Been working on this layout for about a week so far. Features are:
- DIP8 opamps for L/R
- SOIC opamp for ground, either use a high current single opamp, or a low current opamp with a class B emitter-follower in its feedback loop
- Bypassing on all opamps via 0805 ceramics right at the power pins to ground
- 1206 resistors everywhere
- Discrete Diamond buffers for L/R, with crd ccs, all transistors in SOT23
- 2 channel design, eliminates the need to isolate the input/output jacks, and reduces current draw
- No parts requirements that require sourcing from only one place, so no shortages like recently with the BUF634, TLE2426, and OPA551
- PCB measures 49mm x 25mm, so fits my favorite case for portable, the Hammond 1455C801, with 8xAAA batteries for a long runtime, and plenty of voltage swing into the majority of headphones we use portably.
I know the schematic shows a PNP transistor for T1, T2, T5, and T6, but it is really this SOT23 CRD, I didnt have an eagle library part for it, and didnt take the time to adapt one specifically for that part.
Prototypes have been ordered, and should be here in about a week.
I have commented this design at Head-Fi.org but I didn't notice that you have SOT23 output devices. This is not a particulary rugged output stage, easy to break. I have used SOT223 types, 1 A transistors.
An externally hosted image should be here but it was not working when we last tested it.
Thanks for the suggestion peranders, looking at the datasheets for some of the transistors I was looking to use, like the MMBT3904/MMBT3906 complimentary pair, confirms what you said, the SOT223 package can dissipate nearly 3x the power of the SOT23, and reading other places, the SOT23 can only handle 100mA or so, while the SOT223 can do 1A. Because of this, I made some changes tonight, but it was quite hard to route the PCB in the space I wanted, and use these output transistors. Anyways, here is what I came up with. Note the incorrect part is again used for the CCS, both on the diamond buffers, and now replacing the DO-35 package CRD, because I just couldnt make it fit while routed reasonably.
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