I recently acquired a Pioneer AX-10 and found the sound exactly as described in a recent Stereophile review: very good DVD-audio, but SACD and CD is so, so.
Opening up the AX-10 (to the horror of my family) I found a about 12 OPA2134 serving as I/V, filters, and output buffers for PCM1704s. Main channels have a balanced pair of these DACs with one apiece for each surround.
My initial modification was to change all op amps to AD825 (15V rails precluded my favorite AD8610/20) using BrownDog adapters, bypassing all output electrolytic and adding 100 nF decoupling caps for the AD825s. The improvement was noticeable, but there was still some mid freq difference between this unit and a BOW CD player I've used as a reference. At this point, the modified AX-10 played SACD slightly better than an Sony 333ES with a Modwright change.
I then turned to Sowter transformers. I tried various models (the load is a solid state preamp, so their DAC-tube transformer wouldn't work for me): 4935, 8230 and 3603. I tried all 3 types taking the signal off of DAC directly, using the transformer as the only load, using a resistor in parallel with transformer primary, and finally, taking the signal off of the AD825 used as an I/V per the PCM1704 datasheets (they used an OPA627 as I recall). The latter method worked best, and sounded best with the 3603 used as a balanced output transformer. (One warning is not to use the 8230 off of the op amp I/V: its impedance is too low and made the AD825 rather warm.)
So now it's the PCM1704 to AD825 I/V to Sowter 3603 to preamp. I've eliminated many op amps and the soundstage and dynamics are improved. I don't yet have the BOW available for comparison, but the SACD now sounds as good as the DVD-audio.
Regards, Robert
Opening up the AX-10 (to the horror of my family) I found a about 12 OPA2134 serving as I/V, filters, and output buffers for PCM1704s. Main channels have a balanced pair of these DACs with one apiece for each surround.
My initial modification was to change all op amps to AD825 (15V rails precluded my favorite AD8610/20) using BrownDog adapters, bypassing all output electrolytic and adding 100 nF decoupling caps for the AD825s. The improvement was noticeable, but there was still some mid freq difference between this unit and a BOW CD player I've used as a reference. At this point, the modified AX-10 played SACD slightly better than an Sony 333ES with a Modwright change.
I then turned to Sowter transformers. I tried various models (the load is a solid state preamp, so their DAC-tube transformer wouldn't work for me): 4935, 8230 and 3603. I tried all 3 types taking the signal off of DAC directly, using the transformer as the only load, using a resistor in parallel with transformer primary, and finally, taking the signal off of the AD825 used as an I/V per the PCM1704 datasheets (they used an OPA627 as I recall). The latter method worked best, and sounded best with the 3603 used as a balanced output transformer. (One warning is not to use the 8230 off of the op amp I/V: its impedance is too low and made the AD825 rather warm.)
So now it's the PCM1704 to AD825 I/V to Sowter 3603 to preamp. I've eliminated many op amps and the soundstage and dynamics are improved. I don't yet have the BOW available for comparison, but the SACD now sounds as good as the DVD-audio.
Regards, Robert