I'm thinking about building one with 2 channels and later on to convert into 4 channel Beta22. Is it possible?
Thanks in advance
"balanced" headphone cable, 4-pin connector connecting each driver to its own amp board's gnd gets most of the way there with just 2 board build
while only 12 Vpk is needed with the HD800 to reach 120 dB SPL, you could bump the B22 PS to the max +/- 30 V spec for even more overkill factor - dropping the standard output bias to reduce heat still leaves huge current overrating vs the 40 mApk the HD800 would be drawing at 120 dB
adding a second, fully isolated power supply xfmr®ulator board for a "dual mono" build with 2 now completely isolated amp boards carries the overkill theme to another level
for the extreme audiophile cred you could add a real balanced audio input with actual common mod rejection - a pair of really good audio input xfmr from Jensen, Cinemag, Sowter, Lundahl....
will cost about the same as 2 more B22 boards if you get the really good ones - could make up the difference by still using stereo volume pot with xmr input instead of springing for a quad
with galvanic isolation on input, separated driver gnd returns, the HD800 only needing about 1/8th the B22 Class A output, the B22 circuit inherent high PSRR and regulated supplies - there is really little reason to think 4 boards would be technically better - no reason at at all think the differences would be audible "ears only" - ie with controlled blind listening test where expectations are removed
the system interface improvement of input isolation would likely be the biggest objective improvement that may have audible consequence - the separated gnd return wires of a "balanced" headphone cable would give measurable but not likely audible R,L crosstalk improvement over TRS
So although you're very keen on the idea of fully balanced Beta and taking it to the extreme, you think there will be no audible difference between a well built 3 channels and 4 channels Beta22?
I am trying to make the most from the least. i.e, I don't want to spend much.
all of my post was tweaks for 2 channel, 2 board B22 - for the HD800 required Vswing, power there is no need for 4 board bridged output - or for any but a very few headphones - HE-5,6 would be the most popular current production examples
a 4-pin "balanced" cable/connector between headphone and amp can be used with 2 boards, will give measurable improvement in crosstalk, "gnd contamination" that is cited by the misguided "3-channel" fad
I don't expect from the numbers for a 300 Ohm load like the HD800 that there is any chance of hearing the difference of 4-pin connectoin over TRS - but it does improve easily measured numbers, settling for the poorer technical performance of TRS bugs me as an engineer
and I absolutely don't believe any custom cable is needed - just replace the TRS connector with a 4-pin connector, multiple contacts, higher pressure will make all of the contact R much smaller than single point cantilever in most 1/4" TRS
a often cited reason for bridged/balanced output is distortion cancellation
4-channel, bridged output purported distortion cancellation is sensitive to exact matching of components, feedback is high enough in the B22 that the residual distortion result will be determined by 2nd order effects in the gain transistors - stuff that is never measured, matched
I don't expect distortion cancellation to be effective, even if it were, single board B22 even harmonic distortion is orders of magnitude below any demonstration of audibility
so bridged/balance output drive with 4 boards doesn't at all bother my sense of engineering necessity for the HD800 load - 2 amplifier boards is all that is needed - all the mentioned tweeks are for use with a 2 board B22 build
isolated input however...
audio line xfmr isolation on the inputs would affect system level interface/integration issues, breaking ground loops, common mode rejecting chassis power line coupling, the bandwidth limitation could improve RF/EMI immunity
line input xfmr also enable balanced input for higher end audiophile source - but the advantages apply with single ended source too
dual mono PS, split bobbin EI power xmfr, shielding, building in a "power conditioner" may have some similar system effects to the audio line input xfmr - and some unique advantages - but are hard to guesstimate relative importance/cost effectiveness