Dan Echo mod

[Ben N]

There is a mod posted here that corrects the low input impedance (~100k) of the Dan Echo by replacing the input buffer transistor with an op amp and increases the input resistors accordingly.

http://www.e-basteln.de/harp/

My questions:

1) I don't like the idea of soldering wires to an opamp.  Can this buffer upgrade be accomplished as well with a better choice of a discrete tranny, say a jfet?

2) Following buffered effects (boss TU2 tuner, MXR microamp), does it matter?

Thanks, Ben

[Mark Hammer]

Thanks for the link!  Nice pictures.  I figured it used a PT2395 but this confirms it.

[Gus]

Two other easy mods might be

1) remove the 470k 10p
f replace the 1uf electro with a film .1uf to 1uf.
 install a high hfe  transistor in place of the 1815 replace the 220k with a 1meg.

2)  remove the 470k 10pf change the 1 uf to .1 - 1uf  film
remove the 220k
replace the 1815 with a fet.
 connect a 1meg from gate to ground adjust the source R for about 5v at the drain.   gate for base   drain for collector   source for emitter

you will not need the 470k with a film cap.

[Ben N]

Thanks.

Suggest a FET?

Ben

[Ben N]

Five years? Yeah, just about ripe for a bump. 

Soo... Since I posted this originally, my Dan-Echo suffered an injury. It didn't stop working altogether, but it now reduces the volume and adds distortion both in bypass and effect modes. This may have resulted from being hooked up with an 128v pedal (a DOD 500-series flanger, I think). She still echoes fine, though. It actually has kind of a sitarry quality as it echoes. Anyhoo, the fact that it echoes suggests that the delay chip is uninjured, right? So, that means that the input buffer is as likely a culprit as anything, right? Which brings us back to this thread...

I finally sat down and opened this thing up. I find that the In/Out board is as Jurgen describes: a regulated and filtered 5v power supply, a 470k and a ceramic cap across the input, and a 10uf in series with the output. I checked the output of the power supply with a fresh battery, and it is something like 4.99v, so that is ok. I got to the main board, and--whoah! Double-sided is definitely out of my league. I find everything in Jurgen's mod, anyway: the 2sc1815 (it is the only discrete on the board, and right next to the connector) and associated passives, and everything is well-marked. But before I start tinkering, 'specially with unsoldering stuff from a board that's as thick as armorplate, I have some questions:

1) Any other suggestions for where the trouble might lie (and no, I don't have a schematic)?
2) In Gus's first suggestion, to use a high hfe tranny as input buffer: how does hfe affect the tranny's operation as an emitter follower? Also, how does the fet get biased?
3) In Gus's 2nd suggestion, to use an FET: is there any concern that the output impedance of the fet will be too high to work with the switching (CD4013)?
4) There is a 4558 in there; I guess as a mixer/output buffer. Can I assume low enough internal impedances that a change to a 5532 would be beneficial, or should I just leave well enough alone?
OK, thanks, all.

[Ben N]

Bump... anyone?