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Old 21st December 2010, 10:51 AM   #11
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Quote:
Originally Posted by merlin el mago View Post
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 by Andy5112405; 21st December 2010 at 10:59 AM.
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Old 21st December 2010, 11:21 AM   #12
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Quote:
Originally Posted by Andy5112405 View Post
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 ?
Why not ask directly to Erno Borbely borbelyaudio@t-online.de?
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Old 22nd December 2010, 07:57 AM   #13
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As this is a cloned design, I'm sure he wouldn't be too impressed.

I'll ask him though as if I've just found the article in GLASS AUDIO.

Does anyone have GLASS AUDIO 1/97 to see if there were any addendums to the article on the Borbely or any subsequent editions.

Last edited by Andy5112405; 22nd December 2010 at 08:04 AM.
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Old 23rd December 2010, 09:42 AM   #14
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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.
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Old 23rd December 2010, 02:27 PM   #15
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Erno is very kind, I own a lot of his stuff.

Cheers

Felipe
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Old 12th May 2011, 08:04 PM   #16
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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.
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Old 19th May 2011, 03:31 PM   #17
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Originally Posted by Andy5112405 View Post
Click the image to open in full size.
Click the image to open in full size.

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.

Click the image to open in full size.
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....
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Old 23rd May 2011, 09:21 AM   #18
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IDIOT ------ IDIOT

After checking the orientation of the cores of the XLR connectors, I found that they were incorrectly wired.

Problem has now gone away. I have decided to leave the Borbelly with NO NEGATIVE FEEDBACK and an open loop gain of 22dB.
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Old 23rd May 2011, 12:42 PM   #19
AJT is offline AJT  Philippines
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hi,

i find SY's Impasse preamp tops in simpliciity and performance....i am building one although i'll use 6CG7 instead of the 6SN7.....
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