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Nordic 7th November 2006 11:15 AM

Inverted vs non inverted what is the whole story...?
Ok so while I had no communications for a week I built the datasheet NIGC with an LM3875... testing on one speaker sounded okish but when I tested in on my main speakers it sounded reaply plastic to say the very least.

Weak bass, overemphasised top etc... Wanting to finish the amp now and get to the next stage I took the soldering iron to the stripboard and quickly wired up NUUK's IGC.

Now I'd be the first to admit Mauro's My_Rev puts the IGC to shame (appart from amount of bass), but the diffirence between the IGC and NIGC were even bigger.

I'm sure I read somewhere that inverted and non inverted topologies affects the bandwidth, could this be the reason?

I'm aware of the fact the gain in theory should be affected by a pot on the source, but thats exactly what I did, 10k Alps and --- no harm done - NUUK's IGC still makes the datasheet NIGC sound like a fart in the wind at both extremes of the pot (sorry neighbours).

I'm so put off non-inverted gainclones now its not funny.

Have any of you had good results with non-inverted setups?
What is your preferance?

I didn't touch the PSU or anything connected to the power pins during the swapover, only changed signal and feedback resistors and caps.

Nordic 7th November 2006 02:57 PM

On further investigation, it seems there is a problem with the woofer in the one speaker....

AndrewT 8th November 2006 10:56 AM

Hi Nordic,
does that mean you have solved the problem or are you still looking for an explanation or solution?

Stuey 8th November 2006 11:37 AM

Nordic, are you saying that the NIGC has more bass than the Rev C, or that they are similar in this area?

Nordic 8th November 2006 06:34 PM

I'd still like your experiences, as there is a difirence between the 2 setups soundwise...

And yes, I found the problem, time for new speakers :(

I would say there is more bass, but with much less control. I.e. not as tight as the Rev C bass.

AndrewT 9th November 2006 07:10 AM

I think the difference in input impedance may be a major reason for what you are hearing.

The non-inverting usually has a highish value of resistor from line to ground setting the input impedance//with stray capacitance//RF filter//input pin impedance. This resistor usually matches the upper leg NFB resistor.

The inverting input has an inline gain setting resistor that feeds into a virtual earth at the -ve pin of the amp. You cannot usually fit an RF filter here The other impedances in parallel are , I believe, smaller than on the non-inverting side, so they may have a bearing (but smaller) on the total effective impedance seen by the source.
However, the BIG difference is the usually small value of in line resistor. I think this will have a profound effect on how the source copes with the load.

jackinnj 3rd December 2006 01:04 PM

the low input impedance of the IGC makes it imperative that you be careful with the wiring. i was playing around with one yesterday -- and it took up more time than I would devote to a small project -- I got the thing to oscillate at 1.2 MHz with only 10% THD ! OTOH, when you are careful with the wiring an IGC should have THD distortion ~ 0.006%

Nordic 3rd December 2006 01:22 PM

was the effect audiabe?

Nuuk 3rd December 2006 02:49 PM

For the record, "Nuuk's IGC" is actually a design drawn up by Thorsten (aka Wang Chang Thankyou man)! :att'n:

Nordic, I'm currently listening to some OPA541 non-inverted chip amps and they sound gorgeous (bass included)! ;)

Nordic 3rd December 2006 04:04 PM

Lol Nuuk, it may be his, but until he builds a dungeon, too bad for him.

Just using datasheet schematic, or something you made yourself?

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