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Old 2nd March 2012, 05:06 PM   #1
M0UKD is offline M0UKD  United Kingdom
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Default Simple Class A amplifier

Hi guys.

I have been playing with some simple single ended class A amplifiers today. I had some 2SC3281 transistors laying about and I came up with the circuit below. I have made similar circuits before but with MOSFET's.

I have a question. Being a transistor, the input impedance is relatively low. Originally I had a 10uF on the input, but the audio lacked bass. I replaced it with a 100uF and all is OK. I am driving it from a laptop and have tried my iPhone, both which drive the amp nicely. The MOSFET circuits I have built in the past, which are very similar in design, I have only needed a 0.1uF on the input due to the higher impedance.

I don't have any line output source here at the moment, but I was wondering if line out, say from a CD player, would work OK with this type of circuit, or would I have to build some kind of pre-amp to drive the low impedance?

Thanks for any help

John.

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Old 20th March 2012, 11:23 PM   #2
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muy interesante tu esquema..... ahora me pregunto.. cuantos watts de salida posee este amplificador & si es posible confeccionar para el mismo un trasformador de acople?????
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Old 20th March 2012, 11:31 PM   #3
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querido john: claro que tienes que construir un pre amplificador... por supuesto en pura clase A ya que el amplificador es en clase A . tal vez con un trasformador de acople en la salida lo puedas llevar a mas watts & con un capacitor de 1500 micro faradios x 35 volts los bajos serán mas profundos e intensos que con uno de 100.
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Old 21st March 2012, 01:17 AM   #4
iko is offline iko  Canada
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This is an English speaking forum; please translate these two posts to English, or they will be deleted.
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Old 21st March 2012, 01:48 AM   #5
godfrey is offline godfrey  South Africa
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Courtesy of Google translate:
Quote:
your scheme very interesting ..... Now I wonder .. few watts of output has this amp & if possible to make himself a coupling transformer???
Quote:
dear john: of course you have to build a pre amp ... course in pure class A amplifier since the class is. perhaps with a coupling transformer at the output it can lead to more watts & a 1500 micro farad capacitor x 35 volts low will be more profound and intense with a 100.
Wow! I'm impressed the translator copes so well with lousy grammar.
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Old 27th June 2012, 06:08 AM   #6
redsky is offline redsky  Indonesia
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Quote:
Originally Posted by M0UKD View Post
Hi guys.

I have been playing with some simple single ended class A amplifiers today. I had some 2SC3281 transistors laying about and I came up with the circuit below. I have made similar circuits before but with MOSFET's.

I have a question. Being a transistor, the input impedance is relatively low. Originally I had a 10uF on the input, but the audio lacked bass. I replaced it with a 100uF and all is OK. I am driving it from a laptop and have tried my iPhone, both which drive the amp nicely. The MOSFET circuits I have built in the past, which are very similar in design, I have only needed a 0.1uF on the input due to the higher impedance.

I don't have any line output source here at the moment, but I was wondering if line out, say from a CD player, would work OK with this type of circuit, or would I have to build some kind of pre-amp to drive the low impedance?

Thanks for any help

John.
Hi John,

Many Thanks for your schematics. I was build mini ZCA base on multiple BD139 (cause i just have it on hand, and it same like 2SC3281 transistor). But I follow Marks Houston schematic for biasing voltage. See my projects in DIY Audio Projects Forum • Mini ZCA dual mono 2.6 watt.
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Old 27th June 2012, 07:51 AM   #7
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Quote:
Originally Posted by redsky View Post
Hi John,

Many Thanks for your schematics. I was build mini ZCA base on multiple BD139 (cause i just have it on hand, and it same like 2SC3281 transistor). But I follow Marks Houston schematic for biasing voltage. See my projects in DIY Audio Projects Forum • Mini ZCA dual mono 2.6 watt.
Since you like to experiment, here's something you should try.
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Old 27th June 2012, 08:34 AM   #8
MarianB is offline MarianB  Romania
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@godfrey your translation is not acurate at all, you rely too much on google translate!

Quote:
Originally Posted by lvcas vilches View Post
muy interesante tu esquema..... ahora me pregunto.. cuantos watts de salida posee este amplificador & si es posible confeccionar para el mismo un trasformador de acople?????
In english:
Youre schematic is very interesting... now i wonder...how many watts does it delivers? ( how much power output ) ,and is it possible to built a coupling transfomer for it?

Quote:
Originally Posted by lvcas vilches View Post
querido john: claro que tienes que construir un pre amplificador... por supuesto en pura clase A ya que el amplificador es en clase A . tal vez con un trasformador de acople en la salida lo puedas llevar a mas watts & con un capacitor de 1500 micro faradios x 35 volts los bajos serán mas profundos e intensos que con uno de 100.
In english:
Dear John, ofcourse you need to built an preamplifier, and of course it has to be in pure class A since the amplifier is in class A, and with a coupling transformer on the output you could get more watts from it, also with a 1500uF/35V capacitor the base would be much more profound ant intense than that with a 100uF capacitor.

PS: @lvcas vilches, 1500uF capacitor en la entrada? no es mucho? ( @lvcas vilches, 1500uF capacitor on the input? isn't it too much? )
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