line out to balanced in using DI box, help!

[lurcher]

Hi all!

My CD player has a standard line out, my amp has line in and balanced XLR in as well. I want to use the balanced inputs on the amp, I've made my own cables connecting the ground and cold on XLR end and standard phono at the other end. This works fine but the output is at -6db resulting in very low volume.

I've borrowed from the studio a Stageline DIB-102 matching transformer to do things the right way. I made my cables using TRS jack at one end (tip and ring connected only, no shield) and phono plug at other end for CD (tip and ground). The output from the DI is balanced XLR. The sound was really quiet and was next to useless!

On the DI the input is at 50 kohm TRS and the output is 600 ohm. I tried the input at 0db attenuation (max).

Does anyone know if this DI will work with my amp or do I need to join the ring and sleeve of the TRS jack to create a semi-balanced cable or do I use tip and shield of the jack? I know you should only join cold & shield at output but the box does have ground lift to avoid hum.

Sorry if this does not make sense! I really want to know how much better the balanced input is against the standard line in.

Amp is Primare I30, CD is Arcam alpha 9.

Many thanks,

Mark.

[brucek]

My CD player has a standard line out, my amp has line in and balanced XLR in as well. I want to use the balanced inputs on the amp

I doubt that you'll gain much from this setup unless you are presently experiencing a lot of noise as a result of long lines or noisy environment.

You have to know that introducing a $100 DI box of electronics into the circuit will add its own inherent noise. You have to overcome this noise first before you're at a break even state.

The Arcam alpha 9 is a nice CD player with a single ended output. The Primare I30 is an integrated amplifier (which is an amplifier and preamplifier in one package). It allows both balanced and unbalanced inputs. If you have a device that enjoys balanced outputs and is a fair distance away, or in a noisy environment, it would be wise to use the balanced interface. But, if these conditions don't exist, and the source has exclusive unbalanced outputs, it makes very little sense to add noise to the circuit with a DI box...

brucek

[lurcher]

Many thanks for the swift reply brucek!

The reason I wanted to use the balanced in on the amp is because of the increase in detail and imaging.
I now know why it won't work: The CD ouput is 75 ohm and the DI input is 50 kohm! no wonder there is such little signal.
I suppose I better start saving for a new CD player with balanced outputs such as the matching Primare CD31, I better tell the wife that I have to have a new CD player immediately!

Many thanks again brucek,

Mark.

[Wayne A. Pflughaupt]

Mark,

The problem is that unbalanced equipment operates with a lower level signal than balanced. I.e., your amp is expecting a much higher signal than the CD player is delivering.

A direct box isn’t going to change that, since it’s a passive device. The sole purpose of a direct box is to convert an unbalanced signal to balanced. It’s not going to change the level of the signal. In their typical pro-audio application, the gain controls on the mixing console with handle that.

Aside from that, very few direct boxes have audiophile-grade specs. Can’t imagine why anyone would want on in their signal chain.

The reason I wanted to use the balanced in on the amp is because of the increase in detail and imaging.

Be sure and hang on to the receipt when you get your new CD player.

Regards,
Wayne

[brucek]

The CD ouput is 75 ohm and the DI input is 50 kohm! no wonder there is such little signal.

Actually, that's the way it should be. This is called a voltage bridge, where the sending device is acting as a voltage source and almost no current is being drawn.

In fact, a perfect voltage source would have an output impedance of zero ohms at all frequencies. This would result in all the output voltage from the source being dropped across its load, with no voltage being lost to the output impedance of the sending device.

Output impedance is basically the internal resistance of an amplifier seen at its output. The value can be slightly frequency dependant because of reactance caused by inductance and capacitance, but either way, a low output impedance feeding a high input impedance is desirable.

In its simplest terms you can view the internal resistance of a source (CD, in this case) and its load (the D.I box) as a voltage divider. The higher the output impedance of a CD, the more voltage will drop across 'it' instead of its load (the DI). This in effect means less voltage will be received at the load end.

If the input impedance of a load device is not significantly higher than the sources impedance, the signal will be reduced or 'loaded down' and its signal to noise ratio and frequency response will suffer. Certainly the load can become too much for the source to supply adequately.

Generally, a high output impedance requires close attention to cable lengths. The concern is that the high reactance (frequency dependant resistance caused by capacitance) of the longer cable, combined with a high output impedance of the source, creates a low pass filter which adversely affects bandwidth. This distortion of the higher frequencies increases with higher output impedance's.

The interconnects capacitance results in a reactance (frequency dependant resistance) that will roll of the higher frequencies. It becomes more and more a factor, the higher the output impedance becomes. This interface is actually the same as the connection between any source like a CD and the input impedance of a preamp, or a preamp and an amplifier, so the same rules apply there too.

Anyway, there's an old rule of thumb that says, the input vs output impedance of a voltage bridge interface should be a minimum of 10:1. That's minimum. It should be greater in a high end system to ensure high frequency preservation. So, if I fed a power amp with 10Kohm input impedance, I would just be OK with 1000 ohm output impedance.

Anyway.........., what Wayne said is true. Changing to a CD player for its balanced outputs is not likely to reveal any new detail and imaging.

BTW, the Arcam Alpha 9 has an output impedance of 50 ohms.... I happen to own one - beautiful player. Keep it.

brucek