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Question of the amplifier circuit Jean Hiraga's Class-A
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Author:  Ben Lamb [ 23 Feb 2014, 11:21 ]
Post subject:  Re: Question of the amplifier circuit Jean Hiraga's Class-A

Yes they would be ok but I would use BC550/560 myself. Buy a load of them as they are only cheap and match a few nice ones up with hfe around 600. Good amps these and well worth building - good luck.

Author:  MrMckSPAZZY [ 26 Mar 2014, 10:53 ]
Post subject:  Re: Question of the amplifier circuit Jean Hiraga's Class-A

I only have a 30-0-30 transformer. +-42V measured at the supply caps. What changes would I need to make to the design to work on the higher voltage?
Can anyone give more clarification on how to set the bias (1.8k and 500ohm)? The old links are dead.

Author:  Ben Lamb [ 14 May 2014, 05:45 ]
Post subject:  Re: Question of the amplifier circuit Jean Hiraga's Class-A

The main problem with using the 42v is the massive heat that will be produced, I recon you could leave the circuit as it is but will need some low speed 120mm fans on your heatsinks to cool it down.

Author:  suztek [ 28 May 2014, 06:12 ]
Post subject:  Re: Question of the amplifier circuit Jean Hiraga's Class-A

As the previous message suggets ,the heat will have to be taken care of. The bias current is not adjusted by altering the 1.8k resistors. The 47 k resistors will have to be increased to allow a current of 1.5 amps at 42 volts ( not recommended) to flow through the output transistors. Use a higher value 58 k and reduce until the right value is reached.The drivers MUST be mounted on individual heatsinks as they will get extermely hot. The 500 ohms (use a multi turn) should at first be set at midway 250 ohms on each side. Once the bias current has been set, the amplifier should be left on for at least 2/3 hours whilst monitoring the heatsinks temperature and bias current..Do not worry as the DC offset will at first be very high. After the amp has stabilized, the DC offset should be adjusted to below 100 mv, preferably 20-50 mv. I suggets that the bias current be set at 1 amp so as to allow any current increase when the DC offset is adjusted. The sound difference between a bias set at 1,5 amps or 1 amp is not noticable. After a few days of operation re-adjust the DC offset.
Be aware that due to the higher voltages some of the resistors will get hot.
I would stick to +/- 35 volts using a well designed regulated power supply that can handle the currents.
The Hiraga amplifiers in Class A configuration have a unique amazing sound. :arrow:

Author:  djestro [ 28 Jul 2014, 15:23 ]
Post subject:  Re: Question of the amplifier circuit Jean Hiraga's Class-A

Hi,

I'm planning to build the Jean Hiraga's Super Class-A Amplifier amplifier. I'm only 20 years old and still go to school. I already build 2 amplifiers : The Leach amplifier and de JHL (John Linsley Hood (15W) class A amplifier). Now I have a question about the Hiraga. I read on this site the heatsinks run at about 40°C above room temperature, right ? When I do a quick calculation I think that it would be 143°C above room temperature.

Rth (j-->mb) = 0.83°C/W ( Junction to case 2SC5200)
Rth (mb--> heatsink) = 8500 m°C/W (thermal paste)
Rth (heatsink --> ambient) = 0.4°C/W (heatsink Fisher SK47 200x150x40 similar to the ones you used)

Rth total = 1.23°C/W

P = 1.65 * 35V = 58W each transistor so 2 transistors on 1 heatsink makes it 116W
T = 116W * 1.23°C/W = 143°C
T total = 143°C + 25°C = 167°C --> this would be the themperature of the heatsink ?
Do I anything wrong here because the transistors would not survive it I think.

Kinds regards,
Djestro.

Author:  suztek [ 29 Jul 2014, 00:44 ]
Post subject:  Re: Question of the amplifier circuit Jean Hiraga's Class-A

Your calculations may be correct but in practice and in my experience with building a working Hiraga amp, each heatsink should run at +/- 70-75 degrees C with summer ambient temperature at 24-26 degrees C. You should not use more than 35 v for the power supply and set the bias to 1 amp. Higher bias settings will increase the heatsink temperature which you will have to size accordingly. You could also use a small computer fan and monitor the temperature with a simple electronic circuit. Good luck

Author:  djestro [ 30 Jul 2014, 05:47 ]
Post subject:  Re: Question of the amplifier circuit Jean Hiraga's Class-A

I agree with you if the bias current is lower, the heatsink will have to dissipate less power (obvious).
But now I have a different question. Lets say the bias current is set to 1A. We hook up a 8 ohm speaker. at max power (clipping) there will flow 1A(rms) through the speaker. Since it is 8 ohm we can calculate the voltage over the speaker, this would be 8 Vrms. When I calculate this to Vpeak-peak it will be 22.56 Vpp... why then use such a high rail voltage ?

Author:  mwhouston [ 30 Jul 2014, 06:51 ]
Post subject:  Re: Question of the amplifier circuit Jean Hiraga's Class-A

djestro wrote:
I agree with you if the bias current is lower, the heatsink will have to dissipate less power (obvious).
But now I have a different question. Lets say the bias current is set to 1A. We hook up a 8 ohm speaker. at max power (clipping) there will flow 1A(rms) through the speaker. Since it is 8 ohm we can calculate the voltage over the speaker, this would be 8 Vrms. When I calculate this to Vpeak-peak it will be 22.56 Vpp... why then use such a high rail voltage ?

Bias current DOES NOT GO THROUGH THE SPEAKER. The output cap stops that.

Author:  suztek [ 30 Jul 2014, 09:42 ]
Post subject:  Re: Question of the amplifier circuit Jean Hiraga's Class-A

djestro wrote:
I agree with you if the bias current is lower, the heatsink will have to dissipate less power (obvious).
But now I have a different question. Lets say the bias current is set to 1A. We hook up a 8 ohm speaker. at max power (clipping) there will flow 1A(rms) through the speaker. Since it is 8 ohm we can calculate the voltage over the speaker, this would be 8 Vrms. When I calculate this to Vpeak-peak it will be 22.56 Vpp... why then use such a high rail voltage ?

Transistor Biasing is the process of setting a transistors DC operating voltage or current conditions to the correct level (obvious) so that any AC input signal can be amplified correctly by the transistor. A transistor steady state of operation depends a great deal on its base current, collector voltage, and collector current and therefore, if a transistor is to operate as a linear amplifier, it must be properly biased to have a suitable operating point.
So if you use a higher voltage you will obtain more power.

Author:  daan [ 14 Oct 2015, 06:18 ]
Post subject:  Re: Question of the amplifier circuit Jean Hiraga's Class-A

I have just completed building the ± 60w version of this amp. On switch on there was just about no current flow in the output transistors. I then started re-adjusting the 47k resistors until I got ±1.65 amp
flowing on both + and - sides. I am not sure if this is the correct way of adjusting the bias currents or should I do this by adjusting the 1k8 bias resistors. All the transistors are as indicated in the schematic but I did not do any pair matching.

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