Impedance Issue between preamp out (EZ1290) and Audio Interface Line IN (Prism).....Help!

[PatchDog]

Hi if anyone with better knowledge than me could help I'd be very grateful.

Issue:

Plugging my EZ1290 XLR output into my Prism Orpheus Interface Mic input (via XLR) works fine as expected.

But when plugging into the line input of the interface via XLR (EZ1290 OUT) to TRS Jack (Interface IN)) the impedance is clearly mismatched, can't get any decent level from the preamp to interface.

EZ1290 Output transformer (secondary) is wired in series so outputting 600 ohms (not sure if the 1k5 resistor and .01uF cap is changing that though?) I've wired it like this:




My Prism interface states the Mic Preamps have an impedance of 5.5kΩ.
And it's Line Preamps have an impedance of 14.5kΩ.
Could anyone tell me how to fix this? I'd like to connect the Ez1290 via the Line inputs of the Prism Interface.

Do I need to change the secondaries to their lower impedance (150Ω)? Or is that a stupid question?

 

[emrr]

all I can say is it's not an impedance problem, look elsewhere. The mic input probably has some minimum gain value, not unity.

 

[PatchDog]

Ok, thanks for your response, I'm still confused though why it doesn't work. It also happened on my older interfaces some times and with various mixers.

These are more specs of the Prism interface if it helps to point out the issue?

 

[PatchDog]

I also saw that the Line level outputs of the Prism Interface are at 100Ω balanced. Which is why I thought that putting the EZ1290 to 150Ω output would make it work. i.e a higher impedance on the input of the Interface requires a lower impendence of the EZ1290 being sent to it, but I'm playing guessing games with the level of my knowledge.

 

[john12ax7]

It also happened on my older interfaces some times and with various mixers.

This sounds like the problem is with the EZ pre. It may just not have enough gain that is being compensated when you plug into another pre

Do you have test gear? Put a signal into the pre and measure the gain.

 

[PatchDog]

Hey @john12ax7, hmmm the thing is, it works absolutely fine through the Mic Inputs on the Prism, with no gain applied on the prism interface, and there is plenty of gain available on the EZ1290 to go way past clipping on Interface. I hardly ever need to go past 1 or two notches on pre's input switch.

To be more specific about the issue, the EZ1290 isn't just 'quieter' when plugged into the Interface Line inputs, it's extremely thin sounding, almost like a fuzzy thin sound and almost in-audible volume no matter if I set Interface line In to -10dB or +4dB and put the gain on full on the EZ1290.

I don't have test gear unfortunately, unless a DAW that can output Sine waves and a multimeter counts.

 

[kags]

Sounds like the line input of the EZ1290 is not working correctly. Does the Ophelia line out work into other gear? Does the EZ line input work with other sources? Have you tried lowering the Orpheus line output like 30 dB and sending it into the EZ mic input?

Have you tried other cables- or at least tested your cables for correct wiring?

Impedance, as previously mentioned, should not be an issue with either of those devices.

 

[john12ax7]

I was reading it differently. Just to clarify, EZ1290 XLR out to Prism XLR mic pre in works? But EZ1290 XLR out to Prism line in doesn't? If it's TRS have you eliminated a bad XLR to TRS cable?

 

[PatchDog]

Sounds like the line input of the EZ1290 is not working correctly. Does the Ophelia line out work into other gear? Does the EZ line input work with other sources? Have you tried lowering the Orpheus line output like 30 dB and sending it into the EZ mic input?

Have you tried other cables- or at least tested your cables for correct wiring?

Impedance, as previously mentioned, should not be an issue with either of those devices.

Yeah the Prism works fine, line outs to my 500 series and Line ins. No problem. Same with the EZ1290, works just fine.

Just to clarify,

EZ1290 output > plugged into Prism Interface MIC Input with XLR = Perfectly fine all is working

EZ1290 Output > Plugged into Prism Interface LINE Input with XLR to TRS = Fuzzy Thin sound hardly anything audible sound

 

[PatchDog]

I was reading it differently. Just to clarify, EZ1290 XLR out to Prism XLR mic pre in works? But EZ1290 XLR out to Prism line in doesn't? If it's TRS have you eliminated a bad XLR to TRS cable?

Yep exactly this. Also FYI it's the same input on the Prism, it automatically switches from MIC to Line dependent on whether an XLR or TRS/TS Jack is plugged in.

Yeah I'm pretty sure it's not a cable issue but I will check again. But I wire my cables attentively and it's a problem I've run into on different interfaces/mixers/different cables over the years with the EZ1290.

If it's definitely not an impedance issue then I'm wondering if it may have something to do with the 1k5 Resistor & 0.1uF Cap strapped across Pin 2 & Pin 3 of the Transformer/XLR. I used a tiny ceramic cap from the early reversion of the EZ1290, I see the newer versions using Polyester Caps now. Could that potentially be a cause?

