How many VA shoud a transformer be for 200W Amplifier - Page 1
Forum section: electronics / beginners

author: christos
author_rank: Regular Contributor
timestamp: April 16, 2016, 01:45:07 pm
content: Hello,,I'm thinking of building one of ESP projects,,the P101 from Rod Elliotthttp://sound.westhost.com/project101.htmit says that its 200W amp at +- 56 volts,,,does that mean my transformer need to be 200VA or more?Thank you!

author: uncle_bob
author_rank: Unknown
timestamp: April 16, 2016, 02:12:34 pm
content: HiIf your amp design is a full up class A, it will pull full current all the time. Class A by it's nature is about 30% efficient. You can wire that amp up to run class A. Class AB (which is more typical) will get the efficiency closer to 50% at full power and reduces the idle power quite a bit. How much depends on exactly where you set the bias.So, you will need a power supply that will deliver between 600W continuous (the class A version) and 400W intermittent (the class AB version). Power supply transformers are generally derated by about 1.4 for use in a simple linear supply. That gets you to around 900 VA for the class A and maybe 600 VA for the class AB.If your AB design is targeted at a something other than full ratings full time, you probably can drop back on the transformer a bit. If you do decide on full class A, you need the big transformer no matter what.One other caution:That design is set up for FETS running *very* close to destruction. A line spike on an unregulated supply could easily take one out. If you decide to regulate the supply (and go linear regulation), the transformers need to be bigger. You will be dropping voltage (and thus burning power) in the regulators. A *far* more normal way to go is to use bigger (higher voltage) output devices and eliminate any need for supply regulation.Bob

author: 4cx10000
author_rank: Regular Contributor
timestamp: April 16, 2016, 02:31:11 pm
content: No more to add about the VA-rating. I built exactly the same amplifier for many many years ago. If my memory serves me right there was some tweaks to get it work properly. I made a PCB in Eagle at that time and used a big transformer from a scrapped amplifier.

author: christos
author_rank: Regular Contributor
timestamp: April 16, 2016, 02:59:36 pm
content: Quote from: 4cx10000 on April 16, 2016, 02:31:11 pmdoes it worth to build it , or not?,,do you suggest any other amplifier ?my goal is have a nice loud clean sound

author: 4cx10000
author_rank: Regular Contributor
timestamp: April 16, 2016, 03:15:54 pm
content: Well, as I recall it was not that bad. I built it to my son and it is still somewhere in the basement, but frankly, I got so much junk there so I do not have a clue where to start lookingWe even tested it to drive a subwoofer, but as I stated earlier, it was years back and can not tell if it was any good for that. Are you planning to make the PCB by yourself? If not, I'll have a check if I still got the layout in my old Eagle machine. When it comes to suggestions of what to build, I think there is lots of people on this forum that would give you better answer than mineI have made some audio stuff but mostly tube amplifiers.

author: christos
author_rank: Regular Contributor
timestamp: April 16, 2016, 03:29:49 pm
content: Quote from: 4cx10000 on April 16, 2016, 03:15:54 pmi was thinking to buy these pcb's from ESP but cant cuz of the capital controlls here in greece,,wont allow me to make payments to other contries so i started to figure how to make one by my self,,but if its easy for you to get that EAGLE file i would appreciate that .. Thank you

author: retrolefty
author_rank: Super Contributor
timestamp: April 16, 2016, 05:31:05 pm
content: If you read the construction instructions carefully you will find that he states that the 200 VA minimum transformer rating is for a single channel. If building two modules for stereo application it will require either 2X 200 VA rated transformers or a single transformer rated at 400 VA. Looks like a great project as most of his are.

author: jitter
author_rank: Frequent Contributor
timestamp: April 16, 2016, 06:09:19 pm
content: As long as your speakers aren't horribly inefficient, and you plan to use it in a normally sized living room, you probably won't be using it much over as little as 1 W.If buying a 400 VA transformer is a bit too costly, it's worth to consider building the "low power" version of that amp. Obviously this would need a smaller and thus cheaper transformer.Quote

author: christos
author_rank: Regular Contributor
timestamp: April 16, 2016, 07:00:14 pm
content: Quote from: retrolefty on April 16, 2016, 05:31:05 pmyea i got that,but wasn't sure about the transformer,,,i cant understand if a amp says its rated for 200w,,does it mean that the transformer should be 200W?

