Equipment Performance During Warm Up

[March Audio]

@ENIGMA raised an interesting question regarding the performance of DACs during warm up and their long term stability.

 

So, I am performing a test using a Topping DX7pro DAC to investigate.  The DAC is playing a 1kHz tone at full scale and an FFT is taken at time periods to measure its signal level, noise floor and harmonic distortion characteristics.

 

0 minutes (cold)

 

5 mins

 

10 mins

 

45 mins

 

70 mins

 

180 mins

 

The test is running overnight. I will take another measurement tomorrow morning and comment on the results, but you can already see there is negligible change.

 

15 hours

Edited August 10 by March Audio

[Cloth Ears]

11 hours ago, March Audio said:

@ENIGMA raised an interesting question regarding the performance of DACs during warm up and their long term stability.

 

So, I am performing a test using a Topping DX7pro DAC to investigate.  The DAC is playing a 1kHz tone at full scale and an FFT is taken at time periods to measure its signal level, noise floor and harmonic distortion characteristics.

...

The test is running overnight. I will take another measurement tomorrow morning and comment on the results, but you can already see there is negligible change.

Negligible, but measurable. I wonder why the 3rd, 5th and 7th seem to increase, while the others either don't change or are randomly variable.

[March Audio]

49 minutes ago, Cloth Ears said:

Negligible, but measurable. I wonder why the 3rd, 5th and 7th seem to increase, while the others either don't change or are randomly variable.

 

Just added the plot for 15 hours.

 

From the data we can see that signal level and noise do not change beyond normal measurement to measurement variation.  From other data I can see that frequency response has not changed either.

 

Thye only change we see is regarding odd harmonics, 3, 5 and 7, which increase slightly through warm up.  This has stabilised within an hour.  Its simply a very slight change due to the operating points of the DAC and op amps changing as they warm.

 

From an absolute perspective the performance gets worse after warm up!  This is something that people should bear in mind.  After warm up there is no guarantee that a product will perform or sound better.

 

The other thing to bear in mind is that the distortion levels seen are waaaaayyyyyyy below the audible threshold, and there is no way that the changes observed will be audible.

 

A modern, well designed DAC simply does not need to be warmed up to sound its best.  Of course there may well be DACs that are affected significantly enough to have audible changes, but if that is the case you need to ask why their performance is thermally unstable, and if it is a good design that shows this characteristic.

 

The most likely explanation for anyone that believes they observe a change in sound through warm up of a DAC like this, is it's simply their perceptions changing over time.  We are the significant variable here, not the equipment.

 

 

So thats DACs.  Amps may be different and for good reason.  I will take a look at those another day.

Edited August 10 by March Audio

[Addicted to music]

More like the brain is adjusting to like the SQ

[March Audio]

1 minute ago, Addicted to music said:

More like the brain is adjusting to like the SQ

 

Absolutely.  We do acclimatise to sound.  We do change our perception.   After an hour or 2 of listening it will have subtly changed.

 

 

[Addicted to music]

This is how some manufacturers get away with poor performance,   Some claim to have done the measurements and possible show you a nice FFT distortion graph to say they have the measurements done.  By the time the customer gets it,  have it on line in there rig and the minute they turned it on it sounds terrible,  so they asked for a refund!   The standard go here is they then tell you to “burn-in” 400hrs…...    Then you see posts like: “yeah, you need to do a 400hr burn in before you can even listen to it”   Some of the electrolytic caps would have done 40% of operational life if you did 400hrs burn in.   Just remind myself when I got onto SNA where one manufacturer would declare this and everyone on all Audio forums just jump the band wagon with burn-in.    When you see independent measurements done, they are the worst performers ever!  It surprised me why someone would hand over $1000s to get a product across the Pacific listen to it and still keep it.   I’ve been banging on some of these post to say if it doesn’t agree with you in the 1st 5-10mins, send it back,  not worth keeping,  but it falls on deaf ears,  you know; “the ear rules”.  

[March Audio]

15 hours ago, Addicted to music said:

The standard go here is they then tell you to “burn-in” 400hrs…...    Then you see posts like: “yeah, you need to do a 400hr burn in before you can even listen to it”

 

Yeah, burn in is another myth. Obviously we test amps at manufacture, zero hours.  Year later, no change.

 

Speakers do exhibit a small degree of "relaxation" of the suspension and you can easily see it in the impedance plot, but its not a big difference in other measurements, FR or distortion. Nor the actual sound.  A few hours use and the jobs done.

Edited August 11 by March Audio

[Addicted to music]

31 minutes ago, March Audio said:

 

Yeah, burn in is another myth. Obviously we test amps at manufacture, zero hours.  Year later, no change.

 

Speakers do exhibit a small degree of "relaxation" of the suspension and you can easily see it in the impedance plot, but its not a big difference in other measurements, FR or distortion. Nor the actual sound.  A few hours and the jobs done.

