# Adding Coaxial S/PDIF To The M-Audio Transit



## bryman79

The quality of the most recent LAME encoder, coupled with the need for a high quality, portable sound card provided inspiration for this project. In short, it adds a coaxial S/PDIF output to the M-Audio Transit. So why coaxial S/PDIF? Because the TTL signal from the optical TX contains jitter, which equates to reduced sound quality. However, if not properly implemented, the coaxial output can be as much of a hindrance. Thus, it was imperative to reduce both impedance mismatch as well as reflections in the S/PDIF cable, which is the purpose of this post. But please note that for the most part I am conveying what I have learned from other members of this and other forums, such as Jocko Homo, Glassman, and Roibm. I am not an expert, nor do I claim to be. With this post, hopefully others can similarly modify, and potentially improve on the performance of this device.

 To start, here's the stock Rev. G Transit:






 And here's the finished product:





 Here's the schematic that I used:





 Since R1 and R2 function as both a voltage divider and a 75 Ohm impedance load, the values have some import. Given that I don't have access to a scope, I had to use some math. And to more accurately model the situation, the output impedance of the AK4584 should be added to R1. Since the datasheet (see here: http://www.asahi-kasei.co.jp/akm/en/...584/ek4584.pdf) doesn't specify a value, I had to email AK. A rep responded back saying that it was 10-20 Ohms +/- 30%. Jocko suggested I favor the upper part of the range, so I used this info and made an Excel spreadsheet to help me decide on some appropriate resistor values. I can't seem to attach it, but I will email it to whomever wants it.

 As far as components, I used the IRC Tanfilm resistors from Mouser, and Kemet and Murata ceramic COG caps from Mouser and Digikey. The Newava transformer was also a Digikey item. Again, I can email my BOM to whomever wants it.

 Those with experience with circuits of this type advised me to keep component leads as short as possible. Thus, I used SMDs wherever possible, and tried to arrange the leaded components in such a way to accomplish this goal. See here:









 The goal in implementing these modifications was to maintain impedance throughout the signal path, such that reflections would be minimized. As such, attention was paid to both connectors and the length of the S/PDIF cable.

 Regarding connectors... it's a given that if you use 75 Ohm cable, you must use 75 Ohm connectors. However, the quality of the connectors and the termination also affects impedance, thereby affecting sound. Since I haven't tried and tested many connectors myself and didn't want to invest in the tools to crimp BNCs, it was a relief when Jocko suggested Trompeter wrench crimps. These are easy to install (no tools required other than a torque wrench), and provide repeatable connections. And yes, they are a little expensive, but they're bulletproof.

 Regarding cable length... any reflections induced by impedance mismatches can be reduced by choosing a proper cable length. Most people agree that over 2m will suffice, so I made 2m, 3m, 4m and 5m cables from Belden 1505a. FWIW, the 3m cable sounds significantly better than the others with both of my DACs. And yes, the difference is noticeable with my system.

 Okay, so on to the important stuff... how does it sound? Well, the relocated optical sounds identical, which is to be expected. And the coaxial sounds far superior to the optical. I'm not the best at putting what I hear into words, but the music is much "flatter" through the optical than the coaxial. Also, the soundstage seems to be a bit wider using the coaxial. This comparison was done using my AOS DIR1703E Ally, to which I added a BNC bulkhead connector (see here: http://img.photobucket.com/albums/v4...lyWithJung.jpg). I've recently assembled Pedja Rogic's TDA1541A DAC with the CS8414 (see here: http://img.photobucket.com/albums/v4...DA1541ADAC.jpg), but I only bothered to put a coaxial in that unit. Nonetheless, it sounds terrific through that DAC as well. I hope to compare the modified Transit to a good CD player in the near future.

 Regards,
 Bryan


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## Garbz

finally, after all the whohar I started years ago someone has finally assembled something. Good to hear from your sucess. It's good to see some sensible choices in the output stage. Using a transformer you have effectively isolated the DAC from your computer's ground and maintained lower jitter then the TOTX output. 

 Nice soldering work too.


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## Jam_Master_J

This looks interesting. I'm getting a Transit so I might try this if the optical out is unsatisfactory.


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## bryman79

By way of addendum... at the San Jose meet today, I got my hands on a DAC1. I performed an optical/coaxial A/B test and the results were the same as with my Ally. The music sounded flatter and lifeless using the optical.

