DemonfFX AT-DS (JHS AT+ clone?)

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elguidom
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Post by elguidom »

Hey all,
first of all I may be posting in the wrong section, so mods, feel free to move the thread wherever it should be.

So I've read here and there about DemonFX and how they might be copying the cosmetics of famous pedals but not necessarily their schematics. That seems a bit silly and I don't see what they would gain from only doing half of the job.
Also I have read on Reddit someone said that Josh @JHS had (reportedly) had a look at the AT-DS and said it was not the same as their AT+ and did not sound like it at all. I could not find anything from the man himself, though, only second hand hearsay.
All this made me curious so I decided to have a closer look at the AT-DS internals. I'll be comparing with the Angry Andy Plus (AAP) from PedalPCB which I take to be a faithful clone of the AT+, correct me if I'm wrong.

The obvious thing is that it's mostly SMD, so it makes it near impossible to pimp this pedal, unless you are well versed in SMD soldering, which I'm not.
Evidently, the circuit layout at least is different from the AT+ and AAP. There are 2 boards, one hosting the main circuit and pots, the other hosting the jack plugs, footswitches, and LEDs. They are connected by a 7-wire ribbon.
1-boards.jpg
2-mainboard-front.jpg
3-mainboard-back.jpg
4-switchboard-front.jpg
5-switchboard-back.jpg
First I had to determine how the ribbon was connected.
6-Annotated Switchboard.jpg
I tried to trace everything and made it all available here (in red are the connections from the other side of the board, in light green connections on this side that are hidden by components):
7-Annotated Mainboard.jpg
The first thing I noticed was that there was a TL072, which the AAP doesn't have, so it looks like the boost side is not like in the AAP.
I won't be looking closely at how the power supply is handled and how the reference voltage is produced, I expect things are done correctly, and even if it's not the same as in the AAP and AT+ this won't change the tone. The fact that we see caps rated for 25V makes me think that it is fine to use a 18V power supply, unless somebody tells me I'm wrong? The rest of the components (including the ICs and transistors) should be able to handle that much.

Let's have a look at the drive side first, which is more of interest to me.
After a bit of tracing, I was able to determine that the circuit is indeed the same. I cannot be sure about the capacitor values of course, but I don't see why they would bother to do everything right, resistors included, only to end up botching the capacitor values.
You will notice that my unit is missing the resistor R17. I don't know if this is a QC issue with my particular unit or they decided to go without, but I suspect the former, as I believe it is necessary for the input impedance. I'll just wire a regular 1M resistor on the relevant pins of the drive footswitch to achieve the expected result.
Also the resistor in series with the Drive pot is absent, but that should not matter much except if Drive is set to 0, and even so, maybe not much.
The clipping diodes are wired differently but to the same effect, and the EQ pot is wired slightly differently, I guess because it's a log pot and not a reverse log one.
Here's the schematic based on the AAP one, with the AT-DS component names in red.
8-Distortion circuit vs AAP.jpg
So all in all, I believe that we do have the correct circuit for the distortion side.

The boost side is another matter, I have yet to trace it but my first impressions are:
- there's a TL072 that isn't in the AAP, is it part of the boost or here to stabilize the reference voltage?
- it doesn't look like the AAP boost at all
- the transistor Q1 seems connected both to the boost input signal (through C1 and R1) and to the monitoring LED (through R21 and R16). What the heck is that? I cannot read the label very well, looks like 10 and an X. Any explanation is welcome! Any clue what transistor it might be?
Q1.jpg
Q1.jpg (20.14 KiB) Viewed 4877 times
I hope to be able to make sense of the boost circuit soon. If anybody can provide some help or is able to recognize a know circuit, that would help a lot!

To be continued...

