Hi there, i can give you an honest guidance on this project, unlike alot of crap, thats been spoken over the years about this little beauti, the bt928 circuit that is, this unit has been designed for a certain market hence the cheap price. Basic amplification for your average punter. However grosley overlooked, & not really talked about anywhere ive seen, is its circuit, which is basic but thats its trump card, its well designed & performs excellent with the NE5532, the main heart of the system. Changing op amps & major circuit changes is a waste of time. This circuit has been used widely, with various minor tweaks, at certain price points, with top notch results ! I could go on, but we'll finish there for now. For your self & all the modding clan, here as follows are four major modifications you need to do first before anything else ! These make the biggest improvement out of all, & no it isnt the caps & resistors !
Replace the volume control with one of the famous alps blue potentiometers ( 10k direct swap ) ( 20k input signal resistor will need to be decreased )
Take out the tone control, the two green capacitors & overlooked my most ,the link wire which left in, may create signal path iregularities to one channel ?
Buy yourself a good quality: 15v ( 2a or above ) in line switch mode regulated power supply
avail from cpc, rs , farnell ect
This makes a massive improvement ! Sound quality / balance & distortion levels, The power supply eliminates all noise on the rails, & power output improved !
& this is before you change the resistors & the caps !
I would just like to point out that some people have tried to add regulators to the board, this is not necessary, no more power supply mods are required other than a faster diode ( opinions vary ) once you have purchased the above switch mode reg psu.
Also overlooked on all the bt928 mods ive seen are the 101 ceramic capacitors been left in place ! These are nasty & no good for audio tuning frequencies, replace them with polystrene 100pf types.
Doing the four modifications above will set you on the way to having an excellent performing audiophile headphone amp ! There are a many more minor mods but times run out for me now !
good luck !