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Possum  
#1 Posted : Wednesday, January 16, 2019 6:06:58 AM(UTC)
Possum

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While some of us are still waiting for the official BIII 8 Channel PRO, I decided to do open-heart surgery by transplanting another ES9038 chip into an old BIII board for an upgrade to an 8 channel implementation I did some time back.

I can confirm that Legatos still work superbly being driven by the ES9038 in an 8 channel topology. More importantly, the ES9038 is a MEGA UPGRADE to this topology - the old ES9018 sounded flat and lifeless in comparison to the punch and resolution of the ES9038 driving 8 discrete channels.

Better yet, the default settings of the ES9038 chip worked perfectly for this config using 8 channel I2S running up to 192KHz - and meant simply removing the old BIII firmware chip.

The case isn't the prettiest, but aesthetics were not the main concern as this unit is housed in an enclosed cupboard. The primary design criterion was sound quality, and to this end the build features independently powered AVCC and XO power supplies (thus the number of transformers and regulators). This makes a HUGE improvement to the sound quality over the default topology of running all tridents from the same filthy VD power source

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Edited by user Wednesday, January 16, 2019 6:13:42 AM(UTC)  | Reason: Not specified

akras  
#2 Posted : Wednesday, January 16, 2019 6:28:40 PM(UTC)
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Nice...

Did you think about implications of longish wires that go from the DAC to Legatos? This was always a concern to me, but, on the other hand, longer wires may have a beneficial effect: their inductance may form an additional low-pass filter in front of the I/V (you want them to be the same length for balanced inputs though).

Have a look at this nice reactance calculator: http://www.mh-audio.nl/wireinductance.asp

I don't know what is the input impedance of I/V in the MHz range, but if it's still low (it probably is), then you do get a filter, which will make the job of the I/V stage so much easier and likely reduce distortions (as it will have do deal with much less MHz-range gunk coming out of the DAC). If you have a scope that works in the 50-100 MHz range, you can try to compare the signal at the DAC output vs Legato input and see the difference (if there is any)...

Do you use any high-frequency filtering in addition to what is in Legato?

Edited by user Wednesday, January 16, 2019 6:55:16 PM(UTC)  | Reason: Not specified

Possum  
#3 Posted : Wednesday, January 16, 2019 9:23:49 PM(UTC)
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I do indeed have a couple of scopes (100Mhz and 200MHz), and have previously observed the hf gunk on the DAC outputs - which is why all my newer builds now incorporate hand-wound ferrite bead inductors on the DAC outputs (the new Mercury IV apparently has hf filters but the Legato has none).

I did this build originally for a friend sometime ago. It took several days to mod the DAC chip, install additional transformer and regulater, and change the SE gain of 4 legatos to match the ES9038 - so I didn't have any time ledft to make up 8 inductors. Maybe a mod I'll do later along with isolating the USB I2S inputs...

The newer regulators I use have Murata chokes on the AC in and these also appear to reduce DAC noise. In particular, I now use a dedicated regulator/transformer winding for the AVCC (and also clock) power supplies - which significantly reduces DAC noise on the analog outputs. This gives a clearly audible improvement The improvement is so substantial I now consider it to be mandatory (the second pic shows blue/white twisted wire providing the AVCC SR module with clean 5V - the Input pin that usually feeds the reg with filth from VD is disconnected). I highly recommend this mod. It provides a far bigger and obviously noticeable audible improvement over filtering on the DAC outputs alone as it stops a large amount of crap from entering the analog stage in the first place (the AVCC regs obviously cannot remove all the noise injected into the VD supply by the digital/control stages of the DAC, and its why, no doubt, the chip designers completely separated analog and digital gnds and power inputs).


