A new project using cathode follower valve front end with a TDA9274 chip amp.
The valve uses +/-335VDC for B+.
Heater volts are dropped to 12VDC using two resistors from +35VDC.
The valve uses +/-335VDC for B+.
Heater volts are dropped to 12VDC using two resistors from +35VDC.
Hi Nigel,
I've just bought a pair of these off Ebay and should have them connected up today or tomorrow.
I may have to reduce the gain to 24db, the minimum allowed according to the datasheet as I have a couple of 12AX7 valves which have a higher gain than the 12AU7. (change 680r to 1.5k)
My psu is in a separate enclosure connected via a 1m cable with 3pin XLR connectors because I will also be using (not at the same time!) with a Krell KSA50 clone module due from China in a few days.
I've just checked my mains and its a bit on the high side at 248v off load, 244 with the cooker on full so my 25v toroid will produce about 37v off load, 35v loaded.
I'm doing an auditioning comparison of the hybrid chipamp, the Krell and a 30watt Class A PP 6L6GC tube amp I am building.
My speakers are Martin Logan Aeons which are hybrid electrostatics and a tricky load for any amp. Probably I end up using the chipamp to drive the bass unit. and the 6L6GC amp for the electrostatic panel.
A JLH high power (ELS) Class A amp wasn't upto it and nor was an EL34 PP VR70 (Dynaudio) budget tube amp.
Ian
PS it would have been good to find the insulators supplied with the pcb.
I've just bought a pair of these off Ebay and should have them connected up today or tomorrow.
I may have to reduce the gain to 24db, the minimum allowed according to the datasheet as I have a couple of 12AX7 valves which have a higher gain than the 12AU7. (change 680r to 1.5k)
My psu is in a separate enclosure connected via a 1m cable with 3pin XLR connectors because I will also be using (not at the same time!) with a Krell KSA50 clone module due from China in a few days.
I've just checked my mains and its a bit on the high side at 248v off load, 244 with the cooker on full so my 25v toroid will produce about 37v off load, 35v loaded.
I'm doing an auditioning comparison of the hybrid chipamp, the Krell and a 30watt Class A PP 6L6GC tube amp I am building.
My speakers are Martin Logan Aeons which are hybrid electrostatics and a tricky load for any amp. Probably I end up using the chipamp to drive the bass unit. and the 6L6GC amp for the electrostatic panel.
A JLH high power (ELS) Class A amp wasn't upto it and nor was an EL34 PP VR70 (Dynaudio) budget tube amp.
Ian
PS it would have been good to find the insulators supplied with the pcb.
So long as you keep greater than or equal to minimum gain it should be ok.
I designed a TDA7294 hybrid amp a couple of years back and reduced gain right down in the TDA as valve had a lot of gain. The TDA just oscillated badly.
I designed a TDA7294 hybrid amp a couple of years back and reduced gain right down in the TDA as valve had a lot of gain. The TDA just oscillated badly.
That was my initial plan until I spotted the 24db min gain requirement.
I definitely don't want a 100w power oscillator - had one of those with a chinese power amp "no feedback" design. Except it wasn't, they just didn't bother with the feedback resistor so anything connected to the input caused the output to bounce off the supply rails at 100khz.
Fortunately I had thought to use a dummy load or my speakers would have been toast.
I definitely don't want a 100w power oscillator - had one of those with a chinese power amp "no feedback" design. Except it wasn't, they just didn't bother with the feedback resistor so anything connected to the input caused the output to bounce off the supply rails at 100khz.
Fortunately I had thought to use a dummy load or my speakers would have been toast.
I found the TDA was oscillating and radiating back into the valve which of course amplified the oscillations again.
I found only way to tame it was to put 1nf across TDA +/- inputs.
A bit of a bodge but it worked for the small few I made.
I changed my valve to a cathode follower to reduce its gain to 1 on the next pcb revision.
I found only way to tame it was to put 1nf across TDA +/- inputs.
A bit of a bodge but it worked for the small few I made.
I changed my valve to a cathode follower to reduce its gain to 1 on the next pcb revision.
Did you not consider paralleling the triodes?
Apparently it reduces noise & distortion, whilst the gain stays the same.
Apparently it reduces noise & distortion, whilst the gain stays the same.