mxr analog delay: schematics/pcb layout

[spacedog]

hi,
i´m pretty new to this forum as well to the whole electronic diy thing.
my first real project was the 4m tremulus lune fuzzcentral/commonsound and it works really well =)

so now i´m looking for something new.. especially the mxr analog delay.
the fact that these pedals are sold for more than 160â,¬ on eb*y (germany) made me think of building one on my own.

does anyone know where to find the schematics/pcb layout for free?

i already found the circuit boards on ebay.com for about 50$.. but i guess with additional shipping costs it will be more than i could pay.

best,
.g

[pildus]

you can get schematic & board layout for 10$ here:
http://www.schematicconnection.com/store/asp/prodtype.asp?prodtype=66&ph=cat

[spacedog]

thanks. but i already found that link.
unfortunately i don´t own a credit card to pay..
and i do not know.. if i´m willing to pay 10$

ADD: okay.. convinced myseld that 10$ is not too much.
but does someone know if they send schematics only via mail.. or also via email?

[Mark Hammer]

1) Pretty complicated build for a starter.  Partly because of the parts count.  Partly because of the preponderance of static-sensitive parts.  Partly because of the substantial amount of trimpot tweaking needed.

2) The SAD-1024 version is more complicated than such a thing needs to be these days.  The R5106 (or is that 5101?) version is hard to make because the chip is so rare.  Both chip choices will increase the cost of the build substantially.  If you know exactly what you're doing, maybe that isn't such a big expenditure compared to the prices you quote.  On the other hand, it's a lot of money to pay for something that sits in the corner for months while you try to debug it.

3) The maximum delay from the MXR unit is much less than the amount of delay available from much easier to find, much cheaper to buy digital chips.  Once the appropriate filtering is used, digital chips can sound every bit as good as an all-analog setup.

4) The most distinctive feature of the MXR unit was that it used a "tracking filter".  In a clever bit of design, they harnessed the BBD chips *and* some CMOS switches to the same master clock.  The CMOS switches were used to make switched resistor filter sections that would adjust how far up or down the anti-aliasing/clock-noise-reducing rolloff occurred, depending on the delay time selected.  At the time, this was a truly innovative bit of design, since just about every other commercial analog delay then used a single fixed frequency for their lowpass filters.  Sometimes it would be high enough to give more bandwidth but let clock-whine sneak through at longest delays.  Sometimes it would be low enough that clock-whine never ever became audible but the short delays were still too muted for many users' tastes.  The MXR solution gave you as much bandwidth as could be tolerated at each delay time.  Clever.  Small wonder Ross copied that one too.  Do you actually NEED the MXR circuit to do this?  No.  For instance, look at the schematic for the PT80 that Scott Swartz designed ( http://www.generalguitargadgets.com/diagrams/pt80schem.pdf ).  In between points P and R, you'll see a 3-pole lowpass filter built around a single transistor.  Absolutely no reason why those two 10k resistors closest to the transistor could not be replaced with a dual-ganged 25k pot and two series resistors to make a tunable lowpass filter that could adjust the rolloff of the delay section for brighter or muter sounds at different delays.  In fact, it may even be MORE flexible, tonewise, than the MXR circuit.

The bottom line here is that I don't think you should obsess about the MXR circuit quite so much.  It is unlikely to deliver tonal satisfaction as easily or cheaply as some other alternatives for which there are posted layouts, available sources (Small Bear) and lots of useful builder comments.  I won't limit you to the PT-80 but many people have built it and found that it sounds every bit as good to their ears as its analog equivalent.

[Vsat]

Mark,
Repaired a 3xSAD1024 unit once (power xfmr had broken loose from PCB - soldered pins were all that held it in place). Decided not to clone it, altho the concept is neat, the implementation was lacking... 4-pole Butterworth tracking filters are a bit skimpy for getting rid of clock whine while maintaining max bandwidth for a given delay setting. Would be nicer if it used MN3005 and modern switched-cap filters  ... then it would be worh cloning.
Regards, Mike

[ExpAnonColin]

2) The SAD-1024 version is more complicated than such a thing needs to be these days.  The R5106 (or is that 5101?) version is hard to make because the chip is so rare.

It's 5101 for the archives.

-Colin