Thanks linuxguru for your reply. I'm interested in your kit and mailed you some queries regarding the kit along with my contact no.
Regards
Suman Bhattacharya
Is there any consensus on what kind of power supply does the unit need? (It's just tough for everyone landing here to search and end up with so many different values). On paper I could use a 1000VA
because current is like a pool and only drained in when needed.
I'm going to start with a purely theoretical approach and then factor in the practical aspects to arrive at a xfr rating. (And I'm bad at theory so feel free to correct me). And all this is done assuming that power can neither be created nor destroyed and the amp doesn't generate power, it merely transforms power/energy from one form to another even if using myref
Let's "assume" that the voltage requirement is 22Vac. Mauro's schematic states that each channel delivers 40W into 8ohms. i.e. for a stereo unit we need minimum 80W of power input to get the same output (theoretically).
Now P = I^2 * V
This gives us P = I^2 * 22
Or 1.90Amp or current.
Now amend for the power factor which is approx 1.8 on a bad day.
This means we'll need 1.9 * 1.8 = 3.42Amp for a stereo myref. Factor in the power loss of the amp components itself. so 4-5Amp should be sufficient and will "not" be an overkill.
Now here's the challenge.
I've assumed a voltage rating of 22Vac for an 8ohm speaker. Is this correct?
How do I convert this to a VA rating? A formula would be helpful.
because current is like a pool and only drained in when needed.I'm going to start with a purely theoretical approach and then factor in the practical aspects to arrive at a xfr rating. (And I'm bad at theory so feel free to correct me). And all this is done assuming that power can neither be created nor destroyed and the amp doesn't generate power, it merely transforms power/energy from one form to another even if using myref
Let's "assume" that the voltage requirement is 22Vac
. Mauro's schematic states that each channel delivers 40W into 8ohms. i.e. for a stereo unit we need minimum 80W of power input to get the same output (theoretically).Now P = I^2 * V
This gives us P = I^2 * 22
Or 1.90Amp or current.
Now amend for the power factor which is approx 1.8 on a bad day.
This means we'll need 1.9 * 1.8 = 3.42Amp for a stereo myref. Factor in the power loss of the amp components itself. so 4-5Amp should be sufficient and will "not" be an overkill.
Now here's the challenge.
I've assumed a voltage rating of 22Vac for an 8ohm speaker. Is this correct?
How do I convert this to a VA rating? A formula would be helpful.
Is that for a a dual-channel thing? Also it would be good to know where this rule of thumb comes from. Because we are not pulling 80W continuously unless you run it at a peak and listen to pink noise or similar. There's giant caps on a good power supply that store enough to allow good peaks (that eventually translate into PMPO).
With my calculations a 400VA could easily be called an overkill?
Is the thumb rule assuming some overhead for future expansion?
With my calculations a 400VA could easily be called an overkill?
Is the thumb rule assuming some overhead for future expansion?
You don't want to skimp on transformers unless you are on a really tight budget. Secondly you should always think of the 4ohm scenario where your wattage may be higher.You want a wide safety margin. Having said that since it is mainly a class ab amp you can get away with less but I would not go below 200VA. There is a very nice online simulator for power supplies
If you want/need a much fuller analysis of the PSU supplying a Power Amplifier, go and look at Gootee's Threads.Is there any consensus on what kind of power supply does the unit need? (It's just tough for everyone landing here to search and end up with so many different values). On paper I could use a 1000VA
I'm going to start with a purely theoretical approach and then factor in the practical aspects to arrive at a xfr rating. (And I'm bad at theory so feel free to correct me). And all this is done assuming that power can neither be created nor destroyed and the amp doesn't generate power, it merely transforms power/energy from one form to another even if using myref
Let's "assume" that the voltage requirement is 22Vac
Now P = I^2 * V
This gives us P = I^2 * 22
Or 1.90Amp or current.
Now amend for the power factor which is approx 1.8 on a bad day.
This means we'll need 1.9 * 1.8 = 3.42Amp for a stereo myref. Factor in the power loss of the amp components itself. so 4-5Amp should be sufficient and will "not" be an overkill.
Now here's the challenge.
I've assumed a voltage rating of 22Vac for an 8ohm speaker. Is this correct?
How do I convert this to a VA rating? A formula would be helpful.
Or
use a very much simpler "rule of thumb" that results from many builds over many decades.
Use a Transformer VA that is 1times to 2times the total maximum output power of the amplifier/s.
For a 40W+40W power amplifier, this amounts to 80VA to 160VA.
If the amplifier is outputting near maximum power continuously (driving a shaker table, or PA) then the VA ~ 2times continuous output power.
But these duties are probably met more easily using an SMPS as PSU.
