Using a non-isolated wall wart to power a CMoy amplifier

Hey everyone,

First of all, this is my first post so I'd like to just say hi to everyone. It looks like you guys have a wonderful and supportive community here, which I know is not easy with the rise of internet trolls. 

Second, I have a question about converting a CMoy to use wall power on the cheap.  The local thrift store has a few boxes full of wall warts for nearly free, most (if not all) are probably non-isolated. I also already have two wall warts (24v and 12v), so any solution involving these might be a big cost savings.  I've read a bunch of the DIY section here and it seems like the conventional wisdom is to just not use these because the CMoy uses a virtual ground circuit.

I'm not very good at electronics at all, but it seems surprising to me that it's not a trivial modification to make one of these wall warts isolated by tacking on some extra mildly fancy circuitry with a voltage regulator like the LM317. Is this possible?

In my searching I only found these nibbles of useful advice:

I also recall a post about a guy who uses two non-isolated wall warts to make a true ground and +12/-12 volts but I can't seem to find it anymore. 

In searching the forums I found a bunch of references to Tangent's TREAD supply at  http://tangentsoft.net/elec/tread/ but the link seems to be broken. 

Thanks,

-Kris

Thanks tangent and splaz

I can't believe I couldn't find this on your site last night.  I actually think I identified my wall warts incorrectly because I remember seeing a sentence on your site that was like "use an ohm meter to test all possible combinations of input and output pins," which I interpreted incorrectly and found that I had some measurable resistance between the output ground and output positive. The wording on this link you just sent is much clearer so I only tested for resistance between the AC side and the DC side. It turns out there is no connectivity between these two sides.

One of my wall warts looks like it's actually a reasonably fancy 24v switching-regulated power supply because it takes a wide range of input voltages and seems not to drop voltage under load (part# hpa-602425u1).  The other says 12V on the packaging but outputs 14 volts with no load, and 13.5 attached to my CMoy suggesting that it is an unregulated supply. Both of these types of supplies are isolated as I understand.

I tried plugging them into my CMoy and listened to the amp with some crap headphones and both of these supplies generate some pretty loud low buzzing sound that varies with in volume when I turn the pot.  Is this the expected behavior? It seems like the 24v guy should generate higher frequency noise being switch-regulated.

I guess the next logical step is to connect one of these guys to a linear regulator circuit as you guys have suggested to quiet it down.

I've decided to wait on ordering the linear regulator IC until I get my next big batch of parts from digikey or something similar. I was too excited to get this project started so these parts are all the radioshack special (sadface).  I actually switched to a battery power supply and my audio quality was still horrible.  It turned out I had initially connected the LED from the positive rail to the virtual ground. The current draw caused the rails to become very uneven which also caused the op-amp to be very sensitive to any fluctuation in voltage.  I changed that and it sounds fine now! 

I have to be honest, I can't really hear a concrete difference between source-only and CMoy-amplified signals with my old pair of Stanton DJ Pro 2000's (it does get louder). My girlfriend is giving me a set of Sennheiser HD 558s for Christmas, so I'll have to test it with those. However, I get the impression that the 558s are extremely easy to drive too so it might not make a difference for that configuration either.

I've attached some pictures of the build just for kicks. The work is a little messy, but it was fun and satisfying anyway. 

I'll have to make a follow-up post with a short analysis of it using my girlfriends el-cheap pocket oscilloscope (DS0201).

-Kris 

DC Offset:

Green output channel: 8.6mV 

Red output channel: 4.9mV

Here's a 500hz square wave played by my Samsung Galaxy Tab 10.1. The slew rate seems to be about 2.15mV/us

The following is the signal observed on the amplified end of the CMoy from the same source:

The slew rate is a little higher at 2.7mV/us.  I think I may have just made a mistake when placing the markers because it doesn't seem like it should be any higher than the source.