Xotic - AC Booster [ goop-alarm ]  [traced]

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IvIark
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Post by IvIark »

blakbeltjonez wrote:is there anything i can do to get a bit more distortion?
Increase the Drive pot to 1M
"If anyone is a 'genius' for putting jacks in such a pedal in the only spot where they could physically fit, then I assume I too am a genius for correctly inserting my legs into my pants this morning." - candletears7 - TGP

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Post by MullisMan »

I took the original AC and replaced the tranny buffers with a TL072 op amp for in/out buffering. Everything is working great, but I'm not seeing the same volume range that the original AC has. With the volume pot maxed (100kA) i'm getting more than unity gain, but not by much. I'm wondering if it has anything to do with lug 1 of the volume pot being connected to virtual ground? I'm actually using a switched power supply configuration, using a MAX1044 to get 18 volts via a SPDT switch or 9 volts if desired. I'm wondering if I just ran lug 1 of the volume pot to ground would I get a volume increase? :scratch: Any ideas would be appreciated!

Oh and I got my buffer configuration from AMZ: http://www.muzique.com/lab/buffers.htm

I'll include the schem to show what I mean.

Jeremy
Attachments
ACboosteropbuffer.png
ACboosteropbuffer.png (7.18 KiB) Viewed 706 times
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Chewbacca
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Post by Chewbacca »

MullisMan wrote: Everything is working great, but I'm not seeing the same volume range that the original AC has. With the volume pot maxed (100kA) i'm getting more than unity gain, but not by much. I'm wondering if it has anything to do with lug 1 of the volume pot being connected to virtual ground? I'm actually using a switched power supply configuration, using a MAX1044 to get 18 volts via a SPDT switch or 9 volts if desired. I'm wondering if I just ran lug 1 of the volume pot to ground would I get a volume increase? :scratch: Any ideas would be appreciated!
Jeremy
Hi,
I just built an AC and I have the same Boost Problem. Volume Boost occurs only between 3 and 5 on the Vol Pot and not much at that.
So has there any solution been found yet?
I use 2N5088 for the Buffers. Would MPSA18 give me more boost - they are used in the Boiling Point and that one has a lot of volume boost - more than a tube screamer and definitely more than the AC.
Does 18V do any good?
Thanks
Andre

aionios
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Post by aionios »

There are a lot of discrepancies in the AC Booster schematics floating around, some perhaps due to tracing errors but most of them just due to Xotic changing things along the way since they are known to tinker incessantly with their circuits. My thirst for accuracy got the best of me, so I traced a new-production AC Booster to get a definitive trace.

Xotic AC Booster (2019) trace
Xotic AC Booster (2019) trace

There are several changes from earlier schematics:
  • Clipping diode arrangement (see schematic)
  • Drive pot 500kB -> 1MB
  • C6 (clipping stage input cap) 100n -> 47n
Full tracing journal here (includes RC Booster and Bass RC circuits as well):
https://aionelectronics.com/blog/tracin ... c-booster/

Malacandra PCB here (can build any of the three variants) - updated to V2 based on traces:
https://aionelectronics.com/project/mal ... c-booster/
Aion Electronics - DIY PCB projects for classic and obscure circuits

howmuch
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Post by howmuch »

aionios wrote:
26 Jun 2020, 21:16
Image
Hi aionios,

Thanks for the tracings.
That clipping section in the AC Booster doesn't look quite right to me.
D5 is doing nothing and might as well not be there.

It would make more sense if D1 and D4 where reversed as in the second arrangement in the image below.
Then you get 3 diode voltage drops each way - 6 diodes for the price of 5.

Not saying your tracing is wrong - it could be Xotic design or placement.
Or maybe I'm missing something?
Xotic AC Booster (2019) Clipping Question.png
Xotic AC Booster (2019) Clipping Question.png (10.63 KiB) Viewed 230 times

aionios
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Post by aionios »

See attached. You can see that the diode polarity is always cathode down, anode up, for all five of the diodes. The underside photo is small, but you can make out the corresponding jumpers in the upper left. So I still think the schematic I drew is correct, but I do welcome a second set of eyes!
Attachments
Xotic AC Booster PCB - bottom side
Xotic AC Booster PCB - bottom side
Xotic AC Booster diodes
Xotic AC Booster diodes
Aion Electronics - DIY PCB projects for classic and obscure circuits

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Post by mauman »

howmuch wrote:
28 Jun 2020, 12:34

That clipping section in the AC Booster doesn't look quite right to me.
D5 is doing nothing and might as well not be there.

It would make more sense if D1 and D4 where reversed as in the second arrangement in the image below.
Then you get 3 diode voltage drops each way - 6 diodes for the price of 5.
When I model it in LTSpice, there's a pretty big difference between the two diode configurations when the Gain pot is maxed. The aionios diode layout maintains a symmetrical and smoothly rounded waveform, while the howmuch layout gives a greater output, but asymmetrical and clipped at both top and bottom. So something is indeed different. :hmmm: Mike

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Intripped
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Post by Intripped »

while you're modeling, if you want, please try with and without D5 in aionios configuration; i'm curious about the differences

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mauman
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Post by mauman »

My LTSPice runs don't show any difference in the aionios configuration whether D5 is present or absent - the waveform, amplitude, and frequency response look the same at both low and high gain. This speaks to your point about D5 not doing anything. Assuming the trace is accurate, maybe Xotic included their nonfunctional signature in the feedback diodes? I'm thinking of Slartibartfast and the fjords of Norway here... Mike

alexradium
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Post by alexradium »

Typical soldering jockey mojo,like that 470 ohm cc resistor on output

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