JLH 10 Watt class A amplifier

[AKSA]

I sim it with Jfet for input and superpair baxandall for phase inverter. THD is very low

Bimo,

Clearly too little THD based on Bigun's interesting comments.
Sometimes building a good amp takes more detective skills and engineering........

Hugh

 

[AndrewT]

Good idea. If possible, increase C1 (the output cap) by a factor of 4 or 5.

> a better overall design than the later DC coupled ones

If you enjoy the sound of coupling capacitors at input and output.


Patrick

If the input coupling capacitor is a type that does not generate distortion in the audio band AND sets the passband.
Then a lesser type at the output that has a much wider bandwidth than that carefully selected input capacitor will be virtually inaudible.

A 1uF in to Rin of 39k gives a a lower passband limit of 4Hz
Select a 10mF feeding the 8ohms speaker and it has a passband of 2Hz.

this is back to the usual rule of ensuring the INPUT sets the passband.

 

[Bigun]

this is back to the usual rule of ensuring the INPUT sets the passband.

Agreed - or at least, ensuring the OUTPUT cap doesn't set the passband.

p.s. like your signature line !

 

[davidsrsb]

If you enjoy the sound of coupling capacitors at input and output.

Patrick

The output capacitor removes the need for a speaker protection relay, potentially a greater evil.
Some of the soldering seen in photos here would scare me if there was no speaker protection

 

[AndrewT]

The output capacitor removes the need for a speaker protection relay, potentially a greater evil.
Some of the soldering seen in photos here would scare me if there was no speaker protection

Be careful in what you say.
In some the output capacitor must charge up.
That charging current has to pass around the circuit.
Is the Speaker in the charging circuit?

 

[davidsrsb]

Be careful in what you say.
In some the output capacitor must charge up.
That charging current has to pass around the circuit.
Is the Speaker in the charging circuit?

It is and classic amplifiers like the Quad 303 and various Sugdens have had no problems with it

 

[AndrewT]

in your "classic" examples, do the output capacitors charge up after power ON?

 

[Bigun]

I don't think it's an issue, although one could imagine extreme cases where the capacitor was so large that there would be enough energy to damage a fragile driver. The issue is with turn-on / off current surge causes the speaker to move and maybe generate a 'thump' sound.

 

[mosfetDemon]

I don't recall 'switch-on plop' (as JLH called it) being a problem on my JLH69.

 

[dubai2000]

Has anybody tried the following boards? Any comments?
JLH 1969 class A amplifier stereo high quality PCB components assembled ! | eBay

Thx for any feedback.

 

[e_fortier]

I don't recall 'switch-on plop' (as JLH called it) being a problem on my JLH69.

I believe it depends on the speaker itself, my JLH69 is powering some high efficiency 96db Fostex, without the speaker protection it gets pretty loud at turn-on and turn-off. With 87 db with thicker cone you won't hear much.

p.s. My output capacitor is a 4700uF, 35V so maybe such a big value doesn't help with the thump.

BR,
Eric

 

[e_fortier]

Has anybody tried the following boards? Any comments?
JLH 1969 class A amplifier stereo high quality PCB components assembled ! | eBay

Thx for any feedback.

Kits looks OK except for the old rusted 2N3055 output transistors.

May I recommend to use the MJ15003, these are great sounding output device for this amp.

BR,
Eric

 

[dubai2000]

Thanks, Eric, for the input - and the MJ15003 are a direct replacement - I take it.

 

[AllenB]

They can be, all the usual disclaimers. They were for me.

edit: I used ON devices.

 

[abpea]

I have some JLH 1969 and 1996 Update pc boards on the way!

In the mean time I'm looking for the proper power supply to use for either version. On The Class A Amplifier Site Geoff has a section on Rod Elliots' capacitance multiplier power supply that looks easy enough to put together.

Here's the schematic:

An externally hosted image should be here but it was not working when we last tested it.

Geoff says, "The capacitance multiplier circuit has been developed further, by Rod Elliott of Elliot Sound Products, and the results published as Project 15 at the ESP Audio Pages. The modified circuit is suitable for both the original 1969 JLH amplifier (using only the positive half of the circuit) and the 1996 update."

Has anyone used this power supply with the JLH amps and, just to double check, it can be used as either a single or double rail power supply?

Thanks -
Bruce

 

[AndrewT]

The original JLH power amplifier uses a single ended PSU.
Some of the later developments use a dual polarity PSU.

You have to identify which style you have adopted and from there select either a dual polarity PSU, or a single polarity PSU.

 

[Bigun]

Has anyone used this power supply with the JLH amps and, just to double check, it can be used as either a single or double rail power supply?

There's enough PSRR at the output stage to avoid having to regulate the main power rails. Instead, put a cap multiplier on the power rails to the front end of the amplifier as I mentioned already above (TGM9).

 

[Nico Ras]

That is a cap "multiplier"

 

[dubai2000]

Sorry to ask again, but doing some net searching I found the following offer - any comment? Would the case they have put the amp into be large enough?
Hifi 2.0 channel amplifier Electronic filtering hood 1969 Class A amplifier 10W*2 in Haube 1969 kleine Klasse einen verstrker, macht ist nicht gro, aber mit den Raum und mit bcherregal, warm und einlade aus Verstrker auf AliExpress.com | Alibaba

Thx again for any feedback.

 

[Nico Ras]

I would say it is large enough. In my opinion the price is good.