JLH Question

[abpea]

Hello Everyone,

I'm pulling together parts to build the "Final Circuit" in the 2003 JLH Upgrade doc found here: The Class-A Amplifier Site - JLH Class-A Update

I have a couple questions;

1. Can someone give me a pointer to a suitable variable resistor (there are 3) appropriate for this circuit?

2. Is R10 330 Meg Ohm?

Thanks for all your help -
Bruce

[picowallspeaker]

By checking the schematics , the resistor at the collector of the power transistor should be a hi-wattage resistor , 15-20 w ,and the "m" means "milli",so 0,33 ohm . The values of variable resistors are written aside -50 ohm , 200 ohm , 20 kilo ohm .

[abpea]

Originally Posted by picowallspeaker

By checking the schematics , the resistor at the collector of the power transistor should be a hi-wattage resistor , 15-20 w ,and the "m" means "milli",so 0,33 ohm . The values of variable resistors are written aside -50 ohm , 200 ohm , 20 kilo ohm .

Hi picowallspeaker,

Thank you for your reply. I'm glad I asked about the 'm' on the resistor value otherwise I would have gotten something totally inappropriate. Much appreciated.

I was hoping you or someone could point me to a source (item #) for the variable resistors. I did see the values for the VRs on the schematic, just not sure which one to get or where's the best place to get them.

Thanks again -
Bruce

[picowallspeaker]

Hello ! Sure where you pick up your components you'll find those "trimmers" . Those are nothing but potentiometers , little 3-legged round components ,nothing more .

[abpea]

I'm trying to select a mains transformer for my JLH project.

If I'm reading correctly this amp will run at 22V rails at 1.75-2.0 amps.

I found a Plitron transformer (Part # 077018201) with the following specs:

300VA
Secondary: 2 x 35v @ 4.29A

I believe I can make this a center tap for dual rail operation.

I'm planning on building the capacitance multiplier power supply (schematic here: http://sound.westhost.com/project15.htm)

Any reason not to use this transformer?

Thanks -
Bruce

[Greg Erskine]

hi Bruce,

That transformer does not look right to me.

2 x 0-35 will give nearly 50 volt rails assuming a bridge rectifier is used.

regards

[anthony.taylor1967]

Originally Posted by abpea

I'm trying to select a mains transformer for my JLH project.

If I'm reading correctly this amp will run at 22V rails at 1.75-2.0 amps.

I found a Plitron transformer (Part # 077018201) with the following specs:

300VA
Secondary: 2 x 35v @ 4.29A

I believe I can make this a center tap for dual rail operation.

I'm planning on building the capacitance multiplier power supply (schematic here: Capacitance Multiplier Power Supply Filter)

Any reason not to use this transformer?

Thanks -
Bruce

Hi abpea,

According to the parts list, it requires a 50-0-50 300VA Toroidal Transformer, If you need any more info, I can send you the parts list.

Regards

Anthony

[JPeitzman]

The capacitance multiplier needs a 100vct power transformer, does it really eat that much voltage? You should be able to run it off of a 36vct power transformer. At least this is what I do, alas I do not use a capacitance multiplier so consider that. Apparently I am going to have to read up on the capacitance multiplier, it has been forever since I messed with anything solid stage outside of Nelson Pass inspired simple FET circuits, hollow state is taking all my time.

BTW, that resistor, the .33R one, it isn't critical. Anything roughly around that value will work, and in fact you can completely leave it out. I am pretty sure all it is there for is to measure current through the output stage. Simple Ohm's Law, you know the resistance, so you measure the voltage drop across said resistor and you end up with the current through the output stage. Just don't use 330 meg ohm!

Cheers
James

edit: I am also running the original JLH, but with lower rail voltages, around 22v give or take.

[pilli]

abpea,

I also built that exact version of the JLH.

For the capacitance multiplier, I don't know where the 100V center tap comes from. The article on Rod Elliot's page is generic, and for the JLH the voltages are given at the bottom of the page.
I have 19-0-19 V-AC, that rectified and after the cap-multiplier give around 22V DC per rail.

For the variable resistors, I would suggest multiturn trimmers, they allow more precise trimming. They cost less than one dollar.

Concerning the 0.33ohm, I had also understood that it is used as an easy way to measure the quiescent current (btw, I had a 0,27 at hand, and I stopped looking for a calculator to know the exact Iq - I settled on 300mV, as long as the heatsinks don't burn, and the two channels are the same...)

Hope this helps,
pilli

(So I just repeated what the previous posters had already said... oh well...)

_

[abpea]

Thanks everyone for all your excellent advice.

I read Rod Elliot's article a couple more times and understand the circuit better.

I found a 300VA, 2 X 20V @ 7.5A center tapped transformer I should be able to use for this project.

I'll let you know how it comes out.

Now if I could find a nice source for some 10,000uf caps to use as well!!

Thanks again, all your advice is much appreciated -
Bruce