, I restored R8 and I will keep it as is.
And I added a new TODO to check how R27 and R28 could be changed to give more usable volume and gain.
In the meantime, I restored R8 and I will keep it as is.
And I added a new TODO to check how R27 and R28 could be changed to give more usable volume and gain.
Back at it after a year.
I have added an unbuffered FX loop on the little thing, and modified the tone stack to better follow the curve of classic VOX. The pots do not track exactly like the VOX ones, but it's close enough so that we don't touch the surrounding stages.
To make it clear, besides replacing values, R11 was removed, and a wire was added from its C11 pad towards the junction of R12 and P3-pin3
The result is very pleasant. Especially in how the amp responds to dynamic playing, clarity and the highs. Which is what you would want from a wannabe-Vox amp after all.
Keep in mind that the PCB is particularly crappy. It's super easy to lift pads. Especially if you want to experiment with other parts. Perhaps it's best to use some strip pcb or just go point to point
I have added an unbuffered FX loop on the little thing, and modified the tone stack to better follow the curve of classic VOX. The pots do not track exactly like the VOX ones, but it's close enough so that we don't touch the surrounding stages.
To make it clear, besides replacing values, R11 was removed, and a wire was added from its C11 pad towards the junction of R12 and P3-pin3
The result is very pleasant. Especially in how the amp responds to dynamic playing, clarity and the highs. Which is what you would want from a wannabe-Vox amp after all.
Keep in mind that the PCB is particularly crappy. It's super easy to lift pads. Especially if you want to experiment with other parts. Perhaps it's best to use some strip pcb or just go point to point
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Some thoughts on voltages and gain structure.
At this point, my amp is super clean with huge headroom. And effectively it's a 18W amp, since the TDA is working at +-20V at x13 gain, and the unrestricted preamp can saturate it easily even on low gain. This is LOUD.
A first observation is that by increasing the gain past 1 o'clock, the amp gets increasingly darker and even muddy. Something that was more evident when I tried increasing R8 to 100K and 200K to increase the clean gain. There seems to be some compensation by C8 being out of the loop when the high gain button is pressed, resulting in an additional highpass filter. In low gain setting, the cap is in the loop so the opamp compensates for its effect.
This complicates how the gain structure works. And makes it more difficult to rework the thing if we want to add some decent sounding overdrive or distortion. Especially for home volumes, since there is not enough signal level to clip even low-voltage-drop diodes. I will have to see this as a chain eventually.
A first thought is to keep clipping and overdriving between the two first opamp stages and eliminate the overdrive button. And perhaps move it to control the gain of the TDA or of the last opamp stage. The TDA is not stable below 20db gain. And it will be difficult to further reduce the gain without changing it to something like LM1875. Which might be a good idea anyway. C12 is already gone, so we could perhaps control the gain at this stage as well, without too much effort. So that we could use low output for home and boost it when we play with others and we need the full 18W.
Next we would have to perhaps decrease the voltage level of the preamp. So that bigger gain is still home-friendly and enough to bring either the opamps close to saturation/overdriving each other or enough to clip diodes. 9V sounds reasonable and we should be able to transfer some pedal tech easily.
The first thing I checked was R27 and R28. To have a voltage drop of 7V across them, this means that the entire thing is current limited to ~8.5mA. Which looks rather marginal to operate the two 4558 including losses. If we want to go to 9V output, we would have to make them 1K8, which would bring the current to 6mA, which sounds even more restricting. I will have to do some testing and measuring to see how this would work, but I think that some regulators would work better here. They might remove some character since the thing uses the impedance of the preamp to form the voltage. I don't know how significant this is going to be, but probably not enough since opamps should have enough PSRR.
Any thoughts?
At this point, my amp is super clean with huge headroom. And effectively it's a 18W amp, since the TDA is working at +-20V at x13 gain, and the unrestricted preamp can saturate it easily even on low gain. This is LOUD.
A first observation is that by increasing the gain past 1 o'clock, the amp gets increasingly darker and even muddy. Something that was more evident when I tried increasing R8 to 100K and 200K to increase the clean gain. There seems to be some compensation by C8 being out of the loop when the high gain button is pressed, resulting in an additional highpass filter. In low gain setting, the cap is in the loop so the opamp compensates for its effect.
