Parafeed transformer sounds thin

[funk1980]

I'm in the process of designing my first tube preamp. One feature is a balanced D.I. output. For some reason, getting the parafeed tranformer to work correctly is a pain, and my noobness can't figure what's the best practice. The sound it produces is very thin. It's a 50k:1k, preceded bij a 560k resistor, so the anode of the first gain stage is not to heavely loaded, and to reduce the output to line level. I've discovered that putting a small value resistor (3.3k) in parallel with the parafeed solves the 'thin sound' issue, but the outputlevel is reduced as well.
Why does my knowledge and logic fail me here!?
Any help and insight is appreciated.

[DavesNotHere]

the caps are way too small, increase the output cap to 1uf and the cathode bypass to 220uF.

remove the 520k resistor. put a 50k resistor to feed the other preamp circuits instead of direct connection to the coupling cap.

move the transformer to before the output coupling cap, connect the ground side of the output transformer though a 220uf cap

use an o type attenuator on the primary if you need to attenuate the signal into the primary.

[funk1980]

Thanks for the reply. However, the preamp output (directly connected) sounds absolutly fine. The cathode bypass allows for 10Hz and the coupling cap for 3Hz(!), so plenty of bass response with a minimum of dielectrics. The problem seems to be in the way I connected the parafeed transformer and not the gainstage design...

[ErikdeBest]

What parafeed transformer are you using?

Any transformer works best (= widest frequency response) when fed from a source with low impedance. The 520k series resistor is really not a 'low impedance' source.

Are you actually loading the secondary of the parafeed? Or is the load actually 1k? Maybe that parafeed transformer can also work as a 500k : 10k, so you don't need any series resistor at all.

I am mostly guessing, but maybe it helps.

[yagoolar]

What I would do:
- remove 520k resistor
- replace 47k with a choke or CCS
- if gain is not a key parameter, use another noval tube i.e. ECC81 or ECC88 with higher Ia
- use LED bias instead RC - LF response much better.
Search for excel spreadsheet with parafeed calculation or use LT spice. Also see here Parafeed with resistive load

[kevinkr]

To work properly this transformer should be driven by a source impedance of 15K or less. You can accomplish this with a resistive divider, but the losses will be pretty high - for example a 100K from the output in series with 18K to ground and the transformer primary connected across the 18K resistor - this isn't going to sound great and nets you an additional 15dB of loss before the transformer step down ratio is taken into account.(-17dB) If you want to drive this transformer effectively a 12AU7A or better still 6FQ7 would do a much better job if you don't need a lot of gain. (And doesn't need the resistive padding discussed above.)

[funk1980]

Thanks for the replies guys. I didn't realize low source impedance was an issue for these kind of transformers. I'm pretty much fixed on the 12ax7, and I;m afraid aroudn 100k will load down the gainstage to much. so I probably have to find a 'free' cathode somewhere further up the preamp . Back to the drawing board!