How to convert Unbalance -> Balance Input ?

Minh · 16th September 2002, 11:52 AM

Hi.

I've bought this amp a while ago, it's rated at 185Wrms/Side. I'm hoping to convert the unbalance input to a balance one but not sure of the correct configuration.

Your help is must appreciated.
Here's the shematic.

Mambo · 16th September 2002, 12:40 PM

Try this, i took it from the service manual of an Aleph amp.

hugobross · 16th September 2002, 01:24 PM

I've just tried this in simulation, it works fine but I'm not sure about the values of the extra resistors; can someone help me?

Mambo, haven't you forgotten to place a cap in series with R39??

best regards,

HB.

Mambo · 16th September 2002, 01:46 PM

The resistor values are high enough so to not put excessive load in the preceding stage, unless you are using a pre "really" capable of driving lower loads. With the values shown the input impedance is a bout 10K unbal and 25K when balanced.

Putting a cap in series with R39 greatly increases the quantity of DC at the output.

Regards,
Mambo

Minh · 16th September 2002, 02:49 PM

Thanks guys.

Mambo, the circuit works fine in EWB.

Thanks again.

PMA · 16th September 2002, 04:39 PM

You can try following one .. (R = 10k, Rs = 100 Ohm, A1 = A2 = OPA134)

mrfeedback · 16th September 2002, 04:55 PM

Minh,
A car audio DC isolation transformer box works fine.
You may need to load down the secondary to avoid peaking response, and you can add a reversing switch to provide polarity reversal.
Car audio shops, St Lucia Electronics or WesComponents are a source.
$15-$30 for stereo.
I use this on my workbench and it works great - nobody has complained about the sound I'm getting.

Eric.

traderbam · 16th September 2002, 06:08 PM

Hugo's schematic may work ok but bear in mind that the connection between -'ve input and the feedback node will tend to make the amp gain dependent on input impedance. I would think this would be undesireable.

PMA's approach looks better to me (although it adds more circuitry). Although using 2 op-amps is unecessary if you use a single op-amp in adder configuration: plus signal into the +'ve input with an R to gnd. Neg signal to -'ve input through a series R and and R from 've input to output.

Of course you could also configure the amp itself as an adder but the need for stabilizing components and the low resistance from -'ve input to gnd may not be desireable for the inputs (depends whats driving the inputs).

Another way is simply to use only one half of the unbalanced input signal. Ignore the other half. This will work fine although the noise cancelling benefits of the balance line will be lost, but sometimes this is an acceptable compromise.

Good luck,
BAM

djdan · 16th September 2002, 07:25 PM

Minh

Please , give us more information about this amplifier.

It is a "pro" or "hi-fi" amplifier ?
How it work on 4 ohms ?
How it is sound quality ?

Regards ,

PMA · 16th September 2002, 08:08 PM

BAM,

the reason of using 2 opamps (in general) is to maintain the same output impedance of the + and - output. The circuit should not be influenced by following circuitry.

Pavel

16th September 2002, 04:55 PM	#7
mrfeedback diyAudio Member Join Date: Feb 2002 Location: Perth, Australia.	Passive Soloution Minh, A car audio DC isolation transformer box works fine. You may need to load down the secondary to avoid peaking response, and you can add a reversing switch to provide polarity reversal. Car audio shops, St Lucia Electronics or WesComponents are a source. $15-$30 for stereo. I use this on my workbench and it works great - nobody has complained about the sound I'm getting. Eric.

16th September 2002, 06:08 PM	#8
traderbam diyAudio Member Join Date: Jan 2002 Location: Earth	a matter of balance Hugo's schematic may work ok but bear in mind that the connection between -'ve input and the feedback node will tend to make the amp gain dependent on input impedance. I would think this would be undesireable. PMA's approach looks better to me (although it adds more circuitry). Although using 2 op-amps is unecessary if you use a single op-amp in adder configuration: plus signal into the +'ve input with an R to gnd. Neg signal to -'ve input through a series R and and R from 've input to output. Of course you could also configure the amp itself as an adder but the need for stabilizing components and the low resistance from -'ve input to gnd may not be desireable for the inputs (depends whats driving the inputs). Another way is simply to use only one half of the unbalanced input signal. Ignore the other half. This will work fine although the noise cancelling benefits of the balance line will be lost, but sometimes this is an acceptable compromise. Good luck, BAM

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16th September 2002, 01:46 PM	#4
Mambo diyAudio Member Join Date: Feb 2002 Location: Italy	The resistor values are high enough so to not put excessive load in the preceding stage, unless you are using a pre "really" capable of driving lower loads. With the values shown the input impedance is a bout 10K unbal and 25K when balanced. Putting a cap in series with R39 greatly increases the quantity of DC at the output. Regards, Mambo

16th September 2002, 02:49 PM	#5
Minh diyAudio Member Join Date: Nov 2001 Location: Brisbane	Thanks guys. Mambo, the circuit works fine in EWB. Thanks again.

16th September 2002, 07:25 PM	#9
djdan diyAudio Member Join Date: Dec 2001 Location: Romania	Minh Please , give us more information about this amplifier. It is a "pro" or "hi-fi" amplifier ? How it work on 4 ohms ? How it is sound quality ? Regards ,

16th September 2002, 08:08 PM	#10
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	BAM, the reason of using 2 opamps (in general) is to maintain the same output impedance of the + and - output. The circuit should not be influenced by following circuitry. Pavel

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