H7600 Low input volume, malfunction or not?

[ianross]

I just bought an H7600, and I'm experiencing high output volumes and low input volumes.

I have tried all combinations of inputs and many different instruments with the same results.

I have a Bus from my SSL X-desk going into the Eventide with, of course, balanced XLR, and two XLR cables going out into the X-desk stereo FX return.

I am sending a healthy volume, but it reads as about -40db on the H7600. Gain trim and all other input/output settings on the H7600 are at 0.

Is 0 the default for gain trim or should it be higher? 

I have tried, for example, taking a Moog Voyager RME, set it to unbalanced (it has a balanced output option), and then run a TS 1/4 cable into the Eventide H7600. I get the same result. Very low metering for the input. the Output is much higher and louder. 

What's going on? I'm hoping my unit isn't broken.

[nickrose]

Please see my reply to a recent similar query.

http://forum.eventide.com/cs/forums/p/7622/34261.aspx#34261

[ianross]

Nick, I read the post you linked me too. The part about making all the connections doesn't apply to me since, I'm simply going XLR out of the X-Desk cue bus into the Eventide, and then XLR out of the eventide into the FX stereo return. The whole signal path of the X-desk is balanced.

The part about thr 25db of gain headroom might be important though. Can you help me understand it a bit better. Here's my specific setup.

I have two channels going out of a Lynx Aurora into an X-desk channels 1 and 2. I'm then sending those channels to a bus called the cue bus which is going straight into my H7600. On the X-desk I can isolate the cue bus output on the LED volume meter readout. It peaks on average at 12db. 

Now the H7600 meter LED inputs sit at the first LED at -40, and once during the drum loop I have going they peak at the second LED. 

If I turn the cue bus out gain up on the X-desk to hit nearly 0 on the X-desk  meter, then I get a average peak of -12 on the Eventide H7600. 

There doesn't seem to be a way I can get above -12db on the Eventide without really driving into the red on the X-desk which of course overdrives the signal.

It's hard to believe the H7600 isn't meant to display more than a -12db on the inputs meter.

Can you help?

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[nickrose]

It sounds like the H7600 can take much more than your X-desk can give out. Its input clips at about +25dBm. But, your setup is probably OK as is – if the highest level you see on the 7600 is -12, this means that you have 12dB of extra headroom in the 7600 – this is not a bad thing  as some effects can increase the level of parts of the signal.

You could use the LEVELS/inputs/gaintrim control to get a higher level on the 7600 if you feel you need to. See UM p.29 for details.

[ianross]

Thanks so much for you quick responses Nick. We should be able to verify if this is the case as you see it.

I am sending a sine wave from Logic, out of my Aurora 16 and into the X-Desk. The VU on the metering section of the X-Desk reads 8dBu. I am then sending that out to the H7600 and it shows up as -18 on the H7600 (three green LED inputs lit).

Does that sound right? Something seems amiss to me unless this is just how the H7600 is calibrated.

On the X-Desk the VU reads 8dBu, as I said, before being sent to the H7600, but it also reads about 8dBu on the X-Desk VU coming back in from the H7600 with bypass active. I can switch between either signal on the X-desk easily.

However, just to recap, on the H7600 it reads -18 (with three green LEDs lit). 

Why would these LEDs not correspond at all? With the dBu it is receiving or outputing?

---

Does this have to do with the 25db of headroom you were speaking of? I know that my aurora has 16db of headroom at +4. So When I output -16dBfs from Logic, I get 20dBu on the Aurora.

But on the Eventide I get -40ish (with one green LED lit), even though when I come back into the X-Desk from the H7600 (in bypass mode) it shows the same 20dBu.

[ianross]

Here is an even simpler test,

I plugged my Lynx Aurora 16 outputs 11 and 12 into the aurora and took a 1000hz sine wave from Logic at -3dB. I'm feeding that directly into the Eventide and it's showing up as -12 on the H7600 analog in meters.

That's probably as simple as I can explain my problem. How could this be normal?

The output of the Eventide H7600 is going into my X-desk so I can see the VU and it's showing up as much louder than -12. I cannot say exactly since there is an attenuator on the VU meter connecting with the FX return.

Please advice

[nickrose]

The meter readings on the 7600 are relative to its peak level (about +25dBm).  So, a reading of -12 on the 7600 means about +13dBm. There is nothing wrong with your 7600 – your problem is that you are confusing the different indications.

dB is always relative to something else. What this is depends on context.

dBm is relative to 0dB being defined as 0.776Vrms into 600 ohms.

dBu (and dBv)  is relative to 0dB being defined as 1.0Vrms into 600 ohms.

So, you really have to know the context before making these comparisons.

Can't really add anything else.

[ianross]

Could tell me how dBm translates to dBu(v) in terms of level differences with an example. For instance what would +25dBv be in dBu(v)

Or XdBfs into XdBm

If I knew something like this it'd really help me calculate the difference, and make sense.

Thanks for all your help. Lots to research and understand.

[nickrose]

Here are some useful links.

http://www.sengpielaudio.com/dB-chart.htm

http://en.wikipedia.org/wiki/DBu#Voltage

But, on looking at it more closely, I have been guilty of a mistake myself, in that dBu and dBm are the same with a 600 ohm load impedance – I'd said that dBu and dBV were the same, they're not.

So, for your purposes, the 7600's meter readings are relative to +25dBu(m), e.g. -12dB translates to 25-12 = +13dBu.

Why do we do it this way ? My guess is that the amount of headroom is often more important than the absolute level – if you remove the – sign, the 7600 meters display the current headroom.

[ianross]

I've tried to concieve of a simple test to make sure my input meters are reading correctly and I think I came up with one. Let me know if this all looks legit Nick.

A Lynx staff member said the following about the Aurora 16 A/D and D/A:

"The maximum level of 0dBFS at +4dBu trim setting is +20dBu (16dB of headroom)."

So I took a stereo output from the Lynx into the H7600 and outputed a sine wave from Logic at -0.1dB just so I don't clip.

That means that I can subtract 16dB of that going into the H7600. The H7600 reads at -9 with 6 LEDs lit.

Now 9+16=25dB, which corresponds to the 25dB of headroom you spoke of. 

I think this shows what I'm looking for: that my LEDs on the H7600 are working probably and it shows simply the mathematical relationship between the output of the aurora and the metering on the H7600.

Is that right? It's kind of hard to wrap my head around when I think about it. I'm hoping this isn't all just coincidence that the numbers add up.

So it must be, as you said, that my X-Desk really does have a low gain structure, because it seems so far difficult or near impossible for me to get up to -9 on the H7600 through the X-Desk without the X-Desk hitting the read somewhere in its signal path.

[nickrose]

I think you're getting there. You can always boost the gain at the input to the 7600 (and probably cut it on its output) if you're worried, but it doesn't sound like you need to; 9dB of extra headroom in the 7600 is not a bad thing.

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