[Porksoda]

Hello all,

I just picked up a Monica 2 kit, and have been considering a couple mods on it. I did some searching, and didn't find any threads on the subject, so I thought I would just throw it out here. See the following resource links:

TDA1545A Datasheet


Monica DAC (analog stage basically identical to Monica 2)

First off, I wanted to improve upon the power supply arrangement. I am not a fan of battery power, as I have seen some convincing arguements that a good regulated supply is better than batteries under dynamic loading. The convenience of wall power is a big plus aswell.
As it exists now, it uses a current source followed by a string of 4148 signal level diodes. VREF is then taken between two of the diodes such that it is approximately 2/3 of the total voltage. The more I think about it, this really isn't that bad a design. However, this design still has an LM317 in there without any real noise reduction, and the signal diodes form an entirely open-loop voltage supply. I was thinking more along the lines of dual independent Jung super regulators, especially considering the Vref is going to directly influence the noise and dynamic range of the output.
What are your thoughts on the advantages/drawbacks of this sort of power supply modding?

Second, I was thinking of adding some sort of active output stage to this thing. See this thread HERE for a circuit Glassman and I worked on based off of a design by Malcolm. It's a fully balanced, active, and non-inverting I/V stage. The only modification I would make is that GND would be instead connected to VREF, and of course you need to tack on an output buffer stage. Otherwise, you just connect the current output to Iin and you're good to go!
Again, what are your thoughts on this sort of modification and what sort of benefits or drawbacks it would have?

Thanks!


'soda

 

[dsavitsk]

I am a fan of passive I/V stages, so my sense would be to either concentrate on using high quality resistors/caps, or perhaps to work a transformer in there. The reason I like passive is that with active you are amplifying a signal, then attenuating it, then presumably amplifying it again with a preamp. Passive cuts one of these stages out. My experience is that this is a good thing. So, I would stick a Riken Ohm resistor, and an Oil cap instead. I know a lot of people disagree with this, however. If you do go active, I would AB them to see the effect.

The other mod that I think would do the most good would be to put a transformer on the input rather than the caps. Look at the schematic of the AudioNote Kit to see an example of this.

As for the PS, cleaner is always better.

Let us know not only how your mods come out, but how this DAC (modded and unmodded) compares to others.

-d

 

[Porksoda]

Exactly what benefit does the input transformer provide? I am actually planning on having the DAC input go to a microcontroller that switches between a USB input, a coax input, and an optical input, so I am not sure what impact this has on the type of input to use.

Also, with regard to I/V stages, I get the impression that the current output of the DAC is designed to function at a fixed voltage, VREF, and that any impedence on the output introduces distortion. Also, the active stage I will be using is low open loop gain, low feedback so hopefully it will mitigate some of the problems associated with active output stages.

 

[Mod_Evil]

Where you will buy the CS8412? Why do you not use Texas Instruments chips?

... Crystal offers free samples?

Ohhh I'm sorry... I think you will make this circuit... sorry...

 

[guzzler]

Quote:

Originally Posted by Mod_Evil Where you will buy the CS8412? Why do you not use Texas Instruments chips? ... Crystal offers free samples? Ohhh I'm sorry... I think you will make this circuit... sorry...

The Monica schematic Pork has linked to is the old one, and Yeo sold it with the CS8412 anyway. The main differences between the Monica and Monica 2 is that the '2' is asynchronously reclocked, and uses the CS8414 (which Yeo also supplies). This, incidentally, is also obselete, replaced by the CS8416 already.

www.diyparadise.com

 

[dsavitsk]

Quote:

Originally Posted by Porksoda Exactly what benefit does the input transformer provide?

The folks at audionote claim that a good quality input transformer is the key to a high quality DAC. Why? I have no idea, but lots of people at DIYAudio seem to agree. Look at the schematic at http://audionotekits.espyderweb.net/dacschem.jpg for how to do the switching, and how to implement the transformer..

 

[Mod_Evil]

Quote:

Originally Posted by dsavitsk The folks at audionote claim that a good quality input transformer is the key to a high quality DAC. Why? I have no idea, but lots of people at DIYAudio seem to agree. Look at the schematic at http://audionotekits.espyderweb.net/dacschem.jpg for how to do the switching, and how to implement the transformer..

I think the transformer make a isolation... Yes, the transformer is the "key" of the Hi-Quality DAC.

 

[Porksoda]

Does anybody have recommended parts for input transformers?

 

[dsavitsk]

How about an Audionote trans-280

 

[Porksoda]

Well they are based out of the UK, and their prices are a bit high, considering I will need 3 transformers, one for each input. Plus international shipping, blah blah. Too much cash; this is a budget project.

 

[dsavitsk]

I'd search around diyaudio then as there have been discussions about various less expensive options. I think that jocko homo has posted a bit about this topic there.

 

[ble0t]

Yep...Jocko and I had a thread going concerning some Schott pulse transformers, which are now known as Newava. Guess who carries them?

http://rocky.digikey.com/WebLib/Newa...ata/S22083.pdf

 

[Garbz]

The input transformers isolate gear. It's a great way to keep things clean especially if you have a cheap source hooked to an expensive DAC. Or a noisy souce hooked to a sensitive amplifier. Another way is to isolate the receiver from the DAC chip using an ISO150 or something to keep the analogue stage clean.

The CS8416 is a good replacement. But much more difficult to setup without a microcontroller, much harder to work with since the PCBs need tighter tollerences for SMD components, and a high external part count.

Mod_Evil most companies offer sampling, it's a way for them to get feedback on their products and to help companies with designs. Most importantly it often looks in customers which will buy the units by the thousands. Texas Instruments don't have a receiver. They discontinued their 24/96 S/PDIF receiver because it had an error.

 

[Porksoda]

The ISO150 does seem like an interesting option, but I feel that one of the advantages of the transformer is that it isolates the DAC input from the ground plane. Also, how much noise does the ISO150 introduce into the output, and is it an improvement over the unisolated input?

 

[guzzler]

There’s always the option of winding your own transformers. For TTL to S/PDIF output, you need a turns ratio of 10:1 on a toroid core. Philips toroids with 20 turns on the primary, 2 on the secondary work very nicely. For an input transformer, 1:1 winding, say 10 primary, 10 secondary would also work. Probably not as high quality, but would certainly work.