Updated: 11-Jan-03 17:59
Doede's Audio Tweaks...

Tweak of the week: The Sony 777 SACD Player: Transformer Output, By Passing the OPAMP Filter / Buffer Stage

Introduction (March 16th)
Who read my column, now I was already very satisfied with the sound of my new toy, the Sony 777 SACD player. Actually this is an understatement. de "777" actually is VERY GOOD !! I am still breaking my head over the fact how they could make an OPAMP Stage sound so good.... But deep in my heart, I still know what the normal effect of opamp low pass filters is; due to the negative feedback to create a 2nd order filter (there are 2 stages in de signal path !!), you are always too late.... Not having negative feedback normally gives you a cleaner sound and more spatial soundstage. If the OPAMP is very good, the effect will be less. The used Burr Brown OPAMPS (OPA2132) belongs  definitively to this group. So why bother ???? Very simply, I just NEED to try this!! The DDDAC 2000 proves how good a simple Transformer Output can be...... For this purpose I already had 2 Sowter Transformers, the 9360, which is a 1:2 step up with a bandwidth over 100kHz.

Description
This little project is not as simple as it looks like. The DAC used in the 777 is not a PCM63 like with just 1 single ended current output. The PCM1738 used here actually is a stereo DAC, but here it is used in double symmetric output. So you have 4 current outputs per Channel...... To be honest, I still need to think on to to do this best. Do not forget, that the 777, for stereo, is paralleling 3 DAC's (the 3 DAC's you otherwise would use for 5.1 multi channel mode). So per channel you will end up with 12 current outputs to be made 1 single current output.......This will be the next tweak. Back to the DAC stage: In the schematic below you see the first part of the DAC conversion and filtering. You immediately recognize the DAC, the I/V resistors, the I/V converter with the first OPAMP followed by 2nd order Butterworth filter and balanced-to-single-ended-converter. After this stage there are another 2 OPAMPS with a summing amplifier (remember the 3 DAC's) and another 2nd order filter. Than the signal goes to the cinch connectors through an Elco in parallel with a film capacitor. WOWWWW, that is a lot of OPAMP and negative feedback !!

Details on how to do this:
Pictures can tell much more than words, so have a look for your self. Click the little pictures for a large one with more detail.

How does it sound????????
Almost forgotten: How does it sound, bypassing 3 Opamp stages and a lot of components ?? You would expect BIG differences, eh? Well to be honest I did as well...... and? Guess what, the difference is SMALL !! Why? because this analogue stage is so well designed, it hardly messes the signal up. So it is hard to gain giant leaps here. BUT it does sound BETTER and I will not reverse this tweak.  I have been listening to both CD and SACD and the "only" difference it makes, is that the last bit of sharp "SSS", which is so typical for OPAMP feedback is strongly reduced. This means 2 things:

1. The whole sound stage is gaining rest, the music sounds more relaxed (not brown, laid back sound, no no no! it stays dynamic and transparent)
2. As the little details gain on explicit sound reproduction (no more sssss) the whole sound stage opens up a little bit. Focus is getting better.

Update March 17th
That's pretty quick, eh? Well, after I finished and uploaded the article, I have had some brainwaves. So this afternoon I spent some interesting time with my scope/FFT analyzer and a test Disc in the 777....... Before I start spitting my idea (not built yet...) on the final OPAMP less TX output, let me share some interesting measurements on the solution above (which is any way simpler, than what I have in mind for the other one, by the way, so it is still a valid tweak...).

Conclusion ??
This very simple tweak is absolutely worth it. The measurements proof, that every thing is ok. A little drop in Bass ? well I could not tell before I saw this pictures.... or may be I did not play the right CD yet..... but straight till 30Hz is not bad at all.

And now the serious stuff: the Final Solution without OPAMPS (March 17th)
OK, to be able to set up the design on how to get the all the 24 ( !!! ) current signals out of the all the 6 PCM1738's in the 777, we still need to do some measurements first, to know what we have in hand here:

• The internal resistance from the Curentsource (I will spare you the calculations I made based on triple references) is approximately 700 Ohm. There is no hard rule, but you want to load a curentsource with a far lower impedance than its internal !! 600 is FAR too high. ZERO Ohm would be optimum, but than there is no Output-voltage left, so what now? let's go on first and see later

• I definitively do NOT want to demolish all of the audio boards in the 777.  Actually I want the normal Sony Output to be active. Not only for stereo, but also to sustain the 5.1 multichannel output.

• I want to be able to switch between the old and new situation. This can only be done by relays, too many lines need to be connected (24 Iouts remember?)

• I have very good experiences with the Sowter TX 8347, specially made for DAC applications, better linearity and bandwidth. the ratio here is 1:18.7 (I have always found this a very strange ratio.........but who cares?) With this ratio, we only need aprox 100mV or so.

1. We actually use the 47 Ohm resistors which are on the audioboards already and are very easy to access as "patch-point"

2. At the DAC side of the 47 Ohm resistor the Output voltage from the DAC is available IF we ground the other side. You could argue that the minus input from the OPAMP is already ground level..... Yes it is, but it is a virtual ground, so the OPAMP is STILL in the signal path, No, no, no, by the relays we just connect this point to REAL ground. The OPAMP has no problem with that, and by doing so, we more or less ISOLATE the whole OPAMP stage from the DAC, without desoldering or cutting PCB routes.........

