Acoustic 360 Fuzz

Stompboxes circuits published in magazines, books or on DIY electronics websites.
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Post by Barcode »

So, I've been throwing around the idea of doing as Viva ANalog did and build up a pedal version of the fuzz section of the Acoustic 360 bass head (although I don't want the preamp stuff like JC did, I just want the fuzz). Here is the original schematic for the fuzz section:
http://analogguru.an.ohost.de/193/schematics/Acoustic_360_Fuzz_c-fin.gif

And here is my redraw for clarity (used 2n2222 in place of the low gain 2nd stage transistor, 5088s everywhere else, not sure if these are appropriate replacements, but they are close in spec to the original part numbers):
https://i85.photobucket.com/albums/k42/ ... ematic.jpg
Acoustic360FuzzSchematic.png
So I have a few questions. First, what would you guys expect to be seeing voltage-wise on those transistors with the straight 25v biasing? It looks like (from my DMM readings) that the 680K resistor drops the voltage considerably, but that last transistor stage sees the full 25V (presumably gain recovery, high voltage for headroom?).

Also, what changes would you need to make to run this at 9V? I would think you could just run it off 9V, but it seems like the biasing with that 680K would drop the transistors to turn-off levels.

I have a layout for this that I would happily share, but first I'd like to explore a 9V version. Right now I have a 25v charge pump built up and working, so my initial plan was to run it off the original voltage.

Thoughts?

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Post by Dr.Pain-MD »

As a huge fan of vintage ACC gear (I own and love my 140 and 300) I am super interested in this project. I wanted to point out two things. FIrst of all, where does the second pin of the attack control go, it's unspecified. Second of all, when you mention the voltage drop across the 680k resistor, do you mean R4 across Q2 or do you mean R10 which is labelled 68k? As for the 9V operation, I'm sure that if you rebiased the transistors to the same bias points, there would be no problem other than lowered headroom. Let's get some more people on this project, the fuzz on the 360 is a great sound. :thumbsup

EDIT: Mods, maybe you could copy or move this project to the vintage fx section since it would get more views there. That would be nice.

EDIT 2: Never mind, found the second attack pin, d'oh! :slap:

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Post by analogguru »

1.) you don´t need the third transistor - it is only a buffer stage (impedance converter).
2.) the third transistor is running on 25 V.
3.) the other ones are running on ~ 6-7 V. The reason is simple:
Q7 (Q1) is connected as a zener-diode. Therefore you have R39 (R10) for current limiting.

Conclusion for 9V operation:
Change the resistor R39 (R10) to - let´s say for the first test - to 1k.
Remove the output buffer and take the signal from the slider of the gain pot. This should do it.
If you want to add an output buffer, take a standard Boss/Ibanez instead.

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Post by Dr.Pain-MD »

Good points, AG. However, we'd lose the functionality of the attack control if we got rid of the last buffer stage. I'll try breadboarding this one with a clean blend in place of the stock buffer and see how that works out. I'll report back when I get some time to try this out.

EDIT: On a related note, any ideas on getting fuzzes to play nicely with buffers without a ton of extra effort like transformers?

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Post by analogguru »

Dr.Pain-MD wrote: Good points, AG. However, we'd lose the functionality of the attack control if we got rid of the last buffer stage.
If this would be the case, then no Mosrite Fuzzrite (germanium) would work.
Dr.Pain-MD wrote: EDIT: On a related note, any ideas on getting fuzzes to play nicely with buffers without a ton of extra effort like transformers?
If you mean a Fuzz-Face type circuit a 10k - 22k resistor between the (input)-buffer and the input of the FuzzFace should do the trick.
The reason is simple:
A guitar pickup has an impedance of about 10k. The output impedance of a buffer is below 1k. This is too low and confuses the fuzz-face type circuit. By adding the resistor, the input impedance of the fuzz-face circuit is put into a "normal" region.

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Post by Barcode »

Dr.Pain-MD wrote:Good points, AG. However, we'd lose the functionality of the attack control if we got rid of the last buffer stage. I'll try breadboarding this one with a clean blend in place of the stock buffer and see how that works out. I'll report back when I get some time to try this out.

EDIT: On a related note, any ideas on getting fuzzes to play nicely with buffers without a ton of extra effort like transformers?
Yeah, I don't get this either. The Attack control operates completely independently from the output buffer. The attack control (correct me if I'm wrong analogguru) is simply blending between an output directly from the first stage (output from C6) and the heavily clipped output of the next stage (output from C7), so I don't see the output buffer affecting it's operation.

I do like the idea of a clean blend in place of the output buffer, at least for a bass version.

Oh, and yeah, that was a typo, I meant R10.

I didn't figure this should go in the vintage effects section because it never was offered as a separate effect.

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Post by Barcode »

By the way, what are your thoughts on the replacement transistors I listed? They seem close in gain, at least what I could think of off the top of my head.

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Post by Barcode »

Okay, I made adjustments based on the above discussion and discussion on Aron's site, and came up with a quick and dirty test layout. Second eyes please? I think it checks out in ExpressPCB's net checker unless I missed something.
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Post by Dr.Pain-MD »

Barcode wrote:By the way, what are your thoughts on the replacement transistors I listed? They seem close in gain, at least what I could think of off the top of my head.
I just checked a bunch of datasheets and everything is good except Q2 and Q4. I'd use 2N2222s or 2N3904s since 2N5089s have an hfe which is way off from the original transistors in the schemtaic (2N2986). Anyone want to confirm this?