If the build hadn't required SE outputs I would have cut the Legatos in half and stacked 4 halves each side for much shorter runs from the DAC - but that was not an option unfortunately. I hope TPA come up with a modular stackable single channel I/V design for these kind of builds (hint hint)
akras  
#4 Posted : Wednesday, January 16, 2019 9:51:25 PM(UTC)
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I like the idea of inductors on the DAC outputs; I thought about it myself. A potential problem with using ferrite beads is that if there is a noticeable DC current from the DAC to I/V, the resulting magnetic field may saturate ferrite and cause non-linearity (including in the audio band).

Here is a question that I have for you: could you please measure the common mode DC voltage at the Legato INPUT (not output!) when it is connected to the DAC? (I mean the DC voltage between the ground and each of the Legato inputs; I don't have Legato, so I can't do it).

Why I think it's interesting? Because I think different I/V stages behave differently in this respect: Mercury was designed to keep the input DC bias close to AVCC/2 (just like the DAC wants by itself), while IVY seems to pull it to the ground, so there is considerable (and asymmetric) DC current coming to/from the DAC. I wonder what Legato does in this respect...

Edited by user Wednesday, January 16, 2019 9:52:47 PM(UTC)  | Reason: Not specified

Possum  
#5 Posted : Sunday, January 20, 2019 9:22:49 PM(UTC)
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will measure and let you know...
akras  
#6 Posted : Monday, January 21, 2019 5:27:41 AM(UTC)
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Actually, I looked at the schematics, and it should be close to zero. So, Legato is like IVY, and it makes the ES9038 in the 8-channel configuration to output ~8 mA DC, if I'm not mistaken by a factor of 2.

In any case, I thought about inductors in front of I/V, and I would be careful about it. The main issue is the self-capacitance of the inductors. Commercially available air-core inductors resonate at frequencies that are too low to be used after a DAC, and most likely will do more harm than good. A few of ferrite-core ones are acceptable from that point of view, but nonlinearity may be a problem. If there is no DC current through them, I would expect higher even harmonics, but with ~8 mA DC there will be odd harmonics. My very crude estimations suggest that even with the most suitable ferrite-core inductors the harmonics can be in the -100 dB range; with this level, I don't think it has any benefits over the voltage-out mode.

BTW, Legato-III does have a LP filter (~72 KHz it seems).

Edited by user Monday, January 21, 2019 5:38:51 AM(UTC)  | Reason: Not specified

Possum  
#7 Posted : Monday, January 21, 2019 5:56:33 AM(UTC)
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Well, all I know is what I hear.

I am running some ES9028s in mono mode into a Legato and initially there was a pronounced digital 'whistling' noise audible if you put your ear right up to the driver.

The ferrites substantially reduced the 'whistling'. I heard no deleterious effects to either high, mid, or low outputs when ferrites were used. I only ever heard improvements.

I use DSP speaker driver correction, I noticed no change in re-sampling the associated drivers

Make of that what you will...

akras  
#8 Posted : Monday, January 21, 2019 8:42:47 AM(UTC)
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Well, that probably means that even Legato has problems dealing with all the HF gunk coming out of DAC, and the benefits of reducing that gunk outweighed whatever problems the inductors themselves caused (if there were any at all). This is useful information...

I wonder if you can hear the same whistling noises that you mentioned with your most recent setup: 9038 into 8 channels? I understand you don't have inductors in that setup, right? If there is a difference between a 9028 mono (into a single Legato, right?) and a 9038 into 8 channels, I would guess it's because you had double the current (and double the HF gunk) into Legato in the 9028 setup. This would make sense, I think...

Cheers,

AK

Edited by user Monday, January 21, 2019 8:44:16 AM(UTC)  | Reason: Not specified

Possum  
#9 Posted : Monday, January 21, 2019 11:14:42 PM(UTC)
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Agree - BTW I was using a pure sinewave generator as test input into the DAC and didn't notice any appreciable variation in amplitude as I swept thru the aubile frequency band whilst testing that the digital xover was working as expected, nor did I see any new spikes appearing when looking at a fourier xform on the scope (only spikes diminishing).

