The idea here is to discuss best practices for connecting a high end digital preamp or CD/DVD player to a digital, DSP based loudspeaker system. What type of cables to use and why? What is the difference between a mono analog RCA unbalanced cable and a digital 75 ohm RCA S/PDIF cable (they look basically the same from the outside)? What is the difference between a digital AES/EBU XLR cable and a balanced analog XLR microphone cable (they look basically the same from the outside)?
Try to keep the discussion in this thread based on ohm’s law and avoid drifting over into snake oil type topics relating to things like cable risers, etc. Thanks.
There are many different ways to go about making these connections, so I will start things off with a list, together with reasons as to why you would want to choose one method over the other. Remember, I am fairly new at the digital game, so my analysis may be incomplete (or in error). It is based on my recent research into setting up a digital DSP based loudspeaker system using the new Hypex FA123 plate amps.
CD player/preamp unbalanced analog RCA outputs to Hypex DSP unbalanced analog inputs. For this you would need two fairly long (15 to 25ft or so) RCA cables for the right and left channels. This should work OK as long as the analog output impedance of the CD/DVD player (or preamp) is fairly low. For instance, a tube type preamp or CD/DVD player with high output impedance might create a high frequency roll off problem. Another disadvantage is that this adds an additional A/D and D/A conversion stage.
CD player/preamp balanced analog XLR outputs to Hypex DSP balanced XLR inputs. For this you would need two fairly long (15 to 25ft or so) analog XLR microphone cables for the right and left channels. This should work OK as long as the analog output impedance of the CD/DVD player (or preamp) is fairly low. An advantage of XLR over RCA, however, is that they are balanced and therefore have a much lower noise pickup for long runs. They also have lower capacitance per foot. So high frequency roll off, when using a tube preamp, is less of a issue. But analog XLR, just like analog RCA, adds an additional A/D and D/A conversion stage.
CD player/preamp digital 75 ohm S/PDIF outputs to Hypex FA-123 S/PDIF inputs. For this you would need a single, fairly long (15 to 25ft or so) digital 75 ohm S/PDIF cable. About $20 each on Amazon (order placed). My CD player has one S/PDIF digital RCA output, so I will report back my findings when I get my new cables. From what I have read, you should not use a standard unbalanced RCA audio cable for a digital S/PDIF connection. The 75 ohm characteristic impedance of the cable is needed to reduce RF reflective cancellation and insure maximum signal transfer. Another advantage of S/PDIF over analog RCA or analog balanced XLR is that a direct digital connection from source to DSP eliminates one D/A and A/D conversion stage.
AES/EDU digital in/out. Cable looks the same as balanced analog XLR, but the characteristic impedance is 110 ohms instead of 75 ohms. Signal transfer is digital, not analog. I will not be experimenting with this type of cable, because my CD player does not have AES digital outputs. JR’s Oppo BDP-95 also lacks AES digital outputs.
Optical toslink digital in/out. Still researching this method.
HDMI digital in/out. The Oppo has these outputs, but my CD player does not. I will not be researching this method.
Right now, I am leaning toward one S/PDIF RCA digital connection. I ordered three 15 ft. long S/PDIF RCA cables. One cable will run from my CD player’s S/PDIF output to one Hypex FA123’s S/PDIF input. Another 15 ft S/PDIF RCA digital cable will run from this Hypex FA123’s S/PDIF output to the other Hypex FA123’s S/PDIF input. This digital connection between the two Hypex amps is needed to not only transfer the digital audio signal, but also to transfer the remote control signal commands from one Hypex amp to the other. The 3rd 15 ft cable will only be used if I can’t get the digital S/PDIF connections to work properly and therefore need to fall back to a dual analog RCA type connection for the right and left channels.
A) I should probably put together a room sized block diagram of sorts to show the various components and the long cable runs. This will makes things easier to visualize.
B) I need to address the issue of how much gain is needed. Does the connection method have sufficient gain to drive the loudspeaker DSP/power amps to full output? I know that the analog RCA outs from my analog preamplifier can drive the Hypex amps to full output. But I am not sure about the unbalanced RCA or S/PDIF outputs coming directly from my CD player. I have to test this when I get my long 15 foot cable runs.
C) I also need to address the issue of power supply turn-on/turn off thumps and now to deal with them to prevent driver damage. In addition to thumps, you also have turn on/turn-off “squeals” to deal with as the power supplies ramp up and down. The thumps are caused by DC offsets. The squeals are caused by oscillation. The Hypex amps are very well behaved, with only a very small click through the speakers after coming out of protection. They also have no turn off oscillation or thump. But when you shut them off, the power supply capacitors take about 20 seconds to drain off. So, when powering down, I shut the Hypex amps off first, then I wait 20 seconds before shutting off my preamp. Otherwise my preamp will send a turn off type oscillation “squeal” through the speakers. During power up, my preamp does not have a turn on thump or squeal, but just to be safe I always turn the Hypex amps on first, then power up the preamp.
D) I need to address the issue of how the volume level is adjusted with each connection method. When connecting directly to my CD player, for instance, I can only adjust the volume of the loudspeakers using the Hypex FA-123 remote control unit.
