Prism Orpheus connecting to Project Debut Carbon

[PCmusic]

I tested. Basically I don't know how to specifically 'route' the analog input to the out. There is no 'switch'. I guess it's always on.

 

1. I turned up all the volume. Assuming the input is always on, with no cables plugged in the input, the background is slient without any noises.

 

2. I hooked up my cdp through the custom-made RCA-TRS cable to one line input of Orpheus. No issues at all. No background noise.

 

STRANGE!

[PCmusic]

Sorry my DESCRIPTION WAS WRONG ON THE HOME CIRCUIT.

 

I have a B2 Box, dealing with audio only. It is then distributed into different outlets. One dedicated to poweramp. One dedicated to Orpheus. Another one dedicated to other components, with the help of extension box.

 

I have a B1 Box, which deals with ALL ELECTRIC APPLIANCES.

 

Sorry my earlier description is wrong.

 

My PC is connected to an outlet from B1 box. It's earthed, using Consair power supply 550W.

 

The PC is connected to Orpheus through Firewire.

 

I tried disconnected ALL OTHER GEARS (smart TV box, HDMI switch, CDP and so on), with only Orpheus and poweramp left . Still have noises without cables plugged in.

 

Thanks, iago ...

[iago]

Sorry my DESCRIPTION WAS WRONG ON THE HOME CIRCUIT.

 

No, it was OK, I just wanted to make sure that I got it right. No worries!

 

So the Orpheus in stand-alone mode is picking up noise on the instrument /microphone level inputs. Could it be possible that the noise is coming in over the power line? I have a comparable isue since using power line ethernet and MC low output cartridges. When the network is running, the turntable picks up noise through the power connector. The high level inputs are not affected.

 

lI just tried two things, just to be sure that this is not caused by the open inputs.

 

(1) Turned up the microphone input of my own audio interface to maximum gain and turned up headphone volume: noticeable hiss, but no unexpected noises.

 

(2) With cables connected only at the interface end, they may pick up computer noise, depending where I place them on my desktop - they pick up noise mainly through capacitive coupling in this mode.

 

All of it with the computer still connected.

 

You might try to connect the Orpheus to another outlet (different circuit) in your room and check if the noise on the microphone input goes away. If that is the case, you could connect either the Orpheus or (if possible) the offending device through a line filter - they work both ways. Something inexpensive from Schaffner should do.

 

There is also the faint possibility that something is not right with the Orpheus ...

[PCmusic]

iago, thanks for your reply and suggestions. I have try plugging Orpheus in other outlets with no luck.

 

Since I really have no time to further investigate the issue, I just bought an iphono and things work fine. It's not the best scenario but I need to travel and have set up everything before leaving.

 

Will look into the issue later if I have the time. After all, the issue is unresolved ...

[PCmusic]

iago,

 

Have not update the Orpheus condition. Just talked to Prism.

You wrote last time: "So the Orpheus in stand-alone mode is picking up noise on the instrument /microphone level inputs. Could it be possible that the noise is coming in over the power line?"

 

The guy said Orpheus without 'terminated' (connected to any analogue input) will have significant noises for its mic and instrument input. If the analogue inputs are connected, the noise level and nature would become normal.

 

Their answer doesn't solve the problem of Orpheus connecting to the turntable, but they did raise the issue that a 'standalone' orpheus is bound to have noises have its 'unplugged' mic or instrument inputs. Iago, any ideas on this (putting the turntable issue aside)??

[iago]

Hi PCmusic,

 

The guy said Orpheus without 'terminated' (connected to any analogue input) will have significant noises for its mic and instrument input. If the analogue inputs are connected, the noise level and nature would become normal.

 

In principle the guy is right. But the remark refers to unterminated (= one end running free) cables, and yours should be terminated (with the phono pickup).

 

I currently have a similiar device on my desk (Lyra 2), so I did some experimenting. The instrument inputs have 65 dB gain when fully turned up. With nothing connected to the inputs I cannot hear any noise even with full gain on input and headphones.

 

For connection to unbalanced sources I use a RCA to phono plug cable in TS configuration. The cable construction is symmetrical, the screen is connected on the RCA side only. Lying loosely on my desk and connected at one end to the front inputs with gain turned up I get noticable noise and hum - mainly because the screen is floating.

 

 

According to the manual, with RIAA filtering enabled, the gain increases by 16 dB. If you look at the RIAA filter curve, that corresponds to a gain of nearly (65 + 16 + 20) = 101 dB for low frequency signals. Accordingly, the noise level increases considerably, especially hum is pronounced, and signal meter level is near the -6 dB dB mark.

 

If the free floating screen is grounded, the hum (nearly) goes away, white noise stays the same.

 

If I terminate the open end with some coils of banana cable and a 500R resistor, the output gets noticably quieter (level is down to -36 dB at full gain).

 

 

These are the recorded samples:

 

Unterminated connection, no filter

Unterminated connection, RIAA filter

Grounded connection, RIAA filetr

Terminated connection, RIAA filter

[PCmusic]

Thanks. I am going to ask Simon, Prism Orpheus to join this post if he is ok. Your description is extremely methodological and detailed, which I think Prism should have done the same to customers. If he is unwilling for some reason I'll at least let him know your result.

 

iago, is it possible to do the same test for me for unbalanced connections, using mike of Lyra (in the combo input 1 and 2 at the back)? Sorry to write to you on Sunday. And many thanks for this. I hope the mystery could be solved soon. (I actually took Orpheus to the local dealer but with all sophisticated equipment their preliminary analysis of my analogue input is 'NORMAL') ...

[iago]

Yes, I could try the same with the microphone input; it will not be pretty since I only have balanced XLR cables around, but I will rig something up. Give me a few days.

 

I should have commented my results; please allow me to redress this here.

 

Floating shield. In this configuration the cable picks up common mode noise via capacitive and inductive coupling. Since it was reasonably far away from other signal sources, this was mainly crossfeed from the mains. Grounding the cable at the RCA end elliminated that source (in my case not completely, because ground conductance was not ideal).

 

Unterminated cable. With a sink impedance around 1M, I would expect thermal noise with a level of

 

[1] 20 * log (sqrt (1E6 / 500)) ~ 33 dB

 

higher than when terminated with my 500R resistor. This roughly corresponds to the levels I observed.

 

Terminated cable. With terminating resistor R = 500, summer in the city T = 300, and the bandwidth of the PrismSound pre-amp 200 kHz, I would expect thermal noise in an order of

 

[2] sqrt(4 * kB * T * R * f) = sqrt(4 * 1,38E-23 * 300 * 500 * 200E3) = 1,28 uV

 

This corresponds to -116 dBm, where 0dBm (0,775 V) should equal the zero mark in the VU meters. With a mean amplification of (65 + 16) dB, the resulting level should arrive at -35 dBm. Again, a high correspondence to the level I was able to observe.

 

In practice, the terminating impedance would be 47k but total gain would be much lower (around 40-45 dB overall), so noise should be around -72 dB. With a MC system, the impedance is nearer 1/10 of the impedance I used and also you would not use more than 60 dB of overall gain, so I would expect a comparable noise floor. Actually, you should not connect a MM cartridge to the microphone input, it will sound terrible.

 

So maybe it's a cable issue after all, but let's see what the XLR experiment shows.

[PCmusic]

iago thanks so much for your detailed reply. I have invited Simon today. See if he has the time to come to shed light on what we have discussed and your professional analysis.