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

[ruffrecords]

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.

You do not need the 600 ohm resistor across the output. The original Neve circuit has an output impedance of about 75 ohms and will drive any load from 600 ohms upwards. Your problem is definitely not an impedance problem.

As others have said, this is either an cable issue or a design error in the interface which is probably trying to cater for both balanced and unbalanced inputs

Cheers

Ian

 

[ccaudle]

These are more specs of the Prism interface

That does indicate that at minimum gain the microphone input will have 7dB more gain than the line input in -10dBV mode. You will have to compensate for that somewhere.

I also saw that the Line level outputs of the Prism Interface are at 100Ω balanced.

Not relevant, you are concerned about the line level inputs of the Prism.

i.e a higher impedance on the input of the Interface requires a lower impendence of the EZ1290 being sent to it,

No, that is completely opposite, a higher input impedance will cause less reduction in level due to the output impedance of the driving stage.

it works absolutely fine through the Mic Inputs on the Prism, with no gain applied on the prism interface

According to the specs you pasted above it is not possible to apply less than 10dB gain to the mic input. Minimum gain is -1 dBu for full scale digital, line input in low voltage (-10dBV mode) is +6 dBu for full scale. In +4 mode it takes +18 dBu to reach full scale digital.
However, the EZ1290 manual says it has over 70dB of gain available, so there should be no reason to use -10 dBV mode. I have never heard of a Neve design which could not hit 18 dBu, so something else is going on.

TRS Jack (Interface IN))

You do mean the 1/4" input in the center of the XLR connector on the rear panel, or the 1/4" only line inputs next to those on the rear panel, and not the 1/4" inputs on the front panel, correct?

I wire my cables attentively

Can you describe exactly the cable connections on each end, XLR and TRS? The symptoms all point to a cable problem.

 

[PatchDog]

Thank you every one for your super helpful knowledge and suggestions, this forum truly has the best of the bunch on here.

It looks like I made a really dumb error in my XLR wiring on the male pins (output), as most of you suspected (holds head in shame). Humble apologies. Looks like I didn't take into account the 'mirrored' factor of my diagram when I wired this up years ago. I'm re-wiring now so haven't tested but it's got to be that. Rookie error.

 

[Brian Roth]

Thank you every one for your super helpful knowledge and suggestions, this forum truly has the best of the bunch on here.

It looks like I made a really dumb error in my XLR wiring on the male pins (output), as most of you suspected (holds head in shame). Looks like I didn't take into account the 'mirrored' factor of my diagram when I wired this up years ago. I'm re-wiring now so haven't tested but it's got to be that.

XLR pins 1 and 2 swapped?

Bri

 

[PatchDog]

XLR pins 1 and 2 swapped?

Bri

Yes, it was exactly that, Pin 1 & 2 swapped on the XLR outs. Doh!! I've had it like this for over 10 years lol. Very weird that in most situations with my interfaces and mixers over the years it wasn't an issue, at least not audibly.

 

[PatchDog]

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.)

That's really interesting. ~Would you say it would be better to add that 650 ohm over not having it? 20Khz seems like it would be a noticeable difference in comparison. Do you or anyone know how if they had it on the original Neve?

 

[ccaudle]

Would you say it would be better to add that 650 ohm over not having it?

It depends on the characteristics of the specific transformer model you used. You have it connected to your audio interface, I would suggest just measuring the frequency response, that will tell you if an addtional loading resistor is needed or not.

 

[Brian Roth]

Yes, it was exactly that, Pin 1 & 2 swapped on the XLR outs. Doh!! I've had it like this for over 10 years lol. Very weird that in most situations with my interfaces and mixers over the years it wasn't an issue, at least not audibly.

"Modern" equipment almost always has some sort of electronically balanced inputs and outputs vs. transformers. Depending on the design of those circuits, a mis-wired cable may (or may not) create an obvious failure. There are a variety of circuit designs and wiring combinations, so there is no "rule of thumb" if a mis-wired cable will cause any particular type of malfunction.

