Gainclone / chipamp datasheets - newbie questions - please help!

[johnalexwarren]

Side-stepping from my previous posts I'm trying to gain (excuse the pun) a better understanding of the circuits involved before I embark on any more projects. As a bit of background, I am not new to hobbyist electronics as a whole, having had an active interest in it since childhood, but the audio/amplification side is relatively new to me.

I'm looking into building an amp (or amps) based around National Semicon's Overture range. Browsing through the data sheet for these chips, for example the LM1875, 2876, 3886 etc has raised a few questions.

1) In most cases, the data sheets general detail two circuits - 'Typical apllication' and 'Single supply application'. What exactly is the difference between these two circuits and why do they supply both? The 'Single Supply' applications invariably appear to be more complex.

2) Many of the data sheets also give PCB layouts, for example the LM1875 (which I am planning as my first build). Normally these come in two flavours - single supply and split supply. What does refer to exactly?

I would assume it's related to the physical configuration of your PSU, i.e single supply referring to one complete transformer/rectifier/smoothing ciruit per amplifier module and split supply referring to one complete supply circuit split between two amp modules?

3) Leading on from question 2, having done a lot of research into PCB layouts for these amps, I've noticed that the component's values, and/or physical position in relation to the actual integrated circuits, can quite often differ considerably to those specified on the data sheets.

Why is this? Is this simply the hobbyist's own interpretation of the circuit based on their own knowledge? Surely the circuit schematics and PCB layouts advised by the manufacturer would be optimum? Or is that not the case? Are those schematics supplied as basic building blocks, there to be improved? In a nutshell, would the results from National Semicon's schematics be satisfactory to the average audiophile?

If anybody can shed light on any/all of my questions I would be eternally grateful!

Cheers as always!

John

[richie00boy]

Not quite. Split supply is a plus-ground-minus supply, the normal type for audio. Single supply is just positive and ground and uses a capacitor coupled output. OK the input is capacitor coupled as well, but this is normal practice even on a split supply amp as it protects the amp from DC in the source equipment.

[johnalexwarren]

Originally Posted by richie00boy

Not quite. Split supply is a plus-ground-minus supply, the normal type for audio. Single supply is just positive and ground and uses a capacitor coupled output. OK the input is capacitor coupled as well, but this is normal practice even on a split supply amp as it protects the amp from DC in the source equipment.

Ah ok. A split supply is the type I've already researched and built. 1 question down, 2 to go!

Thanks Richie!

[richie00boy]

OK question 1 if you couldn't deciper it from the answer already given! The single supply amp is a little more complex because it has to provide additional bias voltage to the input and DC blocking on the output.

[Redshift187]

Question 2 appears to be the same as question 1.

Question 3: Yes, it is the PCB designer's interpretation. PCB design is an art, and often the manufacturer's design can be improved upon. Designs are always a compromise of some sort, and perhaps the person designing a PCB has different compromises they wish to exploit (cheaper components, less components, exotic components, etc). Sometimes their opinion on what is important is not the same as someone else.

[abraxalito]

Originally Posted by johnalexwarren

3) Leading on from question 2, having done a lot of research into PCB layouts for these amps, I've noticed that the component's values, and/or physical position in relation to the actual integrated circuits, can quite often differ considerably to those specified on the data sheets.

Why is this? Is this simply the hobbyist's own interpretation of the circuit based on their own knowledge? Surely the circuit schematics and PCB layouts advised by the manufacturer would be optimum? Or is that not the case? Are those schematics supplied as basic building blocks, there to be improved? In a nutshell, would the results from National Semicon's schematics be satisfactory to the average audiophile?

The manufacturers and the hobbyist usually have different goals, and hence the application schematic might not be the optimum from the hobby point of view. The manufacturer by and large is interested in presenting the component in the best possible light vis-a-vis the specs - hard facts, numbers. Normally the hobbyist is more concerned about the sound.

As regards applications data, its of variable quality. I've heavily criticized on this forum National's amplifier paralleling appnote, as it contains various errors in relation to component values and tolerances. So to deal with your last question - the audiophile is probably going to pay more attention to details than the apps guys at National, and this will be reflected in the designs. In some cases it seems clear that the apps guys have read things on this forum and incorporated them into their writings - here I'm thinking of CarlosFM's snubberized power supplies.

[johnalexwarren]

OK question 1 if you couldn't deciper it from the answer already given! The single supply amp is a little more complex because it has to provide additional bias voltage to the input and DC blocking on the output

I suspected the two schemtics and PCBs were related, but thanks for confirming!

Question 3: Yes, it is the PCB designer's interpretation. PCB design is an art, and often the manufacturer's design can be improved upon. Designs are always a compromise of some sort, and perhaps the person designing a PCB has different compromises they wish to exploit (cheaper components, less components, exotic components, etc). Sometimes their opinion on what is important is not the same as someone else.

The manufacturers and the hobbyist usually have different goals, and hence the application schematic might not be the optimum from the hobby point of view. The manufacturer by and large is interested in presenting the component in the best possible light vis-a-vis the specs - hard facts, numbers. Normally the hobbyist is more concerned about the sound.

Thanks for your answers guys. Herein lies my problem which is that my knowledge of electronics is almost purely practical. By that I mean that I understand components and what they do, I can translate a schematic over to a physical PCB layout, and I can and have successfully built and tested DIY projects from kits and schematics. However, ask me to actually design more than a simple circuit or alter an existing one, an amp for example, and we reach the limit of my abilities!

So as far as amp projects go, I guess my best bet it to use the layouts which already exist by kind virtue of forum members and website owners. I'm sure after a few builds and a little more research I'll better understand how the circuits work, how the values of the components affect the performance of the amp and how they can be improved.

Thanks again for your help gents.

John.