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Old 17th November 2017, 06:27 PM   #1
Drummer 35 is offline Drummer 35  Spain
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Default lm3886 servo help

Hi all,

I want to draw a pcb of the 3886 using a dc servo, but I have a few questions before pulling the trigger.

IŽll be using the back plane as a power ground for power caps and zobel return, while the front will be the plane for signal ground. There will be pads on each plane so I can put a jumper or resistor in case I get hum or noise.

My question is if the ground at the DC servo net is considered as power ground, or being low current, could be put at the signal ground. R8 is signal ground too?

I plan to use a TL071 as I have some spares, but have read that the LF411 works better. Can I use both of them without changing components values?

How can I calculate values for R4, R5 and R6? I tried to find some info for that but had no luck on it. IŽll be glad if you could send me a link or explain me how to choose the right values for those.

I include the schematic. Saw this one on other thread, but added a zobel and thiele networks and a high cutoff filter at the input.

Please, if you find any error in that schematic let me know.

Thanks a lot for your help,

lm3886 servo schematic 3.jpg
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Old 17th November 2017, 07:10 PM   #2
AndrewT is offline AndrewT  Scotland
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Go to the very old Thread that discusses this in great detail.
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Old 17th November 2017, 07:13 PM   #3
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Drummer 35 View Post
.................. but added a zobel and thiele networks ....................
the Zobel is part of the 4 component Thiele Network.
The Thiele Network is an RC and an LR.
For some values one of the R values becomes zero ohms leaving just three components the LR and a C.
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Old 17th November 2017, 11:48 PM   #4
Drummer 35 is offline Drummer 35  Spain
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Thanks Andrew,

according to what Tom Christiansen wrote:

"There is much confusion about the naming conventions of these output filters. Technically, the RC series network is the Zobel network and LR parallel combination a Thiele network. Occasionally, the Zobel network is referred to as a Boucherot cell as well. In the schematic below, it would have been more consistent with the naming convention to use Lt and Rt rather than Lz and Rz2. As previously stated, there’s much confusion… I will do my best to limit the confusion from here on."

I am not saying you are wrong, I only need to know if that schematic is right and I can start working on it.

IŽll be glad if someone could send me the link to the thread regarding dc servos or explain a bit about how to calculate resitor values.

Thanks!
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Old 18th November 2017, 06:58 AM   #5
AndrewT is offline AndrewT  Scotland
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Tomchr and I have discussed this.
Neville Thiele proposed the stabilising network and it included both the RC (or C) and the LR.
Tom won't listen and as a result his website is still promoting the idea that the LR (alone) is the Thiele Network.
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Old 19th November 2017, 01:56 PM   #6
tomchr is online now tomchr  Canada
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Andrew, I'm perfectly willing to listen as long as you back your claims up with evidence rather than just passing them off as gospel. Just because you say something, doesn't make it right or valid.

You can have a look at Douglas Self, Bob Cordell, and Randy Slone's works and you'll find there's some debate about what the two reactive networks are called. I chose to put a stake in the ground and refer to the L||R as the Thiele Network and the R+C as the Zobel Network. Some refer to all four components as a Boucherot Cell. Others say the R+C is the Boucherot Cell. Yet others, call the R+C a snubber – and they'd be correct in their application of that term as well.

I am certainly willing to take another look at the naming conventions and the wording of my website (the Stability page needs an update anyway), but it's not a high priority at this point. Unless you provide compelling evidence for me to change my naming conventions, I'm going to leave it as-is.
The other thing to consider is what to do if the naming conventions are indeed murky and the topic of contentious debate. What should we do then? Does your word become law or can we agree to have some level of confusion and as long as we define our terms before using them, we can both coexist?

I'm perfectly willing to work with you on this. The ball's in your court.

Tom
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Last edited by tomchr; 19th November 2017 at 02:00 PM.
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Old 19th November 2017, 03:58 PM   #7
AndrewT is offline AndrewT  Scotland
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this may be where it is shown:
Thiele, A. Neville (1975). "Load Stabilising Networks for Audio Amplifiers," Proceedings of the Institute of Radio and Electronics Engineers, 36(9), pp. 297–300. Reprinted Journal of the Audio Engineering Society, 1976, 24(1), pp. 20–23.
I am not a Member so I can't look up the paper.

E.Cherry did a paper discussing the Thiele Stabilising Network in one of the Tech magazines and did a follow up a couple of years later.
I'll try to find the link. It's in a few posts I have made years ago.
There was a Forum download of an extract from the E.Cherry paper, but it would be more difficult to find.
I'll see what I can find.

Electronics World: Jan 1995 and July 1997
Here's the excel, see sheet Zobel
It was written in a very old MS version and does not always load up correctly, but usually legible.
Attached Files
File Type: zip electronicsrev1.xls.zip (371.0 KB, 12 views)
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Last edited by AndrewT; 19th November 2017 at 04:19 PM.
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Old 19th November 2017, 06:08 PM   #8
Drummer 35 is offline Drummer 35  Spain
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IŽm sorry, but I think that thread is deviating...

I didnŽt want to start a debate about how those networks are called.

I need to know if the schematic I attached contains any errors or if I can start working on it.

Thanks again.
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Old 19th November 2017, 09:51 PM   #9
tomchr is online now tomchr  Canada
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Quote:
Originally Posted by AndrewT View Post
this may be where it is shown:
Thiele, A. Neville (1975). "Load Stabilising Networks for Audio Amplifiers," Proceedings of the Institute of Radio and Electronics Engineers, 36(9), pp. 297–300. Reprinted Journal of the Audio Engineering Society, 1976, 24(1), pp. 20–23.
I am not a Member so I can't look up the paper.
Cited by four people, says Google Scholar. That's not exactly a sign of authoritative work. Just saying. In all fairness, it could be the reprint of the paper that's cited four times.

I'm contemplating becoming an AES member. If I do so, I'll take a look at the paper. Sadly, I don't have access to AES through the university.

Quote:
Originally Posted by AndrewT View Post
E.Cherry did a paper discussing the Thiele Stabilising Network in one of the Tech magazines and did a follow up a couple of years later.
I'll try to find the link. It's in a few posts I have made years ago.
I'd appreciate it.

Rather than continuing the OT discussion here, would you just toss me a PM once you dig that up?

Tom
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File Type: png Screen Shot 2017-11-19 at 3.48.14 PM.png (41.5 KB, 238 views)
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Old 19th November 2017, 09:53 PM   #10
tomchr is online now tomchr  Canada
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Quote:
Originally Posted by Drummer 35 View Post
IŽm sorry, but I think that thread is deviating...
I didnŽt want to start a debate about how those networks are called.
I agree. Sorry about that. When I'm being thrown under the bus in public, I do feel a certain need to stand up for myself.

Quote:
Originally Posted by Drummer 35 View Post
I need to know if the schematic I attached contains any errors or if I can start working on it.
At first glance, it looks pretty good. Note that C2 will cause overshoot on the step response. That can be fixed by adding an R+C across R3. This is shown in the LM3886 data sheet (Test Circuit #2 on page 5). I'd go with R4 = 10*R3.
You need supply connections on U3, U4. D1 is not needed if you size the mute resistor (R7) for 0.5-1.0 mA at the nominal supply voltage.

For information on how to design DC servos, have a look at this post: Is this a good LM3886 Kit? (Post #98)

Tom
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Last edited by tomchr; 19th November 2017 at 09:59 PM.
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