Balanced Input as Phase Inverter? - diyAudio
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Old 15th December 2010, 10:04 PM   #1
dre7 is offline dre7  United States
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Default Balanced Input as Phase Inverter?


A recent post got me thinking about this. I am considering building an amplifier utilizing a PP:PP interstage and as little additional stages as possible. I had originally intended using a line transformer at the input to split phase, but I don't have a suitable one in the spares box. Since the source in my system (Behringer DEQ2496) has balanced outs, is there anything preventing me connecting XLR pins 2 and 3 to a dual gang 100k pot and feeding a wiper to each grid in my driver stage? In that case would the center tap be resolved by tying the pot's two ground legs together and using it as the starting point for my signal ground bus, with XLR pin 1 brought over to the ground of my power input and tied to the other end of the bus via an R or C ground lift? I'm also considering using a ladder attenuator for accuracy's sake. Thanks.
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Old 15th December 2010, 10:33 PM   #2
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A good balanced output (not intimately aware of DEQ circuit) shouldn't give you any problems especially through capacitive coupling, but you won't be able to send feedback through it of course and you'll have to be sure behavior is good even common mode and during power cycling or you'll get unwanted signal in/out your PP stages.
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Old 15th December 2010, 11:38 PM   #3
SY is offline SY  United States
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Originally Posted by dre7 View Post there anything preventing me connecting XLR pins 2 and 3 to a dual gang 100k pot and feeding a wiper to each grid in my driver stage?
Any mistracking of sections (VERY likely, especially for an audio taper pot) will cause distortion and compromise common mode rejection. A precision attenuator is better, though not nearly as perfect as an input transformer.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 16th December 2010, 04:56 AM   #4
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How about skipping the dual pot and using a single log pot to shunt between the two phases?
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Old 16th December 2010, 09:57 AM   #5
DF96 is offline DF96  England
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The whole point of a balanced input is to reject common-mode signals. In your case you will be relying on perfect balance in both the driver and output stages, with the common-mode signals finally being rejected in the OPT. A dual volume control will ruin an already poor CMRR. You are creating a lot of problems, and at the same time destroying the very point of a balanced input. It would help if your driver stage itself rejects common-mode signals (e.g. LTP - not separate stages for each phase).
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Old 16th December 2010, 12:09 PM   #6
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Could you use this circuit to come back unbalanced configuration and then proceed normal way ?

Click the image to open in full size.

I once built this differential amplifier circuit for microphone pre-stage to substitute the input transformer.

There you can also see the measured performance.
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Old 16th December 2010, 01:01 PM   #7
jrenkin is offline jrenkin  United States
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Does the DEQ have volume control? Or your source? It seems to me that if you want to use the balanced out of the DEQ, you should adjust the signal before the DEQ output or you will run into all the problems mentioned above.
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Old 16th December 2010, 04:52 PM   #8
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I'm very interested in this. will be doing something simular for a balanced tube based crossover network. I may be able to offer you some test materials. I can provide you with 4 very well wound transformers about half the size of a school eraser (1.5"X1.5"X1.5"). The transformer provides an approx 20:1 ratio. The inductance on the primary is about 27H and the secondary .175H. Using 2 per side will let you move the ground reference. If the 20:1 ratio is a no-go, you can use these as autoformers which would also move the ground reference. shoot me an email if you are interested.
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Old 16th December 2010, 06:22 PM   #9
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I have built totally balanced systems before, for pro-audio installations.
It works just fine. But as already mentioned, all the stages have to be differential and very well balanced. But that is true of all diff amp. Each side needs to track well or be very well matched in order to maintain CMRR.
If you need a volume control, then the best solution is a matched stepped attenuator. Regular swipe pots are rarely matched well enough.

I have also built several PP power amps, using input transformers to split to differential and then used a diff amp to drive the ouputs. Some of the best PP amps I have ever built were done this way. It does increase the number of critical things to get right though.
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Old 16th December 2010, 06:55 PM   #10
Anchan is offline Anchan  United States
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If i understand your original post correctly, I basically have done just what you suggested.
Dual Simple SE's in Push Pull w Differential Input

I used 2 single ended boards from Tubelab.

The amp sounds outstanding.
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