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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, 08:12 PM   #11
blu_glo is offline blu_glo  England
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Default Quad 303 Circuit

This is the Quad 303 I remember.
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Old 10th September 2019, 08:56 PM   #12
gdan is offline gdan  Greece
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Modifications to the JLH 15-20W Class AB amplifier (1970)
Quote:
Originally Posted by Hans Polak View Post
The amp at the left is the Quad 303
The amplifier at the left is the one that J. Linsley Hood describes in the Wireless World, July 1970, article. I don't think it is the Quad.
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Old 10th September 2019, 08:57 PM   #13
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by Hans Polak View Post
Itís the successor of the Quad II tube amp and was Peter Walkers first transistor amp back in the early seventies.

Hans
The circuit in question is definitely that by JLH in published in Wireless World in 1970.

At one time I had that issue in my possession. The additional diode modification to the standard quasi complementary output stage (later recast in form by Baxandall) appeared in a letter from I.M.Shaw in the same issue of that magazine.

Initially I was not attracted by the use of bootstrap connections and the unbalanced look of Linsley-Hood's circuit. This and the preceding introductory article gave rise to a series of challenging letters to the editor and the idea never caught on.

It was recently reported that the latest Rega amplifiers incorporate some of Linsley-Hood's output stage thinking so it is remarkable nearly 50 years on the thinking is not completely dead.

Linsley-Hood's original aim was to devise a circuit to rival the simple Class A circuit Hood had designed but without the penalty of the heat dissipation involved.

That is the context of my current interest since I have a 1996 Class A which has that issue. I will save my comments on the AB design for later.

The Quad 303 which succeeded the Quad II used a quasi complementary triple output stage.
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Old 10th September 2019, 09:05 PM   #14
gdan is offline gdan  Greece
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Modifications to the JLH 15-20W Class AB amplifier (1970)
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Originally Posted by blu_glo View Post
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
I run a simulation with the values for R1, R2, R3, R4 you suggested. The maximum output of the amplifier was +15V and -9V peak. Not much better result.
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Old 10th September 2019, 11:46 PM   #15
mjona is offline mjona  New Zealand
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The dc offset should follow the arrangement for the 1996 Class A circuit where a 7815 regulator was used with a resistor and trim-pot in series from the output connecting to the emitter of the input transistor. For simulation purposes just use a voltage block as for the power supply rails and label it 7815. In real life that will need capacitor to earth to stabilise the output but you don't need to show that in your simulation.

The value of the series resistor trim-pot combination will be a multiple of 10k
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Old 11th September 2019, 12:23 AM   #16
steveu is offline steveu  United States
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This amp ran very high bias currents in both the driver and outputs, which would mean it generated a lot of heat even undriven. This was probably necessary because the compensation is a very large cap that bypasses the output out of the feedback loop so any cross-over distortion is not controlled by the feedback.
Your simulation raises the total supply voltage from 47 to 68 Volts. This is contrary to the need for operation with lower impedance loads while maintaining the thermal dissipation. However the original high bias currents mean the dissipation was much higher than it need be so if you can reduce the bias current to more typical levels then heat may not be a problem. But I have to wonder why you changed the voltage unless you plan to change the power transformer?
Experienced DIYA would completely redesign the amplifier board to a modern circuit but even they might be tempted to just buy existing modules from Apex or China/Amazon. It depends on if you just want a working solution or do you want to "make it your own".

You can reuse the power transformer for a bipolar circuit by creating a ground voltage half way between the 47V + and -, ei make 23.5V ground and float the existing negative. This takes two big ecaps instead of one.

One more thing. Bootstrapping is a great way to maximize voltage swing.

I should repeat what Mooly and Mark said: MOSFETs would be a good way to correct the lack of output current and the two LEDs is wasteful of voltage. If you choose to continue with the 100mA driver current then I would replace the LEDs with small diodes (~1n4148) and replace the emitter resistor with about 6.8 Ohms. Using MOSFET outputs, 25mA driver current would be plenty.

Last edited by steveu; 11th September 2019 at 12:52 AM.
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Old 11th September 2019, 01:09 AM   #17
PRR is online now PRR  United States
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> Your simulation raises the total supply voltage from 47 to 68 Volts. This is contrary to the need for operation with lower impedance loads

Exactly. A conversion from 16r to 4r at the same power suggests 24V, not 68V.

And cutting a lot of resistors to quarter-value.

Basically cutting a townhouse to a ranch house.
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Old 11th September 2019, 02:53 AM   #18
gannaji is offline gannaji  India
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Please see JLLH 20watt Class AB Lateral Mod
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Old 11th September 2019, 03:21 AM   #19
gannaji is offline gannaji  India
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I am interested in assembling this amp. I have the required semis.(current replacements). Can some one help in finalizing the design, PCBs etc ?
Also, transformer, Caps, box and Heatsinks !
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Old 11th September 2019, 03:32 AM   #20
gannaji is offline gannaji  India
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dear gdan
Please post your .asc file and models file.
--gannaji
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