This complicates how the gain structure works. And makes it more difficult to rework the thing if we want to add some decent sounding overdrive or distortion. Especially for home volumes, since there is not enough signal level to clip even low-voltage-drop diodes. I will have to see this as a chain eventually.
A first thought is to keep clipping and overdriving between the two first opamp stages and eliminate the overdrive button. And perhaps move it to control the gain of the TDA or of the last opamp stage. The TDA is not stable below 20db gain. And it will be difficult to further reduce the gain without changing it to something like LM1875. Which might be a good idea anyway. C12 is already gone, so we could perhaps control the gain at this stage as well, without too much effort. So that we could use low output for home and boost it when we play with others and we need the full 18W.
Next we would have to perhaps decrease the voltage level of the preamp. So that bigger gain is still home-friendly and enough to bring either the opamps close to saturation/overdriving each other or enough to clip diodes. 9V sounds reasonable and we should be able to transfer some pedal tech easily.
The first thing I checked was R27 and R28. To have a voltage drop of 7V across them, this means that the entire thing is current limited to ~8.5mA. Which looks rather marginal to operate the two 4558 including losses. If we want to go to 9V output, we would have to make them 1K8, which would bring the current to 6mA, which sounds even more restricting. I will have to do some testing and measuring to see how this would work, but I think that some regulators would work better here. They might remove some character since the thing uses the impedance of the preamp to form the voltage. I don't know how significant this is going to be, but probably not enough since opamps should have enough PSRR.
Any thoughts?
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Some more work on the boring parts of the gain structure
The power stage is a TDA2030A that runs at +-20V and with an 8ohm speaker gives 18W of power. This means 12V p2p at the speaker. And since the chip is running at 13x gain or 22dB, the max input before it clips is ~900mV. For stability and simplicity, I will avoid messing with the power section.
The tone stack is also a given quantity.
If you use stock values, there is a min attenuation of 6dB or 1.99x
If you use my changed values, there is a min attenuation of 9dB or 2.81x
This is the step that kinda defines our gain requirements for the third opamp stage.
We need max ~900mV
So to get max power, we need before the stack:
0.9 x 1.99 = 1.791V for the stock stack and
0.9 x 2.81 = 2.529V for the modified stack
These numbers are already easy to get from the two first opamp stages, and they are nicely placed in a level where we can also hard clip them near their value with various diode combinations if we want. And even get their hard clipping even at max power, while protecting the power stage from clipping the chip.
So in both cases, the 3rd opamp stage can be safely converted into a buffer. We do not really need its gain.
The extra benefit, we avoid further amplifying noise from the previous stages or introducing more.
And we should get much more home-friendly volume control, without losing max power.
So my next mod to test is removing R16, C13, C12, R15 and R17, and bridge the pads of R15 and R17 with some wire.
Note that this means an attenuation of max ~6.5x or 16dB from previous volume settings. It might feel like pushing the gas pedal and not getting enough speed. Max speed will be still the same though.
The power stage is a TDA2030A that runs at +-20V and with an 8ohm speaker gives 18W of power. This means 12V p2p at the speaker. And since the chip is running at 13x gain or 22dB, the max input before it clips is ~900mV. For stability and simplicity, I will avoid messing with the power section.
The tone stack is also a given quantity.
If you use stock values, there is a min attenuation of 6dB or 1.99x
If you use my changed values, there is a min attenuation of 9dB or 2.81x
This is the step that kinda defines our gain requirements for the third opamp stage.
We need max ~900mV
So to get max power, we need before the stack:
0.9 x 1.99 = 1.791V for the stock stack and
0.9 x 2.81 = 2.529V for the modified stack
These numbers are already easy to get from the two first opamp stages, and they are nicely placed in a level where we can also hard clip them near their value with various diode combinations if we want. And even get their hard clipping even at max power, while protecting the power stage from clipping the chip.
So in both cases, the 3rd opamp stage can be safely converted into a buffer. We do not really need its gain.
The extra benefit, we avoid further amplifying noise from the previous stages or introducing more.
And we should get much more home-friendly volume control, without losing max power.
So my next mod to test is removing R16, C13, C12, R15 and R17, and bridge the pads of R15 and R17 with some wire.
