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Old 14th January 2012, 03:24 PM   #121
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Originally Posted by Joshua_G View Post
I believe your DAC has volume control. Does it?
Yep, that's one of its key features - the ability to drive the poweramp directly. The volume control I'm using is an Analog Devices analog DC-controlled attenuation part - so no contacts, no switches and definitely no VCAs
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Old 14th January 2012, 04:22 PM   #122
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Hi,

Quote:
Originally Posted by abraxalito View Post
Yep, that's one of its key features - the ability to drive the poweramp directly. The volume control I'm using is an Analog Devices analog DC-controlled attenuation part - so no contacts, no switches and definitely no VCAs
Yes, that is an interesting part, but it most definitly does have switches, Fet ones at that and under conditions where they are exposed to some serious current.

Still, I'll have to get me some and try them.

I like the combination of analog volume, low-Z in GM stage (open loop it would seem) and TI output. Oh and let's not forget, massive bandwidth which puts the video Op-Amp's I played with in the last millenium to shame. Might be fun.

Shame it does not glow nicely, at least not very long...

Ciao T
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Old 14th January 2012, 08:27 PM   #123
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Default Veroboard TDA1541 Part 2

Folks,

A while earlier before various little waterglass stromes we were discussing the "easy way" to make a decent DIY DAC with TDA1541, without recourse to hacking up kits and with better resulting better performance.

Part 1 discussing the general circuit is here:

Any good TDA1541A DAC kit?

Incidentally, if anyone fancies making a drawing of the circuit discussed I will not object... I lack the time.

The next step is lay out your vero board.

I find it usefull to get either the real parts, or paper cut-outs to serve as stand-ins and to push them around until I get physical relationships that please my sense of symmetry but more crucially minimise relevant (AC) current loops.

And remember PCB's have two sides and often the shortest way to connect two points on top of a PCB with a capacitor is to have the capacitor on the bottom.

Also remember, with prototyping boards we are not limited to using PCB traces - no-one is stopping us running wires or even isolated copper tape over ground-planes to create structures that would as a real PCB need at least four layers, possibly six. Think about playing startreck 3D chess for a bit to get the "flat world" perspective out of your system.

A particular tick I like is to "bridge over" the power lines using an axial choke (looks like a resistor, mostly), spaced above the board using ferrite beads, with each spacer/bead being one of two formulations , the choke itself usually looses it at a few MHz at best, a "low RF" ferrite bead then takes over and as this looses effectiveness a "high RF" ferrite bead (like >100R @ 100MHz) takes over. Any shunt element are on the "far side"of this arrangement.

Guido Tent's document on decoupling contains all we will need for this build, all shunt elements (capacitors, regulators - for AC even series regulators are actually shunts) need to offer a minimal loop area. Modern IC's often have adjecent ground and power pins ideally spaced for 0603 or 0402 capacitors. See the CS8414 photos earlier in the thread for an example - the 0603 size SMD cap's are 1uF, the 1206 size SMD cap's are 10uF.

For those to whom 0603 or 1206 may as well be chinese, these sizes in inch of the capacitor/resistor etc. So 0603 means a part that 0.06 of an inch long and 0.03 of an inch wide. When buying these I tend to buy at least 10 times what I need as soldering them by hand under aggravated circumstances means I loose more than I solder, many more.

Alas, all the chip's in our "DAC requiring I2S source" build are DIP and laid out in ways that are very un-modern and at best force very long loop areas. It cannot be helped, but emphasises even more the need to get the layout right.

So there is little point bothering with 0603 parts or even X2Y ones. John Brown's use of Picogates is definitly the way to go, especially with 4 layer PCB's and 3D Layout, but I can barely see them things and I refuse to hand-solder them (I absolutely draw the line at 0603).

There is a price to pay for using DIY friendly methods. But given what the "commercial competition" turns out we are quite safe even with these old methods.

An important consideration is always "where does the current flow"?

For example, the I2S attenuators draw current from the driving IC.

By making the reclocker IC a 74HC175 Quad Flip-Flop and loading both outputs of each flip-flop equally we avoid most of the problems modulating the powersupply, but the current will still show glitches and it wants to return to the ground pin of the 74HC175, NOT to the TDA1541... So make your arrangements accordingly.

On the other hand, the TDA1541 inputs draw VERY little current, so we can connect them from the I2S attenuators with fairly long and fairly thin wires (don't overdo it to prove a point though).

