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Modifications to the JLH 15-20W Class AB amplifier (1970)
Modifications to the JLH 15-20W Class AB amplifier (1970)
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Old 10th September 2019, 04:45 PM   #1
gdan is offline gdan  Greece
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Modifications to the JLH 15-20W Class AB amplifier (1970)
Default Modifications to the JLH 15-20W Class AB amplifier (1970)

A few years back, I built this amplifier, as described in the article of the Wireless World magazine in July 1970. The schematic is shown in the left picture. My modifications concerned a regulated split power supply and the use of 2N3055H and MJ2955 as output transistors.
Also I used the components values for the 8 Ω load as shown in the article.
I was very pleased with the technical and acoustical performance of the amplifier.
The only "problem" I found was when the load was less than 8 Ω. In this case the maximum voltage output was very different for the positive and negative side of the signal.

Recently, I needed a low power amplifier to drive a 4 Ω loudspeaker and I decided to look again at this amplifier and modify it so that it could drive a 4 Ω load. My intension was to perform as little changes as possible.
The right figure shows the schematic of the modified amplifier which was also extensively simulated in LTSpice.

Any comments or suggestions for improvements are welcome.
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File Type: png JLHood Class AB amplifier - mod.png (78.7 KB, 603 views)
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Old 10th September 2019, 04:49 PM   #2
Ylli is offline Ylli  United States
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..As little modifications as possible??!!!?!?!?
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Old 10th September 2019, 06:23 PM   #3
steveu is offline steveu  United States
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The reason this circuit performs poorly under low-Z load is that there is only one EF stage on the negative side, and you have not changed that feature??? If you want to use the same PCB, perhaps you should consider using Darlington power transistors. This will allow you to take the heat off Q6 and Q8, and regain ~3 Volts of output swing wasted in Q8. Note that any active current source uses some voltage swing compared to the original bootstrap. Many of the best amps use 3 stages of EF. Be sure to cross-couple the drivers if you add EF stages.
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Old 10th September 2019, 07:14 PM   #4
blu_glo is offline blu_glo  England
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The original circuit was primarily designed for 16 ohms speakers which were more commonly used back in the day.

The modified resistor values originally given are for 8 ohms speakers/load.

For 4 ohms my "guess" would be R1 = R2 = 0.22R R3= 270R R4 = 47R, and re-set current (using the 100R pot) for:
200mA (320mW will be in class A)
400mA (for 640mW in class A)

Heatsinks should settle at less than 60 degrees max for safety if on outside of case and touchable.
you can do 15-20 degrees more if they are inside a well ventilated case and not directly touchable.

With the resistor values thus changed there should not be any need to modify the negative side EF stage.

This information is given as guidance only and has not been tested!

Simon

Last edited by blu_glo; 10th September 2019 at 07:32 PM.
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Old 10th September 2019, 07:53 PM   #5
Hans Polak is offline Hans Polak  Netherlands
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The amp at the left is the Quad 303
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Old 10th September 2019, 08:07 PM   #6
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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It seems to me that this kind of circuits aren't suitable for split power supplies, as the DC offset at the output can't be accurately fixed to 0V. Also, 1.5Ω in the emiters are too high to drive 4Ω loads, you are loosing about 40% of the output swing there.

I would try another better configuration, IMHO.
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Old 10th September 2019, 08:17 PM   #7
Mooly is offline Mooly  United Kingdom
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Modifications to the JLH 15-20W Class AB amplifier (1970)
I'm thinking... replace the output pair with lateral FET's. That would overcome the limitation of the driver stage and laterals should also be stable in bias current with the simple bias scheme this amp has.

If you really want to go dual rail then add a simple servo.
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Old 10th September 2019, 08:17 PM   #8
latala is online now latala  United Kingdom
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Modifications to the JLH 15-20W Class AB amplifier (1970)
What do you mean the amp at the left is a quad 303
Trev
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Old 10th September 2019, 08:21 PM   #9
Hans Polak is offline Hans Polak  Netherlands
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Quote:
Originally Posted by latala View Post
What do you mean the amp at the left is a quad 303
Trev
Itís the successor of the Quad II tube amp and was Peter Walkers first transistor amp back in the early seventies.

Hans
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Old 10th September 2019, 09:00 PM   #10
Mark Tillotson is offline Mark Tillotson
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The new design has a lot of volts wasted in the current sink load using two green LEDs in series. Lose one of the LEDs and reduce the resistor accordingly perhaps? This should lead to more symmetrical clipping (which is one of the stated goals).


You have no high frequency decoupling on the PSU rails.
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