Bottlehead Forum
Bottlehead Kits => BeePre => Topic started by: STURMJ on June 30, 2013, 09:23:50 AM
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I set my prebee up without the xlr output. I am hoping to configure one of the inputs as xlr, for my DAC. I know that to get a true balanced input I need a transformer. Can I just copy the output configuration in the manual to the input and get reasonable results? Or would it be better to just direct wire the input without the caps and resistors, untill I get the transformer?
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Generally, you can wire the - pin to ground on the XLR jack if you'd like to use an XLR cable in an unbalanced configuration.
If you'd like to be the first one to install the balanced intput transformers, let us know and we can walk you through it.
-PB
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Im in for installing xformer, what would you reccomend? I've looked at some and decided I didn't know enough to make a choice.
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The Cinemag CMLI-15/15B is probably not a bad place to start, be sure to specify the single 3/8" stud for mounting.
The XLR jacks that you'll need will just be Neutrik D-shell jacks, Parts Express has reasonable prices on these.
Other than that, you'll want some extra resistance across the output of these transformers to load them properly (try 30K).
-PB
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how about a Hammond 140UEX (10k / 10K, 1:1 ) impedance is a little lower then the cinemag (15K) but I can get the Hammonds easily. If lower impedance is preferred I can also get 140TEX (1K/1K 1:1)
thanks for the input!
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The holes in the chassis are designed for a Cinemag, Jensen, and Sowter model. It actually never occurred to us to even include Hammond on the list.
While you could use the 10K Hammond, you would end up spending more money and drilling more holes in the chassis, with the disadvantage that there is no internal shield.
Also, having the transformer mounted through a single hole allows one to twist the transformer while you're listening to it, which should allow you to find the sweet spot where no magnetic hum is coupled (ymmv, this may or may not be an issue).
So, to use the Hammonds, you will need to poke all the wires through the hole in the chassis, twist the transformer around until it's quiet, mark the holes, uninstall the transformer, machine the holes, then reinstall.
For loading the secondary, I'd try a 12K resistor.
-PB
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Are two of the Cinemag/Sowter's needed or just one?
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You will need a pair of transformers.
-PB
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I should have them by the weekend!
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I got this info from David Geren from Cinemag for any interested parties:
The CMLI-15/15B in standard format is $52.78. The threaded bushing option adds on $2.50. UPS ground will be $XX. Send me a copy of the schematic if you want me to see about upgrades. If it is a balanced circuit topology, I have something new that I designed a few months ago that might be interesting.
I know the Bottlehead schematics are proprietary, but I was hoping one of the gents here (PB?) could provide David the info he needs - cinemag[at]att[dot]net - so he can offer Bottleheads the best option.
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The circuit is not balanced. That's why you need the transformer, to create a balanced input. I'm just sayin'.
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Got it, nevermind then.
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I got the stuff! my only question is the load resistor you suggested. Does the 14k resistor go (I presume in serial) with the inbound wires, going in the transformer? Or on the wires coming out of it, or both. (my guess is the wires coming out of the trans. but I want to be sure)
Thanks again
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I got the stuff! my only question is the load resistor you suggested. Does the 14k resistor go (I presume in serial) with the inbound wires, going in the transformer? Or on the wires coming out of it, or both. (my guess is the wires coming out of the trans. but I want to be sure)
Thanks again
Which transformers did you end up with?
I would put the load resistor on the secondary winding of each input transformer.
-PB
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I got the Hammond 140uex 1000k 1:1. I made a nice mounting platform for them. I'll have that done this weekend. And I got some of the nice Dale resistors (14k).
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Excellent, you are beating us to the punch in terms of attempting this modification, keep us posted on how it goes.
-PB
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Load in parallel, not in series.
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Load in parallel, not in series.
just to make sure I got this here's a rough schematic ( as I understand this)
Wire in (pin 2)---------- ----------wire out-----------selector switch
Transformer resistor
wire in (pin3)---------- ----------Wire out-----------selector switch
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Looks good!
You can run a piece of clip lead from pin 1 to the little ground tab on the jack itself.
If the Hammond has a shield, the shield wire can be soldered to the ground lug on the XLR jack as well.
-PB
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Done!
Now just a little "woodworking" since, even though it looks flush with 1/4 form the edge of the plate .......its not :o.
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Nice implementation there!
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Thanks. Now my next question. What effect would increasing or decreasing the load resistor have. It sounds pretty good, but I feel like it there might be some undiscovered potential in there.
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As a generalized remark - a lower load resistance will give a smoother (more well-damped) rolloff of the treble, along with loss of extension. Well suited (for example) to MC cartridges with a hot high end. A larger resistance will gradually expose the HF resonance, providing more sparkle at first and more fatigue eventually.
For specifics, you must either get the manufacturer to give you guidance - only they know the details of the transformer's more subtle parasitics - or do the experiment yourself. Unfortunately the quality fo the resistor will influence the results, so you will never find a conclusive answer, but you will likely get close enough for personal satisfaction.
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Ok then....off to experiment!
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The 14k recommended resistor was close. I tried a 20k today and it sounds sweet again.
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Wanting to communicate with anyone who successfully did a balanced i/o build of the BeePre. I don't have a kit yet and am considering getting one for X'mas. I noticed there is a daughter board with caps on the balanced output in the BH product page photo but absent on unbalanced output photos of BeePre builds by Aaron and Debra. Can anyone chime in on this?
Also like to know if anyone tried with Jensens or Cinemags and what load resistor values did you use.
