Distortion in the Demeter VTBP201 XLR out

[mbrancalion]

Hello everybody.

It's difficult sometimes to find online, Talkbass or not, some knowledge for solving a particular problem.
So i decided to write a new post. Forgive me if this issue has already been covered (i can find nothing.... )

My longtime owned Demeter preamp (old, blackface) with a Jensen transformer DI has given me several trouble during last years. It's a too long history........
I only will say that contacting or trying to contact the factory had no result. I live in Italy.

It started to distort about 6 years ago. An ex-friend now, who is a tech, destroyed part of the circuit, without telling me before, because "there were errors".
A second technician "solved" but i'm not sure that now the unit works 100% as before.


The problem that is remaining is that the sound is lightly distorted (but i don't desire it to happen) in peaks ONLY in the XLR out, through the Jensen transformer.
The 2 normal jack output, for feeding power amps, etc, sound perfect. Only the XLR distorts.

Now, i can see myself that the wire that go to the jack out are the same that then go to the Jensen. So?.........
One possible solution is that the Jensen i somewhat cooked. But how can it be possible that it works welle but distorts only in the high signal peaks......... in my mind it should work or not work........


So, ideas on it?
Thanks a lot.

 

[Redbrangus]

What are you plugging the XLR output INTO?

 

[mbrancalion]

What are you plugging the XLR output INTO?

Into many devices, for example:
- an input of a SoundCraft MTK mixer/soundcard
- an input of a Roland audiocard
- or an input of a FMR RNP mic preamp (that goes in the MTK).

When going into the RNP, the distortion is much less.

(i know how to set the gain at preamp inputs).

 

[Redbrangus]

All I can offer is it IS a line-level out. That's gonna drive any mic-level pre-amp into clipping without some serious attenuation, but it sounds like you know that. It IS possible to drive a transformer into saturation, but I wouldn't think Demeter would hang a transformer on the output that couldn't handle the signal level the device is capable of producing. So I got nothing... . Sorry.

Edit: I will offer this, along with the shrug: On my audio interface, which isn't either of the ones you have, the XLR part of the Combi-connector can't be used for a line input -- it can only be routed through the mic pre. The TRS part has to be used for a balanced line input. Obviously, YMMV.

 

[mbrancalion]

All I can offer is it IS a line-level out. That's gonna drive any mic-level pre-amp into clipping without some serious attenuation, but it sounds like you know that. It IS possible to drive a transformer into saturation, but I wouldn't think Demeter would hang a transformer on the output that couldn't handle the signal level the device is capable of producing. So I got nothing... . Sorry.

Thanks for your reply, but that XLR output has been working without distorting for 20 years, before sounding like it is now.


UPDATE: i've just discovered that when i set the XLR output pre-eq it does not distort..................
I'm starting to suspect that also the last tech did not do a good job..........

 

[bucephylus]

James does respond to phone calls; but, I have no idea how the shop is or isn't responding to emails. He is pretty close to sliding into retirement mode; and his assistants in the shop have been inconsistent. I know that it is an overseas call; but, your best bet might be to try that. I also suspect you would have to send the preamp back to him for corrections. I have had an amp, which became damaged affecting the Jensen out, into the shop for repair within the last 6 months. So, they are still addressing these issues. I realize that is a lot harder when you are separated by oceans. Best of luck.

 

[mbrancalion]

UPDATE:

If someone could be interested (for the same problem):
i bought online a JJ 5751 tube and put it in the position 1 of the Demeter preamp.

The distortion is not completely absent now, but it can be heard only in extreme peaks when setting the gain in the preamp really high.
So i turned at maximum the output volume trim in the unit, and now i can lower the gain knob, so the distortion has virtually disappeared.

At the same time, i discovered that i like that tube very much. Maybe the best tube i've used in that preamp.

 

[musicmansf]

Best bet is calling Demeter around 10 a.m. Pacific time. That's when I've had the best luck contacting him.

 

[mbrancalion]

For the people who continues to say that i should call by phone an american person from Italy, i suggest:
learn the most italian language that you can, and next try to call on phone people at Markbass in Italy, talking in italian about technical issues.
Just try, and then you'll tell me if it's a viable way of communicating

 

[bucephylus]

For the people who continues to say that i should call by phone an american person from Italy, i suggest:
learn the most italian language that you can, and next try to call on phone people at Markbass in Italy, talking in italian about technical issues.
Just try, and then you'll tell me if it's a viable way of communicating

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.

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 Genalex Gold Lion tubes sound absolutely amazing; though I fear for some of the political / supply chain issues going forward.

Best of luck.

 

[mbrancalion]

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.

I took note, in case.........

Thanks for the reply.

 

[Wasnex]

Something is still not right with the circuit. Reverse engineering the circuit after it has been potentially modified is not very practical.

My advice is to find a technician who is qualified to do the work and has access to the schematics. I have no idea if Demeter will release the schematics.

