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Old 06-19-2021, 08:40 AM
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Default Checking a mic preamp without connection to audio interface

If you have a powered mic preamp connected to a mic via an XLR cable but with no further connection to an audio interface (that would typically be via a balanced 1/4" jack pickup cable), shouldn't the VU meter on the channel output level of the preamp become active when the preamp is turned on, the mic is powered with 48V phantom power, and the output level knob for the channel the mic is connected to is turned?
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Old 06-19-2021, 09:05 AM
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If I'm understanding you correctly, yes.
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Old 06-19-2021, 09:41 AM
jim1960 jim1960 is offline
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Quote:
Originally Posted by SprintBob View Post
If you have a powered mic preamp connected to a mic via an XLR cable but with no further connection to an audio interface (that would typically be via a balanced 1/4" jack pickup cable), shouldn't the VU meter on the channel output level of the preamp become active when the preamp is turned on, the mic is powered with 48V phantom power, and the output level knob for the channel the mic is connected to is turned?
I'm not sure because I've never done what you seem to be describing. I'd suggest hooking it up to your interface before making any determination about what may or may not be wrong.
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Old 06-19-2021, 09:51 AM
Brent Hahn Brent Hahn is offline
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Quote:
Originally Posted by SprintBob View Post
If you have a powered mic preamp connected to a mic via an XLR cable but with no further connection to an audio interface (that would typically be via a balanced 1/4" jack pickup cable), shouldn't the VU meter on the channel output level of the preamp become active when the preamp is turned on, the mic is powered with 48V phantom power, and the output level knob for the channel the mic is connected to is turned?
Yes, you should be seeing the meter kicking. A couple things to check out: 1. Does the preamp have an instrument (DI) input, and is there a switch or button that toggles between the mic and the DI? 2. Have you tried a dynamic mic? A failure of the phantom supply is unlikely but possible.
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Old 06-19-2021, 09:53 AM
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Quote:
Originally Posted by SprintBob View Post
If you have a powered mic preamp connected to a mic via an XLR cable but with no further connection to an audio interface (that would typically be via a balanced 1/4" jack pickup cable), shouldn't the VU meter on the channel output level of the preamp become active when the preamp is turned on, the mic is powered with 48V phantom power, and the output level knob for the channel the mic is connected to is turned?
Not sure exactly what you are saying. "become active" when turned on ? do you mean light up,, or move,,, or both ? Light up yes but will move only when an actual signal is being input.

But the VU meters work on mine regardless if the pre is connected to an interface or not.
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Old 06-19-2021, 10:22 AM
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What would that VU meter be measuring? Usually it's output (work being done).
Not sure what a VU meter would indicate when current not flowing through it.
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Old 06-19-2021, 04:57 PM
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So what prompted this is I took delivery on Thursday of a new Sebatron Axis 200 preamp that I purchased from Soundpure as a demo unit.

I connected it to my DAW (Reaper) via my Scarlett 2i4 interface as follows:

1. XLR mic cables from two SDC's (KM184's) to Axis XLR inputs.
2. XLR mic cables from Axis XLR outputs to Scarlett XLR inputs, Scarlett mic gain controls set to 0, Scarlett phantom power off, Scarlett Line/Inst switch set to Line.
3. Scarlett USB feed to PC and DAW setup with two tracks (Channel 1 and 2 on both the Scarlett and Axis).

When I power the Axis up, for each channel the Pad/Gain control knob is set to -15dB (full counterclockwise to the left), Output Level control knob is set to 0, HPF is off, phantom mic power is off. When I turn the phantom power on for each mic channel and turn the Output Level knob to the right, there is no activity on the Axis VU meters and no activity on the track level indicators in the DAW.

Using same XLR cables from mics to the Axis, if I take the Axis out and go straight to the Scarlett and use it's preamps, everything works fine.

I wonder if the Axis is DOA. I'll be in touch with Soundpure first thing on Monday.
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Old 06-19-2021, 05:20 PM
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Quote:
Originally Posted by SprintBob View Post
I wonder if the Axis is DOA. I'll be in touch with Soundpure first thing on Monday.
The only thing I can think of is that perhaps you have your cables reversed (but I'm assuming you checked and double-checked that) or that you didn't turn the gain up high enough to make up for the -15 pad.

Does the red indicator light come on when you engage phantom power?

Short of any of those, I can't think why it would not be working other than the unit being damaged in some way. And since it's a demo unit, that's certainly possible. In the videos on this unit, SoundPure says they have them in use in their studio. A demo from them might have a lot of hours on it.
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Old 06-19-2021, 05:22 PM
Brent Hahn Brent Hahn is offline
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Now that we know that the pre is Australian and you're (probably) American, the only other thing I can think of is that it might be set for 220V operation, when you probably want 110.
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Old 06-19-2021, 05:32 PM
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Quote:
Originally Posted by jim1960 View Post
The only thing I can think of is that perhaps you have your cables reversed (but I'm assuming you checked and double-checked that) or that you didn't turn the gain up high enough to make up for the -15 pad.

