DIY Audio Projects Forum

Continuing my Wood Cheek series (seven in all and still have five) here is an assembled but not fully wired hybrid headphone amp. 4SU front end and OPAMP (like CMoy) backend. External PS is complete with three voltages; HT, filament and B+ for the OPAMP section.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |

Here is a shot from underneath and some updated outer images. I wired the first socket wrong so added another and rewired it to the first correctly this time. Apart from a few poorly soldered socket pin connections on the second socket the amp worked perfectly. The limiting R to the 6V reg has been finalised to 1K which puts just under 10V on the input to the reg to give me a steady and overly well filtered 6.02V at the PT2308. The chip max V is 7V. Chip (Class AB) draws 12mA.

This is the best piece of audio gear I have ever built in 50 years of this hobby. My second pick would be the recently constructed Chuguang 6SN7 50 year treasure, active load, preamp ("Icon"). The preamp outperformed the far more expensive Elite with its PsVane Globes.

With the inputs shorted and my new meter on 200mV scale (displays in RMS) I got 0.0uV of noise on the output with pot (blue velvet) fully open. On my Sennheiser 700s there is no hum, hiss or noise of any kind. The blackest blacks. The current tube is a new Tung Sol gold pin which when flicked has NO ringing at all. Great tube and a pair have sat in the draw years. Music through the amp and 700s is absolutely sublime. I just forget I'm on phones and get totally lost in the music. This amp beats the last two all tube HP amps - Sanctum and Sanctorum absolutely hands down. Stunning is the only word to describe the music. Gain is more than sufficient when you consider in the 4SU stage there is no Ck(s) and the chip stage only has a gain of 3db.

The external PS has a triple Pi choked (5H Hammond) filtering stage; C-L-C-R-C-R-C. There is over 400uf for high voltage caps for filtering. Last cap right at the tube section is a 4uf PIO (snubbed). Very clean DC (near 10,000uf of Pi filtering) on the filaments and triple filtering and regulated DC to the chip. All LESR caps on the chip and the poly output cap from the tube section is fed direct to the chip. Pots on the front so no input on the chip I just share the output cap of the 4SU stage. There is a 100uf LESR output cap from the chip. I thought of a NP but way to big and I'm not sure it would have improved the sound. Very, very happy.

I could go on...

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |

I know how you feel about your pre amp but sometimes dudes out there like my dad dosen't accept new techology like hybrids and only cares about the pure valve stuff

It works well to drive the HPs.

Did some tests -3db into 33omhs is 780KhZ. Low end 40hZ. Square wave under 1K no so hot but good to 100K.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |

Are you using capacitor in feedback network the ground the resistor, so the dc gain is only 1 but ac gain is the set value?
That capacitor could be too small in the op amp so f cut 40hz

The squarewave dosen't matter anyways, aslong as system stable and less than 3 oscillations, unless you have allot of music with square waves, some music does

No cap in the feed back network. Here is the schematic I used.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |

I belive is capacitour couppling plorbem, for the 40hz roll off

The input impedance of op amp is simply these two resistors added or look at inverting op amp tutorials
It could be valve to op amp couppling cap too small or something

You probally wouldn't have make a mistake on simple cut of calculations for output cap to hadphones and valve input cap

But the easist way to make mistake is the input on the op amp, make sure if you have to caps then is series and is just resistor divider formula to get the capcitance

No, as above, I used the output cap of the tube section (0.68uf) as the input cap. So no additional input cap.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |

This is possibly the last thing that can go wrong for a 40hz roll off
You've set all cut of freqencies at 20hz ? and these add up to a -3db at 40hz
-3db one stage then again on other stage, you add the dBs so thats -6db at 20hz if you have two stages and fcut 20hz
(you multiply the two gains at 20hz with -3db less volume (add the dbs in log scale = multiply))

I generally find 5to 10hz cut off to be good

So how do I fix it?

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |