simple preamp from half 12ax7

[erez1012]

hi
want to tray build simple preamp, no feedback, 2 channel from single 12ax7.
I want to connect a R2R DAC to it.
after the preamp i have single ended VFAT (only one transistor)

I'm worried about exit impedance of the pre

 

[zintolo]

What is the output Vpp of the preamp and the input sensitivity and impedance of the power amp?

 

[erez1012]

the DAC output is 1mA on 100 ohm resistor.
plug this to the preamp (in the picture).
I measured the signal at the output in the preamplifier, I do not remember so much, but something in the 5 volt range output.

don't know the impedance of the power amp, but when i build it, the recommendation was that need a preamp with very low impedance and high voltage output.

the reason that i remove the 12ax7 cathode follower it's because i measure high 2nd harmonics and 3nd harmonics in the output.
and without follower i measure only 2nd harmonics, that i like.

 

[TG]

This thing's output impedance is around 100kOhm. Not suitable for almost any power amp, not even remotly appropriate for driving interconnect cables of any length.

 

[erez1012]

OK

 

[TG]

You should try to troubleshoot your cathode follower first before abandoning the whole idea. It might be just not optimally biased for the job and therefore adding distortion.

 

[erez1012]

thanks, i try

 

[nigelwright7557]

I would suggest using 2 valves and have a second stage cathode follower for lower output impedance.

 

[Jeroen211]

If you need the high gain and a low output impedance you have 2 options: add a cathode follower after the 12AX7. You better use a 12AT7 in stead of a 12AX7 for this stage.

The other option is to use a different tube with high gain and low Rp, e.g. a EC86 or D3A (triode switched)

 

[erez1012]

The other option is to use a different tube with high gain and low Rp, e.g. a EC86 or D3A (triode switched)

thanks
do you have schematic for this circuit?
i understand that the V+ and filament are same?
thanks

 

[v4lve lover]

12B4 can also work if you need only 3x gain

 

[erez1012]

maybe for start, i play with bias resistor R11, and measure 1khz in the spectrum till the 3nd harmonic is go smaller.

 

[Bigun]

I believe your best option is to forget the cathode follower, this tube of yours has low gm, it will be unsuitable for low distortion in that position. Use a FET follower, it will be transparent and preserve the sound you like so far.

 

[vinylkid58]

maybe for start, i play with bias resistor R11, and measure 1khz in the spectrum till the 3nd harmonic is go smaller.

Is there a particular reason you want all the gain that a 12AX7 has to offer?

jeff

 

[erez1012]

Is there a particular reason you want all the gain that a 12AX7 has to offer?

no, ~50 its OK.

 

[Jeroen211]

maybe for start, i play with bias resistor R11, and measure 1khz in the spectrum till the 3nd harmonic is go smaller.

You could DC-connect both stages by removing R11, R13 and C3, it will be simpler and better

 

[Jeroen211]

thanks
do you have schematic for this circuit?
i understand that the V+ and filament are same?
thanks

No I don't have ready-to-go schematics but it is a very simple circuit. I've found: Hybrid Amplifier by Andrea Ciuffoli where you can get some ideas for the D3A.

Note that your demand of high gain (~50) is much higher than preamps typically deliver. In fact it is more like an input stage of an integrated amp. I am busy with a similar 'preamp' that should be able to drive a EL84 directly (~7V). I am thinking of using the D3A for it.

 

[erez1012]

I believe your best option is to forget the cathode follower, this tube of yours has low gm, it will be unsuitable for low distortion in that position. Use a FET follower, it will be transparent and preserve the sound you like so far.

Thank you very much, an interesting idea.

Is there a particular reason you want all the gain that a 12AX7 has to offer?

maybe i have miscalculation:
DAC output is +-1mA on 100 ohm resistor, so 0.1v+-
i measure with scope, the power amplifier at full volume is 2.5Vptp
so i need 25 gain

need a new post: Which tube is suitable for 20 gain and low exit resistance?

 

[erez1012]

I found some tubes in the closet that might help:
12au7a
5654w

 

[erez1012]

hi,
add back the cathode follower.
12au7 and 12ax7 as follower the gain is ~10
12ax7 and 12ax7 as follower the gain is ~30
i order 12AT7, try that option too