Just ordered a Mainline and that will be my first BH build. I have a recording studio and am interested in experimenting using the BeePre as a tube line stage to see if it improves my mixes going balanced in and out from my DA converter to the BeePre and back into my mixdown deck. Also thinking of using it to drive my powered studio monitors. Everything sounds good as it is, but I am sure once I put in the Bottlehead stuff it will all sound better.
Any comments and photos will be greatly appreciated.
Best, Dennis
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Wanting to communicate with anyone who successfully did a balanced i/o build of the BeePre. I don't have a kit yet and am considering getting one for X'mas. I noticed there is a daughter board with caps on the balanced output in the BH product page photo but absent on unbalanced output photos of BeePre builds by Aaron and Debra. Can anyone chime in on this?
They simply built their BeePres with an extra set of unbalanced outputs and no XLR output.
-PB
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Just wanted to know is that daughter board with the caps, that I see on the BeePre products page part of the balanced circuit? Just noticed that part looks different. Sorry if I did not make myself clear.
Dennis
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The extra caps and resistors around the output jacks are for the balanced outputs. They are, however, not mounted to a PC board.
-PB
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Hi Paul, So if I opt for the balanced input transformers, I'll need to put a loading resistors across the outputs of the transformers, correct? If I use the recommended Cinemags, what value do I use?
That is why I want to find someone who has put in a transformer and made it work sucessfully. I could run it unbalanced all the way but all my pro stuff runs balanced and the impedance are different. That is probably the reason for the R/C network on the balanced outs.
Thinking more of using as a tube line drive than a preamp to color my stereo bus.
Anyway any advice would be greatly appreciated.
Dennis
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what you see above is Hammond transformers (1k 1:1) mounted on a small piece of aluminum plate. Terminal plates mounted to that as well. That is wired as a input. the stock kit is configured to have one output xlr. I instead used rcas on both outs. The stock configuration has the caps hanging from terminal strips to the xlr pins. I hope this makes more sense.
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22K is a decent start for loading the Cinemags.
There's nothing wrong with feeding the BeePre from unbalanced sources, then using the balanced outputs.
-PB
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Thanks guys!
The reason I want to go in balanced is that my D/A converter is balanced out +4dB. I could just do a balanced to unbalanced cable, but the proper way to do it would be to use sort of an unbalancing transformer that would step down the impedance and levels. Just wanted to do things right. Still thanks. Want to try the Manline then tackle the BeePre.
Dennis
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Want to try the Manline then tackle the BeePre.
I have to correct my post from earlier this week. That might be one of the funniest Freudian slips ever...
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The idea behind the balanced output on the BeePre was to get balanced without the use of transformers. This is because I have always found a transformer to soften the leading edges of the bass notes. At +4 you have tons of signal, and you will be able to kick it up even more through the gain of the BeePre. So if it is possible to take just half of the output swing from the DAC - for example if that output is a differential circuit rather than a floating transformer secondary - it might actually sound a tiny bit better to do that than putting the signal through the transformer and you won't experience much if any loss of the level. The BeePre won't add any of that kind of transformer sound itself, as the balanced output does not use a transformer. Of course this all depends upon what kind of coloration you are looking for. If you are trying to mellow the sound overall the transformer might be what you want. If you are trying to sweeten the top end and flesh out the lower mids a bit but want to preserve the bass punch, no transformer might work better.
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@dcham - here is a link to the Jensen Transformers white paper on interconnection: http://www.jensen-transformers.com/an/an003.pdf - just in case you wanted a more thorough pro-studio perspective!
You can see from the "Typical Balanced Output" figure that it has two outputs of opposite phase, but both are derived from the same internal signal. In this case, as suggested by Doc B, you can use either phase (plus ground) as an unbalanced output at half the voltage, without loss of performance except for the increased possibility of ground loops.
My further comments below are more than anyone has asked for, but I'm putting them here so as to be easier to find in the future by search engine.
A few - mostly old-school - balanced outputs are done by using an output transformer, which may or may not have a grounded center-tap. In this case you can use either phase with the grounded CT, or ground one phase (and un-ground the CT if used) and use the other as "hot" for a 6dB voltage increase. You can't to this with the "typical" circuit because it would be shorting one of the op-amps.
A few digital sources may have dual, independent split-phase DACs per channel. Using just one phase as above would then be using just one of the DACs per channel, losing at least theoretically some increase in performance in fully balanced mode. In that case, an input transformer for the downstream device is needed to obtain true balanced operation, at the prices (as Doc B says) of introducing some transformer sound.
Finally, it is possible to make a balanced to single ended tube circuit without transformers, adding two triodes per channel. Personally I doubt the transformerless advantage would outweigh the effect of added stages, but since we haven't done it that remains just a guess.
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Thanks everyone for the input. Learning as I go. ;-)
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Finally, it is possible to make a balanced to single ended tube circuit without transformers, adding two triodes per channel. Personally I doubt the transformerless advantage would outweigh the effect of added stages, but since we haven't done it that remains just a guess.
I built just such a circuit this week, though we haven't evaluated it sonically.
-PB
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Just re-reading the replies between my sessions here at work. I think I confused everyone, I understand the BeePre has a transformer-less balanced output. No arguments there and I agree that transformers may degrade the sound.
My question was regarding installing the balanced inputs with a transformer, which is what you recommend.
Apologies for all the confusion. Thanks for the patience and answers.
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I understood what you were saying just fine, and attempted to explain that my own philosophy is to avoid the transformer anywhere possible. We offer the ability to install a transformer to receive a balanced signal. But personally I try to avoid the transformer where possible, and in this case I would suggest taking one half of the signal (i.e. a single ended signal) from your DAC and running that into the single ended input if the BeePre - if that is possible. If you must use a transformer then balanced is a good way to go.
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Thanks Doc! Got it now.