The L20 V9 is a bargain, but bias is fixed for kit market
The designer, initials LJM, makes a lot of kit designs for the chinese resellers. He often uses the same basic front end design, copied more or less from Douglas Self's Blameless Amplifier example design. It's well understood and very solid, so why not? He uses fixed bias in most of his amps to keep them simple and fool proof for the kit market. It's his business decision to keep it simple with no adjustments, probably the right solution for the kit market.
The bias in this amp is set by a traditional Vbe multiplier, which then drives MOSFET source followers to the gates of the outputs. So there is something between the Vbe multiplier and the gates, namely the source followers, which adds a variable offset to the bias. In practice it does not seem to be a problem though. The good news is the bias drops with rising temperature as it should, but it is slightly overcompensating. You get to almost zero bias when the heatsink is very warm, which seems ideal. As long as it sounds OK, you should leave it alone.
If you want to have some bias even warm, then the cool quiescent 20ma bias you set may have to be a little higher, say to 30-40ma. With heating, that should drift down to 10-20ma when hot, still perfectly OK for a CFP output stage. This is as good as it gets without a bias servo, which I prefer not to have. Realize that if the bias went the other way you would have destructive thermal runaway. Don't fix something that is not broken.
WHY NO SCHEMATIC?
The schematic for this amp is never published due to copycats in China. There is already a clone of this L20 amp by a competitor named YUANJING, on a red board. LJM has no real defense against this, except putting roadblocks in the way of copycats by not giving out the schematic, or intentionally publishing wrong schematics. That makes it hard for legitimate users and repair guys, but tough stuff. LJM is trying to protect his proprietary design, probably a lost cause in China, but I wish him luck.
I have reverse engineered the schematic of a very similar LJM design and I strongly suspect this one is identical. The front end seems to be a direct copy of Self's Blameless Amp down to component values. It is published in most of Self's books, and was published online too in some articles in Electronics World, a trade pub. So it's prior art and public domain. Still, I totally respect that LJM is supplying good designs at very low prices, and I wish him continued success.
NEED OUTPUT PROTECTION
My concern is that LJM never puts in output short circuit protection in his designs, which means when someone shorts the output they can blow the amp and the speakers which it is driving. That's an expensive price to pay because this won't survive operator error, in my opinion. Operator error is a fact of life. Besides, we actually know how to solve the problem. Output protection circuitry adds a little cost, and some audible artifacts, but it is standard in professional gear. If you don't like it, you can defeat it or remove it, but I personally want it available to protect my speakers.
After some study, I think I know how to add it to this amp, and to it's little brother amps too. I doubt Mr. LJM will design it in, because it does add slight cost and he seems to quibble about every extra part, witness his taking out the Darlington VAS that was in the Blameless Amp, big mistake due to added distortion. This L20 V9 design makes it a little harder because of the MOSFET source followers. I would choose the L20 SE, a similar design which uses bipolar pre-drivers, allowing a simple diode string to to the job.
I published a schematic for the little brother of this amp, the MX50 SE, including original design and mods for output protection. I suspect the front end and driver are similar and the mods would apply directly to the L20 SE, but not the L20 V9 due to the MOSFET pre-driver. If you are curious, that post is here:
http://www.diyaudio.com/forums/solid-state/273821-mx50se-ljm-2015-a.html#post4866083
The designer, initials LJM, makes a lot of kit designs for the chinese resellers. He often uses the same basic front end design, copied more or less from Douglas Self's Blameless Amplifier example design. It's well understood and very solid, so why not? He uses fixed bias in most of his amps to keep them simple and fool proof for the kit market. It's his business decision to keep it simple with no adjustments, probably the right solution for the kit market.
The bias in this amp is set by a traditional Vbe multiplier, which then drives MOSFET source followers to the gates of the outputs. So there is something between the Vbe multiplier and the gates, namely the source followers, which adds a variable offset to the bias. In practice it does not seem to be a problem though. The good news is the bias drops with rising temperature as it should, but it is slightly overcompensating. You get to almost zero bias when the heatsink is very warm, which seems ideal. As long as it sounds OK, you should leave it alone.
If you want to have some bias even warm, then the cool quiescent 20ma bias you set may have to be a little higher, say to 30-40ma. With heating, that should drift down to 10-20ma when hot, still perfectly OK for a CFP output stage. This is as good as it gets without a bias servo, which I prefer not to have. Realize that if the bias went the other way you would have destructive thermal runaway. Don't fix something that is not broken.