For best results get a return authorization and send it to Demeter. I understand this is probably cost prohibitive. So at some point you may need to decide whether you will pay too much or find another preamp that suits your needs. This could involve buying another Demeter.

Perhaps someone in the US could serve as a proxy to communicate with Demeter.

 

[mbrancalion]

Something is still not right with the circuit. Reverse engineering the circuit after it has been potentially modified is not very practical.

In my opinion, there is something different, but not much, maybe one only component.

- My advice is to find a technician who is qualified to do the work and has access to the schematics. I have no idea if Demeter will release the schematics.

- Perhaps someone in the US could serve as a proxy to communicate with Demeter.

I have a dear friend bassist in the USA that always did his best for my troubles with the unit. He was the one who ordered it for me in 1997.
Yet it seems that lately the Demeter's company service is a bit loose.....

Schematics of it do not exist, for what i searched.

I needed to fix many time that preamp. In 2003 i had to install a new transformer, custom made in Italy from specs.
Then in 2007 it had severe problems with constant hums and noises, developed after hundreds of stages and concerts.
At that time an appreciated local guitar amps tech studied the circuit, copied it, and made some modifications to the power board, eliminating ground loops and such (he said). Since that moment, the preamp behaved wonderfully. Background noise and hum disappeared, and sound was perfect, better than before.

Ten years later the subtle distortion began. The tech disappeared, quit his job and refused any form of communication.
So i found another local guitar amp tech and tubes wizard who literally distroyed the power circuit without telling me before. Distortion was not eliminated and the preamp sound was worse that before. I deleted this person from my life.

So i found another more serious local tech that patiently brought back the circuit like MAYBE it was before. And notice that i had photos made by myself of the inside. But they were not enough.
We had the only other bassist in the area that i'm aware of, VTBP201's user, to bring his own unit and we opened and examinated it for replicating the circuit after making some detailed pics and analysis of components.

This new skilled tech made it work almost like before, but the latent distortion in peaks was never eliminated. Then came the covid, and next the tech moved abroad in Morocco and is no more available.

In the meanwhile, i found an unexpected local seller of the VTBP201, older unit like mine, so i bought second hand another one. I still have it.
It's a non-modified original unit, apparently the twin of my preamp. And it sounds (with same tubes, swapping them) different from my unit. It's more sterile sounding. More clinical, it has not the same warmth in the lows. But i bought it for future needs and reference.
And: it has some more buzzing noises (my unit does not anymore). And by feeding the same input signal in both units and recording, their output is out of phase, 180°.

So, the idea that people at Demeter take my unit, restore it to how it was in the beginning, is not appealing.... Because for 10 years of its life it has sounded BETTER than its original form ....


Now, as i wrote in a post above, the problem seems virtually solved. If i'll need it, i have another contact with a professional restoration lab around 100 km from where i live. I had other stuff fixed by them, very skilled and professional. We'll see.



Thanks for writing.

 

[Wasnex]

In my opinion, there is something different, but not much, maybe one only component.





I have a dear friend bassist in the USA that always did his best for my troubles with the unit. He was the one who ordered it for me in 1997.
Yet it seems that lately the Demeter's company service is a bit loose.....

Schematics of it do not exist, for what i searched.

I needed to fix many time that preamp. In 2003 i had to install a new transformer, custom made in Italy from specs.
Then in 2007 it had severe problems with constant hums and noises, developed after hundreds of stages and concerts.
At that time an appreciated local guitar amps tech studied the circuit, copied it, and made some modifications to the power board, eliminating ground loops and such (he said). Since that moment, the preamp behaved wonderfully. Background noise and hum disappeared, and sound was perfect, better than before.

Ten years later the subtle distortion began. The tech disappeared, quit his job and refused any form of communication.
So i found another local guitar amp tech and tubes wizard who literally distroyed the power circuit without telling me before. Distortion was not eliminated and the preamp sound was worse that before. I deleted this person from my life.

So i found another more serious local tech that patiently brought back the circuit like MAYBE it was before. And notice that i had photos made by myself of the inside. But they were not enough.
We had the only other bassist in the area that i'm aware of, VTBP201's user, to bring his own unit and we opened and examinated it for replicating the circuit after making some detailed pics and analysis of components.

This new skilled tech made it work almost like before, but the latent distortion in peaks was never eliminated. Then came the covid, and next the tech moved abroad in Morocco and is no more available.

In the meanwhile, i found an unexpected local seller of the VTBP201, older unit like mine, so i bought second hand another one. I still have it.
It's a non-modified original unit, apparently the twin of my preamp. And it sounds (with same tubes, swapping them) different from my unit. It's more sterile sounding. More clinical, it has not the same warmth in the lows. But i bought it for future needs and reference.
And: it has some more buzzing noises (my unit does not anymore). And by feeding the same input signal in both units and recording, their output is out of phase, 180°.

So, the idea that people at Demeter take my unit, restore it to how it was in the beginning, is not appealing.... Because for 10 years of its life it has sounded BETTER than its original form ....