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Post by RnFR »

looking forward to seeing how this turns out for you. I'm kind of surprised noone has tried this circuit by itself sooner.
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Post by Barcode »

RnFR wrote:looking forward to seeing how this turns out for you. I'm kind of surprised noone has tried this circuit by itself sooner.
Yeah, I'm hopeful. I love the sound of the fuzz in JC's clone of the whole thing, I was surprised also at the lack of any analysis on any of the forums.

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Post by Dr.Pain-MD »

Barcode, have you tried this out yet. I really want to try out the rebiased version as soon as I can, but that may be sometime later this week. Would like to hear if anyone else does before me.

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Post by Barcode »

By the way, it's on the old schematic, but I didn't include it on mine, the pots are 500K.

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Post by Barcode »

Dr.Pain-MD wrote:Barcode, have you tried this out yet. I really want to try out the rebiased version as soon as I can, but that may be sometime later this week. Would like to hear if anyone else does before me.
Not yet, but I plan on etching my board and possibly populating it this evening. I may have a result in a few hours...

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Post by Dr.Pain-MD »

Barcode wrote:By the way, it's on the old schematic, but I didn't include it on mine, the pots are 500K.
Check my transistor suggestions, I think they could be improved/chosen better.
Barcode wrote:
Dr.Pain-MD wrote:Barcode, have you tried this out yet. I really want to try out the rebiased version as soon as I can, but that may be sometime later this week. Would like to hear if anyone else does before me.
Not yet, but I plan on etching my board and possibly populating it this evening. I may have a result in a few hours...
Awesome, post your results when you decide to do your thang. I'm going to breadboard this first, I might try it tonight if I have the time.

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Post by Barcode »

Dr.Pain-MD wrote:
Barcode wrote:By the way, it's on the old schematic, but I didn't include it on mine, the pots are 500K.
Check my transistor suggestions, I think they could be improved/chosen better.
Barcode wrote:
Dr.Pain-MD wrote:Barcode, have you tried this out yet. I really want to try out the rebiased version as soon as I can, but that may be sometime later this week. Would like to hear if anyone else does before me.
Not yet, but I plan on etching my board and possibly populating it this evening. I may have a result in a few hours...
Awesome, post your results when you decide to do your thang. I'm going to breadboard this first, I might try it tonight if I have the time.
Definitely, I may even try some lower gain 2n5133 trannies i have. at least in the 2n2222 position on my schematic. Probably 2N3904s everywhere else.

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Post by Barcode »

Okay, two things:

1) My transistors are wrong way round on the layout. I'm not sure how, I guess my pins must be incorrectly numbered on my custom part in ExpressPCB :(

2) Right now I don't have much sustain, and it's just a little buzzy. Not any real good strong fuzz going on. I'm thinking it's a biasing issue. I'm going to put a trimmer in that R10 spot and play around.

Here are my voltages, they fall pretty much in line with what I expected (except maybe Q4), but the sound is still pretty light and buzzy, not nice and thick like the fuzz demo of JC's:

Q2:
C=6.5
B=2.84
E=0

Q3:
C=.99
B=.6
E=0

Q4:
C=9.52 (Supply)
B=5.65
E=5.32

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Post by Dr.Pain-MD »

Just breadboarded the circuit and played around with it for a while. I had a wrong part in at first and I have more errors in there (I know it), but here are my voltages (I used 2N3904 for all):

Q2 - C = 2.40V
B = 0.64V
E = 0V

Q3 - C = 0.99V
B = 0.61V
E = 0V

Q4 - C = 8.76V
B = 0V
E = 0V

My Q4 is completely messed and it doesn't pass the signal through either pot, but I'm sure it's my fault somewhere (your bias voltages look better than mine). I probed the collector of Q3 and I get a cool muffled, bassy light fuzz with my active bass. It sounds somewhat like JC's demo, but not nearly as strong and it's way too muffled as is. I'm going to leave it for today since I have a lot of other stuff to attend to, but I'll be sure to pick this up again at the end of the week. I'll keep posting though, I'd like to see this worked out. I will also attempt to rebias the transistors on paper first, that should give good results. Not too bad so far.

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Post by Barcode »

Okay, just found a much cleaner schematic drawn by Joe Piazza, and the stock resistor for 25v was 6.8K according to his schem, not 68K as i listed above. As such, for 9v you will need to almost eliminate R10 to similarly bias everything.

I've got the 25V version on my test box right now with the 6.8K, and I'm getting better fuzz, but I think I've got a stage not working properly because when I turn the Attack pot all the way in one direction the signal goes out.

In looking at the way the attack control works (blending in the second stage on full, bypassing second stage on minimum), that tells me the second stage is not passing signal. I'm getting decent enough fuzz that I'm certain once I iron that issue out, we will be in business. I'm guessing the biasing for the second stage 2n1306 replacement needs to be altered, 2n3904 not being suitable as having a much higher gain. The datasheet for the 2n1306 device shows a gain of about 20, so I'm guessing we need to find a better replacement than 2n3904, which has a much higher gain. Since 2n1306 is a Ge device, the biasing resistors for that stage probably need adjusting too.

I'm with you, it's bed time, but I think tomorrow or the next night I will play with that bias point for stage 2 and see if I can bring it to light. I'm pretty sure finding an appropriately low gain silicon device and being able to tie down that bias point for the resistors on that stage will yield GREAT results, judging from the low gain side of the attack knob...

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Post by Dr.Pain-MD »

That's great! Can you post the other schematic for me please? That would be most awesome. Yeah, the 2N3904 was indeed a bit high in the gain department when I was looking at the subs, but it was the closest thing I could think of. I'm with you 100% of the way on this project, the 360 fuzz sounds so gnarly. Let's get it working. :applause:

EDIT: Found the schematic, rotated it and hosted it to post here. Here it is:

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