WHY NO SCHEMATIC?
The schematic for this amp is never published due to copycats in China. There is already a clone of this L20 amp by a competitor named YUANJING, on a red board. LJM has no real defense against this, except putting roadblocks in the way of copycats by not giving out the schematic, or intentionally publishing wrong schematics. That makes it hard for legitimate users and repair guys, but tough stuff. LJM is trying to protect his proprietary design, probably a lost cause in China, but I wish him luck.
I have reverse engineered the schematic of a very similar LJM design and I strongly suspect this one is identical. The front end seems to be a direct copy of Self's Blameless Amp down to component values. It is published in most of Self's books, and was published online too in some articles in Electronics World, a trade pub. So it's prior art and public domain. Still, I totally respect that LJM is supplying good designs at very low prices, and I wish him continued success.
NEED OUTPUT PROTECTION
My concern is that LJM never puts in output short circuit protection in his designs, which means when someone shorts the output they can blow the amp and the speakers which it is driving. That's an expensive price to pay because this won't survive operator error, in my opinion. Operator error is a fact of life. Besides, we actually know how to solve the problem. Output protection circuitry adds a little cost, and some audible artifacts, but it is standard in professional gear. If you don't like it, you can defeat it or remove it, but I personally want it available to protect my speakers.
After some study, I think I know how to add it to this amp, and to it's little brother amps too. I doubt Mr. LJM will design it in, because it does add slight cost and he seems to quibble about every extra part, witness his taking out the Darlington VAS that was in the Blameless Amp, big mistake due to added distortion. This L20 V9 design makes it a little harder because of the MOSFET source followers. I would choose the L20 SE, a similar design which uses bipolar pre-drivers, allowing a simple diode string to to the job.
I published a schematic for the little brother of this amp, the MX50 SE, including original design and mods for output protection. I suspect the front end and driver are similar and the mods would apply directly to the L20 SE, but not the L20 V9 due to the MOSFET pre-driver. If you are curious, that post is here:
http://www.diyaudio.com/forums/solid-state/273821-mx50se-ljm-2015-a.html#post4866083
Thanks slowhands, i found your post very enlightening, since there's not much info on the L20 V9 design.
I love the simplicity of your output protection mod. I guess it's not (directly) applicable to my boards, but it's food for experimentation (in simulation).
I agree that output protection is a must have, if one values his speakers.
I was hoping that there would exist a universal output protection design, something with a small (0R1?) sense resistor, or even better, sensing across the emitter resistors, that could detect overcurrent and DC conditions in one package, but i have not come across something like that. All protection utilizations seem to be dependent on specific output designs...
I love the simplicity of your output protection mod. I guess it's not (directly) applicable to my boards, but it's food for experimentation (in simulation).
I agree that output protection is a must have, if one values his speakers.
I was hoping that there would exist a universal output protection design, something with a small (0R1?) sense resistor, or even better, sensing across the emitter resistors, that could detect overcurrent and DC conditions in one package, but i have not come across something like that. All protection utilizations seem to be dependent on specific output designs...
Have you looked at the Thyristor protection used by Cyrus in their little power amplifiers?
It's not universal, but it does trigger on excess current through either emitter resistor, or on excess bias current (shoot through if frequency is too high). Cyrus use it to turn off the input stage, but it could be used for any of many different protection scenarios.
opportune time to rejoin the thread
Many thanks for long interesting and educational posting.
My new pair of V9.2 boards have just arrived and as soon as my eyesight settles I shall hope to be trying the kit out and definitly with a protection circuit in place .
To me this started as a fun hobby but several failures for various reasons, some unknown have left me slightly less than enthusiastic .Whilst financially this could be described as a disaster, it is after all a hobby which sadly for various reasons I shall soon be withdrawing from.
I do hope any real beginner who thinks that the cheapo alternative is going to be a cakewalk into sonic Nivarda realizes the whole thing is a gamble and possible minefield if you havent (like me) got enough resources or background.
Bear in mind I purchased mine ready assembled.
Hope to report great progress very soon.Also will be selling/ giving away some of these nice but now spare boards.
Dave
.
Many thanks for long interesting and educational posting.
My new pair of V9.2 boards have just arrived and as soon as my eyesight settles I shall hope to be trying the kit out and definitly with a protection circuit in place .
