# help with preliminary calculations

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.

#### 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
Vrms=11v
Vpk= 15v(aprox)
Irms (1ch)= 1.37A

This gives me a rail supply of +/- 15V

Do these calculations look accurate ?

Thanks
J

#### counter culture

8 ohm speaker? OK.

#### tenderland

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.

#### glennb

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.

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.

#### Boscoe

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.

#### AndrewT

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.

#### tenderland

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

#### counter culture

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.

#### godfrey

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.

#### tenderland

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.

#### counter culture

OK, you have the Cordell and the Self books.

The simplest way of understanding the current in the IPS its to think of it as affecting the gain. Look on page 42 of the Cordell book. You will see a simple IPS with current source and resistor loading. The gain of the stage can be understood as the collector load (1k in this case) divided by the emitter resistance. In this case it is the emitter-to-emitter resistance.

The emitter resistance re is 26/Ie(in milliamps). The value of Ie is one milliamp in this case so the resistance is 26 ohms, but doubled to 52 ohms because it is an LTP. It's a transconductance amplifier, and the transconductance goes up with the collector current.

It's generally a good idea to have a high beta in the driven resistor of the VAS, because

1) it loads the the differential pair, and its beta affects its input impedance

2) the overall voltage gain depends on beta, but...

...look at p.95 of the Self Book (4th.ed), you will see that local shunt NFB through Cdom sets the HF gain and controls Nyquist stability, but the LF gain below the dominant pole frequency is ultimately of little importance as long as there is sufficient gain for overall linearisation and the linearity of the VAS is not markedly affected by the IPS transistor type, given a reasonably high beta.

Why not learn to use LTspice and do some simulations, it's a great tool for learning about circuits.

#### tenderland

good stuff

Thanks Counter Culture , thats some good info. I have a trial version of NI Multi sim im trying to do some simulations on. I also down loaded LT Spice.

J

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.