Why must balanced amps (or preamps) be bridged?

Bricolo · 21st December 2002, 07:58 AM

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?

Bricolo · 21st December 2002, 10:16 AM

Here is the diagram (simplified one, from the bosoz pdf)

I don't know what R15 is used for, can someone explain me?

peranders · 21st December 2002, 10:23 AM

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.

kristijan-k · 21st December 2002, 11:26 AM

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

Bricolo · 21st December 2002, 11:30 AM

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?

jh6you · 21st December 2002, 11:42 AM

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

21st December 2002, 07:58 AM	#1
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	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?

21st December 2002, 10:23 AM	#3
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	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. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

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21st December 2002, 11:30 AM	#5
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	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?

21st December 2002, 11:42 AM	#6
jh6you Account Disabled Join Date: Aug 2002 Location: Near to the Pacific Ocean	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