author: christos
author_rank: Regular Contributor
timestamp: April 16, 2016, 07:02:36 pm
content: Quote from: jitter on April 16, 2016, 06:09:19 pmI wont use it in a small living room or any room,,outside only,for the speakers ,I dont have any yet for that power

author: uncle_bob
author_rank: Unknown
timestamp: April 16, 2016, 07:20:48 pm
content: Quote from: christos on April 16, 2016, 07:00:14 pmHiWith a proper set of heatsinks, a 200W amplifier will deliver 200W into the rated load on a continuous basis. That is how the guy describes that amplifier (power into a load). If you are looking for an *output* transformer, then yes, you would get a 200W output transformer. Since your *input* transformer needs to supply both the power delivered to the load *and* the power the (inefficient) amp burns up *and* some interesting losses in the power supply .... you need a bigger transformer than 200W.Also consider that the power the amp burns up needs to go somewhere. 200W into a heatsink is a lot of power, 400W is even more. Unless you like listening to a fan, that's going to be a pretty big heatsink.The design he shows is not all that unusual. It is a very normal way to do up a linear amp with mosfets. If you make some very minor changes it will work with bipolar transistors. I had an amp with a very similar schematic (and power level) back in the 1970's. What makes it unique are the little tweaks here or there. They are all component value issues and not major topology changes. As another poster has observed, you likely will need to mod some of those values anyway.Bob

author: mariush
author_rank: Super Contributor
timestamp: April 16, 2016, 07:27:11 pm
content: christos, use your brains.If the amplifier is meant to output up to 200w per channel, do you think it's going to do it with 100% efficiency?Simple answer is no, class AB amplifiers are usually up to around 70% efficient, but can be lower efficiency.I don't see why you're trying to make an amplifier using that schematic, do you want it simply because the schematic is available and seems easy? Or is it because the chips seem cheap?I'd suggest just going with a class D amplifier... TDA8950 is a decent chip, and there's ready made amplifier boards on ebay, see for example this:http://www.ebay.co.uk/sch/i.html?_odkw=tda8590&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xtda8950.TRS0&_nkw=tda8950&_sacat=0IF you still want class AB, why not go with a couple of LM4702 :http://www.digikey.com/product-detail/en/texas-instruments/LM4702BTA%2FNOPB/LM4702BTA%2FNOPB-ND/1576957There's even application note with the full schematic explained for an amplifier with this chip:http://www.ti.com/lit/an/snaa031a/snaa031a.pdf

author: SeanB
author_rank: Super Contributor
timestamp: April 17, 2016, 10:30:43 am
content: Christos, I would post you a suitable 450VA transformer, as I do have 2, and they were used in a HMA9500 amplifier. Just they are 14kg each. Are you willing to wait 3 months or so for sea post to get them to you?If you want the MOSFETS I also have them, from the same amplifier, but they may or may not be faulty, yours to test. They will come with the TO3 sockets as well and a small piece of SRBP board they used to mount to.

author: uncle_bob
author_rank: Unknown
timestamp: April 17, 2016, 11:24:48 am
content: Quote from: SeanB on April 17, 2016, 10:30:43 amHiSounds like about all he still would need is:Heatsink(s)Enclosure, switches, and connectorsBig filter caps for the power supplySmall (cheap) parts for the rest of the circuit.Still a few things to get, but not a whole lot. Sounds like a very good deal.Bob

author: SeanB
author_rank: Super Contributor
timestamp: April 17, 2016, 11:39:06 am
content: Postage though will not be cheap to Greece though, around $30 per kilo at the latest price I got from SAPO.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2016, 11:45:53 am
content: The power is limited by several factors:The amplifier circuit sets a short time limit Even for a short time it can not deliver much more than 200 W without clipping. The transformer has slow thermal limitation. So it can deliver it's rated power (VA) for a long time, but much higher values for short times of something like a minute. Due to efficiency of the amplifier and rectifier, the transformer would need about a rating in VA of about twice the amplifier output power to get that power out over a long time.Similar the speakers have short time limit for clipping (there rated power) and a considerably lower thermal limit. So you can't sent long time 100 W to a 100 W typical speaker without breaking it.So the power you need for transformer depends on the music you use and the speakers you have.So something like the suggested 200-250 VA transformer sound reasonable or even rather large, as this should allow for something like a 60 W continuous output power - enough to burn typical 100-200 W rated speaks, and still plenty to damage your ears. With a smaller transformer you might want to have some kind of over-temperature protection.

author: christos
author_rank: Regular Contributor
timestamp: April 17, 2016, 12:12:38 pm
content: Quote from: SeanB on April 17, 2016, 10:30:43 ami have a torodial core that is ID 55mm, OD 115mm ,/H 75mmi think its rated for 400VA?

