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Simple asym  to symmetic buffer
Simple asym  to symmetic buffer
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Old 12th August 2019, 12:44 PM   #1
Turbowatch2 is offline Turbowatch2  Germany
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Default Simple asym to symmetic buffer

Hi, maybe someone can give me some qualified advice.

I need symmetrical input signals for a stereo D-amp. Signal source is asymmetric.
Why a summetric signal input?
First, as it is preferred by the manufacturer, he advises to use input buffers and
second, to run one channel inverted to prevent the amp from bus pumping.

I have small, inexpensive stereo pre-amp boards with a NE5532 OP-amp, unity gain, that are inverting.
I plan to place them directly at the RCA input.

Now, my question:
Is there any problem if I run the input signal into one of the pre-amps inputs, inverting it, so I get the negative phase which feeds the D-amp.
Then take the inverted signal and run it into the second channel of the pre-amp, inverting it again, for the positve phase.
As far as I can see, there is no disadvantage and buffering and phase split are done with a minimum of OP-amp stages. The D-amp has 8k ohm input impedance.

I ask because I have not seen such a simple solution so far.
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Old 12th August 2019, 12:47 PM   #2
Zen Mod is online now Zen Mod  Serbia
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Simple asym  to symmetic buffer
you'll find example of that in any book about OP-amps

you need two OP cells to have balanced out , one working in non-inverting, second working in inverting mode

though - what are you asking is already tried solution , just slightly less kosher
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Old 12th August 2019, 12:53 PM   #3
xrk971 is offline xrk971  United States
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Simple asym  to symmetic buffer
Try this - it works well and sounds great. Gives the amp a nice SE Class A sound signature too. Not a bad thing for many Class D amps. Very simple.

Making balanced output from single end

I have even used it as a headphone amp here:

Balanced Drive 2 Transistor SE Class A Headphone Amp

Here is an example of the balanced output from single input:
Click the image to open in full size.

Same for square wave:
Click the image to open in full size.

There is the 2 transistor implantation, above scope shots were done with BC550 and BC560. The headphone amp used KSC1845 and 2SA1837.

Aksa even has a 3 transistor version with lower distortion if that matters to you.

Click the image to open in full size.
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Old 12th August 2019, 01:02 PM   #4
00940 is online now 00940  Belgium
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Quote:
Originally Posted by Turbowatch2 View Post
I have small, inexpensive stereo pre-amp boards with a NE5532 OP-amp, unity gain, that are inverting.
Yes, as long as those preamp are indeed unity gain, it should work without problems and a 8K load is no big deal for ne5532.

Just another point of attention to look for: if you have a HF filter on these boards which is close to the audio band, you might want to remove it in the second channel.
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Old 12th August 2019, 01:04 PM   #5
Turbowatch2 is offline Turbowatch2  Germany
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@ Zen Mod

You did not completely understand my question, maybe read again. I have these boards and both OP-amp cells are inverting and shall stay this way. So not one inverting, one not!

@XRKaudio

I want to use something existing, not design my own buffer.
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Old 12th August 2019, 01:12 PM   #6
Turbowatch2 is offline Turbowatch2  Germany
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Just for clarification, this is the amp module, it is so small it can be soldered directly at the RCA
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Old 12th August 2019, 01:15 PM   #7
Turbowatch2 is offline Turbowatch2  Germany
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@ 00940
good point, I will check this.

For less than 2 € I can not do that board by my self, only have to change a resistor and change the OP-amp for a real one, as these are 99.99% fake.
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Old 12th August 2019, 03:17 PM   #8
xrk971 is offline xrk971  United States
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Simple asym  to symmetic buffer
If you want existing then tough to beat THAT1646.

Ready to use breakout board with bypass caps etc here:

BOB-14003 SparkFun | Mouser
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Old 13th August 2019, 06:11 AM   #9
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by Turbowatch2 View Post
Now, my question:

Is there any problem if I run the input signal into one of the pre-amps inputs, inverting it, so I get the negative phase which feeds the D-amp.

Then take the inverted signal and run it into the second channel of the pre-amp, inverting it again, for the positve phase.

As far as I can see, there is no disadvantage and buffering and phase split are done with a minimum of OP-amp stages. The D-amp has 8k ohm input impedance.

Not a good solution.

Use fully-differential THS4131 wired like this:
IMG_1954.jpg

This will correctly rejects CM signal from the input and feeds your amp with a true symmetrical signal.
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Old 14th August 2019, 01:52 AM   #10
sgrossklass is offline sgrossklass  Germany
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Quote:
Originally Posted by Turbowatch2 View Post
Just for clarification, this is the amp module, it is so small it can be soldered directly at the RCA
Interesting. Do I get this right: input --> 10k --> 100k going off to what appears to be power ground for no good reason ---> 100k || C_ceramic between -in and out?

Do check which grounds (if any) are connected on the board in order to avoid any nasty surprises. With some bad luck the scalpel might have something to do. I would not want my signal ground and power ground to be connected, for one.

What you want is quite doable, it's basically Fig. 1 in Balanced I/O with an inverting input. Depending on levels required and the source's driving capabilities it may be a good idea to go lower than 10k for resistors, for a reduced noise penalty (at around 1 Vrms, I see little problem with 4k7 or 3k3 when using a genuine 5532).
Note that the circuit effectively has 6 dB of gain - if the source already outputs more than enough level and its noise becomes a potential issue, increase input resistor to achieve negative gain as needed. That's the advantage of having an inverting input.
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