Many years ago, one of my studio clients introduced me to the Behringer CT-100 box. It tests a variety of cable combinations. One really cool feature is that it will detect an intermittent connection. I sometimes have had a funky problem at the end(s) of a cable with RCA plugs I soldered on. I can plug the cable in, wiggle the cable at the RCA connector and the box detects the fault....typically an intermittent short circuit. Those can be very tricky to detect with just a simple multimeter.

It also has a very basic tone generator function as well. It's FAR from being even close to a lab-grade device, but is quite handy when "toning out" an audio line in a studio or PA application. Here in the USA, Sweetwater has it for $29.00 with free shipping. UK dealers should have it for a low price as well.

https://www.sweetwater.com/store/detail/CT100--behringer-ct100-6-in-1-cable-tester

Bri

 

[PatchDog]

"Modern" equipment almost always has some sort of electronically balanced inputs and outputs vs. transformers. Depending on the design of those circuits, a mis-wired cable may (or may not) create an obvious failure. There are a variety of circuit designs and wiring combinations, so there is no "rule of thumb" if a mis-wired cable will cause any particular type of malfunction.

Many years ago, one of my studio clients introduced me to the Behringer CT-100 box. It tests a variety of cable combinations. One really cool feature is that it will detect an intermittent connection. I sometimes have had a funky problem at the end(s) of a cable with RCA plugs I soldered on. I can plug the cable in, wiggle the cable at the RCA connector and the box detects the fault....typically an intermittent short circuit. Those can be very tricky to detect with just a simple multimeter.

It also has a very basic tone generator function as well. It's FAR from being even close to a lab-grade device, but is quite handy when "toning out" an audio line in a studio or PA application. Here in the USA, Sweetwater has it for $29.00 with free shipping. UK dealers should have it for a low price as well.

https://www.sweetwater.com/store/detail/CT100--behringer-ct100-6-in-1-cable-tester

Bri

That's a cool little box for the money, I know it well, I ran Behringer tech support for 5 years. I make all their synth videos these days though.

Well cheers for the help again everyone. Success, it's all working perfectly now. Still can't believe I had it wired like that for 10 years oh boy.

 

[Brian Roth]

So glad you have it sorted out! The big clue for me was the very low signal level that sound thin/tinny. I recall my first part-time gig in a "real" studio (2" 16 track Ampex and a MCI desk) 50+ years ago. I was maybe 19 years old, and already had some skills after working with garage bands when in high school.

Anyway, one day I was micing up a band and one mic on the drums sounded....very weak and tinny! My studio mentor immediately recognized the sound and he quickly traced it down to a pin 2 (3?) open circuit in the mic cable. THAT sound was permanently etched into my brain...lol.

Bri

 

[Matt Syson]

The resistor plus capacitor across the transformer secondary corrects the frequency response to a fair degree and then the 604 OHM or whichever value close to 600 Ohms corrects bot frequency and lEVEL 'errors' when that is in circuit. Having that switchable woulmd allow you to hear bother versions of 'compromise' as in or out the true original response depends on the external gear you are connecting it to. the response also changes when feeding LONG lengths of cable (300 Feet or more) so when the gear is used in a fixed unstallation values might need adjusting to get an overall response thqat meets the full requirements. The definition of 'right or wrong' is variable so the 'best compromise' values are used. This of course makes a mockery of all those determined to be 'authentic' when gear is taken out of the original design parameters.

 

[warpie]

Speaking of terminating resitors I have seen some designs that they use as high as 1k2 Ohms across the secondaries. What does the value depend on? The ratio of the transformer?

 

[ccaudle]

What does the value depend on?

I think you are essentially picking the damping for a RLC circuit. The capacitance is distributed across the windings, so probably a bit difficult to measure independently or calculate from geometry, but easy enough to put a variable resistor across the output and measure the frequency or step response at various loading values.

 

[rh001]

Hi!

I agree with others that this is not an impedance issue. It seems like a missing or wrong connection issue. The xlr to 1/4" cable is miswired (like socket 1 and 3 swapped or some other swappage, or a wire is broke. Or a connection in the to or from gear is broken.

Troubleshooting so far seems incomplete. You must do these testes:

1) check cable wiring with ohm meter and make sure socket 1 of xlr goes to shield (aka sleeve) of 1/4" plug, socket 2 of xlr goes to tip of 1/4" plug and socket 3 of xlr goes to ring of 1/4" plug.

