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Old 12th October 2013, 11:05 AM   #51
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by piggybladder View Post
..............Most designs have a zobel at the output which I omitted for 2 reasons. Firstly it wasn't the designer of the pcb's intention but also I came across some designs where the designer added a zobel and then removed it because it had a deleterious effect on the sound. Likewise most design have a 220pf cap across pins 9 and 10 for rf attentuation. I also omitted this primarily because I wanted to see if I could hear any rf on the circuit as it stands, which I can't. If you want to add a zobel the followinglink may prove helpful.

IC Audio Power Amplifiers and Zobel Networks: One Size Does Not Fit All | EE Times
You live dangerously, or rather you let your amp live dangerously.

The EE times article is discussing the effect of a speaker Zobel fitted at the speaker and the effect that combination has on the amplifier.
That is a very different situation from the amplifier output Zobel that is provided at the amplifier to provide a defined HF load that aids amplifier stability.
Fit an amplifier Output Zobel. National define the components for you.
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Old 12th October 2013, 11:32 PM   #52
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Quote:
Originally Posted by AndrewT View Post
You live dangerously, or rather you let your amp live dangerously.

The EE times article is discussing the effect of a speaker Zobel fitted at the speaker and the effect that combination has on the amplifier.
That is a very different situation from the amplifier output Zobel that is provided at the amplifier to provide a defined HF load that aids amplifier stability.
Fit an amplifier Output Zobel. National define the components for you.
To be clear, the output zobel you are referring to is the 4700uf electrolytic, 0.7 mH inductor and 10ohm resistor shown at the output of pin 3 in Fig 3 of the National datasheet, yes?
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Old 13th October 2013, 12:13 AM   #53
gootee is offline gootee  United States
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The Zobel network is a low-value resistor and low-value cap in series, from output to ground. It should be connected as close as possible to the amplifier output, and directly to the load ground.

The Zobel network is sometimes also called a Boucherot cell, although I have usually seen the Boucherot cell referred to as the parallel inductor and resistor that are in series with the output, combined with the Zobel network from output to ground.

By the way, it is not a good idea to wrap the inductor's wire around the resistor, to make the low-value inductor that goes in parallel with the resistor.
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The electrolytic capacitors ARE the signal path: http://www.fullnet.com/~tomg/zoom3a_33kuF.jpg

Last edited by gootee; 13th October 2013 at 12:22 AM.
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Old 13th October 2013, 01:11 AM   #54
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Quote:
Originally Posted by gootee View Post
The Zobel network is a low-value resistor and low-value cap in series, from output to ground. It should be connected as close as possible to the amplifier output, and directly to the load ground.

The Zobel network is sometimes also called a Boucherot cell, although I have usually seen the Boucherot cell referred to as the parallel inductor and resistor that are in series with the output, combined with the Zobel network from output to ground.

By the way, it is not a good idea to wrap the inductor's wire around the resistor, to make the low-value inductor that goes in parallel with the resistor.
Thanks Goatee. I was trying to work out how Andrew T's mods at post 22 to the XY pcb and schematic at post 21 relate to any of the schematics in the National Datasheet. In fact I was looking at fig 3 of the TI datasheet which = fig 2 of the national datasheet.

Think I have it more from Rod Elliott's 3886 schematic. If I have understood this correctly R6 on Rod's schematic = R1 of the aformentioned pcb, (and in my build at least needs to be changed in value to 2R7). C4 on Rod's schematic is the cap which Andrew would have soldered onto the XY board in place of the speaker in/out block terminal and it returns to the XY pcb 0V line. Andrew T would then tap off a line to the speaker from the top of R6/R1 and add a paralelled inductor and resistor in series to the speaker input returning the speaker output to MAG.

A picture speaks a thousand word so I have attached Rod's original circuit and a version of how I understand it relates to post 21 and 22.
Attached Files
File Type: pdf Zobel Thiele.pdf (143.3 KB, 136 views)
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Old 13th October 2013, 02:55 AM   #55
gootee is offline gootee  United States
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I think that the schematic would stay almost the same, except for the addition of a 220 pF-or-so cap in parallel with R5 (R1 on Rod Elliot's schematic), the 22k input resistor.

AndrewT was talking about modifying the PCB, in order to accommodate the schematic. i.e. In order to add the L1 and R5 on Rod's schematic to the PCB.

I think that what you have drawn is correct. But when you say "speaker tap off", it's a little confusing. The speaker will still connect to where it says "OUT1" on your schematic, which will now be a jack mounted in the chassis. The parallel R5 and L1 in your schematic will be dangling in the wire between the output and the speaker jack.
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Old 17th October 2013, 05:33 AM   #56
sudheer is offline sudheer  India
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Hi,
You are looking for ready PCB, or want to make your own ?
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Old 11th November 2013, 03:55 PM   #57
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Originally Posted by sudheer View Post
Hi,
You are looking for ready PCB, or want to make your own ?
Sudheer, after a layoff doing other things, I'm about to make a couple of my own Thiele output network pcbs along Greg Erskine's lines. If they seem okay, i.e. the amps are stable and without any noticeable sound degradation I will try and do the Andrew T mods to the XY boards, if my hands are steady enough (I have a disability which pretty much limits what intricate work I can do). Any hints/tips gratefully received. Thanks.
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