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Input impedance question (bridging two inputs)

bongomania

bongomania

Commercial User
Oct 17, 2005
35,688
3,919
PDX, OR
www.ovnilab.com
Disclosures
owner, OVNIFX and OVNILabs
#1
I have a two-channel active DI, and each of its inputs has a 10Mohm z in. I want to connect one passive bass to both simultaneously. Can I just wire the two input jacks together, or will/could that result in any degradation of the signal or ground loops? If I wire the inputs together is the z in halved per input, or is it unchanged, or is there a formula to apply? Is an active (or transformer-isolated) splitter the best way to do this?

Advise please. :)
 
#3
As Ehque said, the net input impedance would be 5M ohms. But I don't see why you would wish to use both inputs for the same instrument unless you're trying to increase the signal level (I think it would be up by 6 dB). It shouldn't degrade the signal as 5M is still pretty high where loading probably wouldn't become an issue. I don't think a ground loop problem would happen either as long as the split happens very close to the input jacks and not before a long run of two cables. If you do get a ground loop you could simply lift the ground at the second (OR first) input jack.
 
#5
The two channels are completely separate, they do not merge. Each channel is capable of providing a very wide range of gain, more than enough to drive any line level gear or power amp. So I am using one channel as a DI to FOH and the other channel as a preamp into my rack gear and power amp.

The unbalanced direct 1/4" output of this DI is post the entire preamp stage, so I'm finding that running the two channels in series adds a bit of noise.
 
#6
bongomania said:
The unbalanced direct 1/4" output of this DI is post the entire preamp stage, so I'm finding that running the two channels in series adds a bit of noise.
Click to expand...
Probably caused by a ground loop from running two outputs to two destinations. Read this:
[Invalid or Expired Link Removed]
 
#7
Hmm... Certainly good to be reminded about the constant struggle against poor grounding design, and it does light a fire under my butt to reconfigure the grounds of all my gear from signal-gnd to chassis-gnd...

But that's separate from the obvious noise gain involved in running two preamps in series. No?
 
#8
bongomania said:
But that's separate from the obvious noise gain involved in running two preamps in series. No?
Click to expand...
Yes, as each gain stage adds more noise. But since you're driving two devices from the same source chances are you're adding ground sourced noise, especially with one of the sends being unbalanced.
 
#9
Fair enough. Analyzing it further:

I opened up the DI and at least the XLR outs are chassis-grounded. I get it that combining balanced and unbalanced signals is a recipe for noise, but then what's the best solution for what I'm trying to attempt? Let's assume for the moment that what I'm trying to do is a good idea, as that would please me greatly. :) It's a fantastic DI and a fantastic preamp, and it has two discrete channels; I'm just hoping there's a way to drive both from one source without grounding problems.
 
#11
Bill, Bill, Bill. The instructions in the Rane note regarding the right way to use unbalanced connections say to use a DI box to balance the connection. Can you see why that would not make any sense in this application?

There is the Invalid Link Removed, a passive xformer-isolated splitter that outputs two unbalanced signals. Is that the ticket?

Or is there something in the Rane note about making a properly-wired Y cable that I'm just not seeing?
 

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