Or if 1/4" plug is unbalanced type then socket 1 and 3 of xlr should go to 1/4" plug shield and socket 2 of xlr to 1/4" plug tip.

2) test that cable(s) additionally be plugging from/to other equipment.

If the cables pass tests 1 & 2 then the cable is not the problem. Then problem is with with sending or receiving unit, possible the connectors on those units themselves. It seems you can visually inspect the wiring of the sending unit..any wire broken? Jankily soldered? xlr polarity and sheld correct?

On the receiving unit you can do a gerry-rigged test by, with 1/4" cable plugged into that unit and it's gain turned up (but be careful of speakers...keep speaker level low at first) touching with finger/hand, alternately to cable female xlr socket 2 & 3. Obviously you will need a small screwdriver or paperclip or something like that to make contact in the xlr female connector. You should hear a buzz or hum when touching, and should sound the same on both xlr 2 and 3. That's if the 1/4" input is balanced.

If 1/4" input is unbalanced then you should only hear finger hum/buzz when touching xlr socket 2 if cable is correctly wired, and socket 3 if wired out of phase.


If, with balanced 1/4" plug you hear hum/buzz with touch test on only xlr socket 2 or 3 then there is a broken connection in the receiving unit on tip or ring.

Cheers

 

[PatchDog]

Thanks @rh001. Yeah you are correct, I found the issue last night, I stupidly mis-wired the XLR output (I originally built this pre over 10 years ago). Pins 1 & 2 were swapped. Rewiring solved the issue immediately.

 

[Graham Nystrom]

That's really interesting. ~Would you say it would be better to add that 650 ohm over not having it? 20Khz seems like it would be a noticeable difference in comparison. Do you or anyone know how if they had it on the original Neve?

It’s really nice to have the option. Put the load resistor on a switch if you can. Otherwise leave it out and build a cable that has the load resistor in the barrel across pin 2+3.

With it you get flat response and tamer overall gain. Without it you get an aggressive hf boost way up at like 20k. Both are nice depending on your mic and your source.

In the old days, the next circuit in line (eq etc) would probably present the 600 ohm load, so while it didn’t necessarily have the resistor, it also didn’t need it.

What’s even more fun to is to put 3 or 4 load resistors on a switch so you can have options from flat to full HF boost (caused by the unloaded output transformer “ringing“).

 

[ruffrecords]

Neve output stages we designed to drive any load from 600 ohms upwards. Peaks in the response are caused more by driving source impedance rather than load impedance. The Neve class A output stage has a source impedance of about 30 ohms and the Zobel network across the transformer tames any HF peaks caused by transformer interaction with this source impedance. With or without a 600 ohm load, a Neve should be essentially flat out to about 75KHz.

Cheers

Ian

 

[inkster]

Pin 3 hot...lots of companies selling sorta balanced gear including some well known widely used recording interfaces don't like gear with pin 3 hot which I see some times and I still get calls like "my manley sounds weak and thin" and I ask "when did you install the new protools rig?"

 

[kags]

Pin 3 hot...lots of companies selling sorta balanced gear including some well known widely used recording interfaces don't like gear with pin 3 hot which I see some times and I still get calls like "my manley sounds weak and thin" and I ask "when did you install the new protools rig?"

Are you saying that the new Avid interfaces are pin 3 hot? I've installed hundreds of HDI/O, Carbons, and MTRXs and haven't had an issue, but nothing surprises me anymore! I have compared the DB25 analog pinout from the Carbon to the Tascam standard, and it matches, as did the older 192 interfaces.

 

[Matt Syson]

There is a more than subtle difference in the term 'hot' so you HAVE to read the whole story. Hot can mean it has active signal on it which may or may not be in phase with either other outputs or the 'cold' signal which if active could be referred to as hot but phase inverted. Most manufacturers aim for coherent phase response from input to output BUT occasionally things get messed up. Thus pin 2 of an input XLR is now (mostly) trated as being inphase and pin 2 of an OUTPUT XLR of the same gear should also be 'inphase although if it is an EQ the 'phase' may be 'out' at various frequencies) (delay).