Electronics > Repair
Audio… (Amps, THD, channel separation) with analog equipment (scopes, etc.)
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TimFox:

--- Quote from: vk6zgo on December 29, 2024, 10:23:41 pm ---
--- Quote from: CaptDon on December 29, 2024, 05:36:34 pm ---120vac line voltage is around 340vpp.  As to your amplifier 120vpp equal 60 vp x .707 = 42.5vrms and that becomes 225 watts rms with no head room or around 22.5 watts with 10dB of head room all assuming an 8 ohm load.

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There is no such thing as watts rms.
You can have rms volts, & rms amps, but when you multiply one by the other, you get "average power".

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This is one of my pet peeves:  when you start with rms voltage to calculate the power into an 8-ohm load, you get "mean" or "average" power, as you said.
However, there is such a thing as watts rms, it's just useless.  One could lie about the power of ones amplifier using rms power:  for a sine-wave it's (3/2)1/2 = (1.225) x Pavg.
CaptDon:
Yeah, yeah, I get it, RMS volts times RMS current isn't RMS power. The audio industry has used this term for so long it is ingrained in me. And the industry tends to use the term 'average' as an indication of the average amount of musical power being delivered to a speaker and we see speakers with ratings like 300 watts RMS 400 watts peak or 300 watts musical power 200 watts RMS and tons of other weird specs. My calculation maybe should simply be called 225 watts 'balls out' and 22.5 watts with enough headroom to not have the 'hammered shit sound'. I think I will propose that!! 225 watts B.O.  22.5 watts before H.S.S., yup, gonna start writing that spec on all the amps I repair from now on! Sadly, when we allow for 10dB of headroom for today's musical peaks a 1KW 'average' amplifier suddenly looks like a 100W amplifier with adequate headroom for quality listening pleasure. And what was that weird comment by a previous poster "10db headroom set by amplifier gain"? Oh well, it's all subjective to some degree. I don't think I have ever seen an audio amplifier with a rating like "100 watts average", they always state "!00 watts RMS per channel both channels driven" or something like that. I f--king hate these '7 pound 4 kilowatt' stated D.J. amplifiers that would shit the bed trying to do a mere 100 watts 'average' continuous sinewave!!! Cheers mates!!! I think I was playing guitar at '3 watts average' in church today, oh course that was an average guitar player playing an average guitar under average conditions.
TimFox:
When I first became interested in "high-fidelity" equipment (late 1960s), the industry went through a period of ever-more fanciful ratings of power.
Unfortunately, someone in regulation did not understand the meaning of "rms", which is well-defined mathematically, and imposed "rms power" as a legal description.
Before that, there was an imaginative definition of "IHF power":  since vacuum-tube amplifiers had ill-regulated plate power supplies (good regulation is not required for musical application), they allowed the manufacturer to substitute a stiff supply for the static test.  That made some sense.
Past that, some manufacturers went into full-scale lying:  specifying power "+/- 1 dB", which automatically inflated any power rating by 26% (unless you naively thought that was a normal distribution).
However, no manufacturer of whom I am aware tried to get away with using the actual definition of "rms power' to pick up another free 22.5%.
Flogging the dead horse:  "rms" means "the square root of the average value of the variable squared", and is very useful in electronics and statistics.
Alex Nikitin:

--- Quote from: Fried Chicken on December 29, 2024, 07:01:55 am ---The amp is completely anemic and shitty, not like it was before.  It completely lacks power, and the reason for that, I suspect, is the amazing Elna and Nichicon filter caps I replaced with mouser p/n: 598/383LX333M050B052

I hooked up an old sony amplifier thinking maybe there's something else that could be wrong (speakers, preamp, etc.).  OH BOY HOWDY WAS THE POWER BACK.

I'm pretty sure the filter caps I installed are garbage; luckily I still have the originals. Way to big to even test with my LCR meters.  I could charge it and hook up a resister and find the time constant and back calculate RC whatever, but I am disinterested.

I'm just going to put the old caps back in and cry that they're NLA new... manufactured with real pride, gold cursive lettering on black with FOR AUDIO prominently displayed across the front.

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1) Unless the old caps leaked or died in some other fashion (open circuit for example), or lost capacitance, or substantially increased ESR, I would be very reluctant to replace these, especially the power supply ones. There are some cases when a complete re-cap is advisable, but in general it is not a good idea, IMHO.

2) New capacitors need at the very least 24h under power before you could listen to the amplifier, some caps need a week or more. Nothing esoteric, just forming the isolation layer, with measurable changes.  Leave the amp powered up for a couple of days and then listen to it again, before reinstalling the old set of capacitors (which might be a good idea anyway, as long as these measure OK on the value and ESR). Ideally you should run the amp at 1/4 power output (at, say, 1kHz) into a dummy load for these 24-72 hours.

Cheers

Alex
bdunham7:

--- Quote from: Alex Nikitin on December 30, 2024, 07:58:42 pm ---2) New capacitors need at the very least 24h under power before you could listen to the amplifier, some caps need a week or more. Nothing esoteric, just forming the isolation layer, with measurable changes. 

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Do you mean measurable changes that you can observe (measure, with instruments) during amplifier operation or measurable changes if you had the capacitor isolated and were testing with an LCR meter or similar?  I'd be very suprised if you could tell the difference between a capacitor that has been "formed" for 1 minute vs 72 hours in either case, but causing gross changes in amplifier operation seems very unlikely.
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