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Old 5th August 2004, 08:55 PM   #1
zobsky is offline zobsky  India
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Default Naive CCS quesion

Hi All,

Considering a simple resistor loaded cap coupled gain circuit

In various circuits, I see mention of using a CCS to obtain certain benefits

some of these circuits recommend CCS replacing the plate resistor, while others use the CCS to replace the cathode resistor (and cap, if any)


which of these is preferred .. ?

can they be used together .. ?


i ask because i'm considering dropping one the popular IXYS chips in as a CCS in my 12B4 preamp.

thanks
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Old 5th August 2004, 09:08 PM   #2
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Current sources can be used in either position. Their effects differ considerably.
A current source used in place of the plate resistor will increase the gain of the tube towards the theoretical maximum (mu). However, it doesn't sound as good as a simple resistor. The current source will need to be solid state, which will also effect the sound.
Using a current source instead of a cathode resistor is largely unnecessary unless you're building a differential. Differentials benefit greatly from current sources. In this case, the current source can be either another tube or solid state. In either case, keep in mind that you may need a negative rail in order to give the current source room to work. How much of a negative rail depends on the current source design, but it's pretty much a given that a volt or two isn't going to be enough to let a CCS breathe.

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Old 5th August 2004, 09:27 PM   #3
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It's cap coupled. No one said there can't be a coupling cap on the input, such that allows the differential to operate at an elevated potential. No need for negative voltage.

Tubes work for CCsource as well as CCsink, ever seen a mu stage? Even works with triodes...

Combining CCSs is impossible as they will fight each other for the operating point. Even without the tube in circuit, in fact. Whichever draws slightly more current will have near zero voltage across it.

The advantage of a CCS in the cathode of a normal gain (or output) stage is for guaranteed bias and prevents overcurrent situations. Only applies to class A1 amplifiers.

Tim
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Old 6th August 2004, 12:24 PM   #4
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Default Why not use a choke?

The choke takes the place of the plate load resistor and basically acts as a current source since it allows the DC current through (low DC resistance) and a very large load at higher frequencies due to its inductance. Problems can arise requiring large inductance values with higher rp valves, but since the 12B4 has a rather low rp, this shouldn't be a problem... and it doesn't require any solid state or active circuitry! The main problem is that a choke is probably more expensive than an IC current source, and requires space on the chassis.

For more info, see http://www.vt52.com/diy/tips/tips_chokeload.htm

Also, Jim de Kort has made an excel spreadsheet to graphically represent the frequency response and gain of an inductively loaded gain stage at http://www.vt52.com/diy/tips/platechoke.xls
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Old 6th August 2004, 05:46 PM   #5
zobsky is offline zobsky  India
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Default Re: Why not use a choke?

Quote:
Originally posted by audiousername
The main problem is that a choke is probably more expensive than an IC current source, and requires space on the chassis.


i believe you have answered your own question on "why not use a choke"

thanks for the links, though

comments on other posts

Quote:
A current source used in place of the plate resistor will increase the gain of the tube towards the theoretical maximum (mu). However, it doesn't sound as good as a simple resistor.
then why the abundance of these types of circuits, ..is it the search for (largely unnecesary) preamp level gain?


Quote:
Combining CCSs is impossible as they will fight each other for the operating point. Even without the tube in circuit, in fact. Whichever draws slightly more current will have near zero voltage across it.
i thought so, .. just wanted someone to verify that



so, deducing from these posts,.. can i assume that for a simple circuit, . better performance (and sound) can be obtained by using a CCS (sink) in place of the cathode resistor (and cap)

thanks, V
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Old 6th August 2004, 11:50 PM   #6
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Hi,

Quote:
so, deducing from these posts,.. can i assume that for a simple circuit, . better performance (and sound) can be obtained by using a CCS (sink) in place of the cathode resistor (and cap)
Better performance on paper probably, better sound...I'm not at all convinced as far as CCS loading goes, CCS sinking seems to be a different ballgame sonically.

A CF doesn't use a bypass cap so that dreadful component is out of harm's way.
Most circuits can be biased in several ways avoiding cathode resistor and bypass caps anyway.

As with most other forms of art, audio design is a matter of what you use for what purpose.
IOW, horses for courses....

Cheers,
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