 

[Brian Roth]

"Fuzzy Thin sound hardly anything audible sound" seems to be a CLASSIC case of one of the two leads coming from a transformer balanced output not making the path to the two input leads of the balanced destination. Since you indicate your XLR to TRS cable is "perfect" have you tried substituting a different signal source from another device which has a transformer balanced output?

Bri

/EDIT the possibility of one of the "two leads.....not making the path"....means either pin 2 or 3 of the XLR and tip or ring of the TRS.

 

[PatchDog]

Ok thanks @Brian Roth. I'll re check the cables, though it was having the same issue on both channels at same time, and to my memory those XLR-TRS cables I used worked when I plugged in my 500 series, but I'll check and make certain.

 

[Brian Roth]

@PatchDog It is a possibility that something is awry with the actual TRS destination jack itself, hence my suggestion of substituting a "known working" alternative device which also has a transformer balanced output.

Bri

 

[john12ax7]

The other question would be is it actually TRS or is it TS? That would explain it working with some gear but not others.

 

[ruffrecords]

Hi if anyone with better knowledge than me could help I'd be very grateful.

Issue:

Plugging my EZ1290 XLR output into my Prism Orpheus Interface Mic input (via XLR) works fine as expected.

I think this is where you are going wrong. The EZ1290 is a mic pre and its output should be at line level. You should not need to plug it into another mic input to make it work. It should work into a line input.

The key to this is what is the actual output level. Can you get hold of a regular VU meter and wire it to pins 2 and 3 of an XLR? Plug this into the EZ1290 output and feed some signal though it and see if you can get 0VU output it.

When you say you "can't get any decent level from the preamp to interface" you don';t say what signal source you are using, which input of the EZ1290 it is plugged into and what the EZ1290 gain settings are. Can you clarify this?

Cheers

Ian

 

[PatchDog]

Hi Ian, thanks for advising. I understand it has line level output. Plugging into the Mic amp of my interface is just a workaround, it's actually just a generic input with a simultaneous Jack and XLR connector, the interface automatically switches between line or mic amp dependent on if you have an XLR or TRS plugged in.

I have a cheapo ebay VU meter, I'll give it a try.

It doesn't appear to me that it's a gain issue though, since I've always had plenty of gain with a mic or whatever source I've used with it, and without needing to push the gain much at all.

My old mackie soundcard worked fine with it plugged in via XLR (from 1290) to TRS (into the Interface), but an old Mackie Onyx mixer years ago gave me the same issue as now (going off memory), it didn't like the XLR to TRS, I swapped it to an XLR to XLR connection and it worked fine. Which is why I guessed it might be to do with the destination's load.

Both channels of 1290 react exactly the same, all fine via XLR to XLR, once I switch to XLR to TRS (which also switches the interface into line amp mode) I get basically no volume at all, and when I push the gain on the 1290 it just seems to overload the audio in a thin fuzzy manner with no rise in volume, like all the headroom has been eaten up.

 

[PatchDog]

I was also looking at DOn Classics NV73 preamp, basically looks the same as the EZ1290 but he's got an extra 600 ohm resistor shunt on Output Transformer between + and - . Should this be on the EZ1290 as well? They don't include that in the Assembly Doc.

NV73:


EZ1290:

 

[jokeramik]

I also saw that the Line level outputs of the Prism Interface are at 100Ω balanced. Which is why I thought that putting the EZ1290 to 150Ω output would make it work. i.e a higher impedance on the input of the Interface requires a lower impendence of the EZ1290 being sent to it, but I'm playing guessing games with the level of my knowledge.

View attachment 128709

The output impedance of the interface is too high compared to the impedance of the transformer to drive the transformer. The transformer requires an even lower ohm source of around 20 or 30 ohms if you connect the windings in series. If you connect the primary windings in parallel, the source only has about 10 ohms of output resistance.
Best regards!

 

[jokeramik]

I was also looking at DOn Classics NV73 preamp, basically looks the same as the EZ1290 but he's got an extra 600 ohm resistor shunt on Output Transformer between + and - . Should this be on the EZ1290 as well? They don't include that in the Assembly Doc.

NV73:
View attachment 128713

EZ1290:
View attachment 128714

This resistor-capacitor combination is necessary to achieve a correct termination of the transformer in terms of frequency response and correct pulse reproduction.
Best regards!

 

[Graham Nystrom]

That 600 ohm resistors is optional and gives you a flat frequency response when you plug your 600ohm-600ohm "matched impedance" gear into modern "bridged impedance" gear (Hi in Z, low out Z). Without it, you will have more gain and a big HF boost up around 20kHz. Lack of that resistor isn't your issue.

It common to not connect pin 1 at the preamp XLR output, but to let the receiving gear provide the case connection for pin 1. There is something happening, possibly with the combo jack.

The way a line input handles balanced vs unbalanced signals is different depending on the gear.

All signs point to cabling issue. Like is there a transformer input (mic in or line out?) that got phase reversed or something? When checking your cable make sure XLR pin 2 hits the TRS Tip and Pin 3 hits the TRS sleeve. (Use beep mode on your DMM to make sure none of those connections are shorts somewhere in the cable.)