T-network: the better feedback solution? - diyAudio
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Old 9th September 2004, 08:18 AM   #1
Franz G is offline Franz G  Switzerland
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Default T-network: the better feedback solution?

I think it is time to open a separate thread about the t-network for the feedback (we already discussed it in the "Tube with Power IC Output Stage - JLTi" thread).

Tube with Power IC Output Stage - JLTi

What is a t-network for feedback and how does it look like?

Click the image to open in full size.

This is a t-network (avoiding a feedback resistor >2MB). At the output you see a voltage divider (18K/150R) followed by a "classical" feedback combination.

What is the advantage of the t-network?

It is (specially vor inverted gainclones) easy to achieve higher input Z and lower feedback Z, wich results in many advantages (look the first simulations in the thread above), specially when you can reduce a feedbackpath >100K. More advantages here:http://www.diyaudio.com/forums/showt...342#post470342

Is it applicable for all gainclones?

Yes, it is, when your feedback path has a Z over 100K.

Does the feedback path not get too long?

Tss tss tss ! The Z is important, much more than the length.

Here, you can download an updated Excel File to calculate different values and gains.
Attached Files
File Type: zip t-network_for_feedback.zip (16.6 KB, 819 views)
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Old 9th September 2004, 12:43 PM   #2
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It's very appealing to have 47k input impedance with inverting topology without having to use mega-ohms in the feedback path.
Very nice, Franz.
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Old 9th September 2004, 01:09 PM   #3
Nuuk is offline Nuuk  United Kingdom
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Quote:
It's very appealing to have 47k input impedance with inverting topology without having to use mega-ohms in the feedback path.
At the risk of being called a heathen, may I suggest that the greater input impedance may lessen/remove the need/desirabilty for a buffer with the IGC!
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Old 9th September 2004, 01:24 PM   #4
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Quote:
Originally posted by Nuuk
At the risk of being called a heathen, may I suggest that the greater input impedance may lessen/remove the need/desirabilty for a buffer with the IGC!
No, it doesn't.
10k is fine, 47k is better.
It's not the main reason for using an input buffer.
A pot connected directly to the inverting amp will still affect the gain.
And the amp chip likes to "see" low impedance at the input, something that a pot doesn't have.
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Old 9th September 2004, 07:53 PM   #5
Bricolo is offline Bricolo  France
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Since electronics are mostly tradeoffs, I'm curious to see the drawbacks of such an interesting topology
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Old 9th September 2004, 08:09 PM   #6
Franz G is offline Franz G  Switzerland
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Quote:
Since electronics are mostly tradeoffs, I'm curious to see the drawbacks of such an interesting topology
You better ask, what are the drawbacks of an high Z in the feedback path! This is a drawback of the inverting topology, per se.

Did you see, what Shoog has written in a posting, after converting to t-network feedback:

Quote:
I have tried the network on my VBIGC using 10K,100R,10K. I am very impressed the bass seems to benefit the most, been tighter and more integrated with the rest of the music. There are also improvements in the mid (cleaner and more pronounced), and the highs - more natural. A significant improvement over an already excellent amp.
Very good description of the result for my taste.

Franz
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Old 9th September 2004, 08:15 PM   #7
Franz G is offline Franz G  Switzerland
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Shoog asked:

Quote:
I have a question though. In my original implementation I just had the none inverted input to ground (to no significant ill effect). I have not added a 10K to this pin and have effectively used the none inverting pin as my effective small signal star ground. I am a little concerned that the feedback network, via the 100R resistor might be bleeding into the none inverting input and generating positive feedback.
Do I need the 10K to earth, or am I ok as is ??
Just measure the Bias at the output. When it is O.K. let things as they are.

With the recommended Rkomp in my excel sheet, you have the simplest solution, to minimize output bias.

The LM3875 chips are not optimized for minimal bias.

Not a good idea is (an engineer told me), to choose the Rkomp value by a trimmer pot and then replace it by an resistor. There are too much thermal issues involved, the resistors not acting the same like the chip.

When you want to minimize bias at the output, use servo drives.

Franz
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Old 9th September 2004, 10:28 PM   #8
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Quote:
Originally posted by Franz G
Not a good idea is (an engineer told me), to choose the Rkomp value by a trimmer pot and then replace it by an resistor. There are too much thermal issues involved, the resistors not acting the same like the chip.
I suppose you are talking about the resistor from NI to ground?
After playing for some days I adjust the M-T pot several times for 0mv DC.
Then I take it back, measure it, and change it for a resistor with an aproximate value.
Doing this I never have more than 5~6mv DC.
Even in my amp in my main system, it's what I have, and it works alot.
So, the "drift" is minimal, even considering that I used an aproximate value of what the M-T pot was measuring.
I don't consider or bother using servos.

Hey Franz, this is exciting, I will try this T-network on the amp, but I need some time...
I'm fiddling with some "new" chips for a preamp.
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Old 9th September 2004, 11:49 PM   #9
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I think people here may have some misconception about the use of a high value resisstor as the feedback element for an op-amp (or in this case a chip amp).

What you may acheive is reducing the size of the feedback resistor being used when you use a t-network for a similar gain/input impedance combination.

This may give the apearance of two benifits to your circuit design:

one: the lower feedback resistance will alow a higher bandwidth in extreem cases where parasitic capacitances are causing band limiting of the circuit. However, at audio frequencies I doubt that this should ever become a real problem which can't be solved with proper PCB design and circuit layout.

two: this is probably the one which people will talk about the most (but isn't actualy a real benifit, but leads to poorer performance), that the lower value resistors that you are now using in your feedback network are of lower values than before and so will contribute lower noise into the signal path. BUT, what is actualy the case here is that the t-network will lead to a lower overall noise performance for the circuit once you consider it as a whole. This happens because, as you increse the feedback resistor's value, you will be increasing the amount of noise it creates by a factor proportional to the square root of the increase in value (increase the vallue of resistor by a factor of four and the noise voltage it creates will go up by a factor of two). However, what most people will forget, is that whilst the noise is increasing, you are also increasing the gain of the device, so proportinately, the output signal will be getting larger compared to the noise you are creating by using the larger resistor (so in efect, due to noise goin up by a factor proportional to the square of the increace, and the output going up by a factor exactly proportional to the increace, the SNR will actualy be improved by a factor proportional to the square root of the increace in feedback resistor value).

Therefore, in this case (as with many others) it will always be better to use a single high value feedback resistor in place of a t-network (these are only of use in two situations, firstly if you can't get hold of resistors of suficiently high values; and secondly, if you need to increace the speed of the amplifier feedback and are willing to acrifice some SNR performanceto do so) when desiging the feedback around an inverting amplifier.

Anyway, I hope I haven't missed out too much here (I missed out the bit about noise gain when using the t-network, which means that you don't have the same increace in SNR performance as with a single resistor, but I could always try and explain further if people want, but there are some good data sheets on the ti website which cover the topic better than I can) and if people have questions I can try my best to answer.

Andrew.
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Old 10th September 2004, 07:01 AM   #10
Franz G is offline Franz G  Switzerland
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Andrew

May be you are right. I am not an engineer, but was consultet by one...

Two questions:

- did you try it with an LM3875, 3876 or 3886? If not, you should. Takes you less time than to write the text above

- Why is National recommending feedback resistors between 10-100K in the datasheet?

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Franz
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