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Distortion in the Demeter VTBP201 XLR out

#21
bucephylus said:
I think most of us are just hoping you can get your preamp corrected, and trying to suggest a most effective way to get that done. And, your grasp of English is far better than my grasp of Italian. So, there's that.:cool:

I have also at times been a huge fan of the 5751 tubes, and still own quite a collection of them. Here is a nice reference for that kind of tube.
[Invalid or Expired Link Removed]

I no longer use those in my Demeter preamps, partly because I've found really amazing 12AT7 and 12AX7 tubes, which are used in the Demeter platform, depending on the tube position and outputs. In particular, the Red Lion tubes sound absolutely amazing; though I fear for some of the political / supply chain issues going forward.

Best of luck.
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Interested in these 'Red Lion' tubes... are those Genalex? I can find 'Gold Lion' online but not Red.

JJ's have always worked well for me in the old Dem pre.

I'm shocked these old preamps don't fetch more in the used market. Alembic F1-Xs are >1k. I reckon it's only because Alembic bumped their prices on new units aggressively and consistently over the past decade.
 
#22
marc40a said:
Interested in these 'Red Lion' tubes... are those Genalex? I can find 'Gold Lion' online but not Red.

JJ's have always worked well for me in the old Dem pre.

I'm shocked these old preamps don't fetch more in the used market. Alembic F1-Xs are >1k. I reckon it's only because Alembic bumped their prices on new units aggressively and consistently over the past decade.
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Thanks for the catch. Indeed I got my colors confused. Fixed it.

In fact, I've temporarily removed the Genalex tubes from my powered VTBP-201, because there was some kind of issue I haven't sorted out; could be a problem with their somewhat higher output. I've replaced the tubes James had installed when he shipped the modified amp back to me. Don't recall what those are ATM.
 
#23
Do you have to turn up the output level of the preamp more when you're using the QSC to get the same volume as with the Markbass? I know the QSC wants to see about 1 volt of input level to achieve full gain. Also, IIRC, the PLX series amps had a pretty low input impedance, especially if you're using unbalanced cabling with the quarter-inch inputs. Maybe it's possible you're loading down the output of the preamp a little with the QSC...? Checking it with a different power amp might isolate the problem when you get that opportunity.

Edit: Is the XLR output of the Demeter line level? Or is it mic-level only? (That's my interpretation of their skimpy information.)
 
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#27
Redbrangus said:
Do you have to turn up the output level of the preamp more when you're using the QSC to get the same volume as with the Markbass? I know the QSC wants to see about 1 volt of input level to achieve full gain. Also, IIRC, the PLX series amps had a pretty low input impedance, especially if you're using unbalanced cabling with the quarter-inch inputs. Maybe it's possible you're loading down the output of the preamp a little with the QSC...? Checking it with a different power amp might isolate the problem when you get that opportunity.
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For me it's no difference from the MB head.
Anyway, i've used that setup for years and it has never caused me distortion, like this.
 
#28
mbrancalion said:
It has no power amp to drive, for me.
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I don't understand what this means. We ARE talking about this distortion being present when the preamp is connected to the input of a QSC power amp, right? Or did I misunderstand?

For experimental purposes, what happens when you connect one of the high-level, quarter-inch phone outputs on the preamp to the input of the QSC? A regular tip-sleeve instrument cable will work. (Start with the volume controls down -- it's likely to be a lot louder than with the XLR output.)
 
#29
Redbrangus said:
I don't understand what this means. We ARE talking about this distortion being present when the preamp is connected to the input of a QSC power amp, right? Or did I misunderstand?

For experimental purposes, what happens when you connect one of the high-level, quarter-inch phone outputs on the preamp to the input of the QSC? A regular tip-sleeve instrument cable will work. (Start with the volume controls down -- it's likely to be a lot louder than with the XLR output.)
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I think I am confused as well.

If @mbrancalion is trying to drive the line level input of the QSC power amp with the mic level XLR output from the preamp, I think we have discovered the root cause of the distortion.
 
