help with preliminary calculations
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 6th April 2013, 01:28 AM #1 tenderland   diyAudio Member     Join Date: Jul 2004 Location: us help with preliminary calculations Hello , After a few tube amp projects I have decided to build a solid state amplifier. The speaker handles a RMS power of 5 W. Here are the calculations I came up with. P(with reserve)= 15W Vrms=11v Vpk= 15v(aprox) Irms (1ch)= 1.37A This gives me a rail supply of +/- 15V Do these calculations look accurate ? Thanks J __________________ "If every one is making wheels , why make wheels ? " Steve Jobs
 6th April 2013, 01:33 AM #2 counter culture   diyAudio Member   Join Date: Jun 2011 8 ohm speaker? OK. __________________ Now I am Become Death, the Destroyer of Words
 6th April 2013, 01:42 AM #3 tenderland   diyAudio Member     Join Date: Jul 2004 Location: us Input stage Thanks ! Yes 8 ohm. Iam working on the Input stage ( actually bouncing all over) I will have a cc source feeding a diff pair loaded by a current mirror. Where I am running into some confusion is how I determine how much current is needed(gm) flowing from the collector of the diff pair. From what I have read the diff pair and all the other transistors generally have a beta of 100. __________________ "If every one is making wheels , why make wheels ? " Steve Jobs
glennb
diyAudio Member

Join Date: May 2004
Location: Melbourne
Quote:
 Originally Posted by tenderland Hello , After a few tube amp projects I have decided to build a solid state amplifier. The speaker handles a RMS power of 5 W. Here are the calculations I came up with. P(with reserve)= 15W
Its arguable that the power amp should be rated at LESS than than the speakers.

Quote:
 Vrms=11v Vpk= 15v(aprox) Irms (1ch)= 1.37A This gives me a rail supply of +/- 15V
You need to allow a few volts for losses, power supply regulation and OPS saturation.
__________________
Glenn.

Boscoe
diyAudio Member

Join Date: Feb 2010
Location: England
Quote:
 Originally Posted by glennb Its arguable that the power amp should be rated at LESS than than the speakers.
Please ignore this, it's total utter rubbish. Ignore the speaker power rating it means nothing when it comes to music, you want to get an amplifier as powerful as is reasonable otherwise they just clip sound harsh and dull.
__________________
Audio projects and more: gswdh.co.uk

 6th April 2013, 08:30 AM #6 AndrewT   diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Speakers can pass a very low power when it is DC. When the signal is constant amplitude single frequency sinewave they can pass a lot more power, at least double the DC power rating. If the power is delivered as brown noise or pink noise or white noise, then you will find that the maximum power that can be passed is nearly double the constant sinewave. If the power is delivered as a single event pulse, the peak power of that pulse can be nearly four times the constant sinewave power. The ratio of maximum peak power to DC power can easily exceed 10 times. This makes it difficult to determine exactly what amplifier power rating to use, prior to hearing the result with real music and speech signals. Manufacturers will use any of these power definitions for rating their speakers. The honest manufacturer will specify which power rating they are using. The dishonest just adopt any power value that they think will attract uninformed buyers. That 5W could be a sinewave or a noise power rating. It might work with a 1W amplifier without blowing up, equally it may survive many decades being driven sensibly by a 50W amplifier. __________________ regards Andrew T.
 6th April 2013, 01:03 PM #7 tenderland   diyAudio Member     Join Date: Jul 2004 Location: us Power Thanks all, After reading the replies and initial gut feeling. IVe decided to make it 30 W/ch. This will match well With my skill level and budget . Thanks for the input This is much more difficult than building a tube project. J __________________ "If every one is making wheels , why make wheels ? " Steve Jobs
 6th April 2013, 03:24 PM #8 counter culture   diyAudio Member   Join Date: Jun 2011 Recall that beta is not a reliable parameter of transistors, it varies greatly. Therefore design procedures are aimed at developing designs which do not depend on this parameter. The LF gain of the amplifier does depend on beta, but we ignore this as long as there is sufficient gain to eliminate LF distortion by NFB. I'll try and get back to you about the collector current in the diff. pair, I'm a bit busy ATM. __________________ Now I am Become Death, the Destroyer of Words
godfrey
diyAudio Member

Join Date: Nov 2009
Location: Cape Town
Quote:
 Originally Posted by tenderland From what I have read the diff pair and all the other transistors generally have a beta of 100.
Most small signal transistors have beta higher than that. e.g. BC547C (NPN) and BC557C (PNP) have beta > 400. It's generally a good idea to use high-beta transistors for the input diff pair.

 6th April 2013, 06:59 PM #10 tenderland   diyAudio Member     Join Date: Jul 2004 Location: us great help Thanks all, I will try to keep my questions minimal. I have the three popular books by Sloan, Self and Cordell. I am pooring over them. __________________ "If every one is making wheels , why make wheels ? " Steve Jobs

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