PNP JLH 1969 current and voltage measurements

Hello,
I was hesistating to add these questions on the huge JLH 1969 thread, but wasn't too sure how judicious that would be.
Having built my JLH 1969 three years ago with limited knowlege and no measuring tools (things have not evolved that much to be honest ...) I had figured at the time it would be a good idea to put in an analog ammeter to set up the quiescent current and give indications as to possible future faults.
My version is the PNP HOOD 1969 from the usual suspects. I swapped out the input and output caps for better ones. It has an onboard 3055 capacitance multiplier.
- With an 18VAC psu I am measuring +20VDC at the rails. Does this mean that the capacitance multiplier and rectifier are sucking 5V ?
- The ammeter is on the 18VAC lines. I have the current set at 2.2A. I guess this is not the actual quiescent current as the capacitance multiplier must be using some ?
- The meters never really move when the amp is up and running. However, if i tune it up to clipping volumes, the reading sags dramatically. Is this normal ?
Thanks a lot
 

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PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> +20VDC at the rails. Does this mean that the capacitance multiplier and rectifier are sucking 5V ?

Instead of guessing, measure the raw DC out of the rectifier.

Yes, the AC current is not the same as the DC current, and conversion is unclear.

A class A amplifier should draw a steady current idle or loud.

However an abrupt drop of current at FULL output suggests the Idle current is much higher than really needed. Alternatively that your load impedance is higher than was used to figure the best idle current.
 
Thanks for your reply. I will measure the VDC at the rectifier this weekend as I need to mess about with the grounding scheme I am not happy about.
I also need to get some dummy resistor loads to see exactly what happens when it's turned up loud. It's not an enjoyable experience for me or my neighbours with the speakers connected and I dont want to go blowing a tweeter
 

Rallyfinnen

Member
2017-01-22 10:18 am
20V (single rail I suppose) and 2,2A sounds like you are burning more power than needed for maximum output. Around 1A should be enough depending on speaker impedance.
If you have the possibility (computer, sound card, ARTA/REW plus some home made cables), I would recommend you to measure distortion and tune the current vs distortion to achieve best performance.
Depending on the transistors used etc, there is usually a specific current range where the amplifier works best (lowest distortion)
 
I run it with 8ohm speakers.
Unfortunately I dont have the needed equipment to measure distortion ...
What I really need to do I think is find a way of truly measuring the current flow after the capacitance multiplier. The 2.2A I am reading is AC out of the transformer. Seeing as the supply and amplifier are on the same board I suppose I would need to modify a trace, put in a shuntable resistor and read with a voltmeter.

Does a capacitance multiplier draw much current normally ?

My question mainly arised from the fact that my amp runs very cool, even after several hours. People often say this is a relatively hot running amp. I do only have 1 transistor per heatsink though ... and they are very big !

Edit : I dont have the schematic unfortunately (standard china import practice)
 
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