Archon Loop Mod

[Davey]

(With permission from Matt King from PRS to show internal amplifier photos and schematic diagrams)

As some or most of you know that in some of the earlier Archon amplifiers with manufacturing dates prior to November 2014 that they may have a "hissing" issue within the effects loop.
It took some time but I finally tracked down all of the pieces of information to successfully perform the "loop mod" to attempt to quiet down the effects loop in the Archon.
This worked on my April 2014 Archon.

Sorry for the long post, but wanted to provide a little backstory to explain and it's also pic heavy.
Keep in mind, this is not a fix for 60hz hum, ground loops, bad cables or worn/noisy tubes. As Shawn had stated this is only if your particular pedal setup is causing unusable background noise.

This is a copy of a post from Shawn (PRS) regarding the effects loop.
The signal level comes into the amp at a fairly high level, the level is then dropped before it is sent to the pedals (this is done to keep the signal from being too hot, which might fry some pedals). On the return side (when the signal passes back into the amp), the amp boosts the signal up substantially to drive the phase inverter and power tubes. The level we set for the pedal phase is so low (again to prevent any pedals from being fried) that any noise from the pedal will be magnified when the signal is sent from the pedals back into the amp. If the pedal doesn't add much noise, the amp doesn't have much noise to magnify when sending the signal back to the amp. If the pedal introduces a great deal of noise, then the amp will more significantly magnify that noise in the return line level. This issue has been tested with several pedals; it will not happen with all makes and models of pedals. Actually, this is only happening with a handful of pedals. Please keep in mind that the loop circuitry is always active, even when it is bypassed via the footswitch.

The modification basically adjusts the send/return levels so that the amp sends more signal to the pedals, and boosts less on the return side. This will greatly reduce any pedal noise while retaining overall signal level if your particular pedal is causing this issue. The modification is a simple resistor value change, and only requires the replacement of one small component.

Please keep in mind, this is only necessary if your particular pedal setup is causing an unusable amount of background noise. Please also bear in mind that a small amount of hiss coming from pedals is normal (just as hiss coming from cascading tube gain stages is normal).

BEFORE BEGINNING, IF YOU ARE NOT COMFORTABLE OR EXPERIENCED IN WORKING WITH ELECTRICAL PARTS, THEN DO NOT ATTEMPT THIS AND BRING TO YOUR TRUSTED AMP TECH.
MANY PARTS WITHIN AMPLIFIERS CAN HOLD A DEADLY CHARGE FOR DAYS AFTER BEING UNPLUGGED. PROCEED AT YOUR OWN RISK.
I AM NOT RESPONSIBLE FOR ANY INJURY OR HARM TO YOU OR YOUR EQUIPMENT.

OK, so here's the "Loop Mod"

Order/Buy a 47K ohm 1 watt +/- 5% resistor as shown

Disconnect power, remove rear access screen screws (2 on each side)

Then remove outer case to chassis screws (2 on each underside of amplifier case)

Continued next post.....

 

[Davey]

Slide internal chassis from rear of outer housing

This is the point in the process where you want to discharge all capacitors with discharge stick to avoid any possible risk of electrocution.

This is the particular board you want to focus on

Here is the schematic as provided by PRS

And my edited photo illustrating the changes required

Continued next post.............

 

[Davey]

De-solder the C2 Capacitor and the R6 resistor as shown

With your new 47K resistor, I used the old 100K resistor as a template to bend at the proper locations.

Next snip the ends of the resistor legs to match the length of the old one.

Solder the 47K resistor in place where the 100K resistor was in the R6 location

And you're done!
Note: The C2 Capacitor location remains empty

Reassemble the amplifier in reverse order.
Remember if you are not comfortable in doing this, please take to your own amp tech to avoid injury.
Thanks

 

[bodia]

Rock star! Thanks for posting

 

[DISTORT6]

This should be pinned.

 

[shinksma]

Yes, please pin this! I don't use the effects loop on my Archon very much, but part of that lack of interest stems from just a little too much noise in the loop, unless using certain studio-type rack mount units (which seem to have a different impedance, and match the amp better). I need to make this mod, and I am comfortable doing so (I need to get a capacitor discharge stick, and maybe upgrade my ancient Radio Shack soldering iron kit), so maybe one day I will get around to getting it done.

EDIT: anybody recommend a particular type or brand of capacitor discharge stick? If I'm gonna spend $20 or so, may as well get the right one...

