As I stated in another thread, I've recently acquired a PT P1A (bought used). The funny thing is that my stock DI/O does not work with it. The stock DI/O works fine when connected directly to my transport (Sony DVP-NS500V), but it give you the "flashing green light of death" when connected through P1A. What makes this a particularly puzzling thing, however, is that when I hooked up my modified DI/O with a digital input transformer, it works fine with P1A in the chain. So what gives? Does this mean there's too much noise coming in from P1A? Is this P1A defective (I think not though)?
This made me think twice about what exactly a digital input transfomer does. What does it do precisely? (<--- may be a dumb question but here it goes. :)
It sounds like you may have a DC offset problem in the P1A's digital output. If this is the case, the offset is great enough to where the digital stream is picked up as "DC" by the DI/O, and fails to sense the digital transitions. The transformer in such a case would block the DC component, and enable the stream to be seen normally.
I may ask, with the transformer, does the DI/O lock reliably when a digital stream is introduced to its input? Something tells me the inability of the DI/O to lock reliably with a new digital stream may be DC-offset-related as well.
I just put in a .1 uF cap after the digital jack and now both DI/O can lock on. So there could have been some DC offset. I still believe PT should have taken steps to eliminate it from the source end though. Thanks Todd.
DC offset is difficult to control, especially on absolute terms. It could have been fine at the factory, and some component settling after the factory testing may have caused the offset. It could also be caused by differences in line voltages and/or the DC potential of the common ground.
Many higher-end DACs actually have tranformers on their digital inputs. In fact, AES/EBU balanced inputs are intended to be designed this way.
If I wanted to install a AES/EBU digital input on my DI/O, using a Schott 22133 transformer, how do I need to wire it? Right now I'm using a BNC jack with the trasformer. I don't think there will be much improvement with a AES/EBU jack, but I would like to try it. :)
Thanks for your reply, Todd. If DC offset in P1A is the problem, what could have caused it? Because I remember using a "brand-new" P1A and stock DI/O together without problem a few months back. Then I sold my P1A and recently bought another "used" P1A. Now this one doesn't work with sotck DI/O. So is it safe to assume that it is this particular P1A that has the problem (maybe something inside has failed?)?
When connected directly to my transport, all my DI/O (stock and with transformer) work fine and can lock onto the signal in less than half a second (almost instantly). I didn't install small caps at my digital input jack, as I thought it won't be a big problem with the small amount of DC, as described in cfraser's DI/O FAQ page.
and instead of "optional" for the caps (I put them in) I should say "mandatory"? It didn't make any difference for me with them in or out, but since they were already installed...
I suggest you check out the AES paper on the Scientific Conversions Inc. web site for tech details. The summary, an excerpt:
Reasons for Using Transformers in
Digital Audio Transmission
• Greatly improve common-mode noise rejection
• Reduce conducted EMI emission to ensure
• Reduce transmit and receive circuit susceptibility
to EMI interference
• Break ground loops
• Balance input and output
• Match impedance and levels
• Achieve lowest possible recovered clock jitter
Good luck with it.
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