  Quote:


  Originally Posted by *Jam_Master_J* 
_I might try this if the optical out is unsatisfactory._

 

I doubt you will find the optical "unsatisfactory." Depending upon your DAC, the optical may sound very good. However, you will probably find the coaxial to be superior.

  Quote:


  Originally Posted by *Garbz* 
_finally, after all the whohar I started years ago someone has finally assembled something._

 

I owe a big thanks to you too. See why I hate saying thanks... I always leave someone out. 
	

	
	
		
		

		
		
	


	




 My point in thanking was mainly to avoid taking credit for the knowledge and insight that I seek to acquire.

  Quote:


  Originally Posted by *Garbz* 
_Using a transformer you have effectively isolated the DAC from your computer's ground and maintained lower jitter then the TOTX output.._

 

At the meet today, I had a guy ask me why I was so concerned with coaxial when there is an optical port on the device he was feeding into a DAC1. Not even mentioning the obvious reason that the optical cables are so damn fragile compared to coaxial, I responded that there is lower jitter in the coaxial. He laughed and basically said it's a non-issue since the DAC1 attenuates any jitter that the optical may cause. I must be maturing because I bit my tongue and didn't rub it in his face later when I did the optical/coaxial A/B. 
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *garbz* 
_Nice soldering work too._

 

I should send you a close-up pic. Not as pretty. However, I'm confident that the wires are firmly attached and that I did not overheat the chip, so all is well.

 Bryan


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## Garbz

Lol I'd love to see a DAC which is able to attenuate the jitter from all the circuitry beforehand. In theory you need a buffer of sorts. In practice it won't work for the same reason USB audio is inferior to Firewire. Reclocking does some work but generally no solution is perfect.

 Sounds like this guy is a "let's do it cheap and quick and worry about problems further down the line" type of guy 
	

	
	
		
		

		
		
	


	




 Anyway it's nice to see this as a reference, in the future depending on how I finally get my USB->S/PDIF card to interface with my DAC I may just end up using my transit anyway :S


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## hangguy

I have also modified a transit similar to what bryan had done. My impression is, the biggest improvement comes from going to an external power supply. 

 I was using a pulse engineering PE-65612 transformer, which is similar to the Newava S22083, a transformer in molded plastic. I was working on another project and came across Newava S22160, which appears to be a clone of the old Schott transformer. So I decided to give it a try to replace the PE-65612.

 The change that S22160 brought was more details. The highs didn't get any higher and the lows didn't get any lower. The characteristics of the whole presentation did not change at all. It is just that more information was presented. The weak notes like audience coughing in background, the clapping of hands during concerts, and the tapping of foot all of a sudden are right in front of you. The sound stage now is wider, sounds are coming from outside the speakers. 

 I encourage anyone who has the pulse engineering transformer to try the S22160. It's less than $10 and it is available from digikey.

 Next mod is to add a HagClock to the transit and see what improvement it will bring.

 Mike


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## Garbz

OH MY GOD! I am so stupid! This should teach me to read datasheet twice and create circuit once.

 I decided to mod my transit now that I've built a working DAC. The idea was a bit more complex then what is sugested above. I was trying to re-clock the output via a D-type Flipflop. It's rather simple, feed S/PDIF into the Data pin, feed the system clock into the clock pin. Take output from Q and apply the usual resistors, capacitors, transformers etc. 

 After it didn't work the first time I took a closer look and how the transit was laid out. Sys-clock isn't derrived from the 6Mhz crystal at all. Infact the AKM looks like it's setup to use it's own internal PLL. This ofcourse means I can't swap out the master crystal for a better one, goodbye kwak-clock. Moreover the only output for the system clock is the output attached to the serial output. So MCLKO1 is identical to the clock in the S/PDIF in the first place. My mod if it would have worked would have done bugger all!

 The only thing i'm left with is bryman's mod now. All my other mods would be useless! Looks like I'll just build my own USB-> S/PDIF converter the way I like it


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## bryman79

Garbz,

 From what I hear, a new PSU improves sound. I'm planning on using a Jung dropped to 5V on my new one...