Now, as i wrote in a post above, the problem seems virtually solved. If i'll need it, i have another contact with a professional restoration lab around 100 km from where i live. I had other stuff fixed by them, very skilled and professional. We'll see.



Thanks for writing.

Sounds like the "appreciated local guitar tech" definitely performed some mods (for the better).

For the record, I find the stock VTBP2 to sound a bit sterile as well. I preferred the Alembic. Never seen a Kern IP777, but they always seem to sound best whenever I come across some sort of comparison video. T0 my ears, the Kern seems to have a bit more sweetness, focus, and clarity.

Unfortunately there is not much anyone on the internet can do to help you with the problem. Try different tubes, look for obvious defects, bypass any switching jacks like an effects loop, do the chop stick test, etc. If none of that helps, send the amp to a skilled technician who can go through the circuit with test equipment to localize the specific fault. It seems you have probably done all of it.

Unfortunately it's likely some of the mods done by the "appreciated local guitar tech" will potentially make it much more difficult for anyone who is familiar with the unit to repair it. Especially after multiple failed attempts by other technicians.

All I an do is offer my condolences for the damage your preamp has incurred and wish you luck getting a repair that satisfies your needs.

 

[mbrancalion]

I recorded (on a bassless studio track) something by using the new tube:

[Invalid or Expired Link Removed]

 

[Wasnex]

I recorded (on a bassless studio track) something by using the new tube:

[Invalid or Expired Link Removed]

Nice playing and the bass sounds really good. I don't notice any obvious distortion.

Seems to me that if the output of the Pre is clean and the distortion is only occurring on the DI, then at least you have localized the problem. The signal path is usually relatively short after the signal is split off to the DI. Could be in the amp's electronics, or perhaps the custom replacement transformer has developed some sort of fault or was never suitable to begin with. I was under the impression that an off-the-shelf Jensen will drop right in and may have even been OEM.

Demeter VTBP-201 Transformer Info help needed

Here is a post later in the thread that describes how the DI transformer is connected to the preamp (it's not certain this is the factory connection):
Demeter VTBP-201 Transformer Info help needed

Earlier you mentioned that the polarity of the preferred preamp is opposite from the stock preamp. This could be due to a change/modification, or it could simply mean a pair of taps on the transformer are reversed. Could be either the input or the output. It should not impact the sound unless you are blending signal from two preamps...or something similar.

 

[Wasnex]

If you know the signal flow of the amp and can make the distortion visible on an o'scope, it should be fairly easy to localize where the distortion is getting injected into the DI signal path.

However, this sort of problem may be easier to track down using your ears. You could build a "stethoscope listening amp" and use it to probe the circuit.

If you think something like this may be helpful, open this page and scroll a bit over halfway down till you see the image above:
Amp Tools (el34world.com)

 

[agedhorse]

I think you have gotten into this deeper than you needed to, and now you have a mess that's not the fault of Demeter.

Because of the changes and modifications, I would be surprised if Demeter would even work on it.

When I run into units like this, the cost of repairing properly generally exceeds its value by a lot. I won't work on amps that have been modded without reverting to the original build condition, generally this fixes all of the problems that are related to modifying gear.

 

[mbrancalion]

Very busy days here..... i did not reply because i didn't have much time.
Thanks for the replies.

In fact, i won't send it to the Demeter factory.......
For now i think i can use it as it is now, with the new tube. We'll see in the future.

 

[mbrancalion]

I have an update in my distortion issue with the Demeter preamp.............

I'm coming from a very very busy period (i graduated in classical violin). I had only a very spare free time to play at home and record with electric basses.

I own two Demeter VTBP preamps, blackface. The older that distorts more, the new acquired that does not distort.

Well: I was puzzled to discover that "some days" the older unit (= Dem1) does not distort, not even a bit, and some days the newer one (= Dem2) distorts the same identical way.............
I tried my best to isolate the variables, and i scratched my head.......

At the end i found what i would have never thought in 100 years:
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.
I own no other power amp, and at the moment i have no other available for testing.
In few weeks i'm going to a friend house for testing the issue with another PA power amp.

Now, be it power amp's fault, preamps fault, etc: have anyone ever experimented or heard something like this?.......
I can't believe that it took years to find out this phenomenon, but i can't imagine the cause.

I only know that for years and years i used that preamp with that power amp live, and alwyas connected this way and i had never got a distorted sound in the DI to the PA.
Maybe some component in the amp can be old or faulty?.......


Thanks for every eventual insight.
Greetings.



Note:
when i discovered i had 2 working Demeter preamps for recording (eliminating the power amp), i put the JJ 5751 tube in position 1 and a working JJ 803s in the position 2 in Dem1, and it's astounding. Very open sounding and warm and with character.
I put 2 NOS Philips tubes in the Dem2, and it's astounding on its own, but very different: much more "normal '60 discographic" bass sound, not sterile at all.