Maxon - AD900 Analog Delay  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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analogguru
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Post by analogguru »

People like guts? ok, here are some others from our japanese hackerfriends (click on the pictures for high resolution):

Maxon AD-900 guts
ad900a.jpg
ad900a.jpg (11.74 KiB) Viewed 13208 times
ad900c.jpg
enjoy,
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Post by soulsonic »

Hello everyone! I just finished off tracing this great-sounding Maxon delay - the AD-900.
The one I traced is one of the early models with two MN3005. Later ones used four MN3008, but I think that otherwise the circuit is nearly the same. Overall, it's very similar to the Boss DM-2, except with twice the delay time, and a different clock setup. Compare it to Dirk's schematic of the Ibanez AD-99; many similarities. The AD-900 seems to have a similar vibe to the sound as what you'd hear in the DM-2 and Ibanez AD-9... but the higher voltage (it uses a 12vDC adapter) makes it noticeably cleaner and less grungy.
There is one problem I found with the design: the bypass cuts the tails off the repeats when engaged. If it were a true bypass sort of setup, that would be expected, but since it's not, I see no reason not to have the tails there, so I came up with a simple mod to change the bypass so that it allows the tails to naturally fade out when bypassed.

First, here is the schematic:
Maxon AD-900 Analog Delay
Maxon AD-900 Analog Delay
And here are some gutshots:
Image
Image
Image
Here is the cap that is tacked to the trackside ("Cx" in the schematic):
Image

Now, this is how I did the bypass mod:
- Remove the Brown and Light Blue wires that are connected between Pads 13 & 14 and the bypass switch.
- Jumper Pad 13 and 14 together.
- Break the connection between Pin1 of IC1a and C5.
- Connect wires from the bypass switch to the spot that was just broken. The bypass switch now makes that connection when the effect is engaged.
- Put a large resistor (I used 10M) between the two lugs of the bypass switch that the wires attach to, this prevents pops from happening.
- Done!

I also recommend the following:
- Replace R4, R5, R6, R7, R47, R48, R49, & R52 with high quality metal film types that are matched as closely as possible (I matched to within 1/10%). Each different value should be matched, ie; all the 10k should match, the two 2k should match, and the two 3k3 should match.
- Replace C3 and C36 with a matched pair - again matched as closely as possible.
This mod reduces any error in the emphasis/de-emphasis circuit. By matching these components closely you can ensure that the circuit alters the original guitar's sound as little as possible.

If you look at the pics, you can see that Maxon used tiny cheap little electrolytic caps. Replacing these with high quality types can also help the sound very much. Swapping that cheap 4558 for a lower-noise opamp is a good idea as well. I like the way a 5532 sounds with it. I suppose you could try an LF353 if you want to go for an Aqua Puss sort of vibe...

Enjoy! :D
"Analog electronics in music is dead. Analog effects pedal design is a dead art." - Fran

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Post by lolbou »

Cool stuff, thanks! :thumbsup

Thanks for the tips concerning the matching of resistors too Soul...
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Post by Dirk_Hendrik »

Thanks brother Soul,!
Question though.
In all Ibanez/Maxon delay effects I traced there is a way to make certain that the supply power for the BBD's and the BBD bias is regulated. In my AD99 schem that's done with a 8v2 Zener creating "Vbbd". This way it's ensured that when a wrong or slightly wrong adapter is used, giving a too high voltage, the BBD still gets the proper voltage and even more important, the bias is still correct. If not the BBD can get misbiased causing absence of delays. I spend hours on this when I had a delay that did work at home but not in the stage setup. Voltage of the adapter used at home was a 10 volts Ibanez adapter.

Are you sure that zener correction is not there?
Sorry. Plain out of planes.

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Post by Dirk_Hendrik »

Hmmm...

With the pics... does seem to be the case indeed....
Sorry. Plain out of planes.

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Post by Dirk_Hendrik »

Oh BTW soul,

hate to tell you but the schematic you've just posted is so incredibly rocket science like complex that most likely you will get a lot of views but little response ;) Better post another fuzz.

