6V6 line preamp

[merlin el mago]

Salas what resistors do you use for NFB?

 

[Salas]

I had a mixed bag of NOS European 50ppm metal film resistors, Philips Beyschlag mainly.

 

[merlin el mago]

What about the SQ?

 

[Salas]

Just clean and neutral. I simply used dependable resistors that I had in a range of values. I was more keen about establishing the G3X version electrically. Stability, gain, response, THD+N profile. The meat and bones stuff. Volume pot, Ck bypass cap, and Cout are arguably the more influential passive parts.

6V6 tubes themselves make a noticeable difference is my experience. Especially in the G3X where other parameters are more fixed. I could also see it in the THD and noise profiles on FFT analysis when I was exchanging brands and vintages.

 

[merlin el mago]

Yes I noted the big difference between the brown unknown brand & the Ohmite Audio Gold next will try the Mills MRA12 my favorites for more than 30 ohms values, less than 30 ohms are Mundorf M-Resist Supreme 30W.

NFB resistors have also a great impact in the SQ as anode load resistor, all counts....

 

[Salas]

Its a few parts only circuit so everyone who wants to try several passives of his liking is easier to do so. Some will say its trivial some will relish. I only say its a solid circuit don't change the values, just see to have enough working voltage and wattage spec.

 

[merlin el mago]

Yes first working values as designed, for me is a relish....

 

[merlin el mago]

Salas what about to use as headphone amp without NFB and in place anode load resistor a OPT 15K:600 so using 300R cans 6V6 see 7K5 is enough? or hybrid mu-follower from Ale Moglia to reduce output impedance without gain loss to drive direct the 300R cans?

 

[Salas]

I don't know, never used it with OPT for headphones

 

[merlin el mago]

But you will see if it's possible as suggested? with the same voltages & bias?

N.B. Of course if you aren't interested or it's a lot of work let it go Salas.

 

[Salas]

15000:600 is 1:25 impedance and its square root is 1:5 for voltage so if put on a without feedback 6V6 line preamp you get 1:1 or below voltage gain for headphones.

 

[merlin el mago]

Like a buffer? Could be enough for my Sennhs HD600 300R 97dB 1V?

 

[Salas]

Yes, 300 Ohm headphones buffer

 

[merlin el mago]

There is the other possibility to use the hibryd mu-follower from Ale Moglia aka Bartola as anode load.

 

[Salas]

Like a buffer? Could be enough for my Sennhs HD600 300R 97dB 1V?

If you had 2V RMS or higher DAC it would be loud enough

 

[merlin el mago]

Well I want to use noDac + step-up 1:4 + 26 pre hybrid mu-follower loaded + 6V6?

 

[Salas]

There will be much voltage gain but maybe too many tube stages so to obstruct transparency

 

[merlin el mago]

It's a very good reflection. Happened the same reflection using noDac: when used the step-up in place active gain transparency was absolut best but bass lost authority, added 12B4 or 6V6 more bass less transparency, added xfeed to cans less transparency but soundstage more centered & listened better the insturments. engineering are compromises.

26 DHT & 6V6 G3X are both very very transparent devices.

 

[merlin el mago]

Xfeed = crossfeed
Cans = headphones

 

[gorko1]

Hello guys,

After changing original 15db version to original NBF I've got slight low frequency buzz.
Beefore was dead silent...
I have checked all solder joints...
What could be?

BR,
Goran