# 18V Class A

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#### Fusion916

18V, 1A bias. Simple and small transistor count. Discuss.

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#### Joachim Gerhard

I would like to see the circuit diagram.

Schematic.

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• sch.png
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#### Joachim Gerhard

So it does not have over all feedback.

#### AndrewT

the base current of Q3 is of the order of 10 to 15mA.
The bootstrapped R3 is passing about 3mA.
I would be looking at reducing R3 by a factor of 10 and maybe even lower.

#### lineup

Reduce the gain now set by 1.8k/100ohm.
By using 1.8k/330ohm you get lower distortion and a more suitable gain.

#### Fusion916

the base current of Q3 is of the order of 10 to 15mA.
The bootstrapped R3 is passing about 3mA.
I would be looking at reducing R3 by a factor of 10 and maybe even lower.
Can you explain how you determined this?

#### Fusion916

Reduce the gain now set by 1.8k/100ohm.
By using 1.8k/330ohm you get lower distortion and a more suitable gain.
I need the gain since I use some input sources with low line levels.

#### ATAUDIO

What about moving R4 to between the junction of C-E on the CFP Q2-Q3 and the output node(Q5 C)?
This way you will have some local feedback on the whole compound, getting it more linear, and achieving less distortion at the end, I suppose.
The R4 where it is now could be shorted out or set to a convenient 0.1 ohm if its sole purpose is to measure standby current.

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#### AndrewT

the base current of Q3 is of the order of 10 to 15mA.
The bootstrapped R3 is passing about 3mA.
I would be looking at reducing R3 by a factor of 10 and maybe even lower.

Can you explain how you determined this?
determine what base current flows in Q3. Clue what is a range of hFE for Q3 and what is the bias current of Q3?
determine the voltage across R3. What is the current in R3?

#### lineup

AndrewT,
but
your suggestion with increasing current makes no difference in my SPICE.
I think Fusion916 should well stick to using a 324 Ohm resistor there.

Regards Lineup

#### lineup

Alernative circuit with low distortion.

Here is a variant using two NPN 2SC5200 transistors.
The main difference is the FEEDBACK by resistor R9.
This makes amplifier have feedback and the distortion is much lower.

The THD 0.003% is at 1 Watt RMS output.
Max output is 4 Watt with reasonable distortion.

Regards Lineup

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