But, as AG put to my attention on several occasions in the past,
Could you update the schem with the transistor suffixes added? In the case of the 2SC1815's that's most likely "Y's or "GR's"? It does matter.
Sorry. Plain out of planes.

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Post by analogguru »

Dirk_Hendrik wrote:Thanks brother Soul,!
Question though.
In all Ibanez/Maxon delay effects I traced there is a way to make certain that the supply power for the BBD's and the BBD bias is regulated. In my AD99 schem that's done with a 8v2 Zener creating "Vbbd".
....
Are you sure that zener correction is not there?
The reason is simple:
The AD99 uses MN3205 which can handle maximum 9V supply voltage.
When I can believe my eyes, then the AD-900 uses MN 3005 which can handle a supply voltage of 16V maximum.

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Post by Dirk_Hendrik »

No Ag, That's not what I meant.
The 32's need protection not to exceed that 9 volts. That's correct. Same thing for 30's at some 18 volts (datasheet not at hand, laziness*) But not having voltage regulation for the bias circuit still means that when the supply voltage exceeds the voltage on which the circuit was biased may cause misbiasing and therefore absence of delays at all.

The relation between supply voltage and bias voltage for BBD's is in no way linear.

* [edit]datasheet checked, 18 volts absolute max, 16 volts average[/edit]
Sorry. Plain out of planes.

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Post by RnFR »

so THIS is what all those text messages was about? oh big fucking deal. :roll:



just kidding! killer job, you truly are a rocket scientist! :applause:
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Post by soulsonic »

Thanks guys! :D
Dirk_Hendrik wrote:Better post another fuzz.
Your wish is my command, good sir.
viewtopic.php?f=7&t=706&p=76072#p76072
:lol:

About the transistors... all the buffer/filter transistors are "Y" and the two in the clock circuit are "GR". I will double-check later to be sure, but I'm fairly certain about it.
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Post by jupagblkxten »

Is there any significant advantage to be had by using separate clock drivers for each BBD?

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Post by Dirk_Hendrik »

Capacitive loading on the clock lines, causing poor, undefined, clockpulses.
Sorry. Plain out of planes.

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Post by Dirk_Hendrik »

soulsonic wrote:Thanks guys! :D
Dirk_Hendrik wrote:Better post another fuzz.
Your wish is my command, good sir.
viewtopic.php?f=7&t=706&p=76072#p76072
:lol:

About the transistors... all the buffer/filter transistors are "Y" and the two in the clock circuit are "GR". I will double-check later to be sure, but I'm fairly certain about it.
Thanks laddie! I was sure I could count on ye :D
Sorry. Plain out of planes.

http://www.dirk-hendrik.com

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Post by Grendahl »

Any chance at getting a couple more angles, or a straight on shot of both sides of that board please?
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( if < 1 ohm )

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Post by Dan N »

I missed this thread. Thanks, soulsonic!

I did wonder about how to control two 3101's with one pot.
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Post by sevinisthenumber »

Here is a schematic.
Anyway to add an effects loop to this?
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MaxonAD-900.gif
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Post by hiorgos »

One more picture of it, before I send the pedal back to the shop. It had some annoying noise on the repeats, I guess someone messed with the trimpots.

Image

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Post by foo zen trader »

Hey there!

I've got an AD-900 on the bench right now (with 4 MN3008). This is true trimpot madness and I hope I can adjust this little sucker by ear.
Are those CP1, CP2, CP3... "CheckPoints"? I really do wonder what those circular wires are for and help would be much appreciated.
BTW, I will make some gutshots as soon as possible and post them here. If anybody needs special angles just tell me.

Cheerio,

Dennis :horsey:
JABBA DABBA DOOM

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Post by armdnrdy1 »

C30 on the schematic posted above is shown oriented incorrectly.
The negative side of the cap should be connected to pins 14 & 15 of IC2.
This is clear in the component and trace gut shots.

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Post by armdnrdy1 »

While working on a schematic for the (4) MN3008 version of the AD900...and referencing the (2) MN3005 schematic, I noticed another capacitor that is oriented incorrectly.

C18 (1µF) positive side should be facing the input of IC5. The negative side should be facing SR4 Level trimmer.

I have verified this on both AD900 versions.

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