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Old 22nd March 2012, 07:12 AM   #1
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Unhappy Lateral MOSFET power supply and RMS output power.

Hi guys.

I'm a newbie in this forum, so forgive me if my questions seems absurd and also my english which is not my native language. I studied electronics a while ago, well a decade ago. I built then a 120wRMS on 4ohms power amp using the famous hitachi 2sk1058/2sj162.

This amplifier had a tube preamp and after months of good sound it started to oscillate, well, I could'nt find then what was wrong, so I decided to unmont all pcb's, transformers etc, until I could find time and/or better knowledge, I suspected the preamp, later I discovered It was a poor heatsink/mica construction, there was no kepton available 10 years ago in the electronics shop, so the mica did a short which virtually destroyed the mosfets and of course the drivers, but that's not the point here.

Well, my transformer was 40-0-40 250VA, big heatsink, etc, everything according to calculations (+/-56VDC rails) in an old but effective design which used the even oldest hitachi lateral mosfets which came on TO-3 packages (I forgot their names, but then hitachi referred me to the 1058/162 replacements. It worked for several months, sounded great, but the oscillation (also called motorboating when referring to tubes) started and It's been in a box for several years.

About two months ago I decided to work again on the amp, and found that there are other interesting designs for the output stage, for example the P101 which I ordered the PCB and all the components. But something stopped me on my tracks... the power output for this design gives extreme power under my 4 ohms cabinet, according to Rod, the bias of the mosfets has nothing to do with the power output, the DC rails are responsible for that. (I only use 2 mosfets)

I'm looking for my old schematic on the output stage. The p101 gives 150w in 8ohms and I expect it gives around 200w give or take in a 4 ohms load with the same DC rails that my old schematics wich gave 80w in 8 ohms and around 120 on 4ohms (my cabinet is 4ohms rated 120-130w RMS). Maybe I'm a bit lazy about this, I have many books to read and a lot of theoretical questions, sure you folks have a better understanding of what I'm asking which is:

How come that with the same pair of mosfet for the output stage and the same DC rails my old amp gave 120w and the p101 on the other hand doubles this power rating? This is driving me nuts, I can't understand it, the schematic I built several years ago had very detailed information on the power ratings for different DC rails configuration, maybe I need to get back to my microelectronic books from university and read some more I have around, specific on audio pre and power amp stages.
What do you think?
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Old 22nd March 2012, 11:40 AM   #2
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I think it may depends also on the total gain of the amplifier.
Consider that 150 Watt @ 8 ohm with a 200VA 40-0-40 transformer is a peak value (thus supported for a short time), not continuous power.

Regards,

Roberto

Last edited by washburn_it; 22nd March 2012 at 11:50 AM.
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Old 22nd March 2012, 12:01 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

With 56V rails the p101 claims 150W 8ohms short term, whatever that means.
Claimed RMS figures are 100W into 8 ohms and "about" 150W into 4 ohms.

Output power for MOSFET amplifiers is heavily dependent on the Vds vs.
output current curves of the devices, that may explain the disparity.

rgds, sreten.
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Old 22nd March 2012, 12:39 PM   #4
Mooly is online now Mooly  United Kingdom
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A single pair of Laterals of the original or later flat-pak type are really only suitable for around 70 watts/8ohm

A 100watts rms into 8 ohms means an output voltage of around 28 volts AC rms from the amplifier. That works out at -/+40 volts peak to peak. Assuming the laterals saturate at around 12 volts worst case, you need rails of -/+52 volts or more... all too much for a single pair.
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Old 22nd March 2012, 12:43 PM   #5
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That's one of my concerns, I want to talk about RMS power (continuous) not peaks or "musical" power, I think these terms confuse people, me included, my transformer is 250VA.
I did some math this morning following a thread in this forum. My objective is to not damage the speakers, so for a pure resistive load (which is not) of 4 ohms, 120w using ohms law (maybe I'm mistaken here using a DC power equation) P=V^2/R so 120= v^2/4 which solving for "v" i got v=21.9V and solving i=5,4A, around 118W.
Assuming a 60% of efficiency for a A/B class amp, needs a 200W power supply, which can be achieved with mine which is 250VA.

I don't think these equations should be used for RMS at all, since it's pure DC equation and so it's a peak value which is not what I'm after.

The documents I have in the old design stated 100w RMS with 40-0-40 on 8 ohms resistive load, If 150w on 8 ohms is a peak value (DC) as stated, then clearly I'm doing this the wrong way.

I will have to read more about peak power and RMS power.

