Why is L/R often left out?

[TDWesty]

While I've read many interesting threads in the search for my oscillation problem (BrianGT chipamp, single 37VDC supply), I haven't seen much mention or use of the optional L/R parallel output filter as described in the National tech notes. In fact, this design is the first one I can recall which uses the optional filter on the output.

Most designs I've seen, including the one I am using, include the R-C Zobel. But why not the L/R filter? Is it simply to reduce component count for something probably not needed? Might it help solve oscillation where the pre-amp feeding the amp is introducing the oscillation somehow? I'm still trying to understand this thing a bit better.

I know now that my NAD 3020 preamp causes my amp to oscillate. It could well be a mod that I did on it (and I'm thinking of reverting back to stock to find out). But I would also like to understand this better. My house is probably full of RF (at least 4 PCs on at any time, lots of network gear, 2.4G wireless, cellular, etc.), but I really don't know if this is a factor or not.

I do like the sound of this thing - I've been listening to it most of the day on a PC sound card playing the Artur Rubinstein collection, and it sounds quite good... but I wish I could hook it to my main system without it melting down from oscillation! Perhaps I should just get a selector and volume pot, and skip the pre-amp...

Any thoughts on the filter are welcome :-)

[I_Forgot]

In my case, the L||R filter is there to ensure that the amp doesn't oscillate when I connect electrostatic (or any other speakers) speakers. That amp has been in daily use with my ESL-63s and some ESLs of my own construction for about 5 years now. It has never "complained".

I learned a long time ago that a working amp with an L||R filter sounds a lot better than a dead amp and/or speakers due to amplifier oscillations. Why don't others use it? I guess some people have to learn that the hard way.

I_F

[Juergen Knoop]

In fact, this design is the first one I can recall which uses the optional filter on the output.

I used it in all my LM3886/LM3876 amplifiers. I have also seen it in published circuits. But this was at least ten years ago, before the internet discovered the LM and the hype was founded.
Perhaps it has something to do with the gain clon(ing). Someone starts with a specific design and others just copy it.

[TDWesty]

Thanks to both of you - I will search for the needed parts. How critical are the values of the inductor? I assume the resistor should be 1W or 2W? (the zobel resistor I am using is 2W).

[Juergen Knoop]

I used a 2W metal oxide resistor
don´t have the original instructions at hand, but in a different article they recommend:
15 windings of 0,65mm diameter wire on a 5,5mm drill
seems not very critical
Regards

[Juergen Knoop]

of course the drill has to pulled out afterwards, as it is meant to be an air inductor...
I have just looked at the amplifier, I wound the inductor with a larger diameter, app. 8mm instead 5,5mm, and placed the resistor inside

[TDWesty]

Originally posted by juergenk
I used a 2W metal oxide resistor
don´t have the original instructions at hand, but in a different article they recommend:
15 windings of 0,65mm diameter wire on a 5,5mm drill
seems not very critical
Regards

Thanks for this info. Tonight, I found a 2W 10ohm resistor, and wound a coil using 24ga cat5 wire around a 1/4" drill bit, with 15 tight turns. ran the resistor through the core, then added to one amp speaker output.

No luck. :-(

With my 3020 preamp back in the system, the amp with inductor still climbs from 20C to 40C at the heatsink within 4 minutes. With no other change except swapping source back to a PC sound card, the temp drops to 35C within a few minutes, and will stay below 40C for hours.

I can't understand what the 3020 is doing to cause such problems. Perhaps I should just start reversing all the mods I did on it... (basically all the pre-amp mods here, except bypassing balance/mute)
I also have only replaced a few of the dozen or so caps on the regulated PS board for the pre-amp, and have read this board is sensitive to changes in values... I wonder if this could be it? Voltages are a bit higher than spec at +31.5/-26.5 compared with +29.9/-25.6)

I think I'll undo the mods, since I wasn't sure I could hear any change anyway...

[Eva]

The oscillating piece of equipment is very likely to be your modified preamp rather than the chip-amp. High frequency stuff coming out of the preamp is probably what is disturbing the chip amp. You should use an oscilloscope to investigate the problem.

[TDWesty]

Originally posted by Eva
The oscillating piece of equipment is very likely to be your modified preamp rather than the chip-amp. High frequency stuff coming out of the preamp is probably what is disturbing the chip amp. You should use an oscilloscope to investigate the problem.

You are absolutely correct! I just finished un-modding my 3020 back to stock, and the first power up confirmed this - the power on/off 'click' is gone, and temps are now stable at ~32C at low levels. Excellent! :-)

It is interesting that the 3020 power amp section handled the HF fine, presumably the design is 'safer' to deal with problems like this, although I doubt they anticipated newbies like me messing with BEE's design 30 years later... ;-)

Thanks to all for the info on this little wild goose chase - now I can get back to finishing the amp. I am considering adding a pot, and a source selector switch for 4-6 inputs. Any tips on sources for these at decent prices? I can't seem to find suitable stuff at digikey, but I'm probably not looking in the right places...

[I_Forgot]

Originally posted by TDWesty


You are absolutely correct! I just finished un-modding my 3020 back to stock, and the first power up confirmed this - the power on/off 'click' is gone, and temps are now stable at ~32C at low levels. Excellent! :-)

That's the problem with getting modification info off the web - you don't know if the person doing the mod and telling you how to do it actually knows what they are talking about. It becomes a case of the blind leading the blind. You are lucky that it didn't destroy your amp and/or speakers (this time).

I've never understood this "mod culture" stuff. It is one thing for engineers to look at a circuit they understand and try to figure out how to make it better, but where do people who know little or nothing about circuits get the idea that they can start clipping out "extra" parts and swapping op-amps and end up with something better? The mind boggles...! I wonder if people do the same thing with their cars, motorcycles, cameras, etc.

I_F