This very simple and strait forward rule of "current flows in loops and and long loops mean trouble" is missed by many.

As it stands using 1206 SMD parts for the 74HC logic in DIP format (that is parts with pins that go through the board, not SMD) will do.

For the TDA1541 use the same SMD Film Cap's for both decoupling the DEM Pins (14pcs in total) and to decouple the supply pins (3).

Using 0.1uF Panasonic types in 1210 format (stand them on edge to fit) means they are effective to around 12MHz. As the TDA1541 is bipolar current steering based, it does behave rather different from modern CMOS chips in terms of PSU noise and has no real problems with this kind of decoupling, a CMOS chip on the other hand would be a disaster area decoupled like this.

For the TDA1541 all the decoupling capacitors should be placed below the Chip pointing inwards, with a solid ground plane under the chip. Philips has this implemented in some models, but with very poor quality capacitors and a "ground-plane" that looks more like a knitting project:

Click the image to open in full size.

image Courtesy of Lampizator

Translate this arrangement to a solid copper foil ground plane and using SMD Film Cap's soldered directly to the TDA1541 Pins and you are already miles better than anything else out there with TDA1541, save a few very select exceptions. The same idea mostly works for the rest of the logic.

All of this brings us to the ground-plane. Now vero-boards don't have one.

But you can very nice copper foil that is much thicker than what is found on most PCB's and is rolled from solid oxygen free copper instead of being electron deposited on some plastic at arts and craft shops as well at some electronic supply houses.

Above we where experimenting with layouts using vero-board and real parts or paper cutouts to find arrangements that work, now use paper cutouts to work up a ground plane arrangement that leaves all the pins we need to access at least one pad space and at the time links all grounds with as low an impedance (short distance, wide track) as possible.

Once you worked it out in paper (several times if needed), cut two copies out of the copper-foil using an exacto mini knife, with all the sections removed where we need no shorts.

The way this can look is shown by Naim in this Image:

Click the image to open in full size.

image Courtesy of Lampizator

The arrangement of the decoupling capacitors however is way suboptimal, avoid copying this, after all, we want high performance here!

Next, attaching your copper foil ground-plane to vero-board.

You need to tin one side of the copper foil, if the vero-bard is not pre-tinned (nowadays they are often gold-flashed) tin all the pads/vias the copper foil is going to cover as well, except those that will later hold IC pins (yes, planning is definitely required).

Paste you copper-foil on top, start by heating your copper-foil with a big soldering iron in corner, so you can make sure it positioned perfectly and if needed slightly shift it to fit. If you get this wrong you will be needing new vero-board and copper-foil, BTW.

Okay, now we solder the whole surface of the copper foil to the vero-board, which will incidentally also tin the surface and stop it oxidising. do not solder where IC Groound pins will be, we solder that later.

Did I mention using brasso to clean the foil super shiny? - probably not, well do it before you cut and use cotton gloves handling the foil, or not, depending how neurotic you are.

Having pasted one side of the foil use a sowing needle (and thimble) to punch the holes where the future IC (ground) Pins will go. Hopefully you did not solder them, otherwise you might as well re-drill them. Repeat for the other side.

Fit the IC's, decoupling cap's etc. and then run the needed wires and foils for signals and supplies.

If you spend some time and effort on this you will have a result that most 4-Layer PCB's cannot match, in terms of optimum current loops and ground impedance.

Of course, you can also integrate your oscillators (whatever takes your fancy) and other parts to implement whatever interface you like and with isolation in the same manner, many of the parts needed for this are still available in DIP format and while this is by far from optimum, you can easily do better with well implemented DIP parts than most get with badly implemented SMD parts.

Or you can buy one of John Browne's DAC's or if you are more a BIY than DIY type and all of this is a bit much, audition some decent commercial implementations and buy the one that combines affordability (to you) with good sonics (to you)...

Ciao T
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Old 14th January 2012, 08:50 PM   #124
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Hi Thorsten,
Thank you.

Quote:
Originally Posted by ThorstenL View Post
The next step is lay out your vero board.
Should I'd go for it, I'll invest in real 4 or 6 layers PCB.

Quote:
Originally Posted by ThorstenL View Post
Of course, you can also integrate your oscillators (whatever takes your fancy) and other parts to implement whatever interface you like and with isolation in the same manner
I will embark this project only if I'll have a very good SPDIF -> I2S schematic.