3. Through a whole series (24) of 100 Ohm resistors we more or less "add" the 6 positive outputs and the 6 negative outputs per channel. Why 100 Ohm ?? just a round value, could have been 47 or 150 as well. Just a hard wire could also work, but I want to make sure, that the little offset differences in DC at the Current outputs are NOT leading to too high offset currents. 100 Ohm seems a good value to me, so let it be.

4. The positive and negative are now loaded by the TX which makes it single ended and is removing all common mode error signals and filtering the high frequencies as well.

5. The source Output will be 150mV eff. This is all very nicely within a wide range of the supply rails and with the 1:18.7 this will bring the output to 2,8Volt RMS. More than enough for my Line amp.

1. From the DAC perspective, to see if the load is not too high. As you see it is not and the filtering is done smoothly and outside the audio band

2. Perspective from the input circuit from the Line amp. Do we have enough Voltage ?? Do we have a low impedance source to drive the line amp ??

Update March 19th
Just a quick one..... below is the circuit I am going to build, not the easy patch as the other solution, but I have great confidence in the outcome of this Tweak (more project than Tweak, isn't it??) You can click the picture to navigate around at double resolution (or copy and paste it......).

Finalization (January 10th)
I finally find some time to actually start building this tweak, which is quite some work to be honest. The principle is extremely simple: Through a bunch of relays the Current Output from all 6 PCM1738 DAC's are bundled into 4 signals, Left / Right and for both POS and MIN. This combined signal (therefore all the resistors) is being up lifted by the Sowter Transformer, Case ready. The most precise work is the wiring in the Sony 777, not to mention the disassembly of the chassis. I will limit my self now to the image details of the construction and (of course) the sound impression.......

Actually Built Version 2.3 shows here:        Click the circuit for double Resolution !!!!

Or click here to get the full version 2.3 PDF document

The 777 Sound  Motherboard, location very close to the stereo output cinch

The 777 Sound  Motherboard, but now with the relay , switching the output terminal between Factory output (OPAMPS) and the transformer output. The yellow wires are the wires coming from the Comparator circuit which is driven by the headphone volume control....

This is how the PCB is hard wired, All Silver Wire !!!

Preparations for the Silver Wiring of the DAC boards to the PCB...

At the front you can clearly see that the DAC boards # 2 and #3 are taken of the motherboard. This gives room to solder the wires...

Detail of where the signals are being "tapped" and lead to the relays PCB... Very careful soldering here !!! Before you know the small SMD resistors fall of !! Quite some work to get them back.... Speaking from experience, ahem....

OK, at the end, when all 3 DAC boards are wired, this is the result !

The same, but from a different perspective... The Sowter Transformers can still be recognized. They are glued at the chassis, right under the signal wires...

This is how it works with the headphone: When the Phone Level Control is turned clockwise, the LM311 comparator will change state and drive the relays to take the Transformer "position" and the signal LED will shine through the Phones jacket  (just glued the LED behind the 6.3mm jack) CLICK here for details on how to make the connections.....  When turned the other way, the relays  will fall off and the original factory circuit is complete back in place without any interference (well, one relays contact than....)

The burning Question: How does it SOUND ????
When I bought the Sony 777, I was  wondering how OPAMPS could sound so good. Actually they really do in the 777.... So I was very curious if the transformer tweak could do any good here !! The day the tweak   was ready I plugged  in and was a bit disappointed..... there was hardly any difference !!! All this work for nothing and are OPAMPS really ok than ??? I stopped   listening and let the 777 play day and night in the transformer position. After a few weeks this really made a difference now !!! It seems that the MU metal in the Sowter transformers needs quite some time to get burned  in. Good to know for the next time...

As know by now (yes ??) the transformer output can be selected by the former headphone volume control, so I can try it over and over again, real A-B comparison. Compared to the transformers, the OPAMPS sound harsher. Remind: RELATIVE !! OPAMPS can sound much much sharper !!! It is a very relative world we live in.... The 777 with the opamp output is indeed very good, actually I have not heard of any thing better in this price class (and above...) so far. Than you turn the knob and hear how good it becomes with a transformer output. The gain is actually in the pinpoint definition, depth/width of the soundstage and the clarity of the high frequencies, very very smooth, very analogue..... This was definitively worth it !!! For a short while I thought OPAMPS could do the job, but now I know better, they are a limiting factor  at the  end after all.... In the  meantime I do not need the signal LED, Sometimes, when I sit down to listen some music and up goes the eyebrow, what is wrong here??? MMHHHH, missing something, check the LED and yes..... need some turn clockwise.....

Happy Listening and Building !!!!

Doede Douma

IMPORTANT: The information provided on this page is intended as guide for DIY activities and therefore free to copy and or publish. If any one wishes to use any of the information from my WEB site, please make sure to refer and footnote to my URL Link as source! Doede Douma