Need a big transformer...
Bookmark this page:
 Yahoo!
 Google
 Windows Live
 del.icio.us
 digg
 Netscape
|
|
Posted by on March 22, 2008, 4:35 pm
Please log in for more thread options http://www.national.com/mpf/LM/LM3886.html At 68W RMS, assuming 50% efficiency, I'd need at least, what, a 150W transformer, right? And if I build two for stereo, that would be 300W required. 70V secondary, center-tapped, to get +/- 35V. Those things are expensive on Mouser ($60+). Especially considering the LM3886 only costs $7 or so. Any suggestions where I can get such a large transformer for cheap? Thanks, Michael | ||||
|
Posted by Bob Eld on March 22, 2008, 6:12 pm
Please log in for more thread options There is no such thing as an RMS Watt. You mean 68Watts sign wave average. Depending on the service, you can usually get by with about 1/4 of the average sign wave power in the transformer which would be about 70 Watts. 100 Watt transformer would be a little better. Remember music reproduction does not need continuous power. Any transformer can supply four times it's rated power in short bursts. It's a matter of heating or how hot the transformer is allowed to get. For your application a 100Watt transformer is more than sufficient. | ||||
|
Posted by Phil Allison on March 22, 2008, 10:02 pm
Please log in for more thread options
"Bob Eld" >
>mrdarrett@gmail.com >> So let's say I want to build an LM3886 amplifier.
>> http://www.national.com/mpf/LM/LM3886.html >> >> At 68W RMS, assuming 50% efficiency, I'd need at least, what, a 150W >> transformer, right? >> >> And if I build two for stereo, that would be 300W required. 70V >> secondary, center-tapped, to get +/- 35V. >> >
> There is no such thing as an RMS Watt. ** Really ? > You mean 68Watts sign wave average.
** Which is precisely what the term " watts rms " means in audio amplifier specs. > Depending on the service, you can usually get by with about 1/4 of the
> average sign wave power in the transformer which would be about 70 Watts. > 100 Watt transformer would be a little better. Remember music reproduction > does not need continuous power. > > Any transformer can supply four times it's rated power in short bursts. ** The penalty is in the regulation factor, which is four times worse than normal. With a rectifier and capacitor input filter, the current peaks are higher than the simple resistive load case - makes the DC regulation much poorer than the AC figure quoted in the specs for the transformer. Eg: a nominal 100VA transformer has a full ( resistive) load regulation of 10%. So for a peak secondary voltage of 35 volts under load, the off load peak voltage needs to be 39 volts for a resistive load and 43 volts for a rectifier load - which is higher that the absolute max spec for the LM3886. With four times current overload, the AC peak output and DC supply voltages falls drastically, to around 20 volts. > It's a matter of heating or how hot the
> transformer is allowed to get. For your application a 100Watt transformer > is > more than sufficient. ** Not true at all. BTW: The OP has made his usual dumb mistakes. The 68 watt figure applies to a resistive amplifier load of 4 ohms and DC supplies of ** +/- 28 volts ** A transformer with a 70 volt AC secondary ( plus CT) will produce DC rails of +/- 54 volts off load. Way to high - bye, bye LM3886. The transformer he needs is one rated at 200VA with a 25-0-25 volt, 4 amp secondary. This will give him DC rails of +/- 37 volts off load, dropping to about +/- 28 volts at full load. ....... Phil | ||||
|
Posted by Bob Eld on March 22, 2008, 10:48 pm
Please log in for more thread options
>
> "Bob Eld" > >
> >mrdarrett@gmail.com > >> So let's say I want to build an LM3886 amplifier.
> >> http://www.national.com/mpf/LM/LM3886.html > >> > >> At 68W RMS, assuming 50% efficiency, I'd need at least, what, a 150W > >> transformer, right? > >> > >> And if I build two for stereo, that would be 300W required. 70V > >> secondary, center-tapped, to get +/- 35V. > >> > >
> > There is no such thing as an RMS Watt. >
> ** Really ? > > > You mean 68Watts sign wave average.
>
amplifier
> > ** Which is precisely what the term " watts rms " means in audio > specs.
Yes I know, a bull shit term invented by uneducated technicians and marketing types trying to impress the unwashed masses. The term has been misused for many years but that doesn't make it right. RMS means root mean square. Power isn't a root, power is already squared. Only current and voltage can be the root. You, of all people, should know that. It's electricity 101 stuff. | ||||
|
Posted by on March 22, 2008, 11:12 pm
Please log in for more thread options >
> > > > > > > "Bob Eld"
>
> > >mrdarr...@gmail.com
> > >> So let's say I want to build an LM3886 amplifier. > > >>http://www.national.com/mpf/LM/LM3886.html >
> > >> At 68W RMS, assuming 50% efficiency, I'd need at least, what, a 150W
> > >> transformer, right? >
> > >> And if I build two for stereo, that would be 300W required. 70V
> > >> secondary, center-tapped, to get +/- 35V. >
> > > There is no such thing as an RMS Watt.
>
> > ** Really ?
>
> > > You mean 68Watts sign wave average.
>
> > ** Which is precisely what the term " watts rms " means in audio
> amplifier
> > specs.
>
> Yes I know, a bull shit term invented by uneducated technicians and > marketing types trying to impress the unwashed masses. The term has been > misused for many years but that doesn't make it right. RMS means root mean > square. Power isn't a root, power is already squared. Only current and > voltage can be the root. You, of all people, should know that. It's > electricity 101 stuff. You're absolutely right! National Semi uses the nomenclature "continuous average output power". That's what I'd meant to say. My Kenwood is advertised as 50W per channel. Peak... what does that mean? For one second before the amplifier fries itself? For one millisecond? For five seconds? Then a web search (Kenwood itself didn't publish this) shows my Kenwood can put out 22W RMS. Ah. http://www.crutchfield.com/S-OzS0NnnnJC2/App/Product/Item/Main.aspx?I=113mp205&search=kenwood+mp205 Now if they'd said "continuous average output power" that would be more correct..? MD | ||||
| Similar Threads | Posted |
| Transformer | November 7, 2004, 11:17 pm |
| about transformer | February 26, 2005, 10:49 am |
| transformer 110 to 240 | May 6, 2005, 3:07 pm |
| AC to DC Transformer | January 4, 2006, 3:12 pm |
| Transformer | August 22, 2006, 1:50 pm |
| Transformer Help | January 28, 2007, 10:39 am |
| Need a big transformer... | March 22, 2008, 4:35 pm |
| LEI-4 transformer | July 30, 2008, 6:10 pm |
| transformer question .. | August 15, 2004, 12:09 pm |
| Transformer and Frequency | August 17, 2004, 7:41 am |
| Transformer and Frequency | August 17, 2004, 8:56 am |
| Transformer getting hot and burning | November 13, 2004, 5:21 am |
| Transformer Current | November 14, 2004, 6:01 pm |
| 600-ohm to 600-ohm line transformer | January 31, 2005, 2:38 pm |
| 600-ohm to 600-ohm line transformer | January 31, 2005, 4:18 pm |
> http://www.national.com/mpf/LM/LM3886.html
>
> At 68W RMS, assuming 50% efficiency, I'd need at least, what, a 150W
> transformer, right?
>
> And if I build two for stereo, that would be 300W required. 70V
> secondary, center-tapped, to get +/- 35V.
>
> Those things are expensive on Mouser ($60+). Especially considering
> the LM3886 only costs $7 or so.
>
> Any suggestions where I can get such a large transformer for cheap?
>
> Thanks,
>
> Michael