Buffalo Tweaking

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I don't see anybody here investigating mods to the Buffalo Sabre DAC from Twisted Pear. Is there any interest in pooling here such info and peoples experiences?

Has anybody tried a different IV stage?
Has anybody tried some different PS supply configurations?

I have reason for asking this as I'm trying to help a friend realise a John Swenson IV stage (based on Gary Pimm's CCS) for the Dac. He has an IVY & Counterpoint & I thought that this IV stage might make a good comparison.

I'm OK with the JS IV stage as he has given lots of detailed info on it's workings & paramters (schematic attached). Will not be using the follower stage.

I'm looking for help with the following questions:
1.The current output of the buffalo is a high16mA, I think - if operating in SE output is this reduced to 8mA?
2 Do the IVY & Counterpoint use a resistor for converting I to V?
3. What's the best way of getting SE output from the Buffalo?


Any & all help appreciated!
 

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I too was skeptical about the IVY output for my TP Buffalo. Spent a bunch of time isolating power on the dac board for the Saber chip and clock with limited success before I started playing with PS's for the IVY. Ended up switching the ALWSR's I was using to feed the IVY, for Teddy regs. I love those things, it was a big improvement. I had also built a C multiplier circuit suggested by Rod Elliot that is supposed to be used for Class A amps, but tried it in front of the teddy reg. Another audible improvement. In my system the Buffalo is now a monster, I really enjoy listening to it. Waiting now for Teddy Pardo's Super Teddy reg pcb's.
I would be interested to hear some ideas specifically for discrete Buffalo output stage also.
 
I will soon be replacing LCBPS/LCDPS with sigma11s for the Buffalo board and a sigma22 for the Counterpoints.
Haven't decided yet if I'll build two sigma11s to power the 6 and 6.6v rails of the Buffalo board or if it's too much of an overkill and I could just build one sigma11 for the analog rail (and power the digital rail with just the LCDPS).

In any case, I'll report back with my findings.
 
Leo, over on Art of sound forum, has replaced nearly all the PS supplies with Paul Hynes shunt regs:
1 x Z1A-3v3 positive shunt regulator set to 250mA which supplies main 3.3v and onboard Crystek 80mhz XO
1 x PR3G-1v2 positive series regulator supplying Sabres core and VDD1.2v
2 x Z1703v3 50mA positive shunt regulator supplying the Sabres left and right AVCC (VREF)
1 x Z1A-15 150mA positive shunt regulator set to 150mA supplying the +15v of the IVY I/V stage
1 x Z1N-15 150mA negative shunt regulator set to 150mA supplying the -15v of the IVY I/V stage

and he found:
After powering up and having a quick sound check revealed much more detail, it sounded a little light though so I resisted temptation to listen too much at this early stage so left it alone to burn in, a few hours later the sound was far better, bass had much more depth but most of all was the improvement in the midrange !!!
Over the next few days it continued to improve to a point I was struggling to pull my self away from it, focus,detail with extremely clean and clear highs, unlike a lot of digital sources I've heard ,this unit had a total lack of HF distortion and sibilance, no grain!
The biggest plus for me was now the midrange purity which I felt was originally lacking with the standard Sabre, its actually infront of my TDA1541 dac at the moment

For anybody looking to experiment with PS these seem to be the current requirements:
I have measured the current on the outputs of the 6.5v external regulated supplys
153.6mA's for the 3.3v LDO reg (VD)
61.3mA for the 1.2v LDO reg(VA)
35mA for the +/- 15v IVY (+v side measured)
 
Here's the PS schematic:

sabrepic.jpg


Too much mixing of power supplies, I reckon.

The Va supply powering the LM4562, not surprisingly, has a big influence, as this op-amp acts as a voltage buffer for the left & right channel ref pins. The PS to these pins should also be as clean & low noise as possible. Is the op-amp a good idea?

