Voltage noise in opamp inputs.. how much is too much?

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Hey Ted, can you give us some insight into current feedback? I believe you used this topology in some of your equipment, right?
 
Thanks for the test, Ted!

Using 3k3 feedback resistors and 10 channels, the noise attributable to the mix amp would be -106 dB (using NE33078)... Or have I got my sums wrong? It could be 6dB better than that.
I'd say it is just the -126.8 dB figure, no? 10 3k3 resistors in parallel behave like a single 330? :?

Samuel
 
Apologies guys.... I made a mistake in my 'tests' posting; the ic type that came out best is LM33078 (I said it was NE33078 which was wrong). For those who don't know, the LM33078 has conventional silicon front end, and a very simple 'lopsided' output stage; it operates predominantly in class A which is why I like it.
I can't agree with Svart; I think it sounds good, and use it everywhere.... but a discrete amp made of BC109s still sounds better!! :grin:

'Mediatechnology' asked about the feedback resistors in the test circuit... yes they were 3K3; low enough to perform well in a medium impedance environment, and high enough to avoid too much distortion.

Re current feedback.... I did already publish my balanced mix circuit here previously.
I'm not clever enough to find it and point you there, but perhaps one of you flash guys will do that. :grin:
 
woohoo! i'm Flash!!

http://www.theopiumdenproductions.com/balmix.jpg

and

http://www.groupdiy.com/index.php?topic=14512&start=15&postdays=0&postorder=asc&highlight=superbal
 
Hmmm.... I think you may be right Svart. I believe that the LM33078 has a simple lopsided output arrangement, so it would not respond well to being dragged towards one of the rails. used 'au naturel' I think it sounds nice. My philosophy is to avoid driving the chips higher than about 20dB gain; and where they are used as impedance changers or just followers, then there's nothing wrong with good old TL072s! :razz: :guinness:
 
[quote author="Svart"]I run all my opamps in forced class A, possibly a reason the 33078 doesn't sound as good for me..[/quote]

Svart, we've talked about this before - are you still using the simple resistor trick (ie. 10K or so...)? Or are you using any of the other techniques?

I wasn't sure whether to try it, as I thought it maybe seemed a bit like a hack.

Would you recommend it? Maybe I should do a blind a/b.
 
Hey I'm using the simple resistor trick. I tried many of the other types but they are largely cost prohibitive. You can indeed hear a difference between the forced A version and the normal version of a 5532 but it'll be personal taste in the end if you like it or not.

and I used 3k from outputs to -V
 
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