Monday, December 14, 2020

O2 Headamp output board, OPA1688s Part I

NwavGuy left us with a fairly well performing, battery powered headamp. I built mine fairly early on when they got out into the wild.

It's been almost 8 years since he disappeared into the ether and in that time, plenty of new opamps have been released which seemingly (at least on the surface) look like a better deal then the NJM4556s that drive the outputs (lower distortion, offset as well as higher output current capabilities).

Nwavguy himself was fairly certain the output opamps were the bottleneck for the O2's THD figures, AGDR attempted an output booster that was based on LME49600s and OPA827s (Around 14USD in opamps per channel).

Ultimately I feel that the LME49600s were difficult to heatsink properly given the tight internal space. I do indeed think it should provide (at least for a limited time) way more power than the original 4556s were capable of, the headphone mute relay addition is also a double bonus (rid turn on/off thumps and the motorboating thing that happens when the O2 batteries get drained). 

Unsatisfied I went around looking at Opamps released fairly recently. There were 2 that caught my eye.

One of them would be the OPA1688, the other would be the OPA1622. 

Decisions, 1688s doesn't quite have the capacitive drive, the 1622 is in a tiny package that's a pain to solder and bias currents are ~1.2uA, meaning I'd need another opamp stage in front of the 1622 to drive it properly. 

I decided that parallel 2x OPA1688 (4 op amp channels) would be my design choice, my lowest impedance headphones are 32 Ohms and some back of the envelope calculations determined that even at worst case dissipation, the opamps should scrape through and not hit Tj limits. The O2 is/was all about getting performance at a lowish cost, which I think this decision would be in-line to.

External compensation  by increasing noise gain to improve phase margin was figured out with inputs from johnc124 for another project.

Power dissipation calcs:

RL = 32R

Parallel 4 = 128R effective

Pq = 24(0.018)

     = 0.0432W

Io = 6/128

    = 0.046875W

Pdmax = 6(0.046875) + 0.0432

            = 0.32445W

Temp rise over ambient = 0.324(116.1)

                                       = 37 deg C (per opamp)

                                       = 75 deg C (per SOIC dual opamp device)

To be continued in Part II

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