B-Tech BT928 Headphone Pro Amp
I was looking for a headphone amp for the Television and came across this little unit. It's got a volume control, tone control, 3.5mm & 6.35mm headphone jacks, Gold plated RCA plugs and comes with a 15V DC wallwart.
Component ID HERE (page takes time to load)
Under the Bonnet
Nothing particulary stunning under the bonnet. NE5532 Op amp, a bunch of cheap capacitors and a handful of resistors. The potentiometer isn't too bad for the price and the Gold plated RCA sockets not bad either. There is plenty of room to fit more exotic components and I have already replaced the capacitors with ELNA Cerafines. My next move will be to fit a Burr Brown OPA 2134PA in place of the NE5532 and also to replace the resistors with Metal film varieties. There are all sorts of possibilities, remove the tone control, fit an alps blue velvet pot, incorporate a crossfeed filter, remove the 3.5mm jack, upgrade the power supply... the possibilities are endless.
Sound Quality (with stock components)
I had to double check that I had my headphones connected to the BT928 and "not" to the X-Can V2 when I first gave this amp a listen! The sound couldn't be this good from an amp costing only £35? Deep bass, nice midrange with a slightly laid back treble would be how I would describe the sound. I don't see the point of the tone control, turned fully clockwise the sound is almost "flat" but the treble is a bit recessed. Turn it anti clockwise and you lose the treble altogether. This is one device that I will be removing very swiftly!
This is not an X-can or a Sugden headmaster by
any means but it sure has to be an absolute bargain at £35. Ideal
for use with walkmans, TV's, soundcards and Hi-Fi equipment. It will boost
any signal and not insult your ears with a cheap, harsh presentation...
It sounds smooth and it's possible to listen for prolonged periods. If
you're on a real Budget then this is a super little unit worthy of a place
in your system. If you've got £150 though.... buy an X-can V2.
Under the bonnet
The worlds first ever under the bonnet showing of the B-Tech BT928. Those of you who are offended by hardcore electronic components in the flesh look away now!
Further to the above.... I have replaced the resistors with Metal film varieties and have also replaced the OP AMP to an OPA2134PA with quite astonishing results! The total cost of replacing the caps, resistors and OP AMP came to less than £30 and has brought about an improvement in sound quality that puts this amp on a par (even better in my opinion) with the Creek OBH-11.
I'm not yet convinced that the OPA2134PA is an "improvement" over the NE5532, however, but will give it a week in the circuit to prove if it's worthy of remaining. The NE5532 was definitely better in the bass department whereas the OPA2134PA is slightly better in the mid range area. To my ears, the OPA2134PA is a tad more aggressive sounding and I think the NE5532 was more to my liking but, as I say, I'll give the OPA2134PA time to fully burn in before judging it.
The resistor and cap changeover brought an immediate improvement to the sound with the 5532 OP AMP still in place and this tweak alone is well worth doing. I'm going to fit some some IC sockets to the board so I can do some OP AMP rolling without having to desolder / solder each time I replace them.
Future tweaks will include removing the tone control, possibly incorporating a crossfeed filter and a better PSU. Stay tuned!
Update. The OPA2134PA has burnt in and sounds superb.