Avalon 737sp hissing noise

[mixedbitch]

Hi all,

One of my Avalons is having a problem. When I turn it on, all is normal for about 20 seconds, then, it sounds like a relay clicks and I get a loud hissing noise.

I've tried swapping tubes, but it doesn't seem to matter.

Any suggestions of other things to check before I shop it? I don't have the ground link hooked up, but my other Avalon works fine without it.

-- Rob

[mixedbitch]

Ok, I just did a test. I get a level of about -5db on Protools meter when I plug into the Rosetta 800 (any port, any cable). When I go into the 003, it's fine.

Weird.

Any ideas? I'm stumped.

-- Rob

[Saxon-of-a-son]

So, it's your Rosetta? 737sp does have some hiss. That's why I don't use high-gain. Not noticeable on lower-gain and loud enough sources. But yeah, it's my only preamp that really does emit hiss.

[mixedbitch]

No, it's some combination of that particular 737 (I have another one hooked to the Rosetta that works fine) and the Rosetta. None of the other 7 channels of mic pre connected are having the same problem.

It's not really a hiss, more like the meter immediately pegs to -5 db in Pro Tools and I get a signal present light on the Rosetta, even with no input connected.

Edit: Changed thread title to more accurately describe problem.

[AllAboutTone]

I never owned a 737sp that did not produce hiss, also its the only high end piece of gear I have ever used that produced hiss.....hope that helped.

[RKrizman]

This hissing could be coming from all the people here who hate the 737's!

Seriously, though, I love mine and have no hiss problems.

-R

[mixedbitch]

Neither of my 737s hiss, even with high gain (well, unless I crank them to max with high gain). I changed the title of the thread to reflect this

Someone suggested that maybe +.5V DC is somehow coming out when no signal present. Need to get a voltmeter tomorrow and check whether that's true. I'm thinking grounding issue, or a short somewhere.

[mixedbitch]

Ok, finally got around to testing it. The Avalon is definitely putting out +.5v DC with no signal. I guess that means I should probably shop it.

What's perplexing me is why the Rosetta notices it, while the 003 and an ADAT HD24 don't care about it.

Is there something in their input circuitry that measures base level input and uses that as a reference?

[peat]

needs a retube

[synthoid]

Different receiver circuits do indeed react differently (or not at all) to a DC bias. Capactive coupling or transformer coupling will make a DC bias disappear altogether.

-synthoid

[AllAboutTone]

Originally Posted by peat

needs a retube

I don't always believe this, I have had new ones that give hiss, hell I'm on a Sunday morning Avalon discussion, its the lords day, aren't they something better to discuss than Avalon hiss ?

[mixedbitch]

Originally Posted by peat

needs a retube

I tried swapping tubes, same problem. I guess I could try using the tubes from the other Avalon and see what happens. If that's the fix, it's cheaper than having to take it in to be shopped.

[AllAboutTone]

Originally Posted by mixedbitch

Neither of my 737s hiss, even with high gain (well, unless I crank them to max with high gain). I changed the title of the thread to reflect this

When you say this, do you mean preamp only? bypassing the compressor and eq ?

[mixedbitch]

Originally Posted by AllAboutTone

When you say this, do you mean preamp only? bypassing the compressor and eq ?

For tracking, I never use EQ. I use it mostly as a vocal pre, so I pretty much always have the compressor in with some light compression when tracking.

Guess I've never really tried the Avalon without the compression circuits engaged. Well, I've switched it off to get a feel for what it would sound like, but always quickly put the compression back on.

[AllAboutTone]

I don't think you should have hiss with just the preamp on, that is a issue, the eq and compression is where the hiss is always at when in own one, wait 2 or 3......many.

[mixedbitch]

Well, I got it repaired. The tech said that phantom power somehow got applied to the mic input and it blew the input and output boards.

Does that sound right? I would think there would be some circuitry to prevent that sort of thing from happening.

[rogerbrain]

Originally Posted by mixedbitch

Well, I got it repaired. The tech said that phantom power somehow got applied to the mic input and it blew the input and output boards.

Does that sound right? I would think there would be some circuitry to prevent that sort of thing from happening.

i'm no tech...but that sounds wierd... where would the phantom power be applied anyway but the mic input??

[mixedbitch]

Originally Posted by rogerbrain

i'm no tech...but that sounds wierd... where would the phantom power be applied anyway but the mic input??

Sounds weird to me too, but I have a patch bay from the mics to the pre-amp inputs that are half normalled and I might have accidentally put 2 phantom powers on at once. Although I'm pretty careful about my phantom power.