Minima SE Class A amplifier - help please - diyAudio
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Old 11th June 2008, 09:36 PM   #1
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Default Minima SE Class A amplifier - help please

Okay - firstly...

This amp is based around 2n3055 (Siemens) transistor as I have plenty of them in the parts bin.

I wanted to build an amplifier, solid state, as simple as a tube amp.

Design roughly guided by Douglas Self's writings.

I have constructed a crude but operational amplifier but there is a few tiny niggles. The output suffers from "sibilance" like a vinyl record played with a out-of-alignment stylus. Why is that?

Should I exchange the first transistor for a lower current / quiter type?

I call it the "MINIMA SE CLASS A".

Any advice and input highly apperciated.

Dewald
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Old 11th June 2008, 09:44 PM   #2
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Pic of the working prototype.

Yes, I know the transistors gets hot and must be heatsinked - there is a fan blowing over them whilst I test them. Played for nearly 30 minutes with no trouble.

Powered by a 12v lead acid battery.

Source is a CD-ROM with attenuator.

Load is my set of AKG K240 head monitors.

I am very impressed with the sound but I need to straighten out that sibilance...

D
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Old 11th June 2008, 09:54 PM   #3
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base of that left transistor is hungry ;

from where you are feeding it ?

(think about biasing )
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Old 11th June 2008, 10:01 PM   #4
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Quote:
Originally posted by Zen Mod
base of that left transistor is hungry ;

from where you are feeding it ?

(think about biasing )
Engrish please

D
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Old 11th June 2008, 10:06 PM   #5
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Quote:
Originally posted by Zen Mod
base of that left transistor is hungry ;
Should I change the first 2N3055 to a BC550? Will it work? I'll check tomorrow...

D
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Old 11th June 2008, 10:20 PM   #6
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Quote:
Originally posted by GlidingDutchman


Engrish please

D

you must add another resistor from positive voltage - to base ;

first transistor isn't biased ;

later you can try smaller input transistors , but first you need to "repair" prototype
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Old 11th June 2008, 10:31 PM   #7
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Quote:
Originally posted by GlidingDutchman


Should I change the first 2N3055 to a BC550? Will it work? I'll check tomorrow...

D
2N3055 = output tube

Now you do not use 6550 as input tube? do you?
More likely an ECC88 or even ECC83


Now 2N3055 needs some current to drive.
So, BC550C may be too weak.

You need more like one MJE340 or BD139.
They are medium power transistors, which usually refers to as 'drivers'.
this is because these transistors have enough current output capacity
to drive big Power tranistors, like 2N3055.

One BC550C is a small signal transistor, which can feed a driver transistor (BD139/MJE340),
which in its turn can drive 2N3055, output.

If you have MOSFET like IRF610 or IRF510,
they can be used as BOTH input and driver.
Nelson Pass has done this. Some times.

Otherwise I say you try first with MJE340 or BD139. They are NPN.


Lineup regars
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Old 11th June 2008, 10:33 PM   #8
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Like so (see attachment)

But what will (R7) XXX be?
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Old 11th June 2008, 10:38 PM   #9
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Quote:
Originally posted by lineup


2N3055 = output tube

Now you do not use 6550 as input tube? do you?
More likely an ECC88 or even ECC83


Now 2N3055 needs some current to drive.
So, BC550C may be too weak.

You need more like one MJE340 or BD139.
They are medium power transistors, which usually refers to as 'drivers'.
this is because these transistors have enough current output capacity
to drive big Power tranistors, like 2N3055.

One BC550C is a small signal transistor, which can feed a driver transistor (BD139/MJE340),
which in its turn can drive 2N3055, output.

If you have MOSFET like IRF610 or IRF510,
they can be used as BOTH input and driver.
Nelson Pass has done this. Some times.

Otherwise I say you try first with MJE340 or BD139. They are NPN.


Lineup regars
Okay... now its becoming a bit clearer...

I have some "exotic" Motorola transistors somwhere and will try them as driver units.

What is your opinion on the biasing of the first transistor?

D
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Old 11th June 2008, 10:51 PM   #10
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Here they are: Motorola MJE4919 & SJE5300 Transistors

Motorola MJE4919 & SJE5300 transistors

D
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