No.
you use the speaker impedance/s that you are likely to use.
If like me, then I only consider 8ohms severe reactance speakers.
I design for driving those 8ohms speakers.
If you own or have access to 4ohms speakers, or 15ohms, or 1ohm, then that is your target design impedance.
For short term transients, you must design for higher currents than would be demanded by a purely resistive loading.
I design for transient currents of 3times what an 8r0 resistive load would demand, i.e. 100W into 8ohms and for 15Apk
This would be easily satisfied by adopting a 160VA, 35-0-35Vac transformer.
Just an example for MY brief, for MY speaker impedance.
Adjust the brief to suit your speaker impedance and your amplifier specification.
another example:
50W into 4ohms requires an output voltage of 20Vpk and a transient capability of ~15Apk
whereas a different example for 50W into 8ohms requires 28Vpk and a transient capability of ~11Apk.
Adjust the brief to suit your speaker impedance and your amplifier specification.
another example:
50W into 4ohms requires an output voltage of 20Vpk and a transient capability of ~15Apk
whereas a different example for 50W into 8ohms requires 28Vpk and a transient capability of ~11Apk.
Don't forget to caveat this so as to not confuse some though. A toroid that will do 15A is not what you necessarily want. If you look at manufacturers specs you will think you need a 500VA toroid to get 15A! Taking the 2:1 rule of thumb a 120VA transformer is rated around 3.3A RMS continuous which will cause head scratching amongst those new to the sport
As per the schematic mauro has mentioned 24Vac (and he was using 8Ohms like me). It would be good to calculate the power requirements since the LM3886 is working as a current pump. I remember he did mention the VA of his supply though. But I'm unable to find that post. I'm also looking for a formula to convert VA into current. My toroidal just blew up and I'm suspecting a shorted primary. So I'm going to invest into another PSU and I don't want it to be an overkill. Because when such PSUs are turned on, there's a high current inrush and the mains dims/fluctuates for a second (and I'm not comfortable with that). I want a minimal power and "safe" PSU. If I need to expand in the future I'll just buy another. Think "how much power supply would someone be using if selling these units commercially?" 400VA?
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Thanks for scaring me billshurv
So for a PSU to be exclusively used with "my_ref version 3" and exclusively with an 8Ohm load, what is the minimal supply that I can pick up from this page, that would not have a detrimental effect on the sound: Toroidal Transformer Manufaturers | Energy Saving Transformer | Toroidal Round Shape Transformer - Toroidal Transformers | Transformer Manufacturer & Supplier
So for a PSU to be exclusively used with "my_ref version 3" and exclusively with an 8Ohm load, what is the minimal supply that I can pick up from this page, that would not have a detrimental effect on the sound: Toroidal Transformer Manufaturers | Energy Saving Transformer | Toroidal Round Shape Transformer - Toroidal Transformers | Transformer Manufacturer & Supplier
VA = 1times to 2times the total maximum output power, works for all audio power amplifiers.
Finally Mauro says "In the schematic 2 view a simple Power ON speaker delay (2sec) plus -+ DC protections, and power supply. The transformer It's 225 VA 240/25+25Vac, and Diode (Bridge D1, D5) 5A 250V C5000/3300".
http://www.diyaudio.com/forums/chip-amps/54571-my-audiophile-lm3886-approach.html#post610143
Apparently he is using a 25-0-25 xfr rated at 225VA for stereo/dual channels. Apparently he didn't face any issues either since none were reported.
So would I feel sorry for buying a 25-0-25@225VA for my dual channel? What could go wrong? Heat, clipping?
http://www.diyaudio.com/forums/chip-amps/54571-my-audiophile-lm3886-approach.html#post610143
Apparently he is using a 25-0-25 xfr rated at 225VA for stereo/dual channels. Apparently he didn't face any issues either since none were reported.
So would I feel sorry for buying a 25-0-25@225VA for my dual channel? What could go wrong? Heat, clipping?
Thanks AndrewT. That simplifies it. The LM3886 datasheet says "135W Instantaneous Peak Output Power Capability" (since you are referring to the max output power). This means 135W * 2 = 270VA xfr. But this is for a single LM3886.
If you mean RMS, then that would be 80W * 2 = 160VA. Or 224VA in case we are expecting a 4ohm load since Mauro's pdf says "Output power(±37V): 40W/8ohm – 56W/4ohm"
If you mean RMS, then that would be 80W * 2 = 160VA. Or 224VA in case we are expecting a 4ohm load since Mauro's pdf says "Output power(±37V): 40W/8ohm – 56W/4ohm"
There are many formulas. However the phrase 'it depends' Comes into play here. The rule of thumb Andrew has quoted works. If you wish to go outside that then fine, as long as you understand why.