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Hiraga 20W class A
Hiraga 20W class A
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Old 21st May 2019, 11:28 PM   #1141
Silvestre de moraes is offline Silvestre de moraes
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I'm sorry, I only speak in Portuguese, I'm using google translator. Someone could help me adjust the bias of my Hiraga, between base and emitter is with 12.5volts offset 4mv, thank you all from the forum. Silvestre Moraes
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Old 22nd May 2019, 02:39 AM   #1142
Silvestre de moraes is offline Silvestre de moraes
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I need to adjust the voltage between base and emitter that is at 12.5 Volts. Anyone know how many mv would be?
I was able to adjust to 0.4 mv on the output.
See the pictures bellow.

WhatsApp Image 2019-05-21 at 20.16.22.jpeg - Google Drive

Last edited by Silvestre de moraes; 22nd May 2019 at 02:54 AM.
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Old 22nd May 2019, 07:38 AM   #1143
huggygood is offline huggygood  France
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what is your power supply tension (Vdc)?
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Old 22nd May 2019, 07:42 AM   #1144
huggygood is offline huggygood  France
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in fact,you can adjust the bias uniquely with the two resistor 1.8k between the bases of the output transistors .
the potentiometer (500r or 1000r) is for adjust the offset
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Old 22nd May 2019, 07:44 AM   #1145
huggygood is offline huggygood  France
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Old 22nd May 2019, 02:25 PM   #1146
Silvestre de moraes is offline Silvestre de moraes
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30+ -0- 30-dc
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Old 22nd May 2019, 09:04 PM   #1147
Silvestre de moraes is offline Silvestre de moraes
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I need to adjust the voltage between base and emitter that is at 12.5 Volts. Anyone know how many mv would be?
I was able to adjust to 0.4 mv on the output.
See the pictures bellow.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
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Old 27th May 2019, 08:05 AM   #1148
triode_al is offline triode_al  Netherlands
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Some critisism: sloppy (bad) soldering. At least that is what it looks like.
I can see many wires not immersed like a liquid in solder. So there must be a bad contact.

The components have not been cleaned or have been resoldered many timesand the result is connections that are prone to decay, getting worse, for example when warming up, the sound changes or cracks are heard.

I had the same bad style (if it works it works?) and used 'modern' high temperature solder, but very hard to work with.
I since a few years use SAVBIT, expensive but the result is stable now.

Hiraga20 selfmade.jpg
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Old 27th May 2019, 04:29 PM   #1149
huggygood is offline huggygood  France
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i'm ok with Triode_Al ,you have to redo your solders.
"30-0-30vdc it's too high .
this amp (with its original components) is very very sensitive to its supply voltage.
there you are closer to 30w.
the 20w must remain in the band of 18 / 23vdc, (23vdc max) charging.
if you can, try a variac or bucking transformer to lower the voltage on the primary and therefore the secondary.
if that's what I think, you'll be able to set your offset to almost zero with the right supply voltage.
I reviewed a real 20w from "La Maison de l' Audiophile"
which is over thirty years old.
because of the increase in the mains voltage, it was completely out of order.
I made him the complete power supply, added a bucking transform then changed the resistance of polarization and now, he left for thirty years of good and loyal services.
the most important with this amp is the power, you really have to do it properly and do not take too much freedom with the original plans.
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Old 28th May 2019, 06:41 AM   #1150
Ian Finch is online now Ian Finch  Australia
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Apart from supply voltage, you may know that the 13V is controlled by the RD13 zener diodes, which logically are 13V types. Equivalents are listed in supplier catalogues but they are unusual stock items and may be difficult to buy locally.

It's possible to select 13V zeners at the high or low end of their manufacturing tolerance there but you could try placing a common 1N4148 diode in series with a 12V zener in either location but in reverse polarity to it. This will bump the voltage up by 0.7V and may be enough to trim either the upper or lower zener voltage and shift the DC offset. However, I think the better way may be to adjust the emitter resistors in the input transistor pair. These are already offset in value (210Ω v 240Ω), which suggests there was a good reason for it. Much of it has to do with differences between the NPN/PNP input transistors which cause the offset.

In reality, there is no point in trimming to any closer than about 30 mV offset. It just appeals to our sense of precision to have 0 mV offset, just like we imagine symmetrical circuits to be better than asymmetrical
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Last edited by Ian Finch; 28th May 2019 at 06:47 AM.
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