What is your experience with the law of diminishing returns ?

As subjective as it might be. Personally, I have not encountered it yet.
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Actually it is the opposite -- the more optimized your system the more apparent marginal changes are -- there's a law of increasing returns to tweaking
Rather than the oft used 'diminishing returns' I believe it's far better to utilize Ordinal Utility or/plus expected utility from decision theory ... at least in reference to our choices with respect to this hobby, and the OP's question.

+1 @folkfreak 
No experience so far. $10k power cord replacing $1k power cord with $5k preamp or power amp, anyone ?
inna"I have not encountered it yet."
That is because the components comprising your Music Reproduction System must progress beyond budget, entry level, mass produced equipment you have not yet entered the circle, realm, and world of true high performance, high end, tight specification audio.
Clearthink may be onto something.  Depends on where you snap the chalkline for diminishing returns.  And granted my 'why did I do this' with a $500 upgrade may be chewing gum money to someone else.  Then again, I realize that I come from a time when this hobby wasn't about seeing how high the price tag could go.  


30 years ago, I walked into a high end emporium and heard audio "holography", just recently I acquired it. This is the point of diminishing returns; it's impossible to put a dollar figure on getting where I am, that's because money and knowledge are interchangeable; I put a lot of time and study into this endeavor. Many will never get here, many more, like the fox and the grapes, will say "That's not where I want to be".

Once an audiophile gets here, the question is, how do I make it "Mo better"? And the answer is, "I need a pound of money".
You have to reconcile whether to spend big bucks on new speakers et al if you're 65 years old...  

In order for a question of this nature to have any meaning, reference points will have to be established; at least one well known reference point is the "Stereophile rating". I have subscribed to "Stereophile" for years, and I consider their rating system to be valid; for example a component that's rated "Class A" will sound pretty good, that I guarantee.

I try to buy the cheapest component that's rated Class A; I believe components that cost more are entering the land of diminishing returns.

In order to confirm this, I would need to buy more expensive components; until that happens, I can only guess whether or not this is valid.

Inna, if I was rolling in dough, I wouldn't buy a 10K power cord, and since I'm scientifically inclined, I would not even buy a 1K power cord; where's the science?
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So you are saying every time you spent more on something and did a comparison , the more expensive item sounded better ? One in a million odds of that happening . 
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For those of us on a budget - is the new thing REALLY five times better than what I have now?
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Elizabeth . It is in response to the ops question . I can only think he has tried a few pieces to have that experience .
All it took for me to figure out more money does not mean better sound was having the realization that a professional studio does not have such a component called a preamp . I visited a studio when my step father was recording a cd . I was in the control room listening and asked the engineer what preamp they used and he laughed at me , I was picking his brain about hifi for my system . He told me I needed to get a volume pot and I'm good. Took his advice and it has worked out quite nice .
There have been times I have tried preamps since that day 15 years ago . Tried a $7000 marantaz, trusty old volume pot sounded better . Handful of other preamps along the way as well . So glad I came to the realization on whats important to me . It allowed me to buy speakers that were out of my budget 
I think that the issue is in large part, how much and how do you want to teach your ear/brain mechanism to listen for?

Are you teaching your brain to be happy, or are you teaching it to discriminate among cables? At a certain point, sure, you are becoming more and more sensitive, but to what end?

The law cannot be applied if you got the wrong component, including cable, regardless of price.
The system doesn't have to be high end for you to hear even small differences, but it has to be good enough and very sensitive.
No experience or "brain teaching" is needed.

First climber: Isn’t this something?! Here we are sitting on the top of Everest. What a view!

Second climber: Yeah, the view’s OK but we’re only at Base Camp. ⛺️
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"law of diminishing returns" = wallet pain

Just wanted to clarify how I interpret "diminishing returns". Ergo, discussing generally a nebulous supposed universal phenomenon called "diminishing returns" with a subset of the population having a wide socio-economic distribution is fruitless, imo. 
 @david_ten Perhaps IMO less than PhD level Economics attainment, I find utility function science to be poorly defined.. because people ain’t all that rational

neverthe less , what is the Pareto optimal frontier between combinations of $1k or $5k power cords, ditto with amps, or perhaps a $6k amp w factory cord, or ( aghast ) a $6k power cord with gain....

ones mans optimal utility is anothers junk

‘need a pound of money’ 

$100 = 1g.

454g x $100 = $45,400.

this is not enough money to significantly determine where the law applies IMHO.

for someone with a bare bones rig, the cost won’t be a ton to upgrade.

for someone whose boasting a VAC & von S outfit with uber costly snake oil ladden cabling, the cost could be the same or immensely more.

its more about ‘synergy’ than anything else.

there is just too much stuff out there! the main issue though, is every account comes from someone with different ears, from differing settings, rooms, etc., and as such each account becomes ‘subjective’ to the ensemble and the auditioner.

my rule of thumb is much more meager because of a lack of funds and a lack of interest in becoming a new thingy junkie.

the obvious clue is within all the ‘too much stuff’ there is a ‘whole lot of different’ sounding gear, despite it being better or not.

which ‘different’ suits you best?

I’ve only found out  synergy is king. 

I’ve heard rigs that cost overall in retail speak, say 30k or so, vs those whose totals range towards 60K or more, and the one with the most synergy won.

