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Old 1st August 2007, 04:22 PM   #1
lgreen is offline lgreen  United States
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Default Biamp Buffer - Finished

Hello everyone, here is my biamp buffer.

It takes a differential or singled ended (SE) input and gives you-

1. unbuffered differential or SE output
2. buffered differential and SE output
3. variable level controlled buffered differential and SE output.

Just uses a DRV134 to convert the SE to balanced (if you select SE via the switch) There are then 3 dual opamps configured as voltage followers. Any dual opamps can just be snapped in there, though of course they must be unity gain stable. I'm using an LM4632 on the input and an OPA2134 on the outputs.

Also powered via CRC PS feeding Liner Technologies LT1962/1964 ultra low noise surface mount regulators. These things are very very small.

Web Page

Level control is via a cermet trimpot, but you have the option of flipping a switch and inserting a high quality fixed R of any value you want.
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Old 2nd August 2007, 02:06 AM   #2
anatech is offline anatech  Canada
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Hi lgreen,
Your DRV134 needs to be driven with a low impedance source (per the data sheet). I haven't been able to get my mittens on any yet to confirm this.

A previous design error on my part did confirm high noise levels with the previous part. I always drive these with a dedicated op amp or other type of buffer.

-Chris
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Old 2nd August 2007, 09:18 AM   #3
AndrewT is online now AndrewT  Scotland
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Hi,
is the switch S2/S4 shown correctly in the schematic?
It appears to be substituting a series resistor for the upper leg pot resistance, leaving the full pot value in circuit. This will not give the same attenuation before and after you flip the switch.
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Old 2nd August 2007, 09:40 AM   #4
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Quite flexible indeed.
Looking at the circuit, the two lower opamps appear just to buffer the input opamps. Their output is identical to the output of the input opamps. That seems unnecessary?

Jan Didden
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Old 2nd August 2007, 09:54 AM   #5
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Personally, having done some very strange things with opamps, I'd put a 20-100 ohm resistor inline with each of the outputs. It helps stability with difficult loads/cables, and even though the chips do have short circuit protection, it limits the current in such a situation.
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Old 2nd August 2007, 10:03 AM   #6
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Good point Al. Here speaks the voice of experiemce!

Jan Didden
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Old 2nd August 2007, 05:08 PM   #7
lgreen is offline lgreen  United States
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Quote:
Originally posted by anatech
Hi lgreen,
Your DRV134 needs to be driven with a low impedance source (per the data sheet). I haven't been able to get my mittens on any yet to confirm this.
***
-Chris
Thanks chris, this is being driven by my X-CCS-BOSOZ balanced output so I'm not really using the DRV at all, but If I was I would now carefully look at the situation!

Quote:
Originally posted by janneman
Quite flexible indeed.
Looking at the circuit, the two lower opamps appear just to buffer the input opamps. Their output is identical to the output of the input opamps. That seems unnecessary?

Jan Didden
You are right, they are not strictly necessary. I guess the other two at the top left after the DRV are not necessary either since the variable resistance buffer can probably be driven by whatever is connected just as well. (its 50K)

Quote:
Originally posted by pinkmouse
Personally, having done some very strange things with opamps, I'd put a 20-100 ohm resistor inline with each of the outputs. It helps stability with difficult loads/cables, and even though the chips do have short circuit protection, it limits the current in such a situation.
Good Idea, thanks for the info.
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Old 2nd August 2007, 05:15 PM   #8
lgreen is offline lgreen  United States
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Quote:
Originally posted by AndrewT
Hi,
is the switch S2/S4 shown correctly in the schematic?
It appears to be substituting a series resistor for the upper leg pot resistance, leaving the full pot value in circuit. This will not give the same attenuation before and after you flip the switch.
Yes it is shown correctly in the schematic. The intent there was to just switch out the upper leg resistance from the Pot keeping the overall input resistance the same because the upper leg resistance is the R that is in-line with the audio signal, the other R is the R to ground.

So after flipping the switch when you look at the input to ground the full pot value stays in the circuit intentionally, it is 50K.

However you no longer use the upper leg pot resistance as a series resistor in-line with the audio signal. Instead, the in-line resistance is the same resistance via a better resistor.

So now I see that you are right, the resistance to ground before flipping the switch will be different after. Ooooposie!
my bad. In the limited use I have given this to now it has not been a big deal but I'll look at that too. I suspect that these op amps draw so little current that it will not really matter, but then again I'm not the expert-- you guys are!

Thanks for your comment and for taking the time to look at the schematic.


Note- if you look at the boards you see that I also brought out a ground to solder stuff to. The point of this is to allow connection of RC in-line and to ground separatly in case you want to get fancy and put in a simple filter...say to get band limited audio to your woofer/tweeter amp.
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Old 2nd August 2007, 08:11 PM   #9
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hm, shouldn't one of the DRV134 inputs be grounded?
regards
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Old 2nd August 2007, 08:27 PM   #10
lgreen is offline lgreen  United States
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Quote:
Originally posted by juergenk
hm, shouldn't one of the DRV134 inputs be grounded?
regards
Yeah, the problem is that with the drawing software (circmaker) there is no symbol that would correspond to the DRV134 so I had to wing it with a standard opamp picture that does not really apply.
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