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4S Tube Preamp with 12AU7 Cathode-Follower Output Stage
https://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5186
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Author:  lowhifi [ 24 Dec 2019, 19:02 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Hello. This is a preamp i just started to build, I have almost all the parts and it will hooked to a Bryston 10B active crossover with an input impedance of 33k or 20k depending of the website and it will drive my pair of Lacewood V2. I have a Dynaco PAS3 regulated power supply board, can i use it for the 4S. 6v vs 12v for the heater should not be a problem but the voltage for the PAS3 is 330v vs 380v for the 4S and what's the value of the Cc cap will be recommended. Regards.

Author:  ILoveHiFi [ 29 Dec 2019, 17:50 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

20k load with cut off frequency of 5hZ = 1.5uF
Recomended 1.5uF to 2.2uF capacitor

Cutt off frequency set at 10hZ is not recomended, below 5hZ not recomended
https://www.electronicproducts.com/RC_F ... lator.aspx

Author:  lowhifi [ 30 Dec 2019, 17:12 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Thanks, i will try 1uf first and see the result. Using DC for the heaters will be beneficial? The enclosure still need woodworking but later. Regards.

Author:  ILoveHiFi [ 30 Dec 2019, 17:34 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Depends on how your heaters are set up, if the setup in schematic design is poor quality then DC heaters will reduce noise.

Author:  ILoveHiFi [ 30 Dec 2019, 17:46 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

kmakdaddy wrote:
Hi I am a first time builder with nearly zero electronics experience or knowledge. I have started down the path of my first build with this little Preamp. It will be running a pair of Genelec 8020B’s that have a 10k input impedance and a Yamaha YST-515 sub that I am not sure of its specifications.

I have a couple of questions that I hope this group might be able to help me with.

Should I run a 3rd preamp channel for the sub ? If so what should I be thinking about if I do this ? If not and I just run it off the left Chanel of the preamp is there anything that I need to do here ?

Also for the bridge rectifiers on the power supply am I better with the one piece units like I have used or building out of MUR860 diodes ? I am not worried about the few extra $ to change it if the diodes will make any real change.

For the voltage reg on the heater side of the PSU I have scavenged a neat little heatsink / EMI shield to mount it to. Currently I have the regulator insulated from the heatsink should I remove the insulator and have it grounded to the heatsink. The heatsink will be grounded probably back to the - of the heater output.

On the bridge rectifier on the HV side you can see that I have mounted the caps sitting above the rectifier is this a problem at all ?

I have started with building the power supply of the wall wart variety. I have relied on the knowledge of the guy at Jaycar for the finer details of the components. And I know very little about what I am doing so if you don’t mind I would love some harsh yet helpful criticism from the group.

A couple of notes that are fun or relevant.

The board it is all mounted to is 3mm acrylic. I have used a 35w CNC Laser to cut all the mounting holes for the components and I have used the laser to cut tracks into the board and run 27AWG copper wire laid into these tracks to interconnect components. I have run the interconnects in an arc from point to point to avoid any parallel wire runs.


The next stage is to solder it all together and test is and if all goes well I will start on the actual preamp stage.

Thanks for any help or advice that you can offer.

Marc


The diodes helps little bit with sound quality the fast revocery ones allow less supply noise, Using the heatsink to condcut electirc is very bad idea, when heat conddductivity is not a prolbem the heat sink needs to be isolated from device.

Author:  lowhifi [ 30 Dec 2019, 23:36 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

tizman wrote:
Here is the PSUD II sim for the power supply...

L1 is 2H 100ohm but in your preamp you still have the 154M choke who is 175 ohm. Do you mean 100mA. Regards.

Author:  lowhifi [ 02 Jan 2020, 01:55 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

My 4S is functionnal, very nice. There's a noticable hum when there's no music but nothing annoying. The pot is 100k and there's too much gain with a 12ax7 but OK with a 12au7.
At this point it outperform my restored Dynaco PAS3.Thanks Matt.

Author:  mwhouston [ 02 Jan 2020, 02:03 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Try 250K for