 Also, I was planning to use a Tent XO3 on the new one as well. From my reading of the datasheet, you can simply disconnect the 24.576MHz clock from the TAS1020A and insert a new one from the Tent board (e.g., to pins 21 and 18... see page 22 of the AK datasheet). This will limit you to 24/96, but that's fine with me. And then run the SPDIF out as I did in my first one to the Tent board for reclocking, and then into a BNC.

 Regards,
 Bryan


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## Garbz

Correct me if I am wrong (which I very well may be considering my track record the last 2 weeks), but the chip is currently not using an external clock source. It is using the internal PLL. The obvious problem is how do you set the chip to use the external clock itself. My current guess is even if the external clock is connected it will continue using the PLL untill the correct change is made to the CM[0:1] bits in the register if it is not already set to mode 3. My gut tells me it's currently mode 0 or 2. Page 19 of datasheet.


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## hangguy

Garbz,

 What makes you think that the AK4584 is in internal clock mode?

 Mike


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## Garbz

I couldn't for the life of me find where the external clock was comming in. The external clock lines and the 6mhz crystal appear unconnected as I did try to tap them initially without thinking for my reclocking circuit. I just couldn't get a clock pulse from any pin except MCLKO1 which was at the wrong frequency i think.


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## hangguy

Garbz,

 The clock from TAS1020 comes in the AK4584 at pin 21 MCLKI/XTI. Look at the first picture that Bryan posted, that is the second pin from the bottom on the left side. There is a via to that pin and I measured the frequency of that pin using a frequency counter, the clock frequency is 24.576MHz for 2 channel 96KHz PCM, and 36.864MHz for DD/DTS signals.

 Mike


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## Garbz

Umm pin 21 is the second pin from the bottom rights side. Which via is connected to it? It appears unconnected to me, and I couldn't get a clock pulse from it when I tried


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## hangguy

That is the second pin up from the U3 marking on the left side with the orientation in the picture. The via goes to the bottom side of the board to TAS1020a.

 Mike


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## Garbz

Ahhh pin 10. It'll be interesting to see if the clock on that pin is identical to the clock output of S/PDIF for a given sampling rate. I'll try tapping it on the weekend and see if I can get a signal out of it.


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## Garbz

Who needs weekends 
	

	
	
		
		

		
			





 I went ahead and tried it. No sucess. Just so you know what i'm trying to do. I'm using a 74HC74 D type flipflop. The clock from the clock source, and the Data from the S/PDIF line. The output Q should have only the jitter inherent in the clock itself. This is what common reclocking circuits do.

 There is a clock signal on that pin but whatever it is it's not the correct S/PDIF frequency. I'm not going to rule out that I've burnt out the flipflop with constant resoldering, or that I've set up something incorrectly but I will experiment further. Thistime on the weekend 
	

	
	
		
		

		
		
	


	




 .

 Could you do me a favor since I don't actually have a frequency counter or ocilliscope (which would eliminate the guess work). Could you please check the frequency output of the S/PDIF signal? R5 on the schematic or Pin 5 from bottom left. in Bryan's pic.


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## hangguy

Hi Garbz,

 Now I see what you are trying to do.

 A frequency counter would not be able to tell you the freq of your SPDIF data, since it only count edges in a period of time. As the SPDIF data changes so is the freq reading from the freq counter, although the sampling freq does not change. 

 Let me study the datasheet of 4584 over the weekend and see if this reclocking scheme is doable.

 Mike


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## bryman79

Any idea what D1's purpose is? I just received the next Transit to mod... it's a rev. F, with the only noticeable difference being the diode.

 Thanks,
 Bryan


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## Garbz

Noticable difference. Who knows what other changes they made 
	

	
	
		
		

		
		
	


	




 But now that you mention it it does seem kind of out of place. Maybe they realised they simply didn't need it


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## bryman79

Garbz,

 They _added_ the diode going from rev. F (the one I just received) to rev. G (the one you see in the pics).

 Regards,
 Bryan


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## Garbz

Well on that note maybe it was missing 
	

	
	
		
		

		
		
	


	




 Anyway I gave up with the flipflop. There's no clock i can source that seems to work. I'll just go the standard voltage divider cap, the input is to my dac is xformer coupled so i'll leave that out.


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## perco

Is the twisted red and black cables the power mod? Just solder the cables like you did and supply them with +5V and GND? How about the USB power? No problem with sound card needing the USB ground from the computer?


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