The clock is supplied by a spur off the 3.3V main supply & should be separate & a much better regulator than a LDO
 
Those opamps have PSRR > 100 dB I think. The opamps are configured as voltage followers so whatever noise in the input will show up in the output (I think that's why there is a pi filter in front of it). Thus a clean supply at the inputs of the opamps will give you "best-bang-for buck" And this is what has been proposed to use with the "Vref" mod. There is also a circuit based on the Vref, but it is a regulator (not just a reference). This "Vref-reg" will be useful to power the clock.
 
jkeny said:
Diagram of PS mods points:
buffalo_layout.jpg


I would suggest one other hugely important mod - feed the clock with a very low noise separate 3.3V supply - I don't know if anybody has done this & can report?


I don't think you can provide power with a reference. Better provide the reference to the opamp and the opamps provide the power.
 
Glt,
As usual, be careful with these PSRR figures. The neg supply is worse than the pos & at 20kHz this is down to about -65dB - not so great after all! This can be easily bettered by discretes!

Leo, noticed significant differences with different supplies to VA!

Why the necessity for the voltage buffer? Is it just a cost cutting exercise that was on the original Sabre eval board?

I don't think you can provide power with a reference. Better provide the reference to the opamp and the opamps provide the power.
I don't know what you mean by this 3.3V is the reference, what power are you talking about?

edit: glt, can you post the Vref regulator circuit here?
 
An interesting idea was just posted on DiyHiFi by wildmonkeysects in which he surmises that MAYBE the sweet spot is to use V out & ensure your AVCC PS is
a really, really clean, preferrably open loop Avcc. No overshoots, no close phase margins if using feedback, no cheezy ceramic caps, no deriving the reference from Vdd
And this
internally, there is probably a sweet spot for the external load where the Sabre's thermal modulation is minimal. Not convinced it would be all I or all V. Dunno yet. Dunno the internal layout.

In other words forget about I to V conversion & try V out with a clean AVCC - what's there to lose trying this? this could be why Rolls likes the transformer IV solution so much?

Just some things to consider!
 
Over at the other thread, there were two designs: A "Vref" that provided a very clean 3.3 voltage reference and a Vreg based on the Vref that could be used as a regulator. The Vref was a replacement of Va at the input of the opamp. Most people experimented adding Vreg to the input of the opamp, in effect cleaning the voltage noise. You could use the Vreg design to power the analog section directly bypassing the opmap, but I don't think anyone reported doing that.
 
Hi John,

Dan here, from the SKA forum. This is what I'm using for the Analog supply to my Buffalo:

SabrePSU.jpg


Q2 is an MPSA14 darlington. Way, way better than LM317s. This is currently feeding Greg's Mini-reg to provide a nice and clean 3.3V Vref.

The clock: a separate supply with an AD797 based reg feeding a 66MHz oscillator from Guido Tent.

I'm using a Twisted Pair DIR to convert S/PDIF to I2S. Didn't make a jot of difference to the sound, but enabled 192K to work.

Analog stage is an SKpre symmetrical discrete opamp for balanced to SE conversion. The output offset is trimmable, so no coupling caps needed. Superb.

Next up ..... bypassing the LM4562 buffer and feeding the analog supply directly from the Mini-reg. Not at all convinced this will help, but worth trying.

Dan
 
Ah Dan,
I never knew your username here - I'm easy to trace between forums - I use the same username!

Nice! Does this pre-regulation into the Mini-reg improve it by much?

Is that clock supply a PFMflea?

The SKPre is just used for BAL to SE, then IV stage or what?

I'm not sure why the LM4562 buffer is there? Is it to isolate the left from right AVCC?
 
jkeny said:

Nice! Does this pre-regulation into the Mini-reg improve it by much?

Well originally it was pre-regulated by an LM317 as per the standard Twisted Pair setup. I changed to a design similar to the above, but using a Zener diode, which was an improvement. Changing to the LT1031 - plus a couple of refinements - was a further improvement.


jkeny said:
Is that clock supply a PFMflea?

Similar, I use a JFET current source instead of the 7812, plus a couple of other minor changes.

jkeny said:
The SKPre is just used for BAL to SE, then IV stage or what?

Just balanced to SE - no I/V needed.

jkeny said:
I'm not sure why the LM4562 buffer is there? Is it to isolate the left from right AVCC?

The LM4562, together with a reference, forms a regulator. But if your reference is able to supply sufficient current the opamp isn't needed. You need to bypass the 1K + 1uF low-pass filter though.
 
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