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Old 19th December 2010, 01:09 PM   #1
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Default Valve Line Amp suggestions

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Although I'm a very experienced Electronics Engineer I'm new to tubes.

Having built the Borbely EB-894/211 I have a problem with using it.

After studying the circuit diagram I am trying to use it as an Unbalanced Input - Balanced Output amplifier.

In this case Input 1 is shorted to signal ground and the RCA input is applied to Input 2. A balanced output is taken from Output 1 and Output 2.

According to measurements and the circuit diagram, in this configuration the amplifier has an input impedance of 200 Ohms which is playing havoc with whatever I try to drive it with.

Are the suggested values for R1. R2, R8 and R19 wrong or should I be shorting Input 2 and using Input 1 as the unbalanced input?

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Last edited by Andy5112405; 19th December 2010 at 01:24 PM.
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Old 19th December 2010, 02:07 PM   #2
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Yes, place your input on #1, and the common (shield) to #2.
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Old 20th December 2010, 11:08 AM   #3
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That worked fine.

However, I've had to make up custom XLR leads to achieve it.

Pin 1 - Live Input

Pin 2 - Signal Ground

Pin 3 - Signal Ground

Plug Chassis - DEFINITLELY NOT CONNECTED

Plug Chassis is connected to Mains Earth by the metal socket and the metal chassis.

One connected NO PROBLEM. Any more gives you an Earth Loop.

I guesss you can buy Gold Plated XLR Male Sockets where the body of the plug is not connected to its surrounding metalwork.

In my finished design the mains Earth is ONLY connected to Chassis. Signal Ground is never connected to Mains Earth, not even through any fancy circuitry.
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Old 20th December 2010, 12:20 PM   #4
DF96 is offline DF96  England
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That is not a balanced amplifier, as R8 is not equal to R1. A balanced amplifier does not mind which way round you connect it unbalanced. It looks like an unbalanced amplifier, with ground signal subtraction. Strange, because it is close to being balanced but spoilt by one resistor value (and the optional capacitor on one input - strangely called feedback).
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Old 20th December 2010, 12:41 PM   #5
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Quote:
Originally Posted by DF96 View Post
That is not a balanced amplifier, as R8 is not equal to R1. A balanced amplifier does not mind which way round you connect it unbalanced. It looks like an unbalanced amplifier, with ground signal subtraction. Strange, because it is close to being balanced but spoilt by one resistor value (and the optional capacitor on one input - strangely called feedback).
If you change R19 for 1Meg, don't put the feedback cap C8 & change R8 to 1Meg the same value as R1 1Meg you get a symmetric balanced circuit.
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Old 20th December 2010, 01:38 PM   #6
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I guessed as much.

Running it as I suggested in Post 3 the amp has suddenly burst into life with enormous gain.

My Arcam CD36 is now overdriving it into clipping. I've attenuated the inputs to prevent clipping and it's now working fine. Not overkeen on the final result though, my Mark Levison sounds better with the CD36 driving it through a passive pre-amp.

You might ask what I am up to. I'm building a Nelson Pass F4 which will need a pre-amplifier. Will this be better than the ML. Time will tell.
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Old 20th December 2010, 01:41 PM   #7
DF96 is offline DF96  England
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If you are attenuating the input, do you need a line stage at all? Most systems don't need one, unless you want to add a little noise and distortion to your music.
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Old 20th December 2010, 01:42 PM   #8
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If you add a Salas Simplistic High Voltage Shunt Regulator for B+ & a Rod Coleman CCS for filament heaters for sure will sound better.
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Old 20th December 2010, 01:44 PM   #9
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Quote:
Originally Posted by DF96 View Post
If you are attenuating the input, do you need a line stage at all? Most systems don't need one, unless you want to add a little noise and distortion to your music.
If you don't play vinyls surely don't be necessary.
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Old 20th December 2010, 06:48 PM   #10
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The F4 requires nearly +/-20V swing at it's input. A high voltage pre is necessary.

I'm only testing the pre on its own at the moment, the F4 is not quite finished yet.
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