Noise of discrete opamp

[robmil]

Hi All

I in the past built a 990 discrete op amp and while good for line level I found if I substituted the AD797 in my Phonoclone for the 990 things just got noisy! The specs on the 990 say it has about the same noise level as a 797.

Are there any discrete op amp designs that have the 1nV/sq or lower for use in phonostages or mic preamps.

thanks Rob

PS. like the new diyaudio site.

[mctylr]

Just to ask a question to your motivation, not a blanket criticism, but why do you want a discrete (i.e. not IC) op-amp in a line-level (phono or mic preamp) stage?

I thought that most of the appeal of a discrete op-amp is better current (and thus power) handling capability for usage primarily in power amplifier.

-Michael

[robmil]

Hi

I just prefer the 'sound' of discrete op amps. In my line stage the 990 does sound better than a AD797. The 797 sounds 2 dimensional, cold with a very small sound stage. I see that most high end line preamps and mic preamps use discrete and the cheaper units use IC units like 797, 2019 - the usual suspects.

Rob

[peranders]

AD797 is a low noise device. 990 uses LM324 is pretty low noise so I think you have some problems which has to do with working conditions.

[peranders]

Here you have several discrete opamps
http://www.sg-acoustics.ch/analogue_...distortion.pdf The file contians 436 pages, beware.

[robmil]

Hi Peranders

when you say 'pretty low noise' and 'working conditions' do you mean for MC application the 990 just isn't good enough?

R

[peranders]

1 nV/Hz is quite alright for most MC cartridges. Is the noise more than the vinyl noise?

[mctylr]

Originally Posted by peranders

AD797 is a low noise device. 990 uses LM324 is pretty low noise so I think you have some problems which has to do with working conditions.

You mean an LM394 (super-matches transistor pair) which is decent low noise, but from a previous era (i.e. their now at life-time buy from National, and becoming more difficult to source). Not the LM324 quad op-amp with 0.5V/ns skew rate and 1MB GBW which makes it similar to a 741. You had me confused for a minute.

I agree the interaction with the existing circuit (I take it that is what you mean by working conditions) are not as necessarily matching the suitable circuitry for (clone or John Hardy?) 990C op amp to operate to its best ability.

I haven't look at the phonoclone circuit, or closely at the 990C datasheet, but some general recommendations would be to check input / output impedence expectations, see if there are differences in any compensation circuits if they are not both internally compensated, ensure that the existing power source meets the needs (voltage and current) of the discrete op-amp, which may have higher current demands, check for any voltage supply decoupling recommendations in the 990C's datasheet.

I'll admit I'm of the opinion that the idea of swapping ("upgrading") op-amps has gone from a useful idea (replacing an "poor" op-amp (e.g. 1st / 2nd generation IC op-amp) with a much better or improved version, to now randomly swapping op-amps for their "sonic qualities", and even switching between BJT and FET op-amp with no awareness of what they are doing.

I don't mean to criticize robmil, I'm just annoy with the general trend, not with any individuals, often amateurs or enthusiasts with little to no electronics knowledge, end up wasting their hard earned money buying a small horde of various op-amps, including flavors-of-the-week or latest and greatest "black magic" hard-to-find exotic op-amps available only from opportunistic vendors, to try to blindly modify their current equipment.

Good luck to robmil,
Michael

[peranders]

Sorry, LM394 of course.

[robmil]

Hi Michael

No offense taken. I don't have any hard earned money.... it's pretty easy for me to get hold of a lot it. I work and earn a lot, so whatever I wish to do with it is totally up to me

So what if I as well as others want to experiment? Don't we all? Is there anything wrong in swapping ICs? Is it hurting you..... NO!

I've built several mic preamps, modified them using some of my limited electronics knowledge and found them to be better afterwards. I've also built and modified a couple of phonoclones BUT couldn't figure out why the 990 would not work in the input stage section.... do you know why it doesn't?

Peranders - yes the noise level is WELL above the vinyl noise. I use an Ortofon PW Winfeld MC Cartridge.

Rob.