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Vox Pathfinder 10 as a modding platform

Started by dimkasta, May 09, 2021, 05:52:05 PM

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dimkasta

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.

phatt

If you can find space on the panel you can always replace R7 with a pot and re-solder the Leds.
That will give you much more control on how hard the leds (Or diodes if you prefer) activate.
I've never had this amp so just a thought.
Cheers, Phil.

mguzzo

#2
lots of good tips there. I haven't modded any of the circuit on mine yet. I did install a Celestion 8" in place of the 6.5" bulldog speaker. It fit quite well in the cabinet. My son has been using it with his string bass for small gigs (church, musical pit), the clean channel and speaker handle the string bass tone quite well in those small settings. My plan is to replace the 2030 with a 2040 and put in a larger transformer to drive a little more wattage.

what's the difference between adding a 1/4" for driving an external cabinet v. the headphone/line out jack?
this is not the sig you're looking for

phatt

The Difference between the 2 chips is not worth the effort.
Your ears would hardly be able to notice the Difference between 10 Watts and 15 Watts.
Those chips will now likely be hard to find anyway so *Just buy a bigger amp*.
Re the headphone out. Well it's exactly what it says,,
for driving headphones,,, different Z and Voltage.
If you plug in a speaker you won't hear much. :-[
Phil.

joecool85

Great job!  I love modding these small amps, plus the Vox Pathfinder 10 looks mint!
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

dimkasta

Quote from: mguzzo on May 11, 2021, 12:24:36 PM
My plan is to replace the 2030 with a 2040 and put in a larger transformer to drive a little more wattage.

As Phil said, it's not really worth it. The 2030A is already at its max with +-20V, being able to give around 18W. That's plenty loud for a guitar unless you are playing big venues, in which case the limit is the speaker.

The easiest and probably the most effective way to get more output is to add the output jack and hook it into a cab with a more sensitive speaker. Even a 2x12.



LM1875 could be a good candidate to get more power. But it will need a new PSU and probably a beefy heatsink. For now, this is probably "going too far" territory for me.

For bass, the preamp is too mid-focused. Perhaps the FX loop would be a solution for you so that you can bypass the preamp with its highpass filtering and drive the output directly with a bass preamp or pedal. But to be honest for bass I would just get a fender rumble and call it a day.

Quote from: mguzzo on May 11, 2021, 12:24:36 PM
what's the difference between adding a 1/4" for driving an external cabinet v. the headphone/line out jack?

The headphone output has series 1W resistors that limit the power to levels adequate for headphones.

dimkasta

By the way, about R8, I just restored it and I will probably keep it as is

I also added another TODO point, to experiment with R27 and R28 to see if I can get a more usable volume and gain range

dimkasta

After one year, I finally found some time to add the unbuffered FX loop.

It's basically two quarter inch switching sockets.

I did not take a photo and I am a bit bored now, but you can see how it works on my Champion 40.

The right one gets the signal from the preamp, and the left one returns it to the power amp. You can use both independently. And with nothing connected, the thing defaults to pass the preamp to the power amp.



I have not managed to play with it a lot yet. I will post more (including photos) when I have some more time.

dimkasta

OK I have tried the modified tone stack from the other thread.
Without messing with the output of the previous stage and the loading of the next, this gives  a curve that is spot on to the classic VOX. The pots do not track the same, but it's close enough.

More explanation here
https://www.ssguitar.com/index.php?topic=5153.0

What I did was replace the following

R10 - 15K
R11 - Removed and added a wire from its C11 pad, towards the junction between R12 and P3-pin3
R12 - 1K
C9  - 1nF
C10 - 560nF
C11 - 150nF

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The result was spectacular, especially in the clarity of the amp and the highs.

Just be careful with the PCB. It's very crappy and it's very easy to lift pads if you overdo it. If you do not have a proper soldering tool, then perhaps it would be easier to cut the parts and then solder the new ones point to point on top of the potentiometers. Especially if you plan to experiment with different parts.

dimkasta

#9
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. 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?

dimkasta

#10
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.

dimkasta

#11
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.


dimkasta

Short update about my progress. I am currently going too far. I have designed my own PCBs and face plate. I will probably open a new thread when I have some more to show. Unfortunately PCB fabrication takes too long and design changes hurt both in time and money

Just a teaser for now

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