The HiFi chat, build and modification thread!

A very rough and dirty explanation of why Op-Amp slew rate matters - a lot - if accuracy is important to you.

So to add measurement to the above, I took this...



...and did this. This is a 20kHz sine wave. A not particularly tasking signal.



This is the overall picture, difficult to measure with accuracy, so zoomed in...



If you rear the measurements, it can be seen that on the steepest part of a sine wave, to reach 1V (the standard metric for measuring slew rate) it takes 0.5uS. If an op-amp requires 1uS to reach 1V, it cannot reproduce that waveform accurately, and will chase the slew rate as hard as it can producing a sort of triangle wave with a rouded off top.

Ergo, some op-amps are not suitable for audio, but will certainly sound different!

Sorry for boring you, but it came up in email, and I thought a bit of detail on the subject would be useful in dispelling some audiophile mythology about certain op-amp types.

I'll shut up now!

Just noticed the 'saved' date - think the bios battery in the Agilent needs changing!

I had read in forums that diff opamps sound different, and took the plunge about 1 1/2 years ago with the 111.......and it does sound better than the other opamps i had, more anologue was one discription, and i would agree. Just as long as it's safe to use, i see it as, some people prefer vinyl as it adds a certain dirty back ground that can make some music more listenable.........was it sony or someone who tried a system or putting certain background noise in a digitol system trying to sound more easy and anologue ?............perhaps it's something to do with why we like guitar distortion and effects......sometimes a little distortion can add a certain atmosphere or a bridging / blending of sounds our ears like ?

Strangely the main reason i used the 111 was because i found my treble to clinical, too sharp, a bit ovr the top, .........but the 111 also spreads the music apart a bit and makes it more wholesome some how, and less , on or off with instuments comming in and out of the sound stage.........it certainly is deatailed and has a bit more depth/warmth.......but yes it looses a little focus in the highs, but i would say 'soften', but with this kit it seems to be doing it a favour, perhaps it is making up for the shortfalls of my system as it is ?.......maybe a better CD player handle stuff better.....

It will certainly cut HF above about 10kHz as the reduced slew rate means HF transients just wont make it to the amplitude intended by the source due to the slow response, though anything above 10kHz will also be progressively more distorted so maybe the lower amplitude is a good thing! The effect may not be unpleasant - it sort of depends on the harmonic spread and structure of the distortion products.

Your description of the sound is probably what I'd expect, the blurring of sound stage, softening of highs etc. However, two wrongs don't make a right, and you maybe right about source equipment.

I would perhaps take issue with the statement that 'different op-amps sound different', and qualify it by saying op-amps used wrongly or outside of their design parameters (back to the OPA111) sound different. Several different op-amps with similar specifications used in a circuit which operates them within their correct parameters will sound as near as damn exactly the same. I'd be quite happy to bet money on this, when done under double blind conditions and repeated enough to be statistically significant.

It all comes back to whether you want a 'pleasant' sound, or whether you want to hear exactly what is on your CD. Personally, I have to aim for accuracy, as anything else is too hit and miss. Messing with that accuracy such as adding noise to make a high resolution digital medium sound like an outmoded, noisy, low dynamic range, high distortion medium just sounds crazy to me.

I've had a good listen tonight with the AD8610's as i damaged one of my 111's and if I can just sort that hum thing out, i think just shaving off highs with these in , might just make me forget the 111's

OH by the way i looked inside my silly special cables today, the ones that are meant to be cryogenically treated............there's 4 little braided wire , THAT'S IT.......then seperate to two in each pos and return, they seem to have no insulation just an outer covering 4 bare looking braided wires.........hows that work then, surely it should short ???..........if there is a coating it's nats thin :shock:
The pic does not show that you can see the wires through the sheeve stuff......thought pos and return can't touch ?

Yeah, that's a very naff cable - the insulation could be varnish (like transformer wire). Different for the sake of it. Bet the capacitance is sky high...

I prefer a screen rather than some bullsh*tty twisted pair which does precisely nothing to reduce noise in an unbalanced input.

The AD is a much better bet for audio. Very low input noise voltage, much better slew rate (if it doesn't sound bright, then there's nowt wrong with your source components!) but be careful how you use it as it *could* be unstable because of it's very high gain bandwidth and very high slew rate in the wrong application. I imagine it will need to be used in circuits with HF cutoff filtering at the input (properly designed stuff will have) or it *could* oscillate at RF. You'll soon find out as it will get damned hot, and op-amps generally dont.

You're on the right lines with that one though - just don't fall for the audio forum mantra that expensive is always best. They are definitely off the mark if they think that the OPA111 is 'better'!

You wanna get some decent cables...

Dont start. A wire is a wire, whatever carbon-fibre-unobtanium-polarised-cryogenic-99 carat gold you make it out of - unless you particularly feel like changing some laws of physics?

cryogenic...

...damn I love the people who fall for that b*ll*cks!

Almost as much as those who spend a small fortune on a 'fancy' mains lead then plug it into a socket in the wall...

I made my cables for myself; did think about marketing them but there's just so much rubbish out there touted as the perfect cable that I just couldn't be *rsed! Even though mine are about as good as you can get

The sound engineers I work with assured me that once I'd got past the age of 19 all the technobabble is just so much scotch mist; it's a very subjective subject; and that's the main reason that even the biggest manufacturers shy away from 'blind' comparison tests...

...very unscientific I know but I tweaked my designs using the thunder at the beginning of Dire Straits 'Brothers in Arms' as a benchmark; for years I never even knew it was there!

I also have to 'beg to differ' on your 'a wire is a wire' assertion; I've made three versions of my cables...

...one using plain copper wire, one using silver-plated copper wire, and another using pure silver wire; and they all sound different.

I also use only copper and silver in the construction; none of this 'lump of solid brass with a thin plating of silver or even (Gosh!) gold' tacked onto the end of some 'cryogenically treated' OFC copper...