Aion FX Anomaly (Hot Cake)

[Envee]

Hi all,
Having some issues with the Anomaly, and hoping to find some help here.  I'm a long time reader, with a limited understanding of the things you write about. 
I've built three Anomaly pedals from the PCBs and each time I run into the same problem: I can't hear any difference with the "mode" or cream/bluesberry switch on the Anomaly.  I tried different transistors BC557B (higher gain) and 2N3906.  One has the Mad Bean mods.
The pedals sound great, two sound virtually identical to the Hot Cake in non-bluesberry mode [EDIT- the two without the madbean mod].  But there's a big difference in the bluesberry mode function.  The Hot Cake changes quite a bit, but the Anomaly sounds the same.

Trying to think of the best way to set up the issue to make it possible to get help.  I think this post will be mostly background info and links to schematics.  By "background" I mostly mean the work and explanations from Aion and you all here:
https://www.diystompboxes.com/smfforum/index.php?topic=125157
https://aionfx.com/news/tracing-journal-crowther-hot-cake/

Here's a quote of the quote with the quote:

I know when I'm over my head, so I turned to DIYStompboxes to see if someone there could help. Here's an explanation from Rob Strand:

The way it works is when the output rises on the positive swing it lifts the emitter of the PNP transistor. When the output reaches 0.6V above the voltage set on the base a collector current flows into pin 5.

One thing this method does is clip the opamp *without* the opamp's output stage saturating against the positive supply rail. That probably leads to much cleaner clipping.

The purpose of the zener is give to a reference voltage which is 2.7V below the positive rail. When the base of Q1 is connected to +V, Q1 is disabled and the opamp clips naturally. When the base of Q1 connected to the zener the opamp will clip cleanly like I described before at a voltage 0.6V above the voltage on the zener – ie at 2.7 – 0.6 = 2.1V below the positive rail. The zener voltage is probably purposefully chosen so the Q1 circuit can control the clipping *without* the IC clipping.

So, in summary: when Bluesberry mode is engaged, the offset null pin is used to cause the op-amp to clip without hitting the power rails (which is the source of the harshness in non-Bluesberry mode). When Bluesberry is disabled, Q1 is effectively out of the circuit and the offset null pin is not used.

Anomaly schematic:
https://imgur.com/a/IKloLuX


In the next post, I'll probably provide voltages at the pins and some other spots.  My guess is that measuring the voltage at certain points might help solve this.  I'm not sure how much the voltage is supposed to change after the Z2 2.7v zener diode.  Looks like that changes about .95v when the mode switch is flipped.  Then at Mode SW3 the voltage changes about 3.08V, the largest change I found as a result of the mode switch.
I'm also not sure whether voltage at Pin 5 is supposed to change.  Let's not get sidetracked because I think I just don't correctly understand the "collector current" description. 

Thanks!

[Envee]

Here are some hopefully relevant voltages.  The first number is with the switch down towards footswitch (non-bluesberry, right?), and the second number after the slash is "switch up" (bluesberry, I think):

After R12: 8.56v / 8.56v
Z2 [2v7] cathode (connects to R12):  8.56v / 8.56v
Z2 [2v7] anode (connects to R9 and Q1 Base):  7.63v / 8.56v
Mode SW2:  7.63v / 8.56v
Mode SW1:  8.56v / 8.56v
Mode SW3:  7.63v / 4.55v 
Q1 Base (VC): 7.63v / 8.56v
Q1 Emitter (connects to R6 and pin 6): 4.74v / 4.67v
Q3 Collector (connects to pin 5):  0.14v / 0.14v
VB (after R9):  4.74v / 4.63v

Pin 1: 0.14v / 0.14v
Pin 2: 4.75v / 4.67v
Pin 3: 4.32v / 4.24v
Pin 4: 0v / 0v
Pin 5: 0.14v / 0.14v
Pin 6: 4.75v / 4.67v
Pin 7: 8.56v / 8.56v
Pin 8: 0v / 0v

[bluebunny]

A belated welcome, Envee.  I've moved your topic to somewhere with more passing traffic.

[ElectricDruid]

Welcome Envee!

