# Audio amplifier design, basic questions.

#### Coollestersmooth

Joined Dec 18, 2014
72
^^ i was almost ready to give up on this today, as i had no clue how to do the calculations, working them out entirely on my own would have been beyond me. i can only thank you all for keeping the dream alive! im going to go get a good nights sleep due to waaaaaay to much stressing about this the last few days. Tomorrow i shall sit down and run some calculations.

#### #12

Joined Nov 30, 2010
18,224
That's how you learn. Do the math, hook it up, see what smokes, do it over.
Three or four of these and you will be able to do anything you want with a discrete audio amplifier.

#### Coollestersmooth

Joined Dec 18, 2014
72
all caps must be rated for Vcc/2 or just those in the output/feedback loop?

#### #12

Joined Nov 30, 2010
18,224

Vcc/2?
Not likely.
More like Vcc times 2.
The voltage rating on a capacitor is its maximum survivable voltage.
Don't cheap out. Good electrolytic caps only cost a few nickles or dimes.
You wouldn't save a dollar on this if you installed half a dozen risky rated capacitors.

#### Coollestersmooth

Joined Dec 18, 2014
72
ah, ok. read it somewhere. having rechecked it i think it was only for the bootstrap capacitor, which i am looking into atm since its reintroduction in jonys updated circuit in post #114. at least im assuming thats what C3 is.
from what i can gather bootstrap capacitors have to reach a certain minimum value in order to ensure minimal ac across them but i shouldn't need to do too much calculation for it.

#### #12

Joined Nov 30, 2010
18,224
Even if you can prove a certain capacitor normally runs below Vcc/2, you don't play that game with an experimental setup. Every cap should be over-rated so you don't get a face full when you make a mistake.

Safety, dude. Never bet you life that you couldn't possibly pick up the wrong capacitor or connect it to the wrong place.

#### Coollestersmooth

Joined Dec 18, 2014
72
thank you jony, i'm going to have a play in an hour or 2, all going well i should be running simulations later

#### Jony130

Joined Feb 17, 2009
5,539
The positive saturation voltage we can find using this simplified circuit

Vout_max = Vcc - Vce(sat)3 - Vbe4 - VR10 - V_Cout.

So if we assume Vce(sat) = 0.2V; Vbe4 = 0.8V ; V_Cout = 10V and we can find ILmax and Vout_max.

ILmax = (Vcc - Vce(sat) - Vbe4 - V_Cout)/(R10 + RL) = (20V - 0.2V - 0.8V - 10V)/(4.33Ω) ≈ 2A

And from this

Vout_max = 2A*4Ω = 8V

Vsat+ ≈ 2V

Now let us try to find the negative saturation voltage.

But first we need to find a DC voltage across C3 capacitor

VC3 = Vx - Vy = 10V - 220Ω*20mA = 10V - 4.4V = 5.6V

Notice here that now the voltage across R5a is almost constant and equal to VC3 - Vbe6 ≈ 4.8V, And this means that R5a acts just like a constant current source. And this is why C3 also increase Q3 voltage gain.
IL_max for negative half of a cycle we get when Q6 and Q7 enter saturation region.
IL_max_neg ≈ (V_Cout - Vbe(sat))/(R9 + RL) = (10V - 0.8V)/(4.33Ω) ≈ 2.12A

So Vsat- ≈ Vbe(sat) for Q6

Without C3 situation for positive half cycle is exactly the same as before, but for negative half of a cycle the situation looks different

Now without C3 Iload_max_neg for negative half of a cycle is equal to:

Iload_max_neg = (V_Cout - Vbe6)/( (R5a+R5b)/(Hfe6 * Hfe7) + R9 + RL )

So if we assume Hfe6_min = 100 and Hfe7_min = 20 we have Hfe6*Hfe7 = 2k

Iload_max_neg = (10V - 0.8V)/(440Ω/2k + 0.33Ω + 4Ω ) ≈ 2A

What a surprise C3 don't help much, why is that ?? Well because I chose large Q3 current (20mA) (low R5a+R5b) and the current gain of a complementary feedback output pair (Sziklai pair) is quite large. But don't remove C3 from the circuit because as a I said earlier C3 also increase open loop gain (Q3 voltage gain).

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#### Coollestersmooth

Joined Dec 18, 2014
72
so im working through the calculations at the moment, i am just calculating Rin and am wondering, if i plan on putting the input through a 10k variable pot to ground for volume control, this would need to be aloud for in my calculations for Rf1? my gut instinct is leaning towards no since at max output this resistance is 0 Ohms

something like this:

if i am to consider it Rin would become
Rin = Rv + R1||R2

#### #12

Joined Nov 30, 2010
18,224
Which drawing are we working with now?
What happened to the differential pair? If you use a differential pair for the input, the gain control feedback circuit is safely separated from the input signal.

