Maxon - AD900 Analog Delay  [schematic]

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vaccastracca
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Post by vaccastracca »

Does anyone have knowledge of which trimpots can be tweaked to adjust the self-oscillation on a 4-chip version AD900?

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vaccastracca
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Post by vaccastracca »

...or better yet, an explanation of what EACH trimpot does?

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armdnrdy1
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Post by armdnrdy1 »

From this thread:
viewtopic.php?f=1&t=25688

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vaccastracca
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Post by vaccastracca »

Awesome! Thank you so much! :D

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vaccastracca
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Post by vaccastracca »

So, i got the 4-chip AD900 today, and out of the box it didn't self-oscillate at all.
I checked trimpot 11, and it was turned CCW, for minimum repetitions.
I turned it CW, and now it self-oscillates, like any proper analog delay should!
But I have the doubt that one of the previous owners could have messed up with some other trimpots...
There are a few aspects of the repetitions that I'm not sure wether they are normal or not.
So I'll try to describe them....maybe you can tell me if I should investigate more about the trimmer settings or if everything works as it should.

- For instance, when trying to do the "UFO landing" thing, it won't deliver! With the delay time at minimum(fully CCW) and repeat rate at max (fully CW), it will produce a good self-oscillation, but it doesn't last long that way, it soon becomes all mud and only low frequencies, totally loosing any high-frequency content. In the same way, if you start the repetitions from that point and then bring the delay time slower and slower, it will maintain the initial character only for a very short range...even before getting half-way with the delay-time knob, it already - again - looses all the high frequencies...by the time you have turned the knob fully CW, there will be only a big puddle of sub-low frequencies mud (sometimes barely audible!) that is far from the ufo landing trick!

- repetitions start degrading quite early...after 2 or 3 repeats the signal already starts to get "eaten away"...not in a dramatic way, of course, but still noticeable

- as a consequence of what just said, when doing close-to-infinite delays at slow times, as repetitions develope the signal has the tendency to becoming "pulse-like", sometimes in a not too pleasing way...complex phrases can morph into a "tack-tack-tack-tack" thing...

It's hard to explain with words, sorry (especially considering english is not my first languag)! Maybe I should try to do a video or something....but that would be a mess to arrange, so i hope that what i wrote makes some sense and that someone can shed some light on the character of this pedal.
In general, it sounds very nice, clean and warm. A beautiful delay to use for normal things.
But for using it at its extreme settings, it just doesn't seem tweaked right...
I would like to figure out if it's its character. If so, it would mean that it just cannot do what i would want to do with it!
It's also not too encouraging that on Youtube there is not ONE SINGLE video where the pedal gets fully explored in self-oscillation mode! ....maybe it's just not made to do that?

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Post by armdnrdy1 »

I'll be honest with you...
When I calibrated my build...I followed the calibration procedures to the note. I acquired the original documents from the Maxon distributor Godlyke.
The "Repeat" alignment instructions say to adjust trimmer SR11 for the repeats to last ten times and disappear on the eleventh time. Then turn the Repeat knob CW and check for oscillation.
When I adjusted it per factory specs...it did not oscillate.

Another thing that is inherent in this design...the delay is a bit dark sounding.

I received a supply of caps in the post today..and will experiment with changes in the filter section to brighten things up a bit.

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vaccastracca
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Post by vaccastracca »

Well, used in a "normal" way, I don't even find it particularly dark, as a delay.
I have much darker sounding analog delay pedals!!! ...The Maxon is actually one of the brightest I tried so far!
It's the self-oscillation that probably is not what i expected....it never gets really wild...even taken to the extreme, it just tends to become a big ball of low frequencies...
So, the dark character for me happens only when i push the self-oscillation to the max...

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armdnrdy1
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Post by armdnrdy1 »

Maybe "dark" is subjective. This pedal is "darker" than a MXR Carbon Copy.

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vaccastracca
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Post by vaccastracca »

Yes, Carbon Copy it sure much less dark than the AD900...and also much less sweet and organic sounding! Man, I really don't like the Carbon Copy! :P

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Post by vaccastracca »

By the way....i was almost forgetting to ask a very important question!
If i decided to try tweaking a bit the trimmer-pots in my AD900, should I do that exclusively with an oscilloscope or could i go by ear?
I mean, is there the risk of damaging the pedal, when you start touching trimmers?
With old pedals I used to do that all the time, but old gear was sturdier, and generally with much less trimmers!
Also, I don't have an oscilloscope, so going by ear is my only option. And if this way is too risky for the pedal, I would probably leave it like it is.

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armdnrdy1
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Post by armdnrdy1 »

The only trimmer in the AD-900 that could be adjusted per users taste is SR11. (repeat trimmer)

The rest of the trimmers are for BBD bias and balance. Messing with those trimmers will "knock" the delay out of calibration.

The Repeat trimmer is supposed to be adjusted by ear...the rest with scope.

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vaccastracca
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Post by vaccastracca »

OK, then I won't touch it.

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Post by sinner »

vaccastracca wrote:OK, then I won't touch it.
U know u will

Nothing will stop u now

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Post by Dirk_Hendrik »

Sorry. Plain out of planes.

http://www.dirk-hendrik.com

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vaccastracca
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Post by vaccastracca »

Yes, I think you're right! :P
sinner wrote:
vaccastracca wrote:OK, then I won't touch it.
U know u will

Nothing will stop u now

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Post by vaccastracca »

So, i messed with the trimpots a bit, but eventually put everything back like it was (apart from the repeat trimpot that is now maxed out, of course! :P )
I see that tweaking them didn't change the "issues" i was talking about. So I guess i can safely assume that those are not issues but the character of the pedal.
One thing I noticed is that it sounds remarkably similar to my 80s Ibanez AD9...and i mean they are almost identical!!!
The AD9 has more low-end though, which is kept also in the tails.
Is there a trimpot of the AD900 that i could tweak for more low end? ...i guess it's one of the 'balance' ones...

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Post by Dirk_Hendrik »

No.
And yes, there's a strong resemblance between all Iba/Maxon analog delays.

No there's no low end trimpot. And stop touching those trimmers untill you understand what you're doing. Your messing things up and it won't get "better".
Sorry. Plain out of planes.

http://www.dirk-hendrik.com

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vaccastracca
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Post by vaccastracca »

Eheheh...sorry, I'm a long-time trim-messer! :P
Anyway, another question about the AD900.
Does the 2 chip version supposedly have a bit cleaner tails than the 4 chip?
Still comparing the 4 chip AD900 with my old AD9, and i was noticing that the repeats on the AD900 get dirty before the AD9, and are also dirtier overall (with dirty i mean slightly distorted). The AD9 also has better headroom: it handles slightly better hot input signals before clipping.
I was reading somewhere (probably in this same forum i guess) that the more chips are on the mainboard, the dirtier the repeats are. So 2 chips: cleanest. 4 chips: less clean. 8 chips (AD-999): very dirty.

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Post by Intripped »

...or the more you mess with trimmers the more the repeats are dirty...

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