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elguidom
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Post by elguidom »

OK so looking more closely at the connections, it looks like what is connecting R21, R16, and Q1 together might merely be the V+ voltage. I'll have to confirm this, but it's time to go to bed :)

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Post by elguidom »

So here's what I think is the circuit for the boost. I put the values for the resistors according to their label (weirdly, R14 has a EIA-96 label when all others use the more usual 3 digits, maybe they wanted a rare value?).
For the transistor I chose to put an NPN transistor though I have no clue what it actually is, but the connection to V+ to that one pin was the hint I took.
There seems to be some clipping involved with D1 and D2. The series resistors R6 and R8 are odd, maybe they wanted to achieve a specific overall value.
I'm also surprised that the pot is only at the end of the circuit so this circuit can lower OR raise the output level. I have yet to give it a listen and see if there's much overdrive...
If anybody recognizes this topology and can give me the name of a pedal that uses it, I'd be very grateful!
BoostSchematicAttempts.jpg

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plush
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Post by plush »

elguidom wrote: 28 Jan 2023, 14:02
For the transistor I chose to put an NPN transistor though I have no clue what it actually is, but the connection to V+ to that one pin was the hint I took.
It's a jfet and it's pretty much useless in this application.

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Post by elguidom »

plush wrote: 28 Jan 2023, 14:29
elguidom wrote: 28 Jan 2023, 14:02
For the transistor I chose to put an NPN transistor though I have no clue what it actually is, but the connection to V+ to that one pin was the hint I took.
It's a jfet and it's pretty much useless in this application.
Really? Can you elaborate please? How did you know it's a JFet, is it because of the label? What particular model would that be?

And what makes it useless here precisely? I'm not very good at electronics though I understand the basics... Is it because the opamps after that already do most of the heavy lifting?

In any case, thank you for chiming in!

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Post by plush »

elguidom wrote: 28 Jan 2023, 16:52
plush wrote: 28 Jan 2023, 14:29
elguidom wrote: 28 Jan 2023, 14:02
For the transistor I chose to put an NPN transistor though I have no clue what it actually is, but the connection to V+ to that one pin was the hint I took.
It's a jfet and it's pretty much useless in this application.
Really? Can you elaborate please? How did you know it's a JFet, is it because of the label? What particular model would that be?

And what makes it useless here precisely? I'm not very good at electronics though I understand the basics... Is it because the opamps after that already do most of the heavy lifting?

In any case, thank you for chiming in!

Okay, maybe I was too quick to judge
It can be BJT, jfet or mosfet. They will all work about the same in this configuration. The most popular should be a jfet, such as J201, 2m5457, j101, j103, etc.
By "this configuration" I mean a voltage follower, aka a buffer.

In this case, there's no need in placing a voltage follower in front of an op-amp with high impedance input.

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Post by elguidom »

Ah yes, I see.
So basically it's acting as a buffer though there should be no need for it given the rest of the circuit.
Maybe it could be there because, imperfect as it is, it brings its own flavor to the tone?

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Post by plush »

elguidom wrote: 28 Jan 2023, 22:27 Maybe it could be there because, imperfect as it is, it brings its own flavor to the tone?
I doubt.

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Post by elguidom »

Well I fixed the missing resistor R17 and re-assembled everything in order to finally be able to hear the pedal!

But as it turns out, I manipulated the ribbon once too many and it tore down, which I hadn't noticed, until I tried activating the drive side and got no sound.
So I'll have to replace the ribbon; when I reopened the enclosure and removed the boards it almost came undone in my hands, it's now only holding by one or 2 solder joints.
I'll have to rewire the hole ribbon, probably with something else entirely, maybe separate wires or a rainbow ribbon, I'll see what I have.

In the meantime I was able to at least test the boost side. As expected, it is a 'fixed' light overdrive where you can only adjust the volume. It doesn't sound bad, I believe it removes some low end, so maybe it's a treble booster cousin. In any case I don't think that is what the actual AT+ boost does, judging from the schematic from PedalPCB and reviews/demos I have seen of the AT+ on Youtube.
I believe that on the AT+ if you set the boost to its lowest setting you get as much output level but less dirt, whereas here I just get no sound.

So for now it only looks like half a clone, with arguably the important half being cloned.

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Post by elguidom »

Plot twist!

