[diditmyself]

Goodsound, why don't you just run the test with a weaker signal. If it still shows the same distortion pattern there might be some stability issues.

 

[jcx]

op amp psrr is limited by op amp open loop gain, a relation not often explained clearly in textbooks

“A General Relationship Between Amplifier Parameters, And Its Application to PSRR Improvement” E Sackinger, J Groette, W Guggenbuhl, IEEE Trans CAS vol 38, #10 10/83 pp 1171-1181

CiteSeerX — A General Relationship between Amplifier Parameters and its Application to PSRR Improvement

which gives:

1/(CMRR) + 1/(PSRR+) + 1/(PSRR-) = (1/Adiff) * Zload/(Zload+Zout)

as a fundamental relationship for the standard op amp

 

[goodsound]

I did the reverse phase test again but got same results, however -

I ran the regular thd and imd tests with lower signal levels and lighter loads and it did make a big difference to the IMD, (but not as much to the THD).

The weird IMD disappeard when I lowered the volume of the amp as well as when I reduced input signal going into the amp. For this test I even reduced the load to a purely resistive 200ohm load (resistor). But do note that these volume levels and loads are ridiculously light load for an 'amp'.

Also, the volume level at which there is no imd with the resistor, the same imd exists when I simply change it to the headphone - indicating that the reactive and higher load is causing the imd.

It still doesn't prove for sure that its the opamp's drive capability, it could be the supply rail capacitance or its impedance, because of the fact that the Headsix uses the single version of the same dual opamp and does not suffer from this issue.

On another note I measured the input impedances of both the amps. The GV6 came out at ~9k which is pretty standard, but the Headsix unlike what it says on its website wasn't even close to 10k, it came out somewhere around 66k! Not sure what that is about?!

 

[diditmyself]

Can't you draw a schematic of the Go-Vibe and post it here. Since it's obsolete and more or less could be called a generic circuit based on the works of ppl and amb, I can't see why there should be any copyright issues.

I don't have any AD8620 left to test it with the oscilloscope, but I can simulate the circuit and see what shows up.

 

[diditmyself]

Goodsound, some examples of opamps at unity gain http://www.head-fi.org/forums/f6/opa...y-gain-426887/

 

[jcx]

the pcb photo looks like the HeadSix could have input coupling caps - did you really measure Z@1KHz or just DC R?

You could try reading the pot value/part # too

 

[goodsound]

yes the Headsix has input caps and I did measure the AC Z (not DC R).

unable to get the part# on the pot though.

 

[jcx]

then I would look at the Y input cable test with soundcard driving both amps together to see if the load causes the soundcard to distort