Problem with Avalon 737sp (sound clips available)

[astronautry]

Hello all,

I've lurked around this site for years without an account, occasionally solving some issues in the studio by reading through the threads. I couldn't find one for the issue I'm having, so I figured I'd reach out into the ether to see if any of you geniuses (no sarcasm) can lend me some help. I know there are a lot of folks that really hate this unit, but I quite enjoy mine and would like to keep it around.

So here's the issue:

One day I tried to record with it and the output was crackly and nasty, with very little gain. I've tried posting a .wav and .mp3 clip of what I'm hearing, but apparently that's not possible on this site? Anyway, after that session my wife and I moved around the country for a while so it sat in storage. I thought it was just a blown tube. Fast forward about 3 years and I've replaced the tubes, but I'm still having the same issue.

There is no ambient hiss, static, or change in the noise floor, but the signal leaving the unit is very crackly, almost like the bit depth has been reduced to nothing. It also has very little gain. Sounds like I'm talking through a broken walkie talkie or something like that.

In that recording I'm running an SM7b into the mic input on the back with an XLR, the louder clip is with the high gain button engaged, the other is without. On both clips, I'm not using compression or EQ, but actuating those doesn't change the issue. I'm using the balanced output on the back with a TRS into an Apogee Duet and Protools, with no processing.

I have changed cables, checked the mic, and done every other basic troubleshooting I can think of.

So there it is... any ideas?

 

[zackthedog]

We use those in our studios. They are constantly developing noise and having to be fiddled with. Fortunately we have an in-house technician on hand. But it's a common problem with these units. Could be bad capacitors or resistors, or a faulty filament supply, a regulator, or any number of things. I would send it to a tech for repair. I don't think there's too much trouble-shooting you can do yourself, but I could wrong. Maybe someone else has experience with them and can advise.

 

[sbcrockett]

welcome astronautry! first of all, what part of the country are you in?

I'm a recording engineer and have used the 737 for many sessions, mostly piano and some vocal. I think it's a great preamp and if the pair I was borrowing from a friend were for sale, I would probably buy them.

I'm not sure how much luck you'll have acquiring a schematic since Avalon Design is still in business and producing the 737, unless someone has an unauthorized copy and posted it somewhere online. And, for some of the amazing experts here in the tube forum to really help you without access to either the schematic or the unit itself...it could be difficult.

Since you've replaced the tubes yourself, you've obviously had the unit open. If you're careful and aware about high voltages and the dangers associated with them, I would suggest a thorough visual inspection, focusing on the solder connections of the input and outputs. Although the units are built like tanks, a cracked solder joint or even corrosion could be the cause of your issue. I have an AD2022 and I think Avalon has a very nice layout and well designed products so it shouldn't take too long doing a visual. I would also suggest maybe taking a few good pictures of the board layout in case someone here is able to attempt helping with your problem.

Wish I could be of more help. Good luck and hopefully one of the experts will have additional advice.

-Steve

**as to zackthedog's advice, have you contacted Avalon and asked about repair options? just curious what their repair prices/policies are should I ever develop a problem with my unit. Another thought I just had...if you have two units, you could do some side by side voltage measurements and comparisons, if you have the tools and knowledge about working safely with high voltage circuits.