help on 12AU7 Preamp circuit

[siliconray]

Hello all,

I'm planning to add a tube preamplifier for my solid state power amp. the supply is +-25V. I designed a preamp circuit but not sure it will works fine. Can any one help me to check the design? many thanks

 

[SY]

What's the design goal? Easily audible distortion?

 

[DF96]

With the right value for anode resistor the smoothly non-linear second-order distortion created by the valve characteristic could be partly cancelled by clipping caused by grid current. The result would be low THD, at a particular volume level, but awful sound.

Astonishingly, this technique was actually advocated in an IRE Transactions paper in 1955 by J Z Knapp "Low Distortion Operation of Some Miniature Dual Triodes". I guess back then they didn't know how awful high-order distortion sounds.

 

[rock4016]

Your 12AU7 won't like your low voltage.

Steve Bench plotted some interesting curves for a 6SN7 at low volts. Also the 12SX7 might be worth looking at.

 

[siliconray]

I searched the web found a 12V 12AU7 headphone design so I think the 50V won't be a problem
DIY 12AU7 (ECC82) Tube - IRF612 MOSFET Hybrid Headphone Amplifier
NP-100v12: DIY 12AU7 (ECC82) Tube / IRF510 MOSFET Headphone Amplifier

 

[kevinkr]

It also won't like the bias arrangement. Hint: Look at the potential between the grid and cathode - should be a few volts at most. (Need input cap and one more resistor..)

A 6DJ8 would work a lot better here.

 

[kevinkr]

I searched the web found a 12V 12AU7 headphone design so I think the 50V won't be a problem
DIY 12AU7 (ECC82) Tube - IRF612 MOSFET Hybrid Headphone Amplifier
NP-100v12: DIY 12AU7 (ECC82) Tube / IRF510 MOSFET Headphone Amplifier

Nope, and I bet it sounds pretty bad..

 

[siliconray]

But I have another design, 6N11 + IRF510, the sound is fairly good. Powered by single 24V.

 

[siliconray]

wooo
a big mistake. grid shold be tite to VSS, not GND
look at this one
should I add a small positive voltage (vs VSS) to grid?

It also won't like the bias arrangement. Hint: Look at the potential between the grid and cathode - should be a few volts at most. (Need input cap and one more resistor..)

A 6DJ8 would work a lot better here.

 

[siliconray]

I think I should also try with 6N8P(6NS7)

 

[kevinkr]

wooo
a big mistake. grid shold be tite to VSS, not GND
look at this one
should I add a small positive voltage (vs VSS) to grid?

No...

 

[DF96]

Unless the purpose of this stage is to add distortion, there is no reason not to use a decent supply, say 200V.

 

[kevinkr]

I think I should also try with 6N8P(6NS7)

I assume you mean 6SN7 since you reference 6N8P - again not a great choice for low voltage operation.. The 6DJ8 will actually function fairly well with 50V across it, particularly if a CCS or choke is used as the plate load.

As DF96 points out higher voltage operation would be a better choice, and at 200V and above the 6SN7 will work fairly well.

 

[siliconray]

I know high voltage is better but there's no high voltage in the amp, only +-25V,

 

[DF96]

With only 25V, do whatever you need to do in solid-state.

 

[siliconray]

Just for fun. I'm doing tube+class D, see what will happen

 

[DF96]

Will you be running your Class D amp from only 10% of the required voltage for good performance? Now that would be fun!

 

[siliconray]

class D can run from 12V to +-100V..I'm looking for a low voltage tube preamp and I believe there is

 

[dgta]

Just for fun.

OK, but don't put it up for sale on your website, some innocent person might take it seriously 🙂

 

[siliconray]

OK, but don't put it up for sale on your website, some innocent person might take it seriously 🙂

Oh No, I said just for fun, pure personal project...unless it has amazing performance, haha, who knows? Maybe I can send it to a guru for review when it's done. But now I have to learn more about tube. It's a all new domain for a DSP and embedded guy like me. Hope you can help me😱