There's really nothing you can do to balance the amp if it was designed single ended. Your only choices are to continue to run it unbalance, or replace it.
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There is a way & it's very simple to accomplish!
All you need to do is install some balancing transformers on the inputs, assuming you can solder & you have basic electronics skills? Steve McCormack has these parts available. Very likely you can also get them from Michael Percy Audio. A local musician instrument shop is another likely source if you're not seeking anything too esoteric (I wouldn't - based upon the context of your post).
Since the balancing question has been addressed, let me mention some other possibilities that might cause hum & noise.
Having had experience running sound I'd like to throw a few suggestions out. You may already know this but here goes anyway.
Make sure all grounds are grounded, including mics. Using 57's & 58's? Conversely, make sure things that aren't supposed to be grounded aren't, like those 1/4" jacks. For instance, the little plastic insulator has a way of disappearing.
Same goes for all the cables & the snake. Additionally, only use shielded for shielded applications, etc.
Make sure power is in phase. This will help the most when you plug in, as over half the places you'll set-up in will have screwed up AC. If your stuff is oriented correctly, easier to adjust. You probably don't have to now but learn how to get power from panels. Bad AC, dimmer switches & fluorescent lights are the PA's biggest enemy.
Actually Ghostrider is correct. A component must be designed to be balanced or it just won't be balanced.
What Bob is refering to is running a balanced connection between components. And Bob and TWL are correct that this is possible and only requires a set of balanced to single ended transformers. If the noise is caused by pickup in the interconnects then the balanced cables will produce a benifit. If the noise pickup comes from other sources, then the advice from Driver should be helpful.
I think you should use both suggestions for best results.
Both groups are right as far as they go.
1) Balanced outputs can be added to anything.
2) Having XLR outputs does not make the piece balanced. This is a design issue which obviously involves more than just outputs. Balanced circuitry is balanced throughout.
Whether or not this alteration will eliminate the 'hum and noise' is not certain, but your proposed solution is fairly inexpensive and easy to accomplish.
Opinions seem to divided on the possibility of balancing a non-balanced amp . I am no electronics wizard by any means but am not afraid to tinker with the insides of my gear . I never did believe the warning - " No user serviceable parts inside !" Does any one know of a circuit diagram or a place to come up with one ?
Opinions may seem to be divided but Nrchy is correct.(1) You cannot easily balance a non-balanced amp but (2) you can make it compatible with balanced sources.
You also ask: "Does any one know of a circuit diagram or a place to come up with one ?" Assuming you mean adding an input transformer for (2), others will suggest specifics. If you mean (1), then you have to tell us what the amp is and provide the original schematic.