#30
Redbrangus said:
I don't understand what this means. We ARE talking about this distortion being present when the preamp is connected to the input of a QSC power amp, right? Or did I misunderstand?
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You are implying that i connect the DI output of the preamp to the input of QSC power amp. I've never done and said it.



For experimental purposes, what happens when you connect one of the high-level, quarter-inch phone outputs on the preamp to the input of the QSC? A regular tip-sleeve instrument cable will work. (Start with the volume controls down -- it's likely to be a lot louder than with the XLR output.)
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This what i'm already doing, and have always done.
When i do this, the DI output is distorted. When i disconnect the jack from lineout of the preamp to the amp, the DI is not distorted anymore.
And, i repeat, years ago the distortion did not happen.
 
#36
mbrancalion said:
I have an update in my distortion issue with the Demeter preamp.............

when i connect the line out jack (one or the other preamp) to my power amp input, a QSC PLX 1602, then the DI of the preamp distorts in peaks. If i disconnect the jack from the power amp in it plays clean.

I tried the power amp turned on, turned off, with its plug disconnected, by using the second channel input, using an adapter for going into the XLR balanced input: same results.

Now, if i connect the Dem line out into the return in the effect loop of a Markbass head, no problem.
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First, just so others understand the situation, the VTBP-201 has a Jensen Xfrmr MIC level output.

Second, for the OP: I presume these confusing behaviors are heard while your DI is connected elsewhere?

It is certainly strange that the distortion would occur when the preamp output is connected to the QSC and not to your MarkBass Efx Return. That could suggest a grounding issue; but, I see that you tried to test that idea by turning off and unplugging the QSC. I would only ask whether you further unplugged the instrument cable from the QSC, and then unplugged the cable from the VTBP. I suppose I'd presume that unplugging the instrument cable feed from the QSC would solve the issue, since plugging into the MarkBass does solve the issue.

Out of curiosity, how do you deal with the voltage supply? I don't believe the VTBP has a 240VAC supply option.

Otherwise, glad to hear you are narrowing down the possibilities and getting at least some good tones.


One last band-aid if you have performances before resolving the larger issue: Buy a Radial JDI to take a Jensen feed prior to the VTBP. Not perfect but close enough to get by until you figure this out.
 
#37
bucephylus said:
Second, for the OP: I presume these confusing behaviors are heard while your DI is connected elsewhere?
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I'm not sure i understand what you mean.
It's obvious that the DI out is connected elsewhere: i record with it. In the past it went to the PA desk, when i played live.


Out of curiosity, how do you deal with the voltage supply? I don't believe the VTBP has a 240VAC supply option.
Click to expand...

Both units where made in the factory for european voltage and frequency (230 V at 50 Hz).


One last band-aid if you have performances before resolving the larger issue: Buy a Radial JDI to take a Jensen feed prior to the VTBP. Not perfect but close enough to get by until you figure this out.
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I'd never do this ! :)
I like to use the demeter for its sound. I always use it post-eq in the DI output.

Instead, i thought i could try a passive DI between demeter lineout and power amp input.... I had a Monacor at home. At the moment i still haven't found it....... :)
 
#38
Sorry for the delay in updating my own issue ......
I am in a quite busy period, but very recently i started reading this forum again.

I confirm that, using a normal passive DI box between the line output of the Demeter and the input of the QSC power amp, both my VTBP Demeter preamps do no distortion, nor in that output nor in the Jensed DI out of the preamp.

So it's evidently a sort of damage in the inputs of that particular power amp, because for 15 years all of this did not happen.
For when i'll use this system again, maybe i'll put a halfrack parametric eq unit (Speck ASC, which i own) in the middle between Demeter and QSC, so to control room acoustics when needed, further the required buffering.


The troubleshooting of this issue lasted years and cost me a lot.
In the end, my reward is having my preamps that work the way they have to ..... :)


Greetings.
 

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