Unbalanced - Balanced Tube Preamplifier wanted - diyAudio
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Old 17th June 2004, 05:38 AM   #1
bebong is offline bebong  Viet Nam
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Default Unbalanced - Balanced Tube Preamplifier wanted

I've just finish my Son of Zen amp. The next is the preamplifer.

I'm now looking for a nice & easy to build tube pre-amp which can serve as Unbalanced to Balanced.

Anyone can help me?

Thanks in advance.
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Old 17th June 2004, 02:57 PM   #2
benny is offline benny  Australia
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easiest thing to do is to build yourself whatever preamp you want, then use an output tranny at the end of the chain to ballance the signal.

only problem is unless your inputting the two balanced sides of your preamp signal directly into the two seperate sides of a PP amp, you have to use another tranny at the end to unballance signal again if the first stage of the power amp is SE... and considering the likely short run between your pre and power amps, this would serve as an almost completly pointless little exercise. If you are inputting directly into the two seperate sides of a PP amp, then it is a worthwhile, and might i say, essential exercise.

you can also use phase inverters to ballance/unballance signal as well, but seeing as you said "nice & easy" then this will just add another complication to the design, making it a little less easy, so stick with a good quality tranny.

later
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Old 17th June 2004, 09:50 PM   #3
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Hi,

I understand the amp already has a balanced input so a paraphase splitter type circuit or diff amp would do fine provided you can adjust it for best possible signal balance.

That is of course in case you'd want to avoid using a xfmr.

Cheers,
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Old 17th June 2004, 10:25 PM   #4
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Quote:
I understand the amp already has a balanced input so a paraphase splitter type circuit or diff amp would do fine provided you can adjust it for best possible signal balance.

That is of course in case you'd want to avoid using a xfmr.
Frank,

1 of the potential advantages of an O/P trafo is a low O/P impedance. It seems to me that voltage followers should be used to buffer the phase splitter.
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Old 17th June 2004, 10:33 PM   #5
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Hi,

Quote:
It seems to me that voltage followers should be used to buffer the phase splitter.
Absolutely. No argument there...
A little more complex in construction but providing excellent balance is the cross-coupled invertor...Almost as good as a xfmr.

Cheers,
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Old 19th June 2004, 06:55 AM   #6
bebong is offline bebong  Viet Nam
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Thank you all,

To Benny:
- It's very difficult for me to get a good output transformer (the output tranny as Banny advised if I understood what you mean). So, I think the "Phase Inverters" is OK for me.

To fdegrove:
- You right, my SOZ is already have a Balanced Input.

To All:
- Could you upload some shematics for all the type of pre-amp. Phase inverter, paraphase splitter type, diff amp and cross-coupled invertor?
- Please be advised that, I totaly new for tube electronic so a detail schematic would be very appreciate.

Regards,
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Old 19th June 2004, 10:26 AM   #7
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Here's a link to a page with phase splitter schematics. The Schmidt circuit is a diff. amp.

http://www.bonavolta.ch/hobby/en/audio/split.htm
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Old 19th June 2004, 06:00 PM   #8
benny is offline benny  Australia
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Quote:
It's very difficult for me to get a good output transformer (the output tranny as Banny advised if I understood what you mean). So, I think the "Phase Inverters" is OK for me.
yes definately... a quality output tranny is important if going down the tranny path, and if it's difficult for you to source one, then best not to bother with it.

if you want to use a PI, i would suggest a cathodyne phase splitter. it's the simplest one there is... you may hear of bad things sometimes about this PI, about it being unballanced, particularly at high frequencies because of the cathode and anode impedances being different, and there is the potential for this to happen, but only if you are driving into two different impedance loads... as long as impedance of the loads are equal, and equal across all frequencies, then this splitter is ballanced, and might i say a very effective design. Also, different interlectrode capacitances do not really make any imballance worth worrying about as these capacitances are so small their effect on the signal is so minute that you probably wouldn't even hear it, especially considering another factor.... these capacitances have values measuring in pico farrads... so a typical value of say a 12AX7 grid-plate measures 1.7pf... this would effect frequencies probably out of the human hearing range.

In my opinion, it is one of the best designs there are because of it's effectiveness and relative simpleness. Sure, there are more complex PI's around, even something like an Isodyne splitter, which offers an AC ballance control that if set up properly using a scope and sig. gen, will garentee 100% perfect ballance no matter what impedances you're driving or whatever other variable you want to throw at it, but it is not worth the extra time/parts if you want something simple as you say, especaialy seeing as you're probably not even going to be able to hear the difference, and also considering a PI with perfect ballance still doesn't compensate for imballances in the circuit... say you've got a couple of 22nF coupling caps on the output with 5% tolerance... 5% of this is 1.1nF... this creates a potential for 2.2nF drift on each side, and while you can try and compensate with AC ballance in the PI, it can't correct the different capacitances having different reactiveness acording to the frequency. also, put into perspective with interlectrode capacitance of tube... i gave the 1.7pF grid plate example.. put the coupling cap drift value into pF and you get 2200pF.... i'm starting to ramble.

cheers
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Old 19th June 2004, 06:28 PM   #9
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Deriving a balaced signal from a cathode follower with a floating PS(and split load inverters in general) is discussed in a TubeCad article -
http://www.tubecad.com/december2000/page10.html

There is a practical implementation of this approach in Glass Audio 1/2000 "A Universal Phase Splitter" by Ari Polisois
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Old 21st June 2004, 05:32 AM   #10
bebong is offline bebong  Viet Nam
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Thank you all,

Will have a look at links and come back with some more questions.

Rgds,
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