Are you using a TL071, like in the schematic? This circuit depends completely on the behaviour and internal wiring of that op-amp, so using a different op-amp type could really change it.

[Envee]

Thank you very much, both of you! Helpful already.

I hope it's a simple as changing the op amp, but I think I used correct TL071 op amps. I tried two versions of TL071 from two sources. The one I measured with the voltages above came from Tayda, a TI marked TL071CP.

The op amp in the other two boards (mad bean and BC557B) is from Mouser, TL071CPE4:
https://www.mouser.com/ProductDetail/Texas-Instruments/TL071CPE4?qs=odmYgEirbwxTDLQi%2FjVuog%3D%3D&countryCode=US&currencyCode=USD

On Mouser, I used the "compare parts" function with the TL071cp and TL071cpe4, but found no spec differences. The data sheet indicates these are both "next-generation versions of the industry-standard" TL071.

The Aion FX parts list provides Mouser number "595-TL071CP."  On Mouser, that brings up a list of three TL071s. One of those is a TI TL071cp, and another is TI TL071cpe4.

Thanks

[duck_arse]

also a welcome to posting. is that transistor in the pics a BCxxx? if so, it's backwards.

[bluebunny]

Your TL071 should be fine.  I think what Tom was checking was that you weren't using another single op-amp (which would indeed have the same pinout).  Pretty often op-amps are interchangeable, but this seems to be a case where something specific is required.

[Envee]

No, it's a 2N3906.

Thanks for confirmation on the op amp.

[antonis]

Just passed to say "hellow"..

[FSFX]

Just passed to say "hellow"..

Suppose that's why it is called an 'Anomaly' 

[antonis]

Suppose that's why it is called an 'Anomaly' 

Τrès logique..

[Envee]

Just passed to say "hellow"..

I greatly appreciate everyone's attention to this. The arrangement of the two VA positions in the schematic is something I've been puzzling over and working on.  I'm trying to do tests to see what this does and whether my PCB build (and an empty PCB) matches the schematic.  Would you please explain the VA issue a bit more? 

Thanks

[antonis]

Would you please explain the VA issue a bit more? 

(MODE 2 - 1)

VA = VC + 2.7V
VC = VA
VC = VC + 2.7V
----------------------
2.7V = 0

Paradoxical maths piece of cake..

[aion]

Would you please explain the VA issue a bit more? 

(MODE 2 - 1)

VA = VC + 2.7V
VC = VA
VC = VC + 2.7V
----------------------
2.7V = 0

Paradoxical maths piece of cake..

Perhaps could be drawn a bit differently for clarity, but the switch is just bypassing the 2V7 zener in one mode (VC = VA), and in the second mode it adds R11 resistor in parallel with R9 in the second mode (presumably to raise the VB bias voltage when VC is dropped, since VB is sourced from VC rather than directly from VA).

[aion]

Which version of the Hot Cake are you testing against? (assuming it's an original - I couldn't gather whether it was being compared against another clone from elsewhere)

The Bluesberry mode seems to have changed somewhat throughout production. There was a rumored Bluesberry mod that has been in the DIY community for awhile, which I haven't been able to source to an actual trace so I can't say anything authoritative about it other than that the implementation is completely different than the one I traced. And at some point after mine I believe it was changed yet again. So all I can really vouch for is that it represents the unit that I traced... I don't know much about how long this exact schematic was in production. But there have been perhaps over a dozen variations of the Hot Cake through its almost five-decade life, and Paul is definitely a tinkerer, so it could be that he felt that Bluesberry mode was too subtle and changed it without saying anything.

Based on the schematic of the one I traced, and Rob's technical description, it honestly doesn't do a whole lot on paper and may only make a difference at high gain levels, since it has to do with the character of the op-amp when it goes into clipping. I went over that section several times because I'd never seen anything like it. Unfortunately mine was destroyed in the trace so I wasn't able to test the Anomaly against an original, but I haven't heard any other reports of issues or possible corrections - although I would concur that I didn't hear much distinction between the two modes either in my testing. But if it's related to the clipping character (i.e. the shape of the distorted waveform, as opposed to the threshold or perceived gain level) then those subtleties could easily get washed out by the amp or other elements of the signal chain.