To me this started as a fun hobby but several failures for various reasons, some unknown have left me slightly less than enthusiastic .Whilst financially this could be described as a disaster, it is after all a hobby which sadly for various reasons I shall soon be withdrawing from.
I do hope any real beginner who thinks that the cheapo alternative is going to be a cakewalk into sonic Nivarda realizes the whole thing is a gamble and possible minefield if you havent (like me) got enough resources or background.
Bear in mind I purchased mine ready assembled.
Hope to report great progress very soon.Also will be selling/ giving away some of these nice but now spare boards.
Dave
.
Another AAAAAAaaaaargggggghhhh
My new pair of V9.2 boards have just arrived and as soon as my eyesight settles I shall hope to be trying the kit out and definitly with a protection circuit in place .
To me this started as a fun hobby but several failures for various reasons, some unknown have left me slightly less than enthusiastic .Whilst financially this could be described as a disaster, it is after all a hobby which sadly for various reasons I shall soon be withdrawing from.
I do hope any real beginner who thinks that the cheapo alternative is going to be a cakewalk into sonic Nivarda realizes the whole thing is a gamble and possible minefield if you havent (like me) got enough resources or background.
Bear in mind I purchased mine ready assembled.
Hope to report great progress very soon.Also will be selling/ giving away some of these nice but now spare boards.
Dave
.[/QUOTE]
OK Sooner than expected as I had a qualified Assistant helping and observing...
The two assembled boards were unpacked and connected to their individual heatsinks.
The Power supply was tested at 53.1 + and - Then dissapated using power resistor.So switch on was from near zero.
The First board was connected and the 50 watt light bulb used a currcent limiter LIT up for less than a second. No flashes bangs or wallops.
Tested DC offset from output to ground and 43.5 * Volts DC.. ???
Weird thing,,.,,, next I double checked the readings with My DMM, Output to ground input on the board itself and .....
31mv.. ???? . To me this does not make sense ????
Note No signal connected to input but I did check continuity of all the ground points ie chassis / power supply caps and all were connected (bleep), Bemused by this and with great trepidation I removed the power supply from this board and transferred the same three leads to the second board.
NO PROBLEMS. All is ok on that board... and small drifting 31mv offset..
If any one can suggest a quick solution or explanation...Please do as Its another email being fired off to Sep Store....It looks well assembled..I pressume the so called tested board has a fault
Initially I can do some side by side measurements on good / bad board but I am not a happy bunny...
My new pair of V9.2 boards have just arrived and as soon as my eyesight settles I shall hope to be trying the kit out and definitly with a protection circuit in place .
To me this started as a fun hobby but several failures for various reasons, some unknown have left me slightly less than enthusiastic .Whilst financially this could be described as a disaster, it is after all a hobby which sadly for various reasons I shall soon be withdrawing from.
I do hope any real beginner who thinks that the cheapo alternative is going to be a cakewalk into sonic Nivarda realizes the whole thing is a gamble and possible minefield if you havent (like me) got enough resources or background.
Bear in mind I purchased mine ready assembled.
Hope to report great progress very soon.Also will be selling/ giving away some of these nice but now spare boards.
Dave
.[/QUOTE]
OK Sooner than expected as I had a qualified Assistant helping and observing...
The two assembled boards were unpacked and connected to their individual heatsinks.
The Power supply was tested at 53.1 + and - Then dissapated using power resistor.So switch on was from near zero.
The First board was connected and the 50 watt light bulb used a currcent limiter LIT up for less than a second. No flashes bangs or wallops.
Tested DC offset from output to ground and 43.5 * Volts DC.. ???
Weird thing,,.,,, next I double checked the readings with My DMM, Output to ground input on the board itself and .....
31mv.. ???? . To me this does not make sense ????
Note No signal connected to input but I did check continuity of all the ground points ie chassis / power supply caps and all were connected (bleep), Bemused by this and with great trepidation I removed the power supply from this board and transferred the same three leads to the second board.
NO PROBLEMS. All is ok on that board... and small drifting 31mv offset..
