Latest project.
An externally hosted image should be here but it was not working when we last tested it.
The valve is just a simple triode amplifier stage on each channel.
The chip amp (TDA7294) has had its gain reduced so the valve doesn't over drive it in normal use.
The two 12 volt zener diodes on the power pins are to drop the voltage from my +/-45VDC volt power supply so I can test at +/-35VDC.
The chip amp (TDA7294) has had its gain reduced so the valve doesn't over drive it in normal use.
The two 12 volt zener diodes on the power pins are to drop the voltage from my +/-45VDC volt power supply so I can test at +/-35VDC.
It runs off +/- 35VDC so there is about 70 volts across the valve.
The heater is dropped off B+ with a couple of resistors and a 12volt zener.
I have wired the heaters in series so it only takes 150mA.
It sounds very clear.
The total gain is around 50. About 10 in the valve and 5 in chip amp.
The heater is dropped off B+ with a couple of resistors and a 12volt zener.
I have wired the heaters in series so it only takes 150mA.
It sounds very clear.
The total gain is around 50. About 10 in the valve and 5 in chip amp.
The total gain is around 50. About 10 in the valve and 5 in chip amp.
How are you achieving gain 5x in the chip? Datasheet specifies a minimum of 24dB closed loop gain which is about 15x.
The output of the tube from the anode is around zero volts DC and ac is passed via coupling capacitor. The valve runs off +/-35VDC so total volts across valve is 70. Seems to sound fine.
The valve is sort of referenced to -35v but I have decoupled the lower and upper rail using 2k2 and 470uF so there is no noise. With no signal in the output is dead silent.
The valve is sort of referenced to -35v but I have decoupled the lower and upper rail using 2k2 and 470uF so there is no noise. With no signal in the output is dead silent.
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The datasheet doesn't say why min is 24db.
I would guess the input cant take a high voltage ?
In the case of LM1875, the datasheet specifies a minimum gain of 10x to ensure stability. I can't say the reason is the same for TDA7294 but i would follow the datasheet recommendation. Anyway, if your circuit is stable, then carry on.
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