Testing Audio Amp Performance - Page 1
Forum section: electronics / general

author: metrologist
author_rank: Super Contributor
timestamp: February 07, 2025, 08:11:01 pm
content: I wanted to use some old car audio gear to piece together a stereo for my studio.I have a Forum FC-150 (150W) amp and an Evergreen MG-200 (200W) amp to choose from.How can I determine which one is a better choice?I was thinking of feeding in two mixed tones and comparing the input vs output at just below clipping. If that would even show anything, and what frequencies?Other methods?Or I could use both amps since the 10-band eq I'm using splits the sereo input between front and rear, plus it has a sub output and control.

author: themadhippy
author_rank: Super Contributor
timestamp: February 07, 2025, 09:19:47 pm
content: Quotewire em up to the speakers your gonna use and blast out your favorite piece of music

author: metrologist
author_rank: Super Contributor
timestamp: February 07, 2025, 10:18:59 pm
content: That's! Not very scientific.I was thinking of taking photos and opening them up. Would that matter?

author: thm_w
author_rank: Super Contributor
timestamp: February 07, 2025, 10:31:21 pm
content: If you are pushing them to the upper power outputs, hook up a load or dummy load and compare. If you aren't pushing to the limit, I would guess not much difference between them.https://www.eevblog.com/forum/testgear/measuring-thd-on-audio-circuits/https://www.eevblog.com/forum/beginners/how-do-i-measure-audio-amplifier-thd-using-my-gear/https://www.eevblog.com/forum/projects/audio-distortion-measurement-techniques/https://www.eevblog.com/forum/beginners/how-to-use-my-hantek-scope-for-thd/

author: themadhippy
author_rank: Super Contributor
timestamp: February 07, 2025, 10:55:53 pm
content: Quoteyou aint listening to the music with your test gear your listening to it with your ears,and its been the method used by the vast majority of sound engineers when choosing there toys.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 08, 2025, 12:48:34 am
content: I endorse the "wire 'er up and let 'er rip" suggestion.Who cares what a piece of test equipment says about the amp's performance?It's your ears, man, that will tell you whether it's to your liking or not.Two amps, so you can do an A/B test.

author: Shock
author_rank: Super Contributor
timestamp: February 08, 2025, 05:46:49 am
content: Audio analysis goes quite deep. It's best to research terms such as frequency response, total harmonic distortion (THD), signal to noise ratio (SNR), dynamic range, phase response, power output, dampening factor, impedance matching, room acoustics. Automated software tools can also help analyse/interpret the results.But with a couple of old amps it's hardly worth doing much aside from perhaps recapping your favorite.I'd look at which one was more expensive, well built and where it was manufactured. Consideration into design and quality component selection typically leads to better performance outcomes.The most important consideration to me of just using a car amp would be if the amp and speakers have the ability to reach a sufficient volume with adequate treble and bass before you have perceptible audio distortion.Home stereo amps typically have a higher power output (watts rms) than car stereos due to their power supplies and not being limited by space, voltage/current and therefore heat as much.You almost need a first lesson in power output to interpret the specifications and one amp can have inflated peak music power and another could be talking about rms per channel at a specific impedance and thd.So listening and trying different music at different equalizer settings and volumes is often more practical.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 09, 2025, 01:47:19 am
content: Most likely it would be the noise floor and not the maximum power that gets noticed first.

author: SteveThackery
author_rank: Super Contributor
timestamp: February 09, 2025, 11:09:33 am
content: You won't tell much, if anything, by looking at the waveforms unless there is something gross, like clipping, going on.Testing the performance of audio amplifiers requires some expensive test equipment: ultra-low distortion signal generator, and then kit which will measure the total harmonic distortion, etc. Then you've got intermodulation distortion, impulse response, etc.Also, it is all unimportant anyway because the loudspeaker performance totally swamps any minor imperfections in the amplifiers. I'm certain either amp will be fine.

