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.
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 looking We 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 mine I have made some audio stuff but mostly tube amplifiers.
Low Power Version
As shown in the schematics below (figures 1 and 2), the amplifier can be made in high or low power version, and although there is a bit of vacant PCB real estate in the low power design, it is significantly cheaper to make and will be more than sufficient for most constructors. If this version is built (using only 1 pair of MOSFETs), IMO it is essential to limit the supply voltage to ±42V so that it can drive both 4 and 8 ohm loads without excess dissipation. With this voltage, expect about 100W continuous into 8 ohms, and around 150W into 4 ohms. Naturally, dual MOSFET pairs may be used at this voltage as well, providing much better thermal performance (and therefore cooler operation), far greater peak current capability and slightly higher power. This version may be used at any voltage from ±25V to ±42V.
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.
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.QuoteLow Power Version
As shown in the schematics below (figures 1 and 2), the amplifier can be made in high or low power version, and although there is a bit of vacant PCB real estate in the low power design, it is significantly cheaper to make and will be more than sufficient for most constructors. If this version is built (using only 1 pair of MOSFETs), IMO it is essential to limit the supply voltage to ±42V so that it can drive both 4 and 8 ohm loads without excess dissipation. With this voltage, expect about 100W continuous into 8 ohms, and around 150W into 4 ohms. Naturally, dual MOSFET pairs may be used at this voltage as well, providing much better thermal performance (and therefore cooler operation), far greater peak current capability and slightly higher power. This version may be used at any voltage from ±25V to ±42V.
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.
yea 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?
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.
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.
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.
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. if 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.
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 trafo. You want to be safe - nothing puts you off in this hobby like a fire in your house - Wives tend 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.
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.