Mismatched VTB-1s

[Tim Perry]

About 7-8 months ago I bought a VTB-1 brand new from fullcompass, fastforward to 3-4 months ago I bought a used VTB-1 from an online forum. I thought they were both acting normal until last night. I had tried a shiny box ribbon out on the used one couple months ago and I got some hiss on the main gain control when pushed past 50dB. Adding the output level didn't add any more hiss as long as the main gain control was at 50dB or below. Pushed past 50dB there is a lot of hiss. So usable gain on the used preamp is 62dB. So last night, for the fun of it I plug a Beta 57a into the VTB-1 that I bought new. Well it needs lots of gain too, not as much as the ribbon wants, but still needs a good smacking of gain. So thinking that I can only push to 50dB on the main gain control I start there, but I wanted more so I push it all the way to 60dB, and to my surprise, there was no hiss. Now I guess the mic impedence could have something to do with it, but I am guessing that its more mismatched VTB-1s. The used one does seem to be a bit older. I'll have to do some more trouble shooting, but was there a design change at some point that would explain my findings?

[Alan Hyatt]

Tim,

The old one could have a weak or damaged tube. You could also not have the impedance switch set right. 50 ohm for ribbons, all else, 200

Make sure the cables are wired correctly. If you use non balanced, you lose 6dB. The 1/4" and XLR are balanced outputs and should be wired in a balanced mode.

These should be checked first. There is always the possibility the unit has an issue. Make sure you have the right AC adaptor 12VAC at 1000ma. Let me know how it works out for you. We can always check the unit out for you.

[Tim Perry]

Yep had the impedence selectors in the right position. I have a gepco snake with neutrik connectors pluged into the XLR ins of the two units. And the outs have TS 1/4" neutrik connectors with some more of the gepco snake wire which is then pluged into a M-Audio 1010LT's RCA's with those cool neutrik Pro-Fi RCA plugs. I think they are the same AC wall-warts, will check tomorrow. If the tube was acting funny on the one wouldn't the hiss problem go away with the tube blend all the way to the left? I guess I could try swapping the tubes between the two units?

But then I don't know if its worth the trouble, I mean the gain is clean all the way upto 50dB then the hiss comes in. Not so with the other unit that I have, its clean all the way up. Hmmmm, what would it cost for you guys to look at it?

And I will double check my trouble shooting again to make sure its not me or the wiring, or the sound card, or the mics, etc...

Thanks Alan,
Tim

[Mike Rivers]

I plug a Beta 57a into the VTB-1 that I bought new. Well it needs lots of gain too, not as much as the ribbon wants, but still needs a good smacking of gain. So thinking that I can only push to 50dB on the main gain control I start there, but I wanted more so I push it all the way to 60dB, and to my surprise, there was no hiss. Did you compare the two preamps side-by-side, with the same mic and the same gain setting? If there's a difference, then something isn't right with one of them. You might try swapping tubes between them and see if that moves the hiss. If the excess hiss follows the tube, then you probably need a new tube.

It's hard to test preamps for noise just by listening to hiss (though I know that ultimately, that's what's important to you). While the impedance of the microphone doesn't affect the noise level of a tube preamp (not nearly as much as it does with a preamp with a conventional bipolar junction transistor front end) there might be a difference in noise level of the two mics. That's why it's important, at least when benchmarking, to test both preamps under the same conditions.

It would be a good idea to use a stable sound source to set the input and output gain controls. Play a steady tone into a speaker, put the mic on a stand in front of the speaker, and adjust the Input gain (with the meter switched to Input) for 0 VU. Switch the preamp's meter to Output and set the Output level to 0 VU. Then plug the mic into the other preamp, repeat the settings, and verify that the knobs (input and output) are set pretty close to the same gain. Then, without adjusting the gain controls, compare hiss with the same microphone plugged into each preamp, and finally compare the hiss between the two mics.

[Tim Perry]

Ok never mind, I had to be dreaming before.

Here is what I did to trouble shoot my phantom noise issue, I say phantom cause now I am not having any problems. Anyways, I hooked up my beta57a to VTB-1 (A), cranked the input gain and output level to max, and recorded my dehumidfier running two rooms over. So ya hardly anything to record, but a slight low humming. Did the same with VTB-1 (B). Then I ran VoxengoSpan on both tracks. The noise spectrum from the two were nearly identical, maybe a little more here little less there. Putting the mouse cursor over 10K, 5K, 1K, all yield results with in 1-2dB, between the two tracks. Really I think 2dB at any frequency would be on the high side of difference between the two units/tracks.

And just to make sure it wasn't the tube I turned the tube level to half way, and recorded with each unit again. Same results. I then flipped the impendence switch, again same variation between the two.

Only thing I didn't try was a ribbon, but I am more then satisfied with what I did, to say sorry for posting in the first place.

I have two identical VTB-1's, not mismatched at all.

Sorry, sorry, sorry, :o :o :o

Tim

[Alan Hyatt]

Tim,

Well I am glad your sorted out. Its nice to see Mike Rivers here. Mike is wealth of knowledge and a big time user of all sorts of gear, so you got advice from a couple of old salts....

One thing I like about this forum, is you can talk gear, resolve problems, and get things done in a civilized manner without someone attacking the other with a mass of curses and insults...