# Toroidal coil

#### TieBravo

Joined Aug 21, 2021
50
Can you please tell me the toroidal coil details for this circuit? And is it suitable for 8 ohm speakers?

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#### Ian0

Joined Aug 7, 2020
3,463
What makes you think those inductors will be toroidal?
Is this some sort of class-A transistor amplifier?
The size of the inductor will be determined by the amount of power you want it to supply, which you didn't specify.
It will be suitable for 8Ω speakers, if you have the right supply voltage, which will be determined by the amount of power you want it to supply, which you didn't specify.

#### TieBravo

Joined Aug 21, 2021
50
What makes you think those inductors will be toroidal?
Is this some sort of class-A transistor amplifier?
The size of the inductor will be determined by the amount of power you want it to supply, which you didn't specify.
It will be suitable for 8Ω speakers, if you have the right supply voltage, which will be determined by the amount of power you want it to supply, which you didn't specify.
What if the power is 12v?

#### Ian0

Joined Aug 7, 2020
3,463
What ever the power supply voltage for an 8Ω load the inductance must be
$$L>\frac R{2\pi f}$$
which gives 63mH for a 20Hz frequency response.
For a class A amplifier, you need a standing bias current of V/R which is 1.5A, and the peak current will be twice that.
So 63mH @ 3A.
A Hammond195S4 is the nearest I could find.
https://www.hammfg.com/files/parts/pdf/195S4.pdf
As it needs a gap to deal with the standing DC current, it's unlikely anyone will want to make it on a toroid.

#### MisterBill2

Joined Jan 23, 2018
9,467
The circuit shown will burn out the speakers after a fairly short time. Aside from that, it will not sound very good because of the nonlinearity. It looks like a fake yoo toob post.

#### Audioguru again

Joined Oct 21, 2019
3,632
You found a useless audio power amplifier circuit.
The coils cut high audio frequencies.

I changed its transistor and base resistor value for a higher output power.
I simulated it to show that the output power is 1.8W with severe distortion.
The speaker and transistor each heat with 4.5W all the time even when not playing sounds.
The DC in the speaker causes its cone to be pulled over to one side causing more distortion.
The input impedance is less than only 2 ohms.

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#### MisterBill2

Joined Jan 23, 2018
9,467
Then when the speaker coil burns loose the distortion gets really bad. The circuit is total junk.

#### Ian0

Joined Aug 7, 2020
3,463
You found a useless audio power amplifier circuit.
The coils cut high audio frequencies.

I changed its transistor and base resistor value for a higher output power.
I simulated it to show that the output power is 1.8W with severe distortion.
The speaker and transistor each heat with 4.5W all the time even when not playing sounds.
The DC in the speaker causes its cone to be pulled over to one side causing more distortion.
The input impedance is less than only 2 ohms.
Surely you must know that your SPICE simulations mean nothing, because Baranek's law applies. (I'm sure it applies equally to amplifiers as it does to loudspeakers.)
Besides, that inductor is probably wound with silver wire, and that would make the output pure, just like a silver bullet kills a vampire!

#### Ian0

Joined Aug 7, 2020
3,463
Seriously though, there is nothing wrong, per se, with the concept of an inductance-loaded single-ended Class-A amplifier. It's about twice as efficient as a resistance-loaded one - the efficiency goes from "dreadful" to "poor".
It worked very well with valves - in fact, if you make the same circuit with a 300B instead of a 2SC5200, you could sell them for £20,000 a pair (but you would have to be a dab hand at a) cabinet making and b) marketing) because the distortion is still about 10%
But transistors are nowhere near as linear as valves, but if you add a handful of small signal transistors you could get something quite respectable, with a sensible input impedance.
I'll draw out a circuit later, but I bet you'll be put off by the cost of the chokes!

#### MisterBill2

Joined Jan 23, 2018
9,467
That circuit is fundamentally flawed because all of the transistor current flows through the speaker voice coil. It does burn out the speaker. That is why that one generation of transistor car radios had an external transformer mounted on the speaker.

#### Ian0

Joined Aug 7, 2020
3,463
That circuit is fundamentally flawed because all of the transistor current flows through the speaker voice coil. It does burn out the speaker. That is why that one generation of transistor car radios had an external transformer mounted on the speaker.
It’s drawn wrong. The choke should connect to the power transistor collector, and the speaker should connect between that point and ground with 4700uF in series.

#### AlbertHall

Joined Jun 4, 2014
11,524
It’s drawn wrong. The choke should connect to the power transistor collector, and the speaker should connect between that point and ground with 4700uF in series.
That would most cetainly be an improvement.

#### Audioguru again

Joined Oct 21, 2019
3,632
Surely you must know that your SPICE simulations mean nothing.
My simulation shows the severe distortion when a transistor is close to cutoff with poor linearity.
The simulation shows 4.5W in the speaker all the time even though the maximum output is much less at 1.8W.
The simulation also shows the 4.5W all the time in the transistor.
The total heating power is 5 times as much as the output power. What a waste!
I did not show the input impedance because I cannot believe it is less than 2 ohms.

#### Audioguru again

Joined Oct 21, 2019
3,632
It’s drawn wrong. The choke should connect to the power transistor collector.
The choke, transistor and speaker are all in series then the sequence makes no difference.

#### MisterBill2

Joined Jan 23, 2018
9,467
Like I already mentioned, it is a poor design. Changing it to have a cap in series with the speaker makes it a bad design that will not burn out the speaker, but still a poor design.

#### dcbingaman

Joined Jun 30, 2021
444
Surely you must know that your SPICE simulations mean nothing, because Baranek's law applies. (I'm sure it applies equally to amplifiers as it does to loudspeakers.)
Besides, that inductor is probably wound with silver wire, and that would make the output pure, just like a silver bullet kills a vampire!
I would not go so far as to say simulations in SPICE mean nothing. Obviously simulations have their limitations. But Audioguru has some valid points. Even a simple analysis shows the circuit is highly inefficient and has major distortions over an equivalent IC chip or Module designed to be an audio amplifier.

#### Ian0

Joined Aug 7, 2020
3,463
I would not go so far as to say simulations in SPICE mean nothing. Obviously simulations have their limitations. But Audioguru has some valid points. Even a simple analysis shows the circuit is highly inefficient and has major distortions over an equivalent IC chip or Module designed to be an audio amplifier.
SPICE simulations don't mean anything for those people to whom Baranek's law applies!
I got my facts wrong - the silver bullet kills a werewolf, not a vampire.

Here's what you can do if you add a handful of small signal transistors. (If you insist that a handful means five then the BCV61 is a dual.) MY version of Spice doesn't have the 2SC5200 so I substituted an MJL3281A.
Spice predicts 0.02% for the distortion, mainly 2nd harmonic.
0.3Ω is the DC resistance of the choke. The bias sets about 500mV across the choke resistance to give a standing current of 1.8A.

#### dcbingaman

Joined Jun 30, 2021
444
SPICE simulations don't mean anything for those people to whom Baranek's law applies!
I got my facts wrong - the silver bullet kills a werewolf, not a vampire. View attachment 246461View attachment 246462

Here's what you can do if you add a handful of small signal transistors. (If you insist that a handful means five then the BCV61 is a dual.) MY version of Spice doesn't have the 2SC5200 so I substituted an MJL3281A.
Spice predicts 0.02% for the distortion, mainly 2nd harmonic.
0.3Ω is the DC resistance of the choke. The bias sets about 500mV across the choke resistance to give a standing current of 1.8A.
What point are you trying to make? This circuit has nothing to do with the original one under discussion? This does look like a fairly nice amplifier but I could do it with an op-amp without so many parts.