If any one can suggest a quick solution or explanation...Please do as Its another email being fired off to Sep Store....It looks well assembled..I pressume the so called tested board has a fault
Initially I can do some side by side measurements on good / bad board but I am not a happy bunny...
success ? I HOPE
[Q
If any one can suggest a quick solution or explanation...Please do as Its another email being fired off to Sep Store....It looks well assembled..I pressume the so called tested board has a fault
Initially I can do some side by side measurements on good / bad board but I am not a happy bunny...[/QUOTE]
Checking and handlimg the board carefully I have discovered that ONE of the 150Mf caps has a broken leg possibly due to damage in transit, suggested by Sep store ? INTERESTING ....
So I have removed identical cap from the faulty L150W and replaced it into the L20.v9.2
VOILA I now have a working board (I think ? as it measures ok.) and full of confidence will proceed tomorrow with the second board on this adventure...Only lost a day....
[Q
If any one can suggest a quick solution or explanation...Please do as Its another email being fired off to Sep Store....It looks well assembled..I pressume the so called tested board has a fault
Initially I can do some side by side measurements on good / bad board but I am not a happy bunny...[/QUOTE]
Checking and handlimg the board carefully I have discovered that ONE of the 150Mf caps has a broken leg possibly due to damage in transit, suggested by Sep store ? INTERESTING ....
So I have removed identical cap from the faulty L150W and replaced it into the L20.v9.2
VOILA I now have a working board (I think ? as it measures ok.) and full of confidence will proceed tomorrow with the second board on this adventure...Only lost a day....
I becoming less and less enamored (meaning I like it less) with the el cheapo shipping of built boards from China to the US. They often arrive with a bunch of components crushed together. This can lead to component or lead damage like you mentioned.
Also, I prefer to mount my output devices AND the board on a heatsink. Most pre-assembled boards come with the transistors soldered ABOVE the PCB, but I prefer them to be BELOW the PCB.
From now on I am buying kits if I can...
Also, I prefer to mount my output devices AND the board on a heatsink. Most pre-assembled boards come with the transistors soldered ABOVE the PCB, but I prefer them to be BELOW the PCB.
From now on I am buying kits if I can...
Up and running with v9.2
Well the new amps are running through a pair of smaller speakers and have same open character as LJM other designs though a bit more bass punch / tightness and instrument seperation towards that 3d goal.DC offset seems low and its running pretty cool for now.The Big speakers will raise that a bit I guess.
I like to run in for 30 hours before moving on to improvements as they change a lot less after this IMO. Hope this one doesnt change much like the others did.
Doubt if I will need Silmics .... The Electro dc blocking cap is far better than Oscons sound but may be removed and a quality PP cap will replace the very short input lead..then board cap pins linked..
I suppose one good thing has happened on testing.The faulty denon sacd player gave off that high level hiss when misreading the dvda and my current limiting light bulb (AKA the Andrew T ) lit quickly like a Xmas tree.
So its bootstrapped with .3.15 fuse / Slow start / temp sensors and DC sensor switch on/off protection circuit .
If this fails I WILL GIVE UP.
Begining to think the journey was worth it....
Well the new amps are running through a pair of smaller speakers and have same open character as LJM other designs though a bit more bass punch / tightness and instrument seperation towards that 3d goal.DC offset seems low and its running pretty cool for now.The Big speakers will raise that a bit I guess.
I like to run in for 30 hours before moving on to improvements as they change a lot less after this IMO. Hope this one doesnt change much like the others did.
Doubt if I will need Silmics .... The Electro dc blocking cap is far better than Oscons sound but may be removed and a quality PP cap will replace the very short input lead..then board cap pins linked..
I suppose one good thing has happened on testing.The faulty denon sacd player gave off that high level hiss when misreading the dvda and my current limiting light bulb (AKA the Andrew T ) lit quickly like a Xmas tree.
So its bootstrapped with .3.15 fuse / Slow start / temp sensors and DC sensor switch on/off protection circuit .
If this fails I WILL GIVE UP.
Begining to think the journey was worth it....
almost at the end of the long and wandering road.
Begining to think the journey was worth it....[/QUOTE]
Well after a couple of weeks break Ive returned for a fresh relisten and after cleaning up with new PP 100nf caps accross the 52v power input to the 35am bridge I am now more than ever convinced of these amps.......Still no Vishays in yet but i am now starting to get a very realistic exact cymbal sound and even identifying brush strokes on some drummers riffs not heard for years.This is with my lower level smaller speakers....Funnily enough these two 100nf caps still changed sonics during 30 hour run in? 3d depth almost as far back and deep as I remember.