I was looking around for a gutshot of the AT+, they are hard to come by, but I found one. The resolution is not great and it's a bit blurry, but from what I see:
- there are 2 8-pin ICs, one being for the drive side, so the other is for the boost
- there are 2 diodes (D1 and D2) aside the ones for the drive side (D4,D3 and I think the LEDs D5 and D6 which are near the power supply input)
- there's a transistor (Q1)
- the boost pot (bottom left) is the last thing before hitting the output, and it behaves as the AT-DS boost pot
- the part numbers of the AT-DS seem to match the AT+ suspiciously

With all that in mind, I would venture to say that the AT-DS is actually a very faithful (too faithful?) clone of the AT+.
It's actually the AAP from Pedal PCB that has a different boost circuit, presumably to save space and fit in a small enclosure.

Here's the picture I'm using:
at-plus-andy-timmons.jpg
at-plus-andy-timmons.jpg (71.4 KiB) Viewed 4594 times

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Post by elguidom »

A bit more on the subject.
After some investigation and internet browsing I stumbled upon the existence of a (discontinued) Timmons signature Xotic BB Preamp (the AT model). I found the schematic on the aion website, and upon reading that page I finally understood that the booster in the AT-DS (and presumably the AT+) is nothing more than a tube screamer circuit with a fixed tone and gain setting, leaving only the output volume to tweak, from which we can easily infer the caps values in the circuit I traced!

So there you go, the AT-DS is an AT overdrive with a fixed gain+tone tubescreamer as a boost. I expect that the AT+ is the same.

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Post by Aromatic_Brush_6769 »

Off topic entirely, but someone should do a post about their 250. It's all through hole, I just sold mine unfortunately

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Post by mauman »

Aromatic_Brush_6769 wrote: 02 Jul 2023, 03:31 Off topic entirely, but someone should do a post about their 250. It's all through hole, I just sold mine unfortunately
The search engine on this forum is quite good, and it looks like there are lots of posts about the DOD 250:
Search results.JPG

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Post by Aromatic_Brush_6769 »

mauman wrote: 02 Jul 2023, 14:16
Aromatic_Brush_6769 wrote: 02 Jul 2023, 03:31 Off topic entirely, but someone should do a post about their 250. It's all through hole, I just sold mine unfortunately
The search engine on this forum is quite good, and it looks like there are lots of posts about the DOD 250: Search results.JPG
Screenshot_20230703_235943.jpg

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Post by mauman »

Sorry! You said it was off topic, so I assumed...

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Post by txfiddler »

Thanks for the interesting post. I purchased one of these several months ago and it is a pretty cool pedal. I did experiment running it at 18v for an evening. It survived and I liked the sound of it much better. It's not on my pedalboard presently but that is not because it is not worthy. It's just that I usually don't need that much gain for the majority of my gigs and there are other lower gain ones I like better for that job. I'll try it again for a gig I have at the end of the month. Likewise there is a recording project I will try it on as well. I will definitely be running it at 18v.

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Post by WoundUp »

Whoa. Odd that I found this.

It was me on Reddit that said Josh said it sounded nothing like his pedal. It was on a live stream. I forget which one as it was a while ago. Its possible it may not even be listed on the channel anymore.

I've had the AT-DS for quite a while and just got a JHS AT+ today. Doing a side by side comparison, they sound very similar. I think the AT-DS maybe a little more noisy. Other than that, it even sounded similar at similar knob locations, which isn't always the case even between 2 of the same pedal due to tolerances.

I'm torn on whether to keep the AT+ or not. I never tried my AT-DS on 18v and had wondered whether it would work. I've got the AT+ on 18v and it sounds better than at 9v. So I guess its time to put 18v to the AT-DS and see how it sounds.

Thanks for tracing these and making this thread. I really appreciate it as I've wondered about these since before I bought the AT-DS. Also, from what I understand, the boost side was said to basically be a Tube Screamer set at a fixed drive & tone setting. It's got enough mids to be TS based, IMO.

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Post by kev02 »

I would love to know what capacitor to replace to help with more bottom end . I've had it under the magnifying light but having a hard time as I just had eye issues due to medical issues. My son can work on those little guys , I'm an old time amp guy (yes size matters in this case ) 🤣 any help as to what cap to replace with I'm assuming. 1uf like a tube scream would be much appreciated. Thanks in advance .

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Post by kev02 »

C3 is the capacitor. I soldered another. 047uf and help a great deal .

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