Stereo 12au7/TDA7294 hybrid amp

[nigelwright7557]

Latest project.

An externally hosted image should be here but it was not working when we last tested it.

 

[nigelwright7557]

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.

 

[kp93300]

Hello Nigel,
the entire board including the B values works between what range of DC values ?
kp93300

How would you characterize the sound and what is the final gain ?

 

[nigelwright7557]

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.

 

[ballpencil]

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.

 

[nigelwright7557]

I changed the feedback resistor to 4k7. SO 4k7/680r = 7

The datasheet doesn't say why min is 24db. I would guess the input cant take a high voltage ?

I have applied 1 volt peak 2 peak ac voltage to input and amp outputs rail to rail volts OK.

Maybe I just got lucky ?

 

[stocktrader200]

Are you feeding the tube signal referenced to the -35v rail?
I used a voltage tripler to get 140v B+ referenced to ground then there is no ac voltage differential created by amplifier load

 

[nigelwright7557]

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.

 

[nigelwright7557]

I have emailed ST tech help.
Will see what they say about it.

 

[ballpencil]

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.

 

[kp93300]

Hi Nigel,
Received the board thanks.
I notice you reminder not to exceed +/-35 V dc power supply.
I have a 500W transformer with secondary ac voltage of 26 o 26 V .
Is this acceptable ??

 

[nigelwright7557]

Should be OK.
Regards Nigel

 

[nigelwright7557]

Now I think about it my original design had trouble with oscillation. I thought it was a layout problem. I fixed it with 1000pf across inverting and non inverting inputs. I bet the reason why is the low gain causing instability.

 

[kp93300]

I am buying the parts now.
Can i use either 12V or 13 V 5 W zener at D1?
thanks

 

[nigelwright7557]

Yes either are fine.
The heater is spec'ced at 12.6 Volts DC.

 

[kp93300]

resistor values

Hello Nigelwright,
After populating the board, I notice that the spacing for most resistors allowed for 1/2 w values.
Are 1/4 w resistors suitable for these locations ??
Please advise
Thank you
kp93300

 

[nigelwright7557]

I used 0.6 watt metal film resistors.

 

[kp93300]

Several parts in the Bill of material labelled with VO.
I need to populate these for 12 au 7 with tda7294 ser up ?

 

[nigelwright7557]

Yes.
VO means valve version only.

Regards Nigel

 

[nigelwright7557]

Fit all the components except optional components for valve version.