• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Valve Line Amp suggestions

[IMGHTTPDEAD]http://img810.imageshack.us/img810/341/pcb008.jpg[/IMGHTTPDEAD]
[IMGHTTPDEAD]http://img839.imageshack.us/img839/3771/pcb009.jpg[/IMGHTTPDEAD]

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?

[IMGHTTPDEAD]http://img338.imageshack.us/img338/5536/imag0075doc.jpg[/IMGHTTPDEAD]
 
Last edited:
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.
 
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).
 
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.
 
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.
 
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.

Surely R19 = R2 = 100R

R8 = R1 = 1M0

C1 and C9 are recommended to be increased from 0.22uF to 1uF if the output impedance is < 100K.

This pre. operates without feedback so is open-loop gain. Are we missing a trick by the designer ?
 
Last edited:
Well Erno Borbely Replied.

We virtually knew the solution to the problem. But here is his answer.

Hi Andy, a 16 year old design, I am astonished, 
Anyway, I am not sure what you want to do with it.
You are saying that with INP1 grounded the amp is useless. INP1 is the input
with unbalanced source, so why should you ground it? INP2 should be grounded,
with unbalanced source, which is what the low impedance is for.
Of course if you want to use it with balanced source, then R8 should be changed to 1MEG,
like R1.
Does this answer your q.?
Wish you a very nice Holiday Season!
Erno.
 
The Pumpkin/Shuntkys are almost finished now. Still not overly impressed with the colouration that the Borbellys intoduced into the equation. Watch this space and I will compare the two.

They are both EXCELLENT pre-amplifiers but I am using them with some serious High-End Hi-Fi that is showing their weaknesses.

The loser will be up for grabs.
 
[IMGDEAD]http://img810.imageshack.us/img810/341/pcb008.jpg[/IMGDEAD]
[IMGDEAD]http://img839.imageshack.us/img839/3771/pcb009.jpg[/IMGDEAD]

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.

[IMGDEAD]http://img338.imageshack.us/img338/5536/imag0075doc.jpg[/IMGDEAD]

Zen Mod suggested converting the circuit to SuperSymettrical which I have done. Am I right in thinking that the Borbely diagram is labelled incorrectly with +/- inputs being crossed over. It looks as though the + input derives the - ouput ???

If I connect the feedback resistor according to the labelling on the PCB I get nothing. Just a very low level signal with significant HUM. I had to go to work before I had a chance to experiment....