. in this instance, the lesser of these two rigs got my attention and vote. each was very nice. in the cheaper outfit, the dAC/PRE AMP  and amps  were of the same brand. The less costly  rig simply sounded more realistic. tangible. 

since that event the owner of the ‘lesser system’ costwise, has been thru the mill bringing in and taking out a wealth of far more costly gear and I’ve not heard many of his rig’s itterations. but if the buy, try, sell, and repeat syndrome remains ongoing, there is a fly in the ointment somewhere. 

the best sounding rigs I’ve heard in friends homes or at dealerships have had one thing in common, synergy. same same power trains for sure. sometimes even the same digital device of the same brand as the power train.

the audio waters get really murky when we feel, and we often do feel, we know what will work best, or better and detour from going with ‘built in synergy’ and adopting the likewise makers devices into out riggs ass much as possible. albeit, money, or feasibility could be concerns there too.

Same same removes a ton of variables limiting the mystery usually but to loudspeakers. often simply buying a ’turn key’ outfit could be ones destination system. maybe. but then theres the ‘money’ issue again!

I’d need about 20 lbs. of $100s and a lot more paitience than I have to even scratch the surface in divining some sort of answer for this  question  still, it would of course be a subjective one at that.

does money correlate to better? quite often.

just ‘cause its expensive doesn’t mean it will improve things already on hand though. well, not everytime. nor does it mean you will realize evewry bit of its ability either if but that one thing is exchanged.. then it becomes about ‘optimization’ as much as it is about synergy.

visiting the 2019 Fla. Audio Expo which I posted about here, of the more than a dozen rooms, only a few clearly stood out above the rest IMO. curiously, in these few demos, only the wires and speakers differed from the source and power train brand.  one of the most expensive setups wherein it used different sources, cables yet the poewr train was the same brand, did not impress accordingly, again, IMO.

Good luck.   

Law of diminishing returns, thats for peasants and serfs who think of price of something, how crass and crude...!

Without any reference point, this is a tower of "Babel"; it's like asking the question; "How far is it from here to Chicago". Where is here? If we don't know that, what's the point of the question.

I suggested a reference point that I know is valid, and even if you disagree, I would hope that you at least know what it is; so far this is a totally meaningless conversation.
I don't understand the question asked, so if the OP could please elaborate a bit that would help.

If the question is about the direct correlation of performance plotted against price and the point where the performance curve begins to flatten against increased price, I stay away from that transition. I neither have the funds to spend in that region nor do I feel the need to waste valuable resources to gain such self-convincing, minor, improvements.
When I first started upgrading, I could hear the difference.  However, I couldn't always tell if the upgrade was an improvement, just different.  Eventually I learned.

I hit diminishing returns when I couldn't hear a difference after an upgrade.  But there was still a  cumulative effect from multiple upgrades -- like I couldn't hear differences each time when upgrading amp, preamp, and cables, but after all three upgrades, I could hear the sound was better.

So I kept upgrading until I got to a level where my ears/brain had stopped learning and I really couldn't tell any more.
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You guys gotta check it out. Low mass inexpensive systems are where it’s at. No more house AC, AC ground, no more speakers, speaker cables, amplifiers, preamps, interconnects, digital cables, fuses, no more transformers. No more distortion. No more noise. No more pencils, no more books, no more teacher’s dirty looks. 🧐
A different perspective...
Twenty years ago, when I had better ears and a less expensive audio system, I made a visit to the local high-end store. I had $5k invested in used equipment (ARC tubes, Soundlab Dynastats, HGS sub). Listened to my reference music on an $80k new system (Martin Login top of the line bi-amped with four Krell monster mono amps, etc.).

The higher end system definitely sounded a lot better. The 3D soundstage really stuck in my mind. After an hour of listening I walked out of the store with a big grin on my face thinking I didn't really hear $75k of difference.
Cables and power cords are components and as such they can sometimes give as much sound change as active components.
However, speakers might not be good enough, so whatever you replace in the chain give about the same magnitude of change.
Or, your system is a total mess, completely in disarray, and whatever you do it won't matter much - same mess.
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I think that Elizabeth has it right in terms of marginal cost / marginal gain curve.   One might quibble about price point (not a lot).

Then the owner’s financial condition as well as sensitivity to the curve must be factored in.

For myself, I lust after the new 30.7 Maggie’s, but a house downsizing leaves me a room 23 x14, not quite large enough. At the end of the day, I love where I am I terms of my system, but we always see the greener grass smiling at us, LOL.

Also, often lost are the relatively inexpensive tweaks, I.e. room treatments, etc.
Reminds me of the old line, "What's the difference between a $2.00 cigar and a $10.00 cigar?"  Answer:  "$8.00"
One knows then the good old diminishing returns (more money than sense is a better term) has truly set in when one's stereo is to good for Redbook CD's........haha
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I have done what you said in your first post and never have looked back and yes it was worth it.
methinks I could spend about $6000, and for my ears I would have a real kick as Hi Fi system to satisfy my needs.
So, $6001+ is my price point of diminishing returns.

I had musical training for ten years, starting at age 8, so I believe that I have a pretty good ear.
Last year, I had to make a choice between the Magnepan 1.7i's and the 3.7i's.  The 3.7i's are more than twice the price of the 1.7i's.   I auditioned both models at Take5 Audio in New Haven.  While the 3.7i's had a bit more bass extension, I actually liked the high end of the 1.7i's more.  In my opinion, the difference in the sound didn't justify spending more than twice as much.  I've had the Magnepan's for a bit over a year, play vinyl and SACD's, and I'm very satisfied with them.  There is a point in high end equipment where the equipment is so excellent that spending a hell of a lot more doesn't improve the sound appreciably.