author: uncle_bob
author_rank: Unknown
timestamp: April 17, 2016, 12:24:03 pm
content: Quote from: Kleinstein on April 17, 2016, 11:45:53 amHiIn most use environments, an average power of 1W per channel is more than needed. Peak to average on most people's music today is below 10 db, that gets you to 10W. This of course all makes some assumptions about listening environment, music type, speaker efficiency, and a few other things. You can do a fine job of audio for a room off of a USB charger port ...Move over to something like a sub woofer and try to hit a low end of 10 Hz. Do that in a "wide open" floor plan modern house. You have an enormous volume (cubic meters) and that's what drives the power requirement. In this case, you do indeed need 100's of watts of power on an average basis. You *really* have to like low end organ music (or home theater) to need this, but some people do. You also need some really amazing drivers in your cabinet and a good design for that enclosure. This also makes a few assumptions (the enclosure is not the size of a small car ...).There are a lot of variables ...Bob

author: BobbyK
author_rank: Regular Contributor
timestamp: April 17, 2016, 12:25:10 pm
content: Transformers and enclosures are the most expensive parts in hobby amp design, so it's a good idea to try a few things out and read up about amp power requirements before you go further in this hobby. There are two schools of thought here:1. Audiophiles/perfectionists like to feed a sinusoid into their Class-A amp, connect it to 4 (or even 2) ohm loads, drive it at full power, and watch the output signal for clipping. If there is clipping, they will throw more Transformer and capacitors at it!2. Engineers tend to go with class-d or class AB(with a low bias) or chip-amps (for lower powered commercial products). They also know that people generally feed their amps with music which has a far smaller duty cycle than a sinusoid. They also know that it is more reasonable to plan for the average listening experience (i.e. not a full blast) unless they are designing the amp for outdoor rock-concerts.Here is my philosophy:1. find out what load you want the amp to work with (8, 4 or 2 ohms). Look at the SOA of your output devices, and find out how much power they can deliver at working temperature. Assume .2 to .1 duty cycle since you are playing music and not sinusoids (or 0.5 if it's a subwoofer amp). Make sure you know what speakers you will be using with this. If you are using them with a pair of 25 Watt speakers e.g., and you listen at barely 1/10 volume, there is no reason to buy a huge transformer - you can always limit the input signal to avoid clipping.2. Always add proper fuses in the primary AND secondary of your supply. Get the current/Voltage rating for the fuses perfectly right. Then add thermal cutouts (about 65 degC) and bolt them to sinks and transformer. You want to be safe - nothing puts you off in this hobby like a fire in your house - wifey tends not to approve of those.3. For amps similar to your design (2 channels each with two pairs of output MOSFETS, into an 8 ohm load) I have gotten away with anything from 150 VA (for home use with studio speakers) to 750VA for 4Ohm outdoor party speaker boxes at full blast. Yes the thermals might kick in now and then, but that's a good thing - it will stop you from going deaf!4. with the cutouts and proper fusing in each rail and the input I have yet to have a transformer blow on me. Additionally, since we are keeping the cost of the hobby low by being reasonable with Transformer and Capacitor selection, you will have some money to spend on the most important part of the amp: the enclosure! Believe me you will get a lot of enjoyment out of a small good looking amp, and none from a circuit board in a giant crusty old box.

author: christos
author_rank: Regular Contributor
timestamp: April 17, 2016, 12:36:31 pm
content: Quote from: BobbyK on April 17, 2016, 12:25:10 pmAs you said,,If i build it,, theres no way that i will use it in the house,,only outdoor party,,i got one for my pc 50watt,,CHIP amp,,so i would like to build something new. learn something new ,get more experience on HiFi amps.And for the enclosure,have no problem with that cuz ,my friend can build one for me free,,Metal,Wood,Aluminium,,anything i want

author: uncle_bob
author_rank: Unknown
timestamp: April 17, 2016, 01:49:16 pm
content: Quote from: BobbyK on April 17, 2016, 12:25:10 pmHiOne really subtle issue that will get you when doing fuses:Fuses have ratings both for current and for voltage. That's pretty understandable. Voltages that are to high will freak out all sorts of devices. The fuse has a current rating to let you know how to use it. The subtle part is that the voltage rating will be different for AC and DC. The line fuse on your AC coming into the transformer might be rated for 3A at 250V (AC). That same fuse may only handle 28V running on DC. If you are fusing the DC rails on a 72V power supply, you need fuses that are rated to work at > 48V DC. You *can* find that sort of fuse. It's bigger than you would expect it to be. It also is more expensive. The key point - don't assume it's the same part as you used for your line fuse ....You might ask: Is this fuse makers trying to scare us into higher priced fuses? I suppose some of it is. I only looked into the "why" of it when one blew up in my face. It didn't destroy anything, but it was quite a surprise. That was back in the 1960's. Fuses haven't changed a lot since then ...Bob

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2016, 03:22:57 pm
content: The fuses are not that critical here as they are in multimeter. A fuse before the transformer is there to prevent a fire in case of a broken transformer. Fuses after the transformer may protect the transformer from overload or a broken rectifier / amplifier. As a transformer is slowly reacting to overload the fuses can be slow blow type.