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Old 2nd July 2002, 11:11 PM   #1
rljones is offline rljones  United States
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Default Sowter/Pioneer AX-10 Mods

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
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Old 3rd July 2002, 07:44 AM   #2
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Default Re: Sowter/Pioneer AX-10 Mods

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 [/B][/QUOTE]
--------------------------------------------------------

Robert

I did this with a SCDBX940 using the Sowter 9360 600/2.4k unit, and with the AD825 as I/V.Two points:

1. The Sowter adds distortion and picks up (inaudible but measurable) hum. Performance is akind to good analogue preamp and provides a tube like, liquid sound. However, I have concluded that it isn't necessarily better after many comparison trials. The sound depends a lot on any filters (first order) connected to the secondary.

2. Current loading the AD825 to at least 2mA with a cascode circuit connected to the -ve rail brings much better benefit in both the treble, and surprisingly, the bass.

The waveform from the I/V stage looks clean and if one uses a valve preamp, offers the chance to delete most of the following circuitry and perhaps balanced connection with passive lf filtering.

There is a major issue with switching noise with coupling methods that bypass the relays. Modders be careful!
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Old 3rd July 2002, 05:11 PM   #3
rljones is offline rljones  United States
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fmak,

I'm not using any filters on the secondary side. Is this a mistake? I thougth that since the 24-bit DAC is sampling at around 1MHz (per data sheet), that the natural roll-off of the transformer was sufficient, allowing one to thereby bypass the 2nd order Butterworth that are so commonly used.

I'm running the signal from the transformer into my preamp, which uses only a PGA2310 and likes a 600 ohm load (such as the 3603 provides). Do you have other transformer recommendations, considering the output impedance should be around 600 ohms? (Space is also an issue and the Sowter is reasonably small.)

Current loading the (-) rail may be a little difficult (possibly on the adapter?). How did you do this, with a jfet, any bypass caps?

Thanks, Robert
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Old 3rd July 2002, 11:25 PM   #4
dorkus is offline dorkus  United States
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Default weird

i find it interesting that your analog mods improved SACD reproduction to where it is comparable to DVD-A now. i would think that the mods would improve both DVD-A and SACD equally, so the margin between them would still exist. i also thought the AX10's limiting factor with SACD was the conversion of DSD to PCM for the PCM1704 DAC, which no amount of analog tweaking would be able to solve.

also, my exposure to DVD-A is very limited but from what i can tell it sounds very different from SACD. not better or worse (that is a subjective call), but different. but then, if DSD is being converted to PCM internally anyway then i guess it would make them sound similar.
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Old 4th July 2002, 12:18 AM   #5
rljones is offline rljones  United States
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dorkus,

You're correct, all signals, DVD-SACD-CD, in this unit go through the PCM1704s. I don't know where the DSD conversion section is (I don't know digital circuitry and wouldn't know what I was looking at if I saw it).

When I said the SACD section sounded better, I was comparing the same SACDs pre- and post-mods in the AX10 to a Sony 333ES. I didn't have the same recording on both SACD and DVD to make even a rough comparison. Since the AX10 converts all to PCM, even if I do such a comparison, it will still not tell me whether PCM or DSD sounds better. Rather, with the extra stage in the DSD decoding in the AX10, I cannot imagine it could sound better than a top DSD unit. I believe I can say, however, that the modified unit sounds better than before it was modified, and better than the semi-modified Sony (which is not a top-of-the-line unit).

I seem to have read that many currently available SACDs are re-released PCM (CD) recordings, so this may contribute to many of those that I've heard that are only so-so. With so many potential variables at this stage, it's difficult to say one is better than the other. I didn't mean to imply that. All formats were improved with the mods, but I cannot quantitate the amount of improvement for each format. (How many LPs do I have that are only so-so, and variable in terms of recording techniques, artists, venues, etc--yet I still prefer good vinyl.)
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Old 4th July 2002, 12:27 AM   #6
dorkus is offline dorkus  United States
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agreed. comparing formats is pretty tricky because of all the variables in the recording chain. i also agree with you about preferring one format in general despite variables... i have a few crappy-sounding SACDs but i still like SACD better than CD, even if a good CD sounds better than a bad SACD. i think even with the variable recording quality, it is still possible to get a handle on the "sound" of a format if you listen to enough recordings. PCM and DSD sound very different to me. vinyl is great too but i'm a child of the digital age and don't have the patience to deal with it tonearms and needles and brushes.
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Old 4th July 2002, 09:04 AM   #7
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Quote:
Originally posted by dorkus
agreed. comparing formats is pretty tricky because of all the variables in the recording chain. i also agree with you about preferring one format in general despite variables... i have a few crappy-sounding SACDs but i still like SACD better than CD, even if a good CD sounds better than a bad SACD. i think even with the variable recording quality, it is still possible to get a handle on the "sound" of a format if you listen to enough recordings. PCM and DSD sound very different to me. vinyl is great too but i'm a child of the digital age and don't have the patience to deal with it tonearms and needles and brushes.
--------------------------------------------
It is difficult, if not impossible to compare DVDA and SACD on commercially available software. There are decent SACD discs but very few hirez DVDAs. The 192 tracks sound very good but most of the 24/48 discs are ordinary and no better than DAT. This is why DVDA hasn't btaken off - I for one won't buy new DVDAs until the situation improves.

On the other hand, DVDV 24/96 discs do sound very good through a good DAC and comparable in some ways to SACD in terms of improvement over 16/44. Rationally, 24/96 and 24/192 in a video format would have sufficed if there were no need for surround.
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Old 8th July 2002, 08:09 PM   #8
rljones is offline rljones  United States
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Default Listening Follow-up

I was able this weekend to compare the modified AX10 as described above, to a Bow Technologies ZZ-8. Prior to modification, the Bow was decidedly better with regular CDs and even more so when playing HDCDs (which the Pioneer cannot decode).

After the mods, the AX10 played regular CDs better than the Bow with better depth and air. (I had a friend over who has professionally played violin and another who plays clarinet; both concurred). The HDCD discs on the Bow were slightly better as compared to when the same discs were played on the AX10 without HDCD.

Another test was using two copies of 'Nature's Realm' by Water Lily with the modified AX10. One is on DVD-audio, the other, SACD--both of the same recording. Listeners preferred the DVD-audio, but this was very close. Slightly more air about the instruments was noted with the DVD-audio version. However, as mentioned previously, this unit sends the DSD signal throught PCM processing, so it is not a truly fair comparison of the formats, except on this machine.

Regards, Robert
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Old 27th December 2002, 07:14 AM   #9
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Hmm, did you measure 15V rail? If you did you scare me a bit, as I didn?t measure (maybe a big mistake) but trusted the service manual which states its 12 V. I have already replaced the 2134 with the AD8610. I?m very pleased with the result. If the rail is 15V I guess I?m frying the OP-amps sooner then later.

Where did you buy the AX10? I thought all American sold AX10 had the AD DACs and not the BB 1704 as the Japanese version. Can you also run your AX10 as an standalone DAC?
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Old 27th December 2002, 05:28 PM   #10
jam is offline jam  United States
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Robert,

How about following the AD825 with a buffer? I believe Jensen might make a better transformer.

Jam
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