Fat Pants Output Impedance >

[Fastocker]

I recently purchased an Echoplex EP-3 and, while searching the web for some info, I found this post on the MetroAmp forum:

http://forum.metroamp.com/viewtopic.php?f=12&t=29331

In the seventh post down, ClinchFX talks about the EP-Pre and the importance of the output impedance -- read near the bottom of the post . . . can someone help me understand what he says here:

"The output impedance is very important to the phase response of the EP-3 preamp. There's another guy selling PCBs for what he claims is the same thing as an EP-3 preamp, but he just connected a 50K pot to the end of the snippet of schematic that I posted, and that just doesn't cut it."

I haven't done a side by side comparison of the Echoplex with my Fatpants yet as the Maestro needs a tiny bit of servicing first . . .

[madbean]

With all respect to Peter and his product, the FatPants is not the Ep-Pre. While I did use that as a jump-off point there are some differences between the two. I actually spoke to him recently about the Ep-Pre which lead me to take another look at the original Echoplex schematic. The output section of the JFET boost section is a bit unusual in that set-up, where the volume control functions a little differently than a typical one. I have yet to try that setup, but it would be easy enough to adapt the FatPants into something similar.

As far as the phase response as it relates to the output impedance: lacking a proper scope it's not something I ever looked at. I simply used a bread board an my ear to arrive at what the FP has now become. While I'm sure there is something to what he's saying, it doesn't really bother me because I'm not claiming that the FP is a clone of the ClinchFX EPP.

If you want to get a better understanding of what he's talking about, have a look here at a schem for the EP-3: http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/effects/ep3_9068-9450.pdf

Take notice of the 220k resistor in series with the output, then the network of 100k resistors and a 500k pot, where the wiper of the pot is actually connected to ground.

I'd be happy to draw up a mod to the FP for this, if anyone wants to try it out.

[CK1]

Wait, do you think he's referring to you, Brian?

[Fastocker]

Thanks, Brian --- I honestly didn't know if he was referring to you or not, I was more interested in any perceivable sound differences.  I really like the most recent Fatpants I've built --- I put a switch in for C3 to switch between a 22n, a 10n and no cap at all.  22n = high end presence, great for darker amps; no cap = same as base tone, better for brighter amps; 10n = nice 'in between' setting, adds the nice preamp boost plus a touch of high end sparkle and clarity.

If you DO happen to draw up a mod to the FP I'd love to try it out.  I have three V1 Fatpants builds here and wouldn't mind experimenting with one of them.  As I mentioned, I have the real deal here now but I haven't done any side by side comparisons since the Echoplex needs a some minor repairs.  The unit DOES work but likely needs new tape, a cleaning, etc.

[masterlk]

I would be interested!

[maysink]

Me too.

[madbean]

Will do.

[jstbrowsin]

count me in too please.

[madbean]

I haven't forgotten....I will do this as soon as time allows.

[maysink]

Have you still not forgotten? 

[aion]

I started changing around the Fatpants schematic to reflect the EP-3's output section as Madbean described above. Schematic is attached. However, I don't think it's properly adapted, since the junction of R6 and the volume pot (which would normally be connected to the echo portion of the circuit) is not connected to anything here. Something tells me that more compensation will be needed before it has the same output impedance and phase response as the EP-3. Anyone care to comment?