C4S resistor calculation

debk · 2152

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Offline debk

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on: April 28, 2014, 06:37:12 AM
I have been looking at using a C4S in a project I am working on.  From looking at the C4S on the bottlehead schematics I have, it looks like the value of R1 is about 0.85v/I.  R2 seems to set the bias current for the C4S.  How is this determined?

Is the vaule of R1 0.85v/I a calculated value and if so I am curious as to how it was derived.

Thanks for the help

Debra
« Last Edit: April 28, 2014, 07:52:35 AM by debk »

Debra K

Eros 2Phono amp
BeePre2, Psvane ACME 300b
Kaiju, Linlai Elite  300b
Monamour 2a3 amps various tubes
Sota Sapphire, Pete Riggle Woody Tonearm, Kiseki Purpleheart Cartridge
Rega P6 Ania Pro cartridge
Roon Nucleus
MHDT Labs Orchid DAC
Jager speakers


Offline Paul Joppa

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Reply #1 on: April 28, 2014, 10:41:05 AM
I actually use 0.855 volts ... it's the difference between the LED voltage (1.57 volts) and the small transistor's base-emitter voltage. All of these voltages are somewhat temperature dependent.

The R2 must allow enough current into the transistor bases and still keep the LEDs well lit. The MJE350 for example has a hfe of "30 to 240" so it needs as 1/30th to 1/240th of the output current - a distressingly large range! My specs for the 2907A don't even specify a range for hfe, only an average value. In the original writeup, Buddha (John Camille, the first C in CCCCS or C4S) said 2mA is better than 1mA but did not discuss higher current arrangements.

I use 1/3 of the output current for currents up to 5-10mA (with a 1mA minimum), and will go as low as 1/10 of the output current where it is a burden to waste current. Hasn't failed me yet but there's no solid calculation behind that.

Note that R2 often has a high voltage across it. Resistors are specified for maximum working voltage (don't use "dielectric strength"!). In our experience, it is risky to exceed half the rated maximum, so R2 is often multiple resistors in series. The fairly new IRC GS3 series is rated 1000 volts/3 watts and I am beginning to use them for R2.
« Last Edit: April 28, 2014, 01:44:10 PM by Paul Joppa »

Paul Joppa


Offline debk

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Reply #2 on: April 28, 2014, 12:37:37 PM
Paul

Thanks for  the great explanation, I appreciate it!

Deb
« Last Edit: April 28, 2014, 02:42:46 PM by debk »

Debra K

Eros 2Phono amp
BeePre2, Psvane ACME 300b
Kaiju, Linlai Elite  300b
Monamour 2a3 amps various tubes
Sota Sapphire, Pete Riggle Woody Tonearm, Kiseki Purpleheart Cartridge
Rega P6 Ania Pro cartridge
Roon Nucleus
MHDT Labs Orchid DAC
Jager speakers


Offline Paul Joppa

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Reply #3 on: April 28, 2014, 01:44:57 PM
I edited several typos - sorry all!

Paul Joppa