Having read up on the best location for filter caps I was unable to get these physically within 1.5 inches of the bridge and Im not inclined to try changing again.
Neither was I able for now to take the centre grounding of the filters back to the star earth.
(just did not have the green/yellow wire piece long enough but I wonder if Andrew T thinks it really is worthwhile ? ) Will probs try at next change.
Hope to soon remove (his) 60 watt bulb which I can only barely get to light up/flicker during louder playing..
Meanwhile I servicved the Denon 3910 slider rails for the lazer mechanism needed oiling..?
So anyone out there who had their doubts due to any of my negative postings then take a chance.
It was a long road.?
Now on to the upgrades? lol
Begining to think the journey was worth it....[/QUOTE]
Well after a couple of weeks break Ive returned for a fresh relisten and after cleaning up with new PP 100nf caps accross the 52v power input to the 35am bridge I am now more than ever convinced of these amps.......Still no Vishays in yet but i am now starting to get a very realistic exact cymbal sound and even identifying brush strokes on some drummers riffs not heard for years.This is with my lower level smaller speakers....Funnily enough these two 100nf caps still changed sonics during 30 hour run in? 3d depth almost as far back and deep as I remember.
Having read up on the best location for filter caps I was unable to get these physically within 1.5 inches of the bridge and Im not inclined to try changing again.
Neither was I able for now to take the centre grounding of the filters back to the star earth.
(just did not have the green/yellow wire piece long enough but I wonder if Andrew T thinks it really is worthwhile ? ) Will probs try at next change.
Hope to soon remove (his) 60 watt bulb which I can only barely get to light up/flicker during louder playing..
Meanwhile I servicved the Denon 3910 slider rails for the lazer mechanism needed oiling..?
So anyone out there who had their doubts due to any of my negative postings then take a chance.
It was a long road.?
Now on to the upgrades? lol
Is yours a CFP design like this L20 though? The output stage bias current is likely 10-20 mA/pair which may be only in the order of 20% of a comparable EF amplifier output stage.
It's not clear how much other circuitry was also powered by the transformer discussed and how much was protected by the lightbulb during testing either but I would guess that bias current makes up the greater part of the load - perhaps only 4-5W total for a single board but an EF amp could be a 22W load and that becomes significant to a 60W inline bulb, if the load is a stereo pair replete with protection/management circuits.
optimized bias settings ?
Hi Andrew and thanks yet again.
Cuirrently(groan) I have not set up board for adjusment but had considered a series resister to equalize dc offset (7.1 and 10.4) when I replace the cheap resisters with 1% vishays,
Historically on numerous amps I have prefered the sonics of lower class A level settings when I adjusted amps.Dunno why but I have theories.
The grounding wire of the two 100nf filters IS to the nearest point which is the common (as stated by you.).I had been reading up on the later bypassing of caps and noticed some comments which I still had in my head.
The main difference between B4 and after is tje use of pure silver wiring to both caps as I usually find the bypassing of any cable with teflon / dilver IS ALWAYS worthwhile in the end.......Caps leads in particular..
I may bypass that short earth green/yellow to ground later just to see if that also makes a subtle difference...
I also do not use that psuedo snubber idea of just caps accross bridge as this always changes the sonics but not for the better.I found the use of RFI absorbing material beneficial above the bridge a simpler alternative.
So its getting close to just a few limited resistor upgrades (20) and I will be satisfied.
Hi Andrew and thanks yet again.
Cuirrently(groan) I have not set up board for adjusment but had considered a series resister to equalize dc offset (7.1 and 10.4) when I replace the cheap resisters with 1% vishays,
Historically on numerous amps I have prefered the sonics of lower class A level settings when I adjusted amps.Dunno why but I have theories.
The grounding wire of the two 100nf filters IS to the nearest point which is the common (as stated by you.).I had been reading up on the later bypassing of caps and noticed some comments which I still had in my head.
The main difference between B4 and after is tje use of pure silver wiring to both caps as I usually find the bypassing of any cable with teflon / dilver IS ALWAYS worthwhile in the end.......Caps leads in particular..
I may bypass that short earth green/yellow to ground later just to see if that also makes a subtle difference...
I also do not use that psuedo snubber idea of just caps accross bridge as this always changes the sonics but not for the better.I found the use of RFI absorbing material beneficial above the bridge a simpler alternative.