 

You would question continuous changes more than a few hours especially in speakers.

[March Audio]

Looking at an amp right now.

 

Marantz MA6100

 

Output 1kHz @ 4.45 volts into 4 ohm load.  5 watts

 

Update tomorrow

 

0 mins

 

5 mins

 

10 mins

 

60 mins

 

180 mins

 

15 hours

Edited August 11 by March Audio

[Steff]

@March Audio thanks, v. interesting. Also, people are going to ask: what type of amp? And how to interprete the information in the top right hand side box...slightly rising dBFS?

Edited August 10 by Steff

[fas42]

Yes, very interesting how the odd harmonics are steadily rising - I've had issues many times of finding the subjective performance slowly worsening over a time frame of hours ... and this may be a glimpse of a measurement pointing to this behaviour in action  ...

[ray4410]

this is very interesting,from my experience i have found that the performance of an amp after warm up varies from amp to amp,i don't think i have ever had an amp that actually sounded worse after playing for several hours,
i remember years ago talking to the late Peter Stein of ME about amp warm up as any ME amp owners would know they have an operate temp light that comes on when the amp has reached optimal sound,i queried Peter about this and he replied start playing music as soon as you turn it on it will reach the optimal temp quicker
looking forward to see what other members think about warm up and sound quality after several hours of listening.

[gemini07]

1 hour ago, Steff said:

@March Audio thanks, v. interesting. Also, people are going to ask: what type of amp? And how to interprete the information in the top right hand side box...slightly rising dBFS?

I wondered about this as well - especially for tube-based equipment.

 

I used to own an Abbas 2.3SE tube dac and it seemed to sound better after around 30-40 mins from turn-on. This was only my subjective interpretation as I had no way of scientifically verifying any changes. Maybe it was just louder, but I definitely had to drop the volume after an hour or so.

 

Then again, maybe we don't have the knowledge or equipment to measure what we think we hear / like.... 

[March Audio]

2 hours ago, Steff said:

@March Audio thanks, v. interesting. Also, people are going to ask: what type of amp? And how to interprete the information in the top right hand side box...slightly rising dBFS?

 

Hi @Steff sorry, I was just putting the plots in the post late last night.  One more to insert after running over night.  All the details will follow soon.

 

dBfs stands for decibels full scale. Full scale being the maximum input level of the A to D converter used to measure the signal.  This is 0dB.

 

In both the DAC and this amp test, the input range of the ADC (dBfs) is set to be around 4.5 volts rms.  The DAC was outputting its maximum signal level of 4 volts rms.  In the amp test it is outputting about 4.5 volts into a 4 ohm load, which equates to 5 watts.

 

Amplifiers are going to be much more variable than DACs.  For solid state class A an A/B it very much depends upon their design and how effective the circuit deals with temperature change, controls bias etc.  Class D will be far more stable.  Tube amps, although I have never tested one for this, I would expect to change a lot during warm up for obvious reasons.

Edited August 11 by March Audio

[March Audio]

OK, I just added the 16 hour plot for the amp.

 

The amp is a Marantz MA6100.  AFAIK a normal class A/B design. Physically Its a quite unusual monoblock, being very thin and deep.  This pair came up used cheap a while ago and I bought them for something to play with.  This is actually the first time I have hooked them up to an analyser.

 

The first thing to note in the data is the level of the 50Hz mains spuria and its subsequent harmonics which extend right up to 6kHz.  Considering the potential age of the amps this has made me wonder if the PSU caps are lifed.  Quite possible, but this wont affect this test.

 

So we see during warm up the level of both odd and even distortion harmonics rise. No other parameters significantly change.  The 3rd harmonic rises significantly, by about 10dB.  Is this audible?  Well, the absolute levels, even when cold, are not as low as I would like them to be.  After warm up I am certainly "twitchy" about audibility, but with music this is still very unlikely.  Stability has occurred after about 1 hour.

 

The point we are looking at here is the change in performance and the assumption people make that sound gets better after warm up.  We can see this is another example of that assumption being incorrect.

 

What we should not do is assume this is representative of all amplifiers.  It is a different scenario to the DAC shown above.  Some amps may well be carefully adjusted to reach optimum at a fully warmed temperature..  However, again we should not assume that any change in performance during warm up is going to be audible; if it is, just as with the DAC scenario, it implies the design is not thermally stable and you have to consider if it is a good design.

 

I am going to try to get hold of a range of amps to test if I can beg / borrow / steal them.  Next is a class D amp.

Edited August 11 by March Audio

[Addicted to music]

2 hours ago, ray4410 said:

this is very interesting,from my experience i have found that the performance of an amp after warm up varies from amp to amp,i don't think i have ever had an amp that actually sounded worse after playing for several hours,
i remember years ago talking to the late Peter Stein of ME about amp warm up as any ME amp owners would know they have an operate temp light that comes on when the amp has reached optimal sound,i queried Peter about this and he replied start playing music as soon as you turn it on it will reach the optimal temp quicker
looking forward to see what other members think about warm up and sound quality after several hours of listening.