#### Coollestersmooth

Joined Dec 18, 2014
72
this was just tacked on to the front of the previous circuit, was just a quick sketch up trying to add volume control. q1 in that image would be q1 in post #114 in which jony stated that
R1 = R2 = 0.5Vcc/(10 * IcQ1/Hfe_min) ≈ 10V/0.2mA ≈ 47kΩ
Also notice that R1||R2 determine the amplifier input resistance. Rin = R1||R2 = 47kΩ/2 = 23kΩ
And to reduce the "DC offset" RF1 should be equal to Rin
RF1 = 22kΩ

#### #12

Joined Nov 30, 2010
18,224
True, but this is a DC consideration. Use a capacitor to couple the input signal to Bq1 so the volume control does not affect the DC drift rate.

#### Coollestersmooth

Joined Dec 18, 2014
72
like this?

in a similar vein. the value for that capacitor is calculated at 5.33x10^-7F
i have assumed i am using a 1 uF cap as good 50V electrolytic caps aren't available any smaller, think at should be fine as rearranging gives a lower freq at about 5 Hz

#### bertus

Joined Apr 5, 2008
22,306
Hello,

Did you see the link I posted?
( http://sound.westhost.com/heatsinks.htm )
In there this is said:

9 - Thermal Compounds
One of the most common mistakes made by hobby electronics enthusiasts (and quite a few professionals too), is to assume that if a little thermal compound is good, a lot must be better. Absolutely not so! The amount of thermal compound should be exactly that amount which ensures that an air-free join is made between the mating surfaces. If too much is applied it will cause an increase in thermal resistance, since it is not really that good at conducting heat. Generally speaking, any electrical insulator is also a thermal insulator, so the thinner the final composite insulation - including thermal 'grease' - the better.

Having said that, one must ensure that the electrical insulation is sufficient for the applied voltage or disaster will surely follow - usually in a spectacular fashion - especially if high voltages or currents are available.

Although several manufacturers over the years have thought they could get away with using silicone grease with no fillers, don't! It doesn't work, and eventually flows out from under the transistor leaving the thermal connection dry and causing device overheating and failure. Always use a good quality thermal compound, and make absolutely certain that it is non-conductive if the transistors are to be insulated from the heatsink. Some of the specialised compounds for CPU cooling in PCs are electrically conductive, and cannot be used where electrical insulation is needed.

Because you use so little thermal compound, I suggest that you invest in good quality. Some are marginal - they're cheap, but you may find out why after a few years when output devices fail. Both the filler (usually an oxide of some kind) and carrier 'grease' (almost always silicone) must be appropriate for the purpose. You need plenty of filler, and a carrier that is sufficiently viscous to keep the filler in suspension, but is soft enough to allow you to get a thin, even coating.

A quick note on the application of thermal compounds is in order. This is the method I generally use, and it works very well once you have a copious supply of workshop rags or paper towels to clean the mess from your fingers.

Apply a small quantity of the thermal compound to one finger, then gently rub with your thumb to create an approximately even coating on finger and thumb. Now rub the thermal compound onto a washer, held between thumb and finger, ensuring that the coating is just thick enough to be opaque, but thin (and even) enough to ensure that the contact will be absolute on both surfaces (transistor and heatsink). Test your skill until with moderate pressure, you can leave a perfectly formed outline of the transistor, with no blobs or gaps, on the heatsink surface.
Bertus

#### #12

Joined Nov 30, 2010
18,224
like this?

Yes.

in a similar vein. the value for that capacitor is calculated at 5.33x10^-7F
i have assumed i am using a 1 uF cap as good 50V electrolytic caps aren't available any smaller, think at should be fine as rearranging gives a lower freq at about 5 Hz
Use a metal film capacitor.

I get 0.17 uf for -3db @ 40 Hz.

#### Coollestersmooth

Joined Dec 18, 2014
72
hi guys, yes thankyou, i am yet to look into sourcing mica or similar along with some thermal grease. i assume applying it is similar to tim for cpu's in pc's, ie little blob, which should then spread when the two plates are pushed together to give a good air free seal. are sil-pads really that awful?
ill have a further look into caps. thanks #12

#### #12

Joined Nov 30, 2010
18,224
Smear the grease around with a Q-tip so it doesn't have to moosh so far.

ps, I just told takao to send you one of his red capacitors. He will probably e-mail it to you.