dac 1 bnc connector is 50 ohms?

[choariwap]

while reading up on the emprical audio mods for the dac1, the following caught my eye:

"The stock BNC is actually 50 ohms rather than the specified 75 ohms characteristic impedance. "

is this true? i tried looking up the part number of the connector and the manufacturer's site does not specify the impedance rating of the connector. one retailer does specify it as 50 ohms.

anyone can confirm this?

and if so, how hard would it be to replace it with a 75 ohm connector? looking at it, you dont need to remove the pcb, right? just desolder the wire connected to the jack, take out the jack, plug the replacement in and resolder the wire back on, right?

 

[Garbz]

Yep it's 50ohms. One of the reasons I hate it so much. They claim to make all these efforts to keep the digital signal pristine and then make a mistake that leads me to think they really have no clue.

50Ohm BNCs have a white dielectric around the pin for the female, and inside the shield for the male. 75ohm BNCs have no dielectric at all inside the plug for the area where the pin mates with the socket.

It should be a simple case of swap out the part, even if it is pcb mounted.

 

[jsiau]

The connector is 50-Ohms. 50-Ohm connectors are far more durable than 75-Ohm connectors due to the extra dielectric material surrounding the center pin of the BNC. For this reason, it is common practice to use 50-Ohm BNC connectors in 75-Ohm systems when the signal bandwidth allows it.

The short interruption of the 75-Ohm transmission line is only significant for frequencies that are much higher than any contained in a digital audio signal. The 50-Ohm connector would be a factor for signals having a wavelength of 2 inches or less in coax (about 3 GHz). A 192 kHz digital audio signal transmits data using a clock that is 64 times the sampling frequency (192 kHz * 64 = 12.288 MHz). 3 GHz is the 244th harmonic of 12.288 MHz and does not exist in a 192 kHz digital audio signal. If it did, the box probably would not pass FCC and CE emissions tests.

Changing the connector would reduce the durability of the product and would have absolutely no effect even at 192 kHz.

 

[Garbz]

Yet it does, and there were posts of TDR results for the line as well as jitter results to prove the difference over at diy-hifi. The termination is the biggest influencer of sound in "digital cables". If terminated properly all digital cables would sound the same.

Also as far as durability counts, the shield of the BNC mates before the centre pin, so unless you frequently jam something other than a BNC plug into the 75ohm socket your chances of breaking something are minimal. I don't even look at the thing when plugging mine it. Just wiggle and turn till it mates, and nothing is wrong with it. Just don't jam a screwdriver in there

 

[jsiau]

Quote:

Originally Posted by Garbz Yet it does, and there were posts of TDR results for the line as well as jitter results to prove the difference over at diy-hifi. The termination is the biggest influencer of sound in "digital cables". If terminated properly all digital cables would sound the same.

A TDR uses a very fast (<1 nS) rise time pulse in order to see very short irregularities in transmission lines. The 50 Ohm BNC will be visible on a good TDR, but it has no effect on the much much slower rise time of digital audio signals.

Also, keep in mind, the 50-Ohm BNC connector has nothing to do with the termination of the transmission line. The 50-Ohm BNC is simply a very short interruption in the 75-Ohm transmission line. The termination is still 75-Ohms, and the line appears reflection-free at digital audio frequencies.

It appears reflection-free because the 75 to 50-Ohm (cable to connector) transition produces a reflection that is exactly opposite the reflection caused by the 50 to 75-Ohm (connector to stripline) transition. These two equal and opposite reflections are separated by two times the propagation delay of the connector or by only about 0.08 nS. This time delay creates a notch in the frequency response at 6 GHz. A digital audio signal has no significant energy at 6 GHz so the return loss is not important at 6 GHz, and therefore the connector impedance is not important.

 

[Garbz]

And yet the results are measurable on the analogue outputs. There's no sense arguing with me here. I have not got this type of measuring equipment or knowledge of high speed digital transmission to write a counterclaim. I am just re-itterating the results I have seen posted on diyaudio and diyhifi, along with my own listening tests which did get an audible improvement when I put correct 75ohm BNC connectors in my DAC, and at a $5 mod for something done proplerly giving an IMHO an audible improvement, there really is no reason not to do this.

 

[jsiau]

And yet the results are measurable on the analogue outputs. QUOTE]

If such measurements exist for the DAC1, I have not seen them. Many converters are very sensitive to small changes on the digital inputs, the DAC1 is not.

We are equipped with Audio Precision SYS-2722 test stations and have tested a variety of digital cables and connectors, and have never been able to measure a difference in performance at the analog outputs of the DAC1. In some cases we can see differences in jitter prior to the UltaLock(tm) jitter attenuation circuit. The UltraLock(tm) system is very effective at attenuating any jitter that may be caused by cables and/or connectors.

In the back of the DAC1 manual there is a test plot showing that the insertion of 1000 feet of Cat5 cable. The digital signal is severely attenuated, the wave shape is distorted, and the eye pattern is virtually non-existent. Nevertheless, the UltraLock(tm) system recovers the data (with bit-for-bit accuracy) and removes the cable-induced jitter to the extent that no change is measurable at the analog outputs (with measurement limits that extend 150 dB below normal output level).

Any changes noted in your setup are probably due to slight gain changes or the inability to quickly switch back and forth between the two connectors.