[Envee]

Thanks so much!  The line "since VB is sourced from VC rather than directly from VA" gave me a perspective on that R9/R11/VB section that helped me understand a lot more. 

[Edited for clarity] I was initially testing my builds against a Hot Cake that I bought used at least a year or two ago.  Now I am comparing my builds to a Hot Cake that I bought brand new this year.  I never compared a build to an internal switch version like the one you traced.  But I do remember being able to hear a difference on the internal switch version of the Hot Cake.

With each build I spent a long time listening for differences at various settings (high/low volume, drive, presence, XLF).  Also tried using a looper before the Anomaly (guitar ->looper-->Anomaly--> amp).  This is just to say that I am trying to exhaust possibilities before asking Forum questions.  I do appreciate questions or suggestions about possible settings/options that might make a difference.

As I wrote before, the Anomaly can sound identical to the non-cream setting of the current Hot Cake.  I love it in that setting enough that it replaced one of my Hot Cakes. 

Your explanation, Aion, also clarified how the switch bypasses the 2v7 zener on the VA path.  Thinking about pin 7 reminded me of another description from Rob (quoted below) from the older thread.  I'll have another look through the values discussed below and also think about biasing at VB and pin 3.  Seems like my build may not match up, and I'll read and check further.

https://www.diystompboxes.com/smfforum/index.php?topic=125157

With the switch in "standard" mode, where the base connects to +V, the 82k also connects to +V.     That configuration matches the 2003 schematic, so it should give some confidence of being correct.   It's not unreasonable that "standard" mode is an option.

If we say standard mode is an option then I see only two options that make sense:  the connection you have on the schematic, and another one where pins 1 and 2 of the switch are swapped.   In that second option the 220k would connect to +V.   

Given the zener is operating at low current it might only have a drop of 2.2V instead of 2.7V

For the way you have the schematics now the opamp will bias in the region of 3.46V to 3.74V and the clip point will be 6.9V to 7.4V.    So that means the opamp is biased to about 0.50 times the clip-point.    Exactly half way.   Looks fairly deliberate.

For the alternative  connection the bias point be higher,   around 4.40V to 4.63V and the clip point will still be 6.9V to 7.4V.  So a that means the opamp is biased about 0.63 times the clip point.     Not an unreasonable value.

The difference should be audible on a side-by side comparison.

[PRR]

Try an OLD TL071.

TI has done a dirty deed. They released a totally new design under the moniker of a 45 year old design. It's not even the same kind of FET. They did-away with offset pins (which may be the secret trick on this pedal).

The new chip may even a be a work-alike in 99% of TL071 designs. But I see a transistor routing from output to offset. That's too weird for TI to ever anticipate.

Mouser keeps their stock fresh, and may not know the part changed. I'd look to Small Bear (under new name) or Jameco (an old-line shop).

[Envee]

Interesting, thanks.  Hey, as I said it'd be great if I can just swap the op amp. 
And yes, Small Bear is great.  Very happy when it opened again under Synth Cube.  They're still very helpful there.

[duck_arse]

Try an OLD TL071.

TI has done a dirty deed. They released a totally new design under the moniker of a 45 year old design. It's not even the same kind of FET. They did-away with offset pins (which may be the secret trick on this pedal).

The new chip may even a be a work-alike in 99% of TL071 designs. But I see a transistor routing from output to offset. That's too weird for TI to ever anticipate.

Mouser keeps their stock fresh, and may not know the part changed. I'd look to Small Bear (under new name) or Jameco (an old-line shop).

whatever happened to the LF351, did they go out of production in all the NS/TI company shuffles? if so, they should still be good for the offset pin as a direct replacement for TL071.

[PRR]

Yes, LF351 too. I remember the TL07x series being significantly simplified (cheapened) from the LF parts. Worked a lot alike but LF cost more. I thought at some point they quit the LF and put that number on the TL0 parts. The 2008 ST datasheet for LF351 sure looks like a TL071 schematic, but I forget details.