DAC direct out, not enough gain, help me build a driving stage.

[50AE]

I'm using the AKM4396 DAC direct out without any cap, canceling the +2,5VDC offset with more (-2,5V) bias on the input tube.

The problem is, the direct out unbalanced outputs about 1V maximum and this is not enough. The driver is E180F. The stage gain is about 44. -44 volts aren't enough to drive the output tube 6P45S. I need -75V swing there for full output.

I can get interstage transformers to use the balanced output and hence get 2V, which fixes my problem, but I don't have money now to get high quality ones. This way I will get rid of the offset as well, two birds with one stone.

So I came up with the idea of two stage drivers. The problem is, I want it to be direct coupled, no cap in the way. But doing so, I have to cathode bias the second tube with much voltage to cancel the 1st stage anode voltage.

... so I have to put a cathode capacitor which I don't want to do. I hate cathode bypass.
... I cannot make fixed bias
... I cannot put cathode battery bias (too muc batteries)
... I cannot use cathode resistor only, the gain will be too low, the output impedance will be very high and won't drive the 6P45S
... Or I can buy a higher gain penthode like D3A, but I don't have time and money to do so now.


So what can I do? This is where your help comes.

I could:

1. Use LEDS, if 12V or more ones exist, I could make a chain
2. Use zeners, which I heard that are noisy, except lower voltage ones I heard, from whom I could make a chain
3. Use voltage regulator tube?

http://store.picbg.net/pubpic/EF/D6/bb55ee9da052efd6.PNG

 

[211hojbjerg]

Hi high gain,
Gain at 44 looks as triode connected E180F. For more gain and keep at one gain stage, you can connect the E180F as pentode and much more gain.
Ra 10kohm, Ia 15mA, you'll roughly double your gain. If that's too much go for s lower Ra.

Best regards
Karsten

 

[50AE]

The problem is, it will be have very high output impedance, not sure if will drive the 6P45S well, which has high miller in triode mode.

Also I don't like the look of pentode curves

 

[211hojbjerg]

Hi,
Output imp will app same as the value as the Ra.

Best
Karsten

 

[Rod Coleman]

You can use my shunt-cascode circuit.

With E180F [triode connected] S ~21mA/V
Use load resistor 10K.

Gain ~ 210

No cathode capacitor needed.

Sound easily beats 2-stage driver.