Peter Stein took advantage of every BJT parameter,   His view was that the die should operate at a specific temp were they operated optimum, this is where it was most linear…hence the reason for active cooling.

Edited August 11 by Addicted to music

[Addicted to music]

18 minutes ago, March Audio said:

OK, I just added the 16 hour plot for the amp.

 

The amp is a Marantz MA6100.  AFAIK a normal class A/B design. Physically Its a quite unusual monoblock, being very thin and deep.  This pair came up used cheap a while ago and I bought them for something to play with.  This is actually the first time I have hooked them up to an analyser.

 

The first thing to note in the data is the level of the 50Hz mains spuria and its subsequent harmonics which extend right up to 6kHz.  Considering the potential age of the amps this has made me wonder if the PSU caps are lifed.  Quite possible, but this wont affect this test.

 

So we see during warm up the level of both odd and even distortion harmonics rise. No other parameters significantly change.  The 3rd harmonic rises significantly, by about 10dB.  Is this audible?  Well, the absolute levels, even when cold, are not as low as I would like them to be.  After warm up I am certainly "twitchy" about audibility, but with music this is still very unlikely.  Stability has occurred after about 1 hour.

 

The point we are looking at here is the change in performance and the assumption people make that sound gets better after warm up.  We can see this is another example of that assumption being incorrect.

 

What we should not do is assume this is representative of all amplifiers.  It is a different scenario to the DAC shown above.  Some amps may well be carefully adjusted to reach optimum at a fully warmed temperature..  However, again we should not assume that any change in performance during warm up is going to be audible.

 

I am going to try to get hold of a range of amps to test if I can beg / borrow / steal them.  Next is a class D amp.

 

24 minutes ago, March Audio said:

 

 

The first thing to note in the data is the level of the 50Hz mains spuria and its subsequent harmonics which extend right up to 6kHz.  Considering the potential age of the amps this has made me wonder if the PSU caps are lifed.  Quite possible, but this wont affect this test.

 

id be interested in how far these caps deviated from the specified ESR.

[March Audio]

2 minutes ago, Addicted to music said:

 

id be interested in how far these caps deviated from the specified ESR.

 

Both amps show similar.  I dont know their origin/usage so they could have been left on 24/7, (facepalm).  I will be ripping the apart and checking the caps when I get a chance.

Edited August 11 by March Audio

[March Audio]

1 hour ago, Addicted to music said:

 

id be interested in how far these caps deviated from the specified ESR.

 

@Addicted to music    Yep, they are Bug*****!

 

 

 

 

[sir sanders zingmore]

Just for a single data point, would it be possible to measure this into a real speaker? 

 

[March Audio]

1 minute ago, sir sanders zingmore said:

Just for a single data point, would it be possible to measure this into a real speaker? 

 

 

No Problem, will do.

[March Audio]

1 hour ago, sir sanders zingmore said:

Just for a single data point, would it be possible to measure this into a real speaker? 

 

 

This is a very good demo of why testing into resistive dummy loads, as some "scientific" websites do, is simply not adequate to characterise amplifier performance.

 

To be fair this was a bit quick and dirty and I would like to do it a bit more controlled, but.......

 

Marantz MA6100       4.5 volts output (5 watts into 4 ohms)  20Hz to 20kHz sweep

 

4 ohm resistive dummy load.

 

"Nominal"" 4 ohm speaker

 

 

 

Of course impedance dips in the speaker load and changing phase angle will cause more power to be dissipated by the amp, hence more distortion.  You can argue that using a resistive load equivalent to the lowest reactive load of the speaker gives you the same info, but it is more complex than that.

 

 

This is a Class D amp.  No names mentioned 

 

4 ohm load

 

 

 

Speaker

 

 

 

 

Edited August 11 by March Audio

[Stereotech]

It would be interesting to see what those first Marantz plots look like with the fresh main filtering caps.

 

If you have time that is.

[March Audio]

Just now, Stereotech said:

It would be interesting to see what those first Marantz plots look like with the fresh main filtering caps.

 

If you have time that is.

 

Been ordered, probably wont get here for a week

[MLXXX]

We see quite a difference in the above graphs between a THD figure taken at only 1kHz (a traditional measurement) and THD tested across the audible spectrum.  In the graphs there is not too much difference at 1kHz between the dummy load and the speaker but there's a big difference at low frequencies. Of course speaker distortion tends to be very high and to swamp amplifier distortion.

 

 

I note that using a speaker for repeated measurements may lead to inconsistencies in the measurements if a driver is driven hard such that its voice coil resistance increases.   You'd need to try to ensure the voice coil was at the same temperature each time you took your readings.   Mind you, who would want to be anywhere near a speaker being driven hard with a test signal?!!