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Originally Posted by ThorstenL View Post
Or you can buy one of John Browne's DAC's or if you are more a BIY than DIY type
John has no DAC to sell at the moment. His latest version is still being developed.
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Old 15th January 2012, 04:35 AM   #125
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Quote:
Originally Posted by ThorstenL View Post
Yes, that is an interesting part, but it most definitly does have switches, Fet ones at that and under conditions where they are exposed to some serious current.
Chapter and verse please. I have found none from reading the datasheet so would appreciate a heads-up. Perhaps there's something I missed
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Old 15th January 2012, 05:12 AM   #126
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Quote:
Originally Posted by abraxalito View Post
Chapter and verse please. I have found none from reading the datasheet so would appreciate a heads-up. Perhaps there's something I missed
If we are talking about the same AD RF attenuator, have you looked at the distortion? I don't mean to be an impedement to creative thinking but the last thing we want is to add an attenuator that has more distortion than a typical 12AX7 tube, but hopefully we are talking about two different IC's.
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Old 15th January 2012, 05:18 AM   #127
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Originally Posted by regal View Post
If we are talking about the same AD RF attenuator, have you looked at the distortion?
I've done more than look at it, I've measured it (in single-ended output mode). Its of the order of 0.05% at full output, falling to bellow the FFT plot's noise at lower levels. Which pretty well agrees with the datasheet - predominantly 2nd harmonic, 3rd harmonic about 6dB down on 2nd. Now I'm running the part with differential output, I have yet to measure but expect to get 0.03% of relatively pure 3rd harmonic at full output.

Quote:
I don't mean to be an impedement to creative thinking but the last thing we want is to add an attenuator that has more distortion than a typical 12AX7 tube, but hopefully we are talking about two different IC's.
I'm not familiar with tubes myself. In any case the THD figure is way less important than the IMD - even for normal audio band duty. Here we have an amp exposed to extreme levels of RF into the 10's of MHz.
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Old 15th January 2012, 05:24 AM   #128
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Hi,

Quote:
Originally Posted by abraxalito View Post
Chapter and verse please. I have found none from reading the datasheet so would appreciate a heads-up. Perhaps there's something I missed
AD60X or similar I presume? BTW, there are options in AD's lineup with more attenuation range and other interesting features? Look at the equivalent schematic shown for the attenuator in the datasheet.

There is no magic, it is a simple switched ladder and what do you presume they use as switches?

Ciao T
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Old 15th January 2012, 05:37 AM   #129
regal is offline regal  United States
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Quote:
Originally Posted by ThorstenL View Post
Folks,

A while earlier before various little waterglass stromes we were discussing the "easy way" to make a decent DIY DAC with TDA1541, without recourse to hacking up kits and with better resulting better performance.

Part 1 discussing the general circuit is here:

Any good TDA1541A DAC kit?

Incidentally, if anyone fancies making a drawing of the circuit discussed I will not object... I lack the time.

Thorsten, thankyou for taking the time to post this useful info. I was going to try one of the the protoboards with a power /ground plane on top and bottom and each hole "isolated" with a gap. But sounds like your technique would be cleaner than a swiss cheese ground plane.

Still the biggest hurddle is finding a means to have the masterclock in the DAC, with the I2S source galvanically isolated.

Would it be nuts to proposed galvanic continuity all the way from the usb ground thru the DAC and provide galvanic isolation on the analog side with good transformers?

If we are careful with keeping the current loops of all our decoupling small maybe this is a decent route for the DIYer?

Or likely still no better than using wm8805 spdif for the galvanic isolation from the computer/transport and giving up on a real master DAC?

Also you haven't talking about the analog section, I posted my thoughts in the big thread. In my head I can't accept that a single common gate mofset is suitable solution for an ultimate DAC. I mean 6 years 400 posts working on the digital side and we are going to dump the output into a single mosfet and expect this is the ultimate DAC? I suggested some brainstorming for step-up transformer I/V + tube as an alternative but he didn't understand the point I was making.

What are your thoughts on the analog side with regard to I/V conversion with attention to minimizing compliance voltage adding distortion?

Last edited by regal; 15th January 2012 at 05:40 AM.
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Old 15th January 2012, 05:47 AM   #130
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Quote:
Originally Posted by ThorstenL View Post
There is no magic, it is a simple switched ladder and what do you presume they use as switches?
I can only suggest that you return to the datasheet and digest it better. Its a ladder for sure, but where do they say its a 'switched ladder' ? Chapter and verse please.
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