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How do I convert two SE amps to a balanced amp?

post #1 of 18
9/14/09 at 5:57am
Thread Starter 
Hello again everyone, I'm back with another question.

I'm a bit indecisive at the moment. I have been contemplating building a Balanced B22 for a while now, but I also have a desire to build a balanced, transformer coupled all tube amp (as discussed in a previous thread). I am to the point now where I am trying to decide, and I have another question.

Someone mentioned that I could wire up two 2 channel amps to make a balanced amp. My question is - how do I do this?

What I am thinking I am supposed to do is wire L+ and R+ to Pin 2 and Pin 3 of one driver's XLR socket - but what happens to L- and R-?

I would of course do the same to the other (identical) amp for the other driver.

If that's all that's needed apart from changing the input, is this safe to perform on any amplifier, or will component changes be required as well?


I have a Heed CanAmp that I am in love with and I'm now also contemplating buying another and making a balanced amp out of the two.

Any help would be appreciated.

Ryan
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post #2 of 18
9/14/09 at 12:36pm
Quote:
Originally Posted by rshuck View Post
Someone mentioned that I could wire up two 2 channel amps to make a balanced amp. My question is - how do I do this?
Vitamins.

Quote:
What I am thinking I am supposed to do is wire L+ and R+ to Pin 2 and Pin 3 of one driver's XLR socket - but what happens to L- and R-?
Nothing.

If you're using a two channel unbalanced amp, there should be no L- and R-. Just L, R and ground.

Quote:
If that's all that's needed apart from changing the input, is this safe to perform on any amplifier, or will component changes be required as well?
It's not safe to say "any amplifier." But generally speaking, yes, most amplifiers can be bridged this way. And "bridging" is really the more appropriate term here over "balanced."

k
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post #3 of 18
9/14/09 at 12:51pm
Bridged can sometimes mean purpose built balanced. Purpose built boards for balanced in and out. They could be used with an unbalanced signal input, but just throwing two unbalanced amps together, one for left and one for right, isn't the same.
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post #4 of 18
9/14/09 at 1:14pm
Quote:
Originally Posted by digger945 View Post
Bridged can sometimes mean purpose built balanced. Purpose built boards for balanced in and out.
In my opinion, a bridged amp should mean nothing more than a bridged amp. To call a bridged amp "balanced" creates a rather confusing corruption of what "balanced" has traditionally meant and what its primary purpose is (i.e. the rejection of common-mode noise). A bridged amp does not provide any common-mode noise rejection at all. In fact, a bridged amp will simply amplify any common mode noise along with the signal.

Bridged amps have been traditionally called bridged amps, whether in professional audio, home audio, and even car audio.

Quote:
They could be used with an unbalanced signal input, but just throwing two unbalanced amps together, one for left and one for right, isn't the same.
Yet that's precisely what most every "balanced" headphone amp out there does.

k
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post #5 of 18
9/14/09 at 1:26pm
Let both ends of the transformer secondary tap float when driving the headphones.. presto - cheap 'balanced' output
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post #6 of 18
9/14/09 at 1:37pm
Quote:
Originally Posted by TimmyMac View Post
Let both ends of the transformer secondary tap float when driving the headphones.. presto - cheap 'balanced' output
Yes, if you're talking about a tube amp. Though the secondaries are typically grounded for safety purposes. Don't want any involuntary electroshock therapy.

And of course you can simply add an output transformer to a single-ended solid state headphone amp and pesto - also a cheap "balanced" output.

k
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post #7 of 18
9/14/09 at 2:10pm
if coming from a true balanced DAC, you can "technically" feed +R and -R to each channel of the first amp. You do same for the second amp by feeding -L and -L.

However, I must warn you that you can easily create ground loops as you can tying the grounds of two separate amps. As long as you know how to trace GND from your PSU all the way to your ouput, then it may be worth a shot. However, I see no real benefit in doing that as it will be more expensive overall.
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post #8 of 18
4/24/10 at 2:59am
I have a true balanced DAC. I have 2 tube amps of the same type with matched tubes. Can I create a fully balanced system by connecting the R+ R- R(Gnd) to one amp and connecting L+ L- L(Gnd) to the second amp?

After that I would resolder the headphones to have L+ and L- outputs of the amp connected to the left driver and R+ R- to the right using a 4-wire cable.
Is it ok? Can I simply omit original grounds (R(Gnd) and L(Gnd))?
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post #9 of 18
4/24/10 at 3:32am
Completely depends on the amp.

You can certainly run an amp in differential mode and have 2 amps together running in differential mode to create a bridged amp of sorts. Balanced implies the actual amp combines the channels to subtract the noise. Well, that's my "simple" understanding, I'm sure there's a much more correct answer out there.
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post #10 of 18
4/24/10 at 4:14am
In my understanding if you drive the driver coil in full balanced mode, than noise subtractions occurs at the driver itself, not at the amp.

My theory, plz correct me if I'm wrong:
You get a + and an inverted - signal from the DAC through the amp, and connect the + to one end, and - to the other end of the coil. In this case all the picked up noise will be canceled out, because noise has the same phase at both end of the driver coil without any voltage difference.

But this is only a side effect, the sketched system should have better stereo separation and other pleasing features.

If it can be carried out without burning the tubes or the can

My question still stands: is this wiring ok?

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post #11 of 18
4/24/10 at 4:34am
You sell both and buy a real balanced amp.
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post #12 of 18
4/24/10 at 10:06am
Quote:
Originally Posted by Steve Eddy View Post
A bridged amp does not provide any common-mode noise rejection at all. In fact, a bridged amp will simply amplify any common mode noise along with the signal.
If you measure across the load, how would you know? The error is common to both phases and cancels across the load.


Schwao:
That is the basic idea of making a 4 channel amp. Depending on your system things may go awry so be prepared to test everything.
Quote:
Originally Posted by KingStyles View Post
You sell both and buy a real balanced amp.
There are very few balanced headphone amps. The VAST majority are 4 channel amps.
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post #13 of 18
4/24/10 at 11:45am
Quote:
Originally Posted by nikongod View Post

Schwao:
That is the basic idea of making a 4 channel amp. Depending on your system things may go awry so be prepared to test everything.
Every advice what to consider and what to pay attention for would be great!
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post #14 of 18
4/25/10 at 9:07am
Quote:
Originally Posted by nikongod View Post
There are very few balanced headphone amps. The VAST majority are 4 channel amps.

Is this configuration considered as "balanced" ? A way to achieve balanced outputs with single ended input ?
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post #15 of 18
4/25/10 at 12:28pm
Quote:
Originally Posted by schawo View Post
Every advice what to consider and what to pay attention for would be great!
Just be careful with how you do or dont (heh) ground things. Thats always been the biggest stumbling block to my efforts at bridging amps.

Quote:
Originally Posted by bidoux View Post
Is this configuration considered as "balanced" ? A way to achieve balanced outputs with single ended input ?
Id include it Its a very neat circuit.

The article on head-wize doesn't say how to hook up the feedback to the circuit, or the DC servo for the second half but its still cool.

Another great balanced headphone amp design is j4cbo's M4 amp which was posted up a few months ago Here
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