New preamp schematics

andrei · 17th January 2006, 03:00 AM

Here is a neat little trick to try for your next preamp project:

arnoldc · 17th January 2006, 03:03 AM

looks like a circuit done in switchcad... what are the results?

alexistheo · 17th January 2006, 03:08 AM

would that arrangement introduce positive feedback?

andrei · 17th January 2006, 03:11 AM

The cool thing about this gain stage is that it draws constant current from power supply, and so does not require cathode bypass capacitor. Load drive ability is pretty good, too. I've drawn the component values for russian 6N30P tube, but almost any other small-signal triode can be used (5687 comes to mind).

For 1V input, simulation gives THD ~0.5% (second harmonic mostly). As the bias is at -7V, the headroom available is very large.

Comments?

andrei · 17th January 2006, 05:06 AM

Despite appearances, there is no feedback to cathode resistor. The symmetry between two stages makes the current drawn by common cathode amplifier (U1) and cathode follower (U2) to be the same in value, but in anti-phase, and to sum up to a constant current through cathode resistor (which provides a constant bias without capacitor). The current drawn from power supply is this constant current plus the (small) load current, so one can use a very high impedance power supply with a small decoupling capacitor.

arnoldc · 17th January 2006, 05:26 AM

has this been put into a prototype yet? i have some 5687 tubes here waiting to be used...

andrei · 17th January 2006, 05:31 AM

No, simulation only so far. I have an integrated amp, so I don't really need another preamp, but I'm tempted to layout a PCB for this just out of curiousity...

arnoldc · 17th January 2006, 05:41 AM

i can make a proto of this point-to-point after my 12B4 pre is done.

3.3uF output (coupling) capacitor ok with this?

Tony · 17th January 2006, 05:50 AM

Quote:

As the bias is at -7V, the headroom available is very large.

this is at the grid of the input right? this is ok i guess as it can handle a 2volt rms signal easily.

andrei · 17th January 2006, 06:06 AM

Output coupling capacitor value together with load resistance will set DC cutoff frequency. 3.3uF into 10kOhm load gives cutoff at 5Hz, which is more than enough. Even 1uF will do. Same goes for PSU decoupling capacitor, BTW. I would go for a smaller value capacitor with better dielectric. You can get away with no electrolytics alltogether with this circuit. To get a quiet power supply, one can use a solid-state CCS (like IXUS IXCP10M45S).

Re:bias There is about 7V DC drop across R2, so the grid of the tube U1 is biased at -7V relative to cathode. Both stages draw 20mA idle each. (This is for 6N30P, for 5687 the resistor values would be a bit different.)

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17th January 2006, 03:03 AM	#2
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	looks like a circuit done in switchcad... what are the results?

17th January 2006, 03:08 AM	#3
alexistheo diyAudio Member Join Date: Jun 2004 Location: New York City	would that arrangement introduce positive feedback?

17th January 2006, 03:11 AM	#4
andrei diyAudio Member Join Date: Dec 2002 Location: Canada	The cool thing about this gain stage is that it draws constant current from power supply, and so does not require cathode bypass capacitor. Load drive ability is pretty good, too. I've drawn the component values for russian 6N30P tube, but almost any other small-signal triode can be used (5687 comes to mind). For 1V input, simulation gives THD ~0.5% (second harmonic mostly). As the bias is at -7V, the headroom available is very large. Comments?

17th January 2006, 05:06 AM	#5
andrei diyAudio Member Join Date: Dec 2002 Location: Canada	Despite appearances, there is no feedback to cathode resistor. The symmetry between two stages makes the current drawn by common cathode amplifier (U1) and cathode follower (U2) to be the same in value, but in anti-phase, and to sum up to a constant current through cathode resistor (which provides a constant bias without capacitor). The current drawn from power supply is this constant current plus the (small) load current, so one can use a very high impedance power supply with a small decoupling capacitor.

17th January 2006, 05:26 AM	#6
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	has this been put into a prototype yet? i have some 5687 tubes here waiting to be used...

17th January 2006, 05:31 AM	#7
andrei diyAudio Member Join Date: Dec 2002 Location: Canada	No, simulation only so far. I have an integrated amp, so I don't really need another preamp, but I'm tempted to layout a PCB for this just out of curiousity...

17th January 2006, 05:41 AM	#8
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	i can make a proto of this point-to-point after my 12B4 pre is done. 3.3uF output (coupling) capacitor ok with this?

17th January 2006, 06:06 AM	#10
andrei diyAudio Member Join Date: Dec 2002 Location: Canada	Output coupling capacitor value together with load resistance will set DC cutoff frequency. 3.3uF into 10kOhm load gives cutoff at 5Hz, which is more than enough. Even 1uF will do. Same goes for PSU decoupling capacitor, BTW. I would go for a smaller value capacitor with better dielectric. You can get away with no electrolytics alltogether with this circuit. To get a quiet power supply, one can use a solid-state CCS (like IXUS IXCP10M45S). Re:bias There is about 7V DC drop across R2, so the grid of the tube U1 is biased at -7V relative to cathode. Both stages draw 20mA idle each. (This is for 6N30P, for 5687 the resistor values would be a bit different.)

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