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Backert Labs Rhumba preamp

Hello,
I am thinking of going to an active line stage for sometime now. I have a separate thread for that and people have suggested some good choices. The reason for this thread if that, while I have all along said that  don't want tubes in the system, a few have convinced me that I should try tubes in the system, especially with ProAcs.
With that said, I researched a few tube preamps, that have XLR out and the Backert Labs Rhumba is the one that has only 2 tubes, which makes me comfortable to "deal in tubes".

I would like to hear from Backert Labs Rhumba owners who moved from Solid State preamplification to this preamp. What are the differences you heard? Any improvements?

I read a review on the Stevehoffman forum and the user mentioned that there is a "hum" in this preamp. This bothers me and hence I decided to check on this forum, if anyone has experienced this with the Rhumba in their system. One poster on this forum also confirmed the "hiss":
https://forum.audiogon.com/discussions/backert-preamps-any-users

So I was wondering, if there are any users who have recently purchased this preamp and what is their experience with the hum/hiss issue.

BTW, I blame Ralph of Atma-Sphere, Almarg  and my dealer for making me consider tube amps :-) Can't believe I am creating this thread.

Thanks!
128x128milpai

Showing 7 responses by atmasphere

atmasphere's avatar
atmasphere
11,615 posts
 
:)  Glad to hear its worked out!
atmasphere's avatar
atmasphere
11,615 posts
 
And how does one acquire an immunity to interconnect cables?  Is it from a consistent exposure to same that you build up a natural resistance to them?  Sheeeesh!🤔😝
Proper inoculation I guess.
however, my encounter with preamp MP-1 is otherwise. In exactly the same setting, all variables are fixed and nothing changed (ie. interconnects, power etc..), i get hum from MP-1 , but i don’t experience hum after i swap to single ended pre. At a friend place, the same MP-1 preamp function just as great.
@philipwu The most likely cause is that the amp you were trying to drive wasn't actually balanced. It may have had the XLR connector (an example of an amp like this is the Lamm 1.1, also any VTL amp equipped with an XLR input), but if the circuit inside wasn't balanced you would totally get a hum. The reason is that the output of a balanced source occurs between pin 2 and 3 of the XLR, and **ignores** ground. If that's connected to a single-ended input, essentially the unused pin (usually pin 3 when this problem shows up) acts like an antenna for noise because the circuit isn't completed! There are several solutions, certain adapters can be made to work, line transformers and its often possible to modify the amp slightly so that it will accommodate a balanced input.

I frequently show at audio shows with speakers that are 105db 1 watt/1 meter and do it with no hum.


atmasphere's avatar
atmasphere
11,615 posts
 
It also doesn't necessarily mean a balanced connection or a fully balanced design circuit (i.e MP-3) will give you lower noise or have any advantage over single ended design. Fully balance design circuit will also susceptible to hum problems too.
Just a correction here, since this statement is essentially false.
The advantage that balanced operation has is several- immunity to interconnect cables, and the system was also devised to prevent ground loop hum and it works quite well! "Fully balanced design circuit" (sic) is far less susceptible to hum problems- it is better able to reject noise in the power supply and far more able to reject hum from close by power transformers and the like. It is also possible to get lower noise, since if the balanced design is also differential, careful use of well-designed constant current sources will reduce noise while also increasing gain and reducing distortion.
The first bit: immunity to interconnect cables, is likely the most important aspect, since audiophiles are well-known to pay serious money for interconnect cables in search of the cable to rule them all. In effect, cables are the 'hidden cost' of operating single-ended preamps.
atmasphere's avatar
atmasphere
11,615 posts
 
So, I am not sure the difference between "Balanced" Vs "true Balanced" that Ralph often refers to.
'True balanced' is not a thing I say so much as I harp about supporting the balanced standard.  The circuit does not have to be balanced to do that properly- it can be single-ended as the Rhumba is, and use an output transformer as the Rhumba does. Transformers can drive balanced lines quite well.
In my opinion, if you are using quality interconnects and your run is not abnormally long, a typical meter interconnect, then a balanced or unbalanced connection is not going to play a big difference in sound quality.
The whole point of balanced line is that the interconnects don't have to be expensive (which might be the same or not as 'quality'). What this means is that you have the same benefit if the cable is 6" or 60 feet, which is to say it will sound right and cheap cables should sound the same as expensive ones, regardless of length.

atmasphere's avatar
atmasphere
11,615 posts
 
@sbank Thanks- I often wonder what people mean when they use the word 'voice' in that manner. I've often interpreted as some sort of tonality? - but it could mean something else. FWIW we don't do anything to compensate our circuits for bandwidth, and while we do listen to the gear of course, we don't do that until we've optimized for least distortion and maximum headroom.

From what
atmasphere's avatar
atmasphere
11,615 posts
 
The MP3 may be really good, but is currently out of my price range and also comfort (no. of tubes) range.
@milpai The Rhumba is a line stage. If you get the MP-3 as a line stage, it only has 4 tubes. The remaining tubes you are probably thinking about are for the optional phono section. The MP-3 has a polarity inversion switch.

atmasphere's avatar
atmasphere
11,615 posts
 
The hum you read about was not inherent in the preamp, but was caused by the higher output impedance of the preamp combined with poor interconnect cable layout between the preamp and amp. Once the guy on the stevehoffman site got the layout right, the hum was gone.

While it does have an XLR output, it does not support the balanced standard. If it did, the hum with the interconnect cable layout would not have been an issue.
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