Those on the front are RCA outputs from a simple preamp circuit…to go to a AVR or another amp.
The green leds are the +/- power rails and the led thats off is a clipping indicator, gain is 1x, 2x, 4x and 8X. Never use the 4 or 8 x positions.
The build is documented in detail at diyaudio.com
and here: https://www.audiocircle.com/index.php?topic=125353.0
Best new stuff:
- · Up tp +/-16Vdc power rails with adjustable regulators for up to a 11Vpeak swing. Useful for high impedance headphones.
- · Lower noise voltage regulators, LT1963A and LT3015. Probably won’t make any audible difference, though.
- · Twice the output current capability and power dissipation - 280mA per channel. Useful for low impedance and low sensitivity headphones.
- · 4 NJM4556A chips to handle the current and +/-16Vdc dissipation, two per channel. Uses the SIP 8 pin inline version, NJM4556AL.
- · NJM2068 replaced with OPA627, which is now in a feedback loop with the NJM4556 chips to null out DC offset and reduce distortion even further. DC output offset voltage should be around 0.3mV = 300uV per channel.
- · Has input RCA jacks and output ¼” Neutrik jack in addition to better (Switchcraft) 3.5mm jacks.
- · Bass boost circuit – switchable on/off.
- · Rotary gain switch with 4 gain settings.
- · Relay-based no-thump circuit that waits 2 seconds to switch in the headphones and then drops them out quickly on power switch-off.
- · Should have even lower background noise than the O2 headphone amp at high gain settings. 4 layer PCB with full middle ground plane.
- · Volume pot is on the input now rather than the middle of the circuit, so it can attenuate “hot” sources as much as needed. Still no pot turning noise.
- · Coupling cap is on the input, 4x as large to work with the 10k pot vs. 40.2k resistor in the O2, to block all incoming DC from the source.
Some of this changed during the build discover process, but for the most part it has what Rocket Scientist mentioned before he dis-appeared.
No batteries, both 1/4" and 3/5’’ head outs, more power, lower noise etc…it has even driven a set of Klipsch high efficiency speakers to fairly high levels! Updated to better MTBF parts, like the low cost push buttons and switches in the original O2.
A complex build, lots of small parts…but very rewarding…excellent operation.