 

[bodia]

This should be pinned.

Yes, please pin this! I don't use the effects loop on my Archon very much, but part of that lack of interest stems from just a little too much noise in the loop, unless using certain studio-type rack mount units (which seem to have a different impedance, and match the amp better). I need to make this mod, and I am comfortable doing so (I need to get a capacitor discharge stick, and maybe upgrade my ancient Radio Shack soldering iron kit), so maybe one day I will get around to getting it done.

EDIT: anybody recommend a particular type or brand of capacitor discharge stick? If I'm gonna spend $20 or so, may as well get the right one...

I PM'd Jamie to see if he would pin it

 

[DISTORT6]

I PM'd Jamie to see if he would pin it

It’s a very common question around here.

 

[bodia]

It’s a very common question around here.

Sure is

 

[DISTORT6]

Thank you, Jamie.

 

[bodia]

Thank you, Jamie.

Ditto! @JMintzer

 

[Davey]

Ditto! @JMintzer

Thanks @bodia for doing that. and Thanks to @JMintzer

 

[bodia]

Thanks @bodia for doing that. and Thanks to @JMintzer

You’re welcome!

 

[boardn10]

Thanks for posting. I have a 2019 and the loop has been my only complaint. I may show this to my tech!

I wish they had this corrected in the build or included send and return levels on the amp.

 

[boardn10]

This is my favorite amp to date. For the loop, I run an Ebtech hum eliminator and it's awesome. Silent now and I ditched my ISP noise gate.

 

[Goran Lorencin]

Thanks man for the detailed presentation, did not know the Archon also had FX loop issues, could you draw parallels to the MT15 issues?

I must say I am a bit disappointed to hear that, it looks like PRS made quite a few amps with FX loop issues but still went to put them on the market. Is that bad QC, bad design or does that happen to any other manufacturer?

I wish someone did this for the MT15!!!

 

[Davey]

Thanks man for the detailed presentation, did not know the Archon also had FX loop issues, could you draw parallels to the MT15 issues?

I must say I am a bit disappointed to hear that, it looks like PRS made quite a few amps with FX loop issues but still went to put them on the market. Is that bad QC, bad design or does that happen to any other manufacturer?

I wish someone did this for the MT15!!!

Thank you.
You might have missed it in my first post that it is not an issue with the Archon effects loop. The issue is in some particular pedals that cause the background noise.
It's not bad QC or design, the loop itself is fine and most don't have a problem with the loop unless a specific pedal they have added in the loop causes an issue.
To help out, PRS decided to revise the send and return levels to help with noisy pedals.

Remember, a small amount of noise from pedals is normal as is some noise from gain stages.

I cant speak to the MT15 as I dont own one.
Hope this helps.

 

[Goran Lorencin]

Ok, I think I understand, but please define background noise, is it always present when the pedal is ON, no matter if you have the tuner ON and there is no signal from the guitar passing trough? Cause I have this issue with the Supermoon reverb pedal in 4 cable method, that just by turning it ON, having it in the loop, without and signal going trough it, it induces quite a noticeable amount of noise/hiss/high pitch squeel and it could be it is not compatible with the MT15, it seem like it is an electronic issue as it comes it even though I have a Sentry noise gate in the chain. I don't know any more, should have another MT15 to A/B test ... I just love the sound of the pedal, but hate the noise even more. But I'm glad you found a solution, and I will keep on searching for mine.

 

[boardn10]

For my Archon 100, it didn't matter what was in the loop, even just a patch cable. I had too much noise.
I ended using an Ebtech hum reduction unit and it's been amazing. Everything is now dead silent.i just sit it on top of thr head and it's the last thing before the loop return.

 

[Goran Lorencin]

For my Archon 100, it didn't matter what was in the loop, even just a patch cable. I had too much noise.
I ended using an Ebtech hum reduction unit and it's been amazing. Everything is now dead silent.i just sit it on top of thr head and it's the last thing before the loop return.

So just a hum eliminator huh? Been looking around and they're quite cheap in the US if you can find an Ebtech, here in the EU they charge 200-250 eur for a Morley stereo hum eliminator (Ebtech nowhere to be found, sold out), but I think that is a crazy price for such a simple device and saw mixed reviews, some say it works like a charm and others that it does not and it only changes the frequency of the hum. So ... I don't know, will try to find someone who's got one and try it. But I think that my issues are not that severe as yours were. Thanks for sharing.