Aion's Beta preamp : Treble pot issue

[eh la bas ma]

Hello,

I am having a hard time debugging this circuit, although it should be quite simple, I hope.

I am building the Sunn Beta Lead/Bass preamp. I'd like to have both versions on one board, so I opted to add two extra 4PDT toggleswitches to be able to change 7 values in the circuit.
Musikding sent me one of the old boards, so I did the "version 1.0 C-MOS fix" as instructed.

https://aionfx.com/app/files/docs/beta_preamp_documentation.pdf

The only issue I have is that the Treble pot is acting strangely in the last few bits of the last quarter of rotation. It's happening in a narrow spot, just before being set fully CW. There is a slight increase of volume, just before the signal fades and is covered by white noises. In other words, all controls are ok except the treble control when it's set fully CW. Almost the whole rotation is fine, and the pot does what it's supposed to do.

Both Lead/Bass toggleswitches sound like everything is working fine. However I had a doubt about how to wire R23 to the toggleswitch : one side is grounded, so i am not really sure if my wiring is allright.
For every components on the toggles, I found the pad with the incoming signal, wired a blue wire going to the middle 4PDT terminal. Then, both Lead and Bass components soldered at the outer terminals, with their remaining leg connected with a wire soldered to the output pad. For R23 Both remaining legs are wired to the grounded pad, I hope it's correct ? I tried to invert both wires for R23, and didn't notice any obvious change.

I wonder what could cause the Treble pot to go off the rails in the final part of its rotation ?

What's happening, according to the schematics, when the Treble is set fully CW ?










Every help and every suggestions are welcome !

[GibsonGM]

What taper is the treble pot?  Calls for B (linear) taper; just checking. 

Totally suspect the additional switching and those 7 parts are somehow part of the problem, LOL...go back over the switch routing...follow the path thru the switching that the treble circuit is using.  (Does it do this on any switch setting?)   Those R's need to go to ground, absolutely...

[eh la bas ma]

What taper is the treble pot?  Calls for B (linear) taper; just checking. 

Totally suspect the additional switching and those 7 parts are somehow part of the problem, LOL...go back over the switch routing...follow the path thru the switching that the treble circuit is using.  (Does it do this on any switch setting?)   Those R's need to go to ground, absolutely...

Thanks for your reply !

I used a B100k, i ordered a full kit from musikding. Treble issue is always present, whatever the toggles settings.
Everything is fine until I reach 95% of rotation...


I also found out that an other experienced builder has a similar issue on the same build (newest pcb v1.1):

https://forum.pedalpcb.com/threads/aion-beta-bass-preamp.15718/#post-199422

Could it be a design flaw ?

[antonis]

Could it be a design flaw ?

You could check it by adding a 10k or so series resistor just right after Treble pot lug 3..

[ElectricDruid]

While the fact that someone else has had the problem suggests there might be a common cause, it sounds most like a dodgy pot, tbh.

One way it can happen is if the pot gets exposed to too much voltage at some point and a lot of current flows. That can burn holes in the carbon track and leave dead spots. That typically happens very close to the ends when the resistance is getting small and the current goes up.

Perhaps there's some potential failure mode of this circuit that makes that more likely, I don't know. But I'd start by taking the pot out and testing it with a multimeter to check you get solid readings around the whole scale. If you don't and it still goes weird at the end, you know the fault is the pot and not the circuit. If the pot's fine, we need to keep digging!

HTH

[eh la bas ma]

Could it be a design flaw ?

You could check it by adding a 10k or so series resistor just right after Treble pot lug 3..

Thanks !

The 10k fixed it !

[eh la bas ma]

While the fact that someone else has had the problem suggests there might be a common cause, it sounds most like a dodgy pot, tbh.

One way it can happen is if the pot gets exposed to too much voltage at some point and a lot of current flows. That can burn holes in the carbon track and leave dead spots. That typically happens very close to the ends when the resistance is getting small and the current goes up.

Perhaps there's some potential failure mode of this circuit that makes that more likely, I don't know. But I'd start by taking the pot out and testing it with a multimeter to check you get solid readings around the whole scale. If you don't and it still goes weird at the end, you know the fault is the pot and not the circuit. If the pot's fine, we need to keep digging!