Hi Bill, I don’t have much to add here except regarding the ‘thumps’ and ‘squeals’. I’m not sure my Hypex FA502 amps feature either of these issues, though perhaps any squeal would be minimal given they are powering subwoofers only.
Signal was from standard subwoofer RCA cables, perhaps 12 feet long. My preamp (NAD M12) has two subwoofer outputs so daisy chaining (if even possible with RCA) was unnecessary. I used the auto-on/off feature on the Hypex amps so they would activate when they sensed a signal.
Thanks for the info, Danny. Just to be clear, the “squeal” that I was talking about in my post above was being generated by my preamplifier (the front end of a KLH 100wpc integrated amp) as it powered down, not by my Hypex FA123’s. The Hypex amps are perfectly quiet during shut down. Just like you have done, I plan to program my Hypex amps to auto-on/off when they sense a signal.
To address this issue, I added an 18uF PP protection capacitor in series with each tweeter. I adjusted the schematic to compensation for the capacitor, so the on-axis frequency response flatness is the same as before.
Most BNC cables used for video production hardware are 75 ohm. We had miles of it back in the SD days connecting switchers, monitors and Betacam decks. You see them these days for SDI connections between gear. They are low-loss and used for long camera runs - 100s of feet. These cables work great for digital audio. You can grab some nice Canare cables on eBay for cheap. Just grab a few BNC to RCA adapters. Most test equipment uses 50 ohm cables - don’t use those.
Thanks for the info, Tom. I was not aware of this. I have a large supply of BNC to RCA plug converters on my test bench for amplifier testing work. But all of these are 50 ohms; not 75. I’ll keep an eye out for the 75 ohm BNC versions on my next parts order go round.
Also, did you see the Mr. Carlson’s lab video about using standard 75 ohm coax (RG6) antenna cable for audio connectors (see link below)? He put a pair of cheap RCA converters on the ends of some antenna coax and converted them into low noise audio interconnect cables. These cables would also work as a digital S/PDIF cable. The only downside that I can see is that antenna coax is generally not very flexible.
Rec’d my cables yesterday from Amazon. Three 15 foot long, 75 ohm digital coax types with RCA’s on each end. Two of them are “Cable Matters” brand and measure 463pF (31pF per foot) on my DMM. The other one is an “EMK” brand and measures 511pF (34pF per foot).
I tested all three cables on my Sony DVP-NC685V CD/DVD player and Hypex plate amps. I compared S/PDIF digital verses analog RCA connection hookups.
Overall gain is 6dB higher with the S/PDIF digital connection verses the analog RCA connection.
Both connection types can easily drive the Hypex amps to full output. The S/PDIF digital connection requires the use of the Hypex remote control to adjust speaker volume level.
The analog RCA connection allows volume control adjustments using either my KLH analog preamplifier or the Hypex remote.
One interesting thing to note is that the RCA analog connection rolls off the treble above 10kHz and is down a full 5dB at 20kHz compared to the S/PDIF connection.
I am at a loss to explain what is causing this roll off. I measured the output impedance of my Sony DVP-NC685V CD/DVD player’s RCA analog outputs and it came out to a very low 834 ohms. So if I connect these long cables to the analog outs, the -3dB half power high frequency roll off should be over 370kHz (per the standard formula 1/(2pi x R x C)). So high cable capacitance or high CD player output impedance cannot be the cause. The Sony player’s spec sheet says that CD’s are plus or minus 0.5dB from 2Hz to 20kHz and that the RCA analog outs can deliver up to 2Vrms into a 10k ohm load. The Hypex analog RCA input impedance is listed as 54k ohms (I measured 55.8k ohms on my DMM). Any thoughts as to what could be causing this unexpected roll off? The red and black curves should be identical!
NOTE: In the graph above, there is an overall rising response above 4kHz. This is a separate issue from the cable roll off problem. The rising response above 4Hz is probably due to either measurement error on my part or due to the fact that the VituixCAD software package filter blocks do not translate perfectly to the equivalent filter blocks that are used in the Hypex HFD software package. This is a known issue and solving it involves a trial and error measurement, tweak, measurement, tweak, etc., process until a good match is finally obtained (per other threads on diyaudio).
I ordered two sets of speaker cables from PE for my Hypex plate amps. Got them yesterday. Took 20 days to get them. Not stocked, special order only. Very good quality cables, the price seems reasonable for what you get. (Since I’m cheap, I was going to DIY my own cable sets, but I could not find a source for the white 4 pin and 2 pin plugs (see pic)).
The cables are four foot long and can easily reach my drivers. Going forward, these cable sets will make it possible for me to pop the Hypex FA123 amps in and out of several builds. I may also build external boxes to house the amps at some point in the future. Decided to go this route and see how it works out. Built in plate amps seem to make on-site hookups go faster with less problems.
Thanks. Went on the JST web site and discovered a large variety of connectors available. Now I have to figure out which type was used. Spacing measures 4mm. The pins measure approx .05 inches in diameter, but this varies a little because the pins are not exactly round. Looks like either the JST VH, XL or PSI series. These series are rated for 10 to 12 amps and can take up to 16 awg wire. The printing on the wires says 16 awg 300V. Feels like PVC insulation.