So its getting close to just a few limited resistor upgrades (20) and I will be satisfied.
most of my amps are EF outputs. I have not checked what the Crimson amplifier does at switch on via the MBT.
Have you guys seen my post #57 in the "ebay amp L20 power amplifier" thread ? Also note that post #53 gives a schematic of the L20 v9 ( the link says v7 but its v9.2 ). There are mistakes in the schematic but I've pointed these out. Finally here's a bit of a puzzler I don't understand, doesn't stop the amp working but degrades its performance - maybe someone can throw some light on this; details in the post.
ltspice models for 2SD1047 and 2SB817
Hi all
I simulated L20 version with all bipolar transistor and I got a good result, but I used 2SC5200 and 2SA1943 output devices and it showed a little oscilacion that need 100PF capacitor in first transistors of darlingth EF3.
Anyone has spice models for 2SD1047 and 2SA817 to share?
Thanks in advance
Hi all
I simulated L20 version with all bipolar transistor and I got a good result, but I used 2SC5200 and 2SA1943 output devices and it showed a little oscilacion that need 100PF capacitor in first transistors of darlingth EF3.
Anyone has spice models for 2SD1047 and 2SA817 to share?
Thanks in advance
The Chinese copies of 2SD1047 and 2SB817 that you probably refer to (original Sanyo parts are long obsolete) are generic types from several Chinese manufacturers though they are unlikely to perform the same as each other and not really like the originals either.
You would have to choose one brand/manufacturer from several for consistent results and you still then need to find someone who has made models based on that particular manufacturer's part, if still available and the marking genuine. I think the chances of all that are very slim.
Hi and thanks for posting.
Yes the switch on safety board does seem to take up a lot of the juice initially and then very little to nothing after its running on the back up speakers (89db) in a smallish room..
The output protection is seperate and not in the "60" watt bulb circuit)
Meanwhile I have now connected the BIG speakers at estimated 85/6db sensivity (max) and the light is probably 3 times the brightness at peak and just flickering at times.I assume this means both channels are using up the near 60 watts ? at this moment. No sign of any great heat yet but it is difficult to serious listen with the light brightness on at this moment .
I have yet to connect using heavier Naim speaker cable (lower resistance?) but dont expect much of a change.
Raffa,
The Mains Bulb Tester is generally ONLY used to prevent damage due to wiring errors.
It is of little use to power a working amplifier.
I'll assume the 60W bulb has hot and cold resistances of 960ohms & 90ohms
If the bulb is glowing dimly it may have a raised resistance of 200 to 300ohms.
Glowing brightly could be 300 to 500ohms.
Let's make another assumption.
You are powering a working amplifier and the bulb is glowing brightly, so the resistance could be 400ohms.
The transformer is receiving ~80% to 90% of it's rated voltage and the bulb is passing ~90mArms.
The input to the transformer is ~18VA (0.09Amps & 204Vac)
But this is all guesswork based on many assumptions to get here.
To confirm the input to the transformer you would need the Voltage at the transformer and use a current monitor to find the current flowing.
Then you would guess at the efficiency of the transformer at this lower than rated voltage to arrive at the PSU input power.
I can't see how this is much good in helping you.
It could help to monitor the amplifier supply rails.
Better to just power on the amplifier direct from the Mains and listen to the music/audio.
The Mains Bulb Tester is generally ONLY used to prevent damage due to wiring errors.
It is of little use to power a working amplifier.
I'll assume the 60W bulb has hot and cold resistances of 960ohms & 90ohms
If the bulb is glowing dimly it may have a raised resistance of 200 to 300ohms.
Glowing brightly could be 300 to 500ohms.
Let's make another assumption.
You are powering a working amplifier and the bulb is glowing brightly, so the resistance could be 400ohms.
The transformer is receiving ~80% to 90% of it's rated voltage and the bulb is passing ~90mArms.
The input to the transformer is ~18VA (0.09Amps & 204Vac)
But this is all guesswork based on many assumptions to get here.
To confirm the input to the transformer you would need the Voltage at the transformer and use a current monitor to find the current flowing.
Then you would guess at the efficiency of the transformer at this lower than rated voltage to arrive at the PSU input power.
I can't see how this is much good in helping you.
It could help to monitor the amplifier supply rails.
Better to just power on the amplifier direct from the Mains and listen to the music/audio.