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Output current of this CAT SL-1 schematic ?
Output current of this CAT SL-1 schematic ?
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Old 11th February 2005, 04:22 PM   #1
Tea is offline Tea  Hong Kong
diyAudio Member
Join Date: Aug 2002
Location: Hong Kong
Question Output current of this CAT SL-1 schematic ?

Hi all, I want to DIY a tube preamp to drive my vintage MC240 tube amp AND my AKG 271s headphone, since my headphone is only 56 ohm so I need a high current and low impedance output preamp.

My question is :

1: What is the output current and impedance of the following CAT SL-1 schematic ?

2: How do I know the output current and impedance for difference schematic or which part should I check ?

3: If this schematic do not have high enough current or low enough impedance, can I modifly it ? If yes which part should I modifly ?

Thank you very much for the help.
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Old 14th February 2005, 12:23 PM   #2
eborgard is offline eborgard  France
diyAudio Member
Join Date: Oct 2004
Output impedance of the LS-26 should be around 80 Ohms, and maximum output current is around 5mA. So this one is not for you (also it does sound very good; it is actually the one I am using).

Output impedance in this case is affected by the 50 Ohms series resistor + depends on tube and feedback (basically, the 100 Ohms output impedance of the tube is "virtually" reduced by the amplifier feedback) .

Current capability depends on the tube used: the 6922 will give you roughly 10mA, ie 5 mA bias + 5mA current.

Additionally, driving a 56 Ohm load requires a very large capacitor in the output (cutoff frequency is f= 1/ (2*PI*R*C) where R = 56 Ohms).

Finally, this preamp has a huge (26dB,i.e. x20) gain.

So it is not for your headphone!

If you really want to use this type of schematic, then you would have to change the 6922 for a large double triode (ex 6AS7 ) or for two pentodes (SV83/ EL84) + use electrolytics for the output cap.

You'd be much better off using a triode SE amp with a right chosen output stage transformer.

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