JLH 15 watt class a question

[Buhl]

Hi there
Will try to build the famous JLH class A amp (new 1996 version) - but theres a few things in the diagram I'm in doubt about:

The only high power resistor is the R10 0R33 ?

Is the middle leg of the 7815(connected to R4) left floating - it is not connected to 0volt or ground ??

Why is the output of the 7815 going to the other channel - cant I just build two seperate channels like the schematic ?

Is there any components where special quality is worthwhile - caps/resistors ?

Thanks !

Buhl

[Buhl]

forgot the schematic.....ups !


Cheers !
Hans

[Calebay]

http://www.tcaas.btinternet.co.uk/jlhupdate.htm[/URL]

[Geoff]

Hi Buhl

Originally posted by Buhl
The only high power resistor is the R10 0R33 ?

Correct. The dissipation in R10 is 1.33W so use either a 5W or 7W resistor (or three 1R 2W in parallel).

Is the middle leg of the 7815(connected to R4) left floating - it is not connected to 0volt or ground ??

The middle leg of the 7815 and R4 are connected to 0v (ground).

Why is the output of the 7815 going to the other channel - cant I just build two seperate channels like the schematic ?

JLH did not like using two components when one would do (or could be shared between channels). You can use separate 7815s for each channel. One word of caution though. The current draw from the 7815 is around 0.5mA per channel. Some 7815s have been found to be unstable at this current though, unlike the LM3x7 etc, the datasheets do not specifiy a minimum current draw. I suggest you add a resistor from the 7815 output to ground (ie in parallel with C3), sized to give a 3mA to 5mA curent through the 7815 (ie something between 3k0 and 4k7)

Is there any components where special quality is worthwhile - caps/resistors ?

Yes.

Resistors R2, R3 (signal input) and R6, R8 (feedback network). The signal also appears across R5 & R9

Capacitors C1 - I suggest a good quality polypropylene (preferrably film and foil rather than metallised film) or, if you can afford it, a paper-in-oil.

C2 - polystyrene. C4 - a good quality electrolytic (preferrably bipolar)

Please note that there is an error on the schematic that you posted. The negative end of C4 should go to 0v (ground) and not to the -ve rail.

Geoff

[Buhl]

Thank you Geoff and calebay - seeing the "updated" schematic I think I will go for that one - the idea of a CCS instead of a chip voltage regulator seems more "right" for me....
The link for the updated version gave me a lot of things to work with - thank you !

One thing - how nessecary is it to use active regulation of the PSU - I was thinking of skipping it i favour of a C-R-C-C(44000uF-0R5-44000uF-44000uF) filter.......?

Cheers !
Hans

[Geoff]

Originally posted by Buhl
One thing - how nessecary is it to use active regulation of the PSU - I was thinking of skipping it i favour of a C-R-C-C(44000uF-0R5-44000uF-44000uF) filter.......?

The 'Updated' version is more tolerant of supply rail fluctuations (ie the quiescent current and output dc offset do not vary as much when the rail voltage changes) so you should be OK with a C-R-C supply, as opposed to active regulation.