One interesting observation is that I originally noticed the increased noise when upgrading from a 9018 to a 9028 (mono mode) - the 9028 was noisier but it does not output more current I believe. I attributed the noise increase to the increased processing power of the newer chip.

The noise is not really audible listening thru headphones (mind you, the Legato headphone buffer circuit does contain an inductor) - its only audible thru the amp. The inductors reduced the noise down to the level usually expected from any amp. In this case, a bit of phased white noise you can just hear if sticking ear up to the tweeter. I've had this same noise from an amp even when the DAC is off. I attributed it to the input leads acting as aerials (as unplugging the leads made it all-but vanish).

So the ferrite inductors appear to be quite effective IMHO.
akras  
#10 Posted : Monday, January 21, 2019 11:52:39 PM(UTC)
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Originally Posted by: Possum Go to Quoted Post
..I originally noticed the increased noise when upgrading from a 9018 to a 9028 (mono mode) - the 9028 was noisier but it does not output more current I believe. I attributed the noise increase to the increased processing power of the newer chip.


Probably: 9028 draws considerably more current than 9018, hence more ground bounce when used on the same board; I guess some of that finds its way into the analogue section...

Possum  
#11 Posted : Tuesday, January 22, 2019 12:04:46 AM(UTC)
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A large part of the DAC is essentially a computer performing real time signal processing. It's using more current because it has much more processing grunt than the 9018. More logic gates switching in the MHz band = more HF noise. Its not current per se, but current being switched at high speeds that generates the noise...



stewart  
#12 Posted : Sunday, March 29, 2020 5:15:21 PM(UTC)
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Very nice build Possum. I understand that for AVCC if I wanted to power it independently of the DAC power supply using an additional power supply, but using the trident regulation as you've done, I would omit the IN pin in the AVCC and supply external power. It looks like you have left the GND pin, so the external regulator would be directly connected to the DAC GND. I also see you've left the DVCC pin in, I understand this doesn't do anything anyways so likely fine to leave.

I had thought you would do this on both sides of the AVCC module, but I see in the examples you've posted, you've only done this to one side of the regulator. I thought the regulators are independent and power the L/R sides?

Edited by user Sunday, March 29, 2020 5:18:10 PM(UTC)  | Reason: Not specified

Possum  
#13 Posted : Monday, March 30, 2020 3:06:30 AM(UTC)
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I bridged the 'In' on both sides with a wire underneath the AVCC module that is not visible in the photo

(both IN pins are disconnected from the main board of course)

I would also recommend this approach with the XO trident powering the clock as it makes a further audible improvement to sound quality.

Once you've done that you might as well power the main board with 3.3v and only use the 1.2v trident
stewart  
#14 Posted : Friday, April 24, 2020 3:43:49 PM(UTC)
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Possum, I hope you're doing well and staying safe. I know that the L/R AVCC draws about 100mA on each side and the VDD_XO draws 25mA in the Buffpro38 DAC. If you are using double regulation (i.e. feeding power to the shunt regs) I would plan to provide 5V@ 100mA into each side of the AVCC regulator and 25mA into the VDD_XO regulator.

If I want to omit the regulators and power the AVCC and VDD_XO channels directly I would provide 3.3-4V into the AVCC and 3.3V into the VDD_XO, but would these still draw 100mA and 25mA respectively, or is the current draw less without the shunt regulators?

Also, when using double regulation and feeding the AVCC and VDD_XO regulators, I assume you've left the GND and DVCC pins (for the AVCC module) connected so that you're feeding the IN and GND pins from external power and the GND, OUT and (for AVCC) the DVCC pins are all connected to the DAC board.
Possum  
#15 Posted : Wednesday, April 29, 2020 8:51:07 AM(UTC)
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Current draw will be more with shunt regs as they shunt current by their very nature (50ma?)

Double reg will theoretically be better depending upon your power source, so if you have the TP regs you might as well do this - otherwise I get stellar listening results from the DIYINHK LDO regs
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