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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 11th September 2019, 06:42 AM   #21
gdan is online now gdan  Greece
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Modifications to the JLH 15-20W Class AB amplifier (1970)
Quote:
Originally Posted by Osvaldo de Banfield View Post
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.
I have built the original circuit with split power supply and the DC offset at the output is very stable and I use the amplifier every day for some years now.
The emitter resistor is 0.47Ω in my new modified circuit. Maybe I have to decrease this to 0.22 Ω. I will check that.
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Old 11th September 2019, 06:43 AM   #22
gdan is online now gdan  Greece
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Modifications to the JLH 15-20W Class AB amplifier (1970)
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Originally Posted by Mooly View Post
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.
This is a nice idea. I think I will try it!
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Old 11th September 2019, 06:53 AM   #23
gdan is online now gdan  Greece
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Modifications to the JLH 15-20W Class AB amplifier (1970)
Quote:
Originally Posted by Mark Tillotson View Post
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.
Nice idea! One green LED is enough.
I run the simulation with only one LED and decreasing the resistor to set the same current. This gave only 0.2 more Volt at the negative side.
But I will keep your proposal.
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Old 11th September 2019, 07:03 AM   #24
gdan is online now gdan  Greece
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Modifications to the JLH 15-20W Class AB amplifier (1970)
Quote:
Originally Posted by mjona View Post
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.
I have also built a lower power version of the JLH 1996 Class A amplifier and the only problem is than runs very-very hot. Especially here in Greece it is not possible to operate it during the hot days of summer.
So I was looking for another solution and I built the original JLH Class AB (1970 circuit). It runs without much hot and I cannot hear any difference with the Class A (1996) circuit.

regards
George
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Old 11th September 2019, 07:18 AM   #25
gdan is online now gdan  Greece
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Modifications to the JLH 15-20W Class AB amplifier (1970)
Quote:
Originally Posted by steveu View Post
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.
As I mentioned in post #1, I used split power supply even for the original circuit of JLH 1970 amplifier.
Also I want to keep the original amplifier, and I want to built a new one with the suggested modifications.

Yes, I agree it is a good idea to decrease the voltage of the power supply, for a 4 Ω load. But I have found that the maximum peak voltage that I need for my loudspeaker is about 25-26 Volt peak during loud passages of music. So that is why I use a voltage of +/- 34 Volt and accept the greater heat consumption.

I will keep only one LED for the current source.

thanks
George
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Old 11th September 2019, 07:22 AM   #26
gdan is online now gdan  Greece
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Modifications to the JLH 15-20W Class AB amplifier (1970)
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Originally Posted by gannaji View Post
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 !
I will design a PCB when the amplifier design is finalised. But this it will take some time I think!

I will post also the LTspice files later.
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Old 11th September 2019, 07:23 AM   #27
Hans Polak is online now Hans Polak  Netherlands
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Quote:
Originally Posted by gdan View Post
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.
Look here
https://www.vintageshifi.com/reperto...ers-Manual.pdf
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Old 11th September 2019, 07:31 AM   #28
gdan is online now gdan  Greece
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Modifications to the JLH 15-20W Class AB amplifier (1970)
Quote:
Originally Posted by Hans Polak View Post
You can read the original article of JLH here :

https://americanradiohistory.com/Arc...ld-1970-07.pdf

See page 321 of the magazine.

I see some similarities but the Quad is much more complicated.

regards
George
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Old 12th September 2019, 02:01 AM   #29
steveu is offline steveu  United States
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Another option is to add an pair or large power transistors to the existing circuit. 25 years ago, I did this to a Sansui G-4700 receiver that I was given because it had blown up. You change the existing emitter resistors to about 2 Ohms and drive the added transistors (c/w heat sink) from there. I used a 2n3773's and 2.2 Ohms. Note this was quasi addition to a complementary circuit so the lower 2n3773 was driven from the PNP collector. Depending on the bias, this gives only fair THD but indestructible output. My old receiver has since been shorted and abused with no failure. The idea is a bit like current dumping. It would be a great way to handle low-Z loads.
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Old 12th September 2019, 03:14 AM   #30
gannaji is offline gannaji  India
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Dear steveu,
Can you kindly add your suggestion to the .asc file, appended below.
Attached Files
File Type: asc JLH1970AB original mymod.asc (6.9 KB, 19 views)
File Type: txt mymodels.txt (14.8 KB, 14 views)
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