Why must balanced amps (or preamps) be bridged? - diyAudio
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Old 21st December 2002, 08:58 AM   #1
Bricolo is offline Bricolo  France
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Default Why must balanced amps (or preamps) be bridged?

I mean the R15 resistor on the BOSOZ diagram

I can't see what it is used for.

Can't one line amplify +in, and the other amplify the -in independantly, and use the potential between -out and +out for output

without any connection between the +in -out and the -in +out lines?
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Old 21st December 2002, 11:16 AM   #2
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Here is the diagram (simplified one, from the bosoz pdf)


I don't know what R15 is used for, can someone explain me?
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Old 21st December 2002, 11:23 AM   #3
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R14 and R15 are only for "pull down". In case the inputs aren't connected to anything the gates of the input transistors would float and this can be very nasty, especially for your speakers. MOSFET's and JFET's most ALWAYS be connected to something!

The stage is a differential stage which means that the + input has influence on the - output and vise versa. Connect signal to + and you will get a non-inverted signal at - output without any signal at - input.
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Old 21st December 2002, 12:26 PM   #4
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Hello,

The resistor labels are little fuzzy to read on the
circuit diagram above, and I think that the Peranders
wrongly located the R15.

Actually, the R15 resistor serve for setting the gain
of the Balanced Line Stage preamplifier.

If you don't really need high gain, than use a little
higher value for the R15 resistor ( 200-500 Ohm ).


Best regards,
Kristijan Kljucaric
http://web.vip.hr/pcb-design.vip
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Old 21st December 2002, 12:30 PM   #5
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peranders, you told about R13 and R14

I want to know about R15 (the one in the middle)




kristijan: this resistor seems to adjust something like symmetry, how can it influe on gain?
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Old 21st December 2002, 12:42 PM   #6
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R15 controls the voltage gain.
Gain in dB is approximately 20 x log((R1+R2)/R15).

Reference: http://www.passdiy.com/pdf/balzenpre.pdf

JH
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