HTH

Thanks,

I measured the pot alone, red probe on middle lug, black on lug 3. Smoothly goes from 102.9k to 5.6R as I slowly turned it CW. Nothing strange to report here.

The noises were quite frightening, fast popping noises followed by loud decaying white noises, and popping noises again... like the speaker is going to be spat out of the room in smokes.

Order and good morals are restored, thanks to this 10k resistor.

[antonis]

Thanks !
The 10k fixed it !

Well done..

Now, delete that 10k resistor and make R31 12k to 15k..
(it should alter HPF's corner frequency but let it be for the time being..)

P.S.
If it works, you'll not have to mode the PCB..

[eh la bas ma]

Well done..

Now, delete that 10k resistor and make R31 12k to 15k..
(it should alter HPF's corner frequency but let it be for the time being..)

P.S.
If it works, you'll not have to mode the PCB..

I soldered the 10k in series with the 3.3k at R31.

When Gain is high, above 12' : No white noises, no pops, there is a volume boost and some low or low-mid frequencies can appear just before fully CW. Fully CW there are a bit of white noises mixed with the signal. Feels like something is gating/saturating inside the distorted signal.

However, with Gain at low setting, Volume high, turning up the treble above 95%, there is a volume boost with some compression, then a popping noise appears, followed by a volume loss and some white noise when i reach fully CW.

Edit : I went back with 3.3k at R31, 10k on treble pot lug 3, all is fine both in low gain and high gain settings, so far. I'll try to put it in the box, see if it still works.

Edit 2 : Everything is fine in the enclosure, still no issue with Treble pot. A second builder told me he has a similar issue with his Aion's Beta build, that makes 3 of us, so far. Something is probably a bit off with the circuit, but a simple resistor between lug 3 and the pcb solves it.

Thank you Antonis !

[matopotato]

The noises were quite frightening, fast popping noises followed by loud decaying white noises, and popping noises again... like the speaker is going to be spat out of the room in smokes.

Maybe you invented a new type of effect that no-one thought of yet?
You'd be surprised what some pedals will do "out there" and people who like just that thing. And the smoke effect will be the killer.
You'll be richer and famouser now ;-)

[aion]

I've heard a couple reports of the treble whine on 10, but it definitely doesn't happen all the time (certainly not on mine) so it's possible it's related to power filtering or wiring or something, possibly component tolerances as well, or something rig-dependent. When volume or an EQ control is on 10, it's more likely to bring out stuff that wouldn't happen in normal use since it's running at the edge of its max performance.

[bluelagoon]

If it runs smooth on treble all the way down to 5R6 then you should be able to just replace R31 with a 5k resistor which should be enough to counter the artifact noise. The less added resistance you put in there the less treble response you lose.

[eh la bas ma]

I've heard a couple reports of the treble whine on 10, but it definitely doesn't happen all the time (certainly not on mine) so it's possible it's related to power filtering or wiring or something, possibly component tolerances as well, or something rig-dependent. When volume or an EQ control is on 10, it's more likely to bring out stuff that wouldn't happen in normal use since it's running at the edge of its max performance.

If it runs smooth on treble all the way down to 5R6 then you should be able to just replace R31 with a 5k resistor which should be enough to counter the artifact noise. The less added resistance you put in there the less treble response you lose.

Thanks for your replies !

I can get a  better understanding about the possible causes of the issue.

In the first post's pictures, the two 4PDT might seem slightly suspicious. I've put all the caps on one 4PDT and all the resistors on the other one. Wired and soldered with great care, isolated from the enclosure, etc.

I would even humbly recommend to add them on your next v1.2 layout, because the Bass mode is also really interesting with guitars, and having to choose between Bass and Lead versions is too hard : both are great.

The treble pot has a very wide range, even with the 10k resistor. I don't have the impression that it is missing some effectiveness.
Quite the opposite, it's still can reach really high settings, with much broader range than Bass and Mids controls.

On the other hand, I tried to increase R31, up to 13.3k or so, and the issue was still there, in the same rotation's zone.

Best option, at least on my build, is the 10k resistor between Treble lug 3 and the pcb pad : There is no problem anymore and still an impressive range of action, much more than enough. When fully CW it does sound like it's "at the edge of its max performance".