Hiraga "le monstre"

[Duco]

Bought 2K2 potmeters, they didn't have 2k5. Soldered them parallel to R2 and R3 on places R4 and R5.

Only when I change R5 does the DC-offset drop. I can get it lower than 10mV but it sound awfully horrible. When I change R4 nothing happens.

When DC-Offset is 4V with speaker connected it sounds all right, not perfect and has a slight bee in the box. But that just gets me right where I began.

-Duco

[Duco]

A bit general question:

To reduce the irritating hum, is it better to use a switching power supply instead of the torodial transformer and connect the capacitor bank to the outputs or use a torodial transformer and use power voltage regulators?

-Duco

[triode_al]

another tip: Increase R6/R9. In the original it was 2k-2k voltage divider (about 12V rails).
With 24V it would be 3k9/2k and with 34 volts I suggest 10 to 12Kohms.

(I have a cascode input up and running with 20k/5k on 37 V rails and that works like a charm. A wrong value ends up with sonic impact, I found out.)

You should not hear the hum when it is balanced right.

Did you check the Idss of the input pair to ensure they very well matched?

[triode_al]

Originally Posted by Duco

Bought 2K2 potmeters, they didn't have 2k5. Soldered them parallel to R2 and R3 on places R4 and R5.

Only when I change R5 does the DC-offset drop. I can get it lower than 10mV but it sound awfully horrible. When I change R4 nothing happens.

When DC-Offset is 4V with speaker connected it sounds all right, not perfect and has a slight bee in the box. But that just gets me right where I began.

-Duco

Duco,
I'm just pondering about this:
the best way is to start with both R4/R5 (the two pots now connected) at zero, and then iteratively increase the resistance until you have the required DC current (idle) that can be measured across R11/R12. There should be a second voltmeter on the output.
You could but do not need to have a resistance of 50 ohms on the output.

Take the top as example: Q4 needs 0,6 volts Vbe and R11//R12 take is 1 amp*0,47 (isn't that your total resistance?) so at the emitter of Q1 you should have about 1,0 to 1,1 volts. Having 0,8x volts there will give small bias current of just 0,5 amp.

The nice thing about this procedure is that it works too with different classes of Idss of the fets.

I hope you have a good pair of Q4 with ample current capability and very lineair Hfe: with higher voltage these can easily saturate.

Buy the way - - How do you connect a speaker when there is 4 volts on the output?

[Simeon Kostadinov]

Hello,
Has anyone tried using NJW0281 and NJW0302 for the output stage in Le Monstre? I am thinking of changing only these from original schematic. I know Sigurd Ruschkow is using these but his amplifier is running at more than 2 amp idle current. How about using these devices at lower current (~0.8A). They has higher Cob than the original Toshiba transistors, but I am not sure if this affects the sound at all. Any impressions ? Comments?

Thanks,
Simeon

[stefstef]

Hey ,i am new at the forum.I just bilt the 8 watt amplifier Hiraga.
Wheni put power on the circuit ther is a problem.On the negative site i misure a voltage on the 1k resistor but not on the positive site.Any a idee what cold be wrong?

[permaneder]

Originally Posted by Duco

@triode_al
Thanks for your comment

You say, mount in addition a 2k5 potmeter parallel to the R2=1k and one parallel to the R3=1K?

This way a have 3 potmeters and I'll be able to adjust the bias the a proper value?

Yours,
Duco

The higher the IDSS of the JFETs the higher the bias. Jean Hiraga operated FETs of Y suffix and an IDSS of significantly lower than 2,5mA.

Try to find transistors of the lowest possible IDSS and decreace the value of the 1k resistors. It is preferable to match the pair close but not a must.

To adjust a proper bias of say 600mA, substitute the resistors by 1k Pots trimmed to 500R. Let the amp warm up and measure the voltage across the collector resistors of both transistors in the output stage (V is equivalent to A). If the bias is too low, increase the values of both (!) trim pots to the same value and vice versa whether it's too high. Allow the amp to warm up again and control the bias again. It might be necessary to repeat the procedure more than one time. Once you have found the proper bias, measure the trimmed value of the pots and change them with fixed resistors of that value.

Voilà, your Le Monstre will work stable for many years!

[Simeon Kostadinov]

Hey folks,
Can you give me an advice if the On Semi 0281/0302 transistors are sutable substitution of the original output transistor?

[Simeon Kostadinov]

Hey folks, can you tell me if the On Semi transistors 0281/0302 are sutable substitution for the original output transistors ?

[ChicagoJTW]

If the bias is too low, increase the values of both (!) trim pots to the same value and vice versa whether it's too high.

If your transistors aren't perfectly matched, though, having matched bias resistors on each side won't balance. I try to match Iq rather than R.