I'm using the female XLR connections on the back of the Axis as inputs and the male XLR connections as the outputs. I've turned the output level all the way to the right with no change at all.

Does the red indicator light come on when you engage phantom power?

Yes

Short of any of those, I can't think why it would not be working other than the unit being damaged in some way. And since it's a demo unit, that's certainly possible. In the videos on this unit, SoundPure says they have them in use in their studio. A demo from them might have a lot of hours on it.

I'll certainly be having that discussion with them. The unit looks perfect, not a mark on it.
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Now that we know that the pre is Australian and you're (probably) American, the only other thing I can think of is that it might be set for 220V operation, when you probably want 110.
There is a switch and it's set to 115VAC.
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Old 06-19-2021, 06:59 PM
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I don’t think this would cause the problem you’re having, but you should connect the preamp output to the Scarlett 2i4 using a 1/4” plug at the 2i4. Using a xlr at the 2i4 runs it into the mic preamp which can handle only a 3.5 dbu max signal (even with switch set to line).
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Old 06-19-2021, 07:18 PM
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So if everything works by taking the Seb out of the equation then the problem is definitely with Seb I have never used one so just guessing
First are you letting the Pre's tubes warm up ? say 5 or more minutes ?
If so :::
#1 what about Bad tubes ???? is there any venting slots where you can see if the tube/s are glowing inside the pre amp (without having to take the cover off) ?


It is strange it really almost sounds like maybe the phantom power is not getting to the mic circuit But if the light is on then it should be ???
Have you tried a dynamic mic and see if you get any signal in the VU meter's ???
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Old 06-19-2021, 10:04 PM
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Quote:
Originally Posted by rick-slo View Post
What would that VU meter be measuring? Usually it's output (work being done).
Not sure what a VU meter would indicate when current not flowing through it.
If the mics were truly "hot", I would have thought this would have been evident on the VU meters.

Quote:
Originally Posted by ChuckS View Post
I donít think this would cause the problem youíre having, but you should connect the preamp output to the Scarlett 2i4 using a 1/4Ē plug at the 2i4. Using a xlr at the 2i4 runs it into the mic preamp which can handle only a 3.5 dbu max signal (even with switch set to line).
I don't have a female XLR to 1/4" plug cable to try that. Based on nothing else working, probably is not the problem.

Quote:
Originally Posted by KevWind View Post
So if everything works by taking the Seb out of the equation then the problem is definitely with Seb I have never used one so just guessing
First are you letting the Pre's tubes warm up ? say 5 or more minutes ?
If so :::
#1 what about Bad tubes ???? is there any venting slots where you can see if the tube/s are glowing inside the pre amp (without having to take the cover off) ?


It is strange it really almost sounds like maybe the phantom power is not getting to the mic circuit But if the light is on then it should be ???
Have you tried a dynamic mic and see if you get any signal in the VU meter's ???
Tried letting the unit warm up for 10 minutes, still dead as a doornail on the VU meters with the output gain turned up. You can feel the mics getting warm though putting you hand on the cover. The phantom power light for both channels comes on also.

I'll try a dynamic mic but it's a moot point as I record solo guitar with SDC's and LDC's.

The more I think about it, something is definetly wrong with this unit. It's going back to Soundpure. At least I have the new Motu interface coming in on Monday to start working with.
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  #14  
Old 06-19-2021, 10:42 PM
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Quote:
Originally Posted by KevWind View Post
So if everything works by taking the Seb out of the equation then the problem is definitely with Seb..................................
Have you tried a dynamic mic and see if you get any signal in the VU meter's ???
Kevwind, you pointed me in the right direction. Plugged a dynamic mic into Channel 1 and the VU meter came to life. Pulled out an inline external 48V phantom power supply and ran the KM184 through it to Channel 1 with the Axis 48V power off and the VU meter again worked. There must be a bad transformer in the Axis that is affecting 48V power on both channels even though when I switch the Axis 48V power on the indicator light comes on. Looks like it's going back to Soundpure for exchange or refund for sure on Monday morning.

Thanks!
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Last edited by SprintBob; 06-20-2021 at 01:07 AM.
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Old 06-19-2021, 11:10 PM
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Quote:
Originally Posted by SprintBob View Post
There must be a bad transformer in the Axis that is affecting 48V power on both channels. Looks like it's going back to Soundpure for exchange or refund for sure on Monday morning.
Bummer that they didn't test that demo unit before they sent it but at least you have the problem figured out. Kudos to Kevin for thinking of trying a dynamic mic.
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