One possibility which crossed my mind this morning is mount the p101, buy a dozen or so of 10w resistors and give it a try with the dummy load wich is not 100% accurate but close, and with the oscilloscope I can check the output wave and estimate the real power that the mosfets are giving just before clipping.

Thanks for the reply.
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Old 22nd March 2012, 12:58 PM   #6
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Quote:
Originally Posted by sreten View Post
Hi,

With 56V rails the p101 claims 150W 8ohms short term, whatever that means.
Claimed RMS figures are 100W into 8 ohms and "about" 150W into 4 ohms.

Output power for MOSFET amplifiers is heavily dependent on the Vds vs.
output current curves of the devices, that may explain the disparity.

rgds, sreten.
Thanks, this makes sense with the documentation of my old design, exactly the same figures.
I will mount a dummy load with 10w resistors (at least a dozen, maybe more) and check with the oscilloscope using a sinusoidal wave at 1Khz once it's working properly, because I rememberl this is the way we checked power output back in the late 90's university labs.
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Old 22nd March 2012, 01:56 PM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by tallerseverino View Post
My objective is to not damage the speakers,
Hi,

Your looking at it the wrong way. Tweeters in speakers are usually blown
by too small an amplifier. The other drivers are at risk with continuous
test tones causing them to exceed their thermal limits and the bass
units at risk with simply too loud heavy bass causing mechanical
damage and/or overheating of popping (from the gap) voice coils.

With music program clipping mildly on peaks, (i.e. as loud as gets
as long as the speakers can handle the bass), average power into
the speakers is far less than the amplifiers RMS power rating.
Music spends 80% of the time at less than 20% of maximum output.

You can't prevent damage by matching speaker and amplifier power.
You can use an amplifier with enough output to blow the speakers
if they are abused quite safely if you are sensible with the volume
control depending n the type of music program being played.

e.g. String quartets can be taken to clipping level on a very big amplifier
with small speakers, but that is not a good idea at all with house music.

rgds, sreten.
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Old 22nd March 2012, 02:38 PM   #8
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The speakers are not for hifi, so no tweeter, crosssover or anything like that. Two 12" (8 ohms each, 4 in parallel). In theory the speakers should exceed the rms power of the amp because rms is sinusoidal, a square wave gives a lot more power than a sinusoidal signal, so I was taught to mismatch the power, but in the opposite way, the speakers always have to be rated bigger than the amp (I've blown tweeters because of this, the amp gave 150w + 150 w rms, the owner claimed the speakers were rated 200w, the day after, I noticed the tweeters weren't working, I opened them and saw the real rms value (80), so I had to replace those tweeters).
I do know that keeping in mind (and ears) volume is a good way of knowing if you're driving the speakers too much, but I don't want to take that chance. I'll keep reading. Thanks.
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Old 22nd March 2012, 08:44 PM   #9
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Post More maths

Did some more maths, lets see what you think:


120W over 4 ohms (DC peak claculation)
P=V.I
I=V/r
P=V^2/R
120= v^2/4
480 = v^2
v=21.9V
i=5,4A

OK, let's see these value son RMS:

VRMS = 0.7 Vpeak
Vpeak(max)= 22V (taken from DC peak point)
Vrms = 15Vrms
Irms=15/4 =3,75Arms
Prms= 82,5Wrms
Idc =Irms.sqrt(2) = 5,4A

Someone also said that these lateral mosfets were to give arount 70w in 8 ohms, it's well in the figure, 70w on 8 ohms and 82w in 4 ohms is a good match, it kind of give the maths and replies meaning.

So we are narrowing down, that peak we wanted to achieve is really 82,5w RMS... on 4 ohm pretty lower figure taken to the claimed 220w (discovered as peak, not real rms )

Las thoughts: We are building based on a 120w rated speaker and giving that peak, transformed in 82,5 RMS, if designed so we have a peak"dangerous" for the speakers: 220W, well that was dv, speakers are rated rms, so wuite sure my 120w rms rated cabinet can handle 82,5w RMS, I must say I'm happy with the idea, building another unit I can easily have a stereo unit.

Im still to prove that these mosfets can handle that and comes from a reply saying the Vds of the lateral mosfets being a lower number, 12. Exicon makes a very good one lateral mosfets in TO-3 packages, rated 250V and. VDsatmax being 12V, well, question number n for today:

Is this Vds(st) a limiting factor on the design?

We can't have more than 12V on Vds before it satturate?
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Old 22nd March 2012, 11:56 PM   #10
sreten is offline sreten  United Kingdom
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Hi,

You don't understand most of the concepts you are talking about.

You really don't have a problem and are looking for one.

rgds, sreten.
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