author: Sorama
author_rank: Frequent Contributor
timestamp: February 09, 2025, 11:59:59 am
content: Quote from: metrologist on February 07, 2025, 10:18:59 pmMentioning the rated power without mentioning the load is not very scientific either…

author: metrologist
author_rank: Super Contributor
timestamp: February 09, 2025, 05:55:51 pm
content: Quote from: Sorama on February 09, 2025, 11:59:59 amThe load? They're not very heavy, but, ironically, one is 56 oz and the other is 53 oz, including rounding errors.I'm sure I'd test them with a four ohm power resistor, but the higher power unit lists two ohm min. and the other lists four ohm, 1% THD, 30-30k. I believe the general rule amongst automotive stereo systems considers a nominal 4 ohm impedance speaker, but I have seen one ohm impedance subs.I feel the aluminum is important here. Black. Black anodized. But now I remember having a gold anodized one. Ah, that might have been my 200W HF amp.I agree with the noise floor being a prominent concern, as well as the sound producing device dominating the equation. I was considering connecting them to my AR-4x speakers, but wanted something a bit more robust.Disclaimer: I've had both eardrums cut open and the inner material vaporized with lasers, and implants inserted. This was better than the alternative, but they're not very good at qualifying sound - quality.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 09, 2025, 08:04:35 pm
content: Quote from: metrologist on February 09, 2025, 05:55:51 pmWell then, there's your answer.If the goal is to suit your own listening requirements, then a listening test is plenty good enough, sans any test equipment.If you're trying to accommodate other people's hearing, then that's a different matter.Of course, you can always have someone else conduct a listening test.

author: Sorama
author_rank: Frequent Contributor
timestamp: February 10, 2025, 05:29:40 pm
content: Quote from: metrologist on February 09, 2025, 05:55:51 pmNot that load…The impedance of the load/speakers when measuring power!

author: Shock
author_rank: Super Contributor
timestamp: February 11, 2025, 12:46:29 am
content: In electronics, a load refers to a circuit or device that consumes power from another circuit or device. In the context of audio amp testing the load is typically used to describe whats connected to the amps output. Which could be speakers or a resistive load.To measure power output from an audio amplifier it is common to use a purely resistive "dummy" load instead of an actual speaker.A speakers impedance can change with frequency and signal amplitude due to their inductive and capacitive properties, so a purely resistive load is a more consistent measurement for baseline power.It also saves damaging your hearing as you can perform other full volume tests in silence.

author: metrologist
author_rank: Super Contributor
timestamp: February 11, 2025, 05:05:52 pm
content: looks like I forgot to type that I would use a power resistor for the test, but the most likely candidates are wire wound. Or maybe I can use a length of hot wire or connect it to my HO generator.I am warming up to the idea of just using the FAC-150 since it has more ink on the label.Also, I downloaded that ARTA software and have several mics with tested frequency response curves I could use.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 23, 2025, 10:22:12 am
content: Quote from: metrologist on February 11, 2025, 05:05:52 pmMore ink on the label? ? ? ? ?

author: Gyro
author_rank: Super Contributor
timestamp: February 23, 2025, 11:58:31 am
content: I have to agree with the others, do an A/B listening comparison. You're not going to hit the (genuine?) power limit of either amp, so it comes down to low level performance.Measurements are generally for when you're designing something, not choosing between two completed items (unless you're harboring ideas of modifying them).

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: February 23, 2025, 11:02:15 pm
content: Quote from: metrologist on February 11, 2025, 05:05:52 pmYou're already applying more science than 98% of audiophiles, well done.

author: metrologist
author_rank: Super Contributor
timestamp: February 24, 2025, 05:00:31 pm
content: LoLYes, the FAC-150 label provides frequency response of 30-30kHz and <1% THD, plus peak and RMS power.They wouldn't put that in writing unless they could back it up.But it is a car amp and not the best specs anyway. I mean modern amps (and speakers) seem to be about the kilohertz - like up to 50kHz...I remember when I bought this amp, it was paired with a set of Blaupunkt Velocity 650 component speakers, sized to match. I still have those and am considering making speaker boxes and use them as bookshelf speakers.

author: deadlylover
author_rank: Frequent Contributor
timestamp: February 25, 2025, 02:14:47 pm
content: Yeah if they go loud enough, next in line would be to check for hiss/noise at idle and for any turn on thump. (in case you want to incorporate auto standby or such)In this economy... idle power draw too haha.

author: metrologist
author_rank: Super Contributor
timestamp: February 26, 2025, 01:09:08 am
content: Now we're talking