Note that this means an attenuation of max ~6.5x or 16dB from previous volume settings. It might feel like pushing the gas pedal and not getting enough speed. Max speed will be still the same though.
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Yeah I just did the change and the character did not change at all (I already had removed C12)
Again, this mod is about removing gain from the third stage, making it a buffer
So 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.
The volume is now much more easy to control for home use. I can even get some overdrive from the first stage (sounds meh) and I can keep it on civil levels, something that I could not do before. It can still get loud if you max it, but it will be much more civil.
This mod also probably makes the change of P4 to audio taper irrelevant
Keep in mind that the overall volume will be lower than previously, since I plan to increase the overall output of the 2 first stages, but have not done it yet.
If you use your p10 on loud occasions, do not do this mod yet.
Again, this mod is about removing gain from the third stage, making it a buffer
So 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.
The volume is now much more easy to control for home use. I can even get some overdrive from the first stage (sounds meh) and I can keep it on civil levels, something that I could not do before. It can still get loud if you max it, but it will be much more civil.
This mod also probably makes the change of P4 to audio taper irrelevant
Keep in mind that the overall volume will be lower than previously, since I plan to increase the overall output of the 2 first stages, but have not done it yet.
If you use your p10 on loud occasions, do not do this mod yet.
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At this point I am probably going too far. Small progress update is that I am designing new PCBs and faceplate. Unfortunately, PCB fabrication takes time and design changes hurt both in time and money. I will open a new thread when I have more to show.
For now just a teaser
For now just a teaser
Hello. Excuse me, I am a noob and can't speak technical english very well. You mean you added a send/return loop too? Could you exolain how to do this, where on the amp and what to buy? I really appreciate your work!
Sorry it has been a while, and I do not have the original pcb any more.
It was basically two switchable quarter inch sockets. The wiring is not complicated, but it's easy to get it wrong.
If I remember correctly, I added it before C14.
The first socket acts like a preamp OUT.
When nothing is plugged in, its switch is closed, and it gets the signal directly to the amplifier. When you plug a cable, the switch opens, and the signal goes to the cable.
The second switch acts like an amp IN.
When nothing is plugged in, the switch is closed and it gets the signal from the internal preamp (through the first socket's closed switch). When you plug in a cable, the switch is open, and the signal now comes from the cable.
The jacket pin of both sockets is connected to ground. Keep in mind that with complex pedal boards without floating PSUs, you will probably get ground loops.
I would also avoid plugging stuff with the volume high. There is no muting with this simple arrangement and you will probably get lots of thumping
It was basically two switchable quarter inch sockets. The wiring is not complicated, but it's easy to get it wrong.
If I remember correctly, I added it before C14.
The first socket acts like a preamp OUT.
When nothing is plugged in, its switch is closed, and it gets the signal directly to the amplifier. When you plug a cable, the switch opens, and the signal goes to the cable.
The second switch acts like an amp IN.
When nothing is plugged in, the switch is closed and it gets the signal from the internal preamp (through the first socket's closed switch). When you plug in a cable, the switch is open, and the signal now comes from the cable.
The jacket pin of both sockets is connected to ground. Keep in mind that with complex pedal boards without floating PSUs, you will probably get ground loops.
I would also avoid plugging stuff with the volume high. There is no muting with this simple arrangement and you will probably get lots of thumping
Thank you, I really appreciate your reply. Floating PSU is a power supply? It means it is not connected to other part of the wire or ground, right? Is there any way to add that, and how?
This part was referring to your pedals. If you use 9V batteries, you have nothing to worry about. If you are using some kind of PSU for them, then just make sure that it's a floating one. Most modern ones are
But it's still fun to watch from here!😉
I'm trying to avoid hacking my Vox Pathfinder 15R any further (I want to change the clipping board) as I'm also suffering from PCB damage due to the number of times I've taken it apart and put it back together again. Reverb is now intermittent🙄
Hmm...I'll have to check the tonestack vs the classic Vox curve....
Just wanted to say thanks to dimkasta, as a result of the info in this thread I have installed a FX loop in my Pathfinder 10. To confirm for anyone it works well just before the C14 cap. I've put a circle around the leg I unsoldered if anyone wants to copy my method.
