Upgrading Priceton Chorus with LM3886 chipamp

[nashvillebill]

Hi, just found this forum, what a great site!

I've recently acquired a late 80's early 90's Fender Princeton Chorus (red-knobs) which uses UPC1188H chipamps.  Rated power is 25wpc (this is a stereo chorus amp) into 8 ohms, using 27vdc power supply rails.

Although this is a great sounding little amp, it needs more oomph for live playing.  The chipamp.com kits look very appealing, it appears that I could even keep the 27 volt rails...merely sub out the power amp section, for minimal cost.

Any thoughts?

[LJ King]

I highly doubt that swapping one chipamp for another is going to give you more power. Chances are pretty good that the output will be limited by the current capability of the power supply.

More volume can be achieved much easier and probably cheaper by using more efficient speakers - I'm assuming you still have the stock Fenders?

[nashvillebill]

I've got new Eminence "Rajun' Cajun" speakers on order, they should be more efficient.

I do agree the stock Fender power transformer would be far too feeble.  The good news: the schematic shows the Fender transformer secondaries at 20VAC, yielding rail voltages of +/- 27VDC.   And the LM3886 chipamp datasheets appear to indicate the 27volt rails would be good, they should allow 4 ohm loads.


So...I pull the stock Fender transformer and replace it with a 220VA, 20-0-20 toroid from Antek.  The new secondaries would be the same voltages as the Fender, making it a "drop-in" replacement, meaning I wouldn't have to mess with the power supply sections for the preamp stages of the amp (they're derived from the 27 volt rails).

Make sense?  I wind up buying:
-two LM3886 PCB's from chipamps.com, along with the components to populate them
-a toroid from Antek for about $35 shipped.
-bigger heat sinks if I can't find anything in the shop

With any luck the total mod would cost less than $75.

Or I merely upgrade the speakers and live with the amp as it is, but what fun is that? 

[metalhead]

 :oYes!
This sounds interesting! Please keep us informed how its turning out. Any ideas how to improve the pre-amp section on these amps. I think there is room for improvement  in the distortion section as the case usually is in solid state fenders. I have done some thinking but no practical solution yet.

[nashvillebill]

Well, today I put the new speakers in, and the amp does sound a lot peppier.  It may be a while before I gig live with it though    

Unfortunately, I don't see an easy way to put a bigger transformer in the amp, and I don't want to butcher up the chassis.  So I guess I'll hold off on the "more power" mod, at least until I get a few more live gigs with it.  I'm strictly using it for acoustic, so perhaps the power will be okay.

As far as the distortion: the new speakers seemed to really help the sound, both clean and distorted.  I was able to get some decent ZZ Top sounds, and some good heavy blues distortion that's good for SRV stuff.  This isn't a heavy-metal amp.  I use a Yamaha Magicstomp pedal with my other amp (a Peavey Classsic 50 all-tube) for my electric stuff anyhow.

If someone really wanted to experiment, they could pull the TL072 opamps and put sockets in, then try subbing different opamps.  An OPA2134 is also a FET opamp, so it should work--it's a higher-performance opamp.   A 5532 is also a very high performance opamp but it's a BJT.