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).

https://www.diyaudio.com/forums/headphone-systems/244473-o2-headamp-output-booster-pcb.html

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.

https://www.diyaudio.com/forums/headphone-systems/293247-opa1688-super-cmoy-2x-9v-real-ground-headphone-relay-pcbs-7.html#post4762471

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

The Danger Zone: O2 Headamp output board, OPA1688s Part II (fillwithcoolblogname.blogspot.com)

Klipsch Promedia GMX 2.1 Hiss & Weak headphone output

Klipsch made some questionable choices with regards to the amount of gain and how they drive the headphone outputs. Mine had bad volume and sub control potentiometers so since I was gonna dive in and replace those, I might as well fix the hiss and headphone section.

Per the stock design, there's an insane 7.5 gain (just on the headphone section) regardless of where the volume control position is at, even with it off, which just makes the speakers hiss constantly (I can hear them at my sitting position) even with no / grounded inputs, I have never used the volume control past the 1/4 mark.

I've traced out just headphone section (bottom right of the control pod PCB), which is the biggest contributor to this hissing. No idea why they went with output resistors and 7.5x gain, with such a insane amount of output resistance the voltage driving the headphones just gets dissipated in them. I put in 1kHz sine wave and measured ~120mVp-p but only ~50mVp-p on the headphone side of the resistors. 

Here's what I did to fix all that. In the course of messing with this I killed the PT2308 while probing around (opps), they're kinda hard to come by in a DIP package, so I was either gonna have to put one on a SMD converter board or put a different opamp in there.

I went with a NJM4556 (which has more output current capability than the original), a gain of 1.5 (which was plenty loud on my headphones), removed and shorted out the 2 output resistors, increased the output capacitors to 470uF, upped the input capacitors to 6.8uF (probably overkill) and changed the feedback capacitor to 150pF (prevents HF from being amplified). 

Even with the stock 33pF feedback capacitor, I didn't manage to get the opamp to oscillate. A good test is probably to stick a nice sharp square wave onto the input and see if it's gonna ring a bunch, I might do that if I experience issues. 

With the changes in place. I can only hear a slight hiss if I place my ears right up to the speakers and only from the tweeter. I no longer have to only use the bottom 1/4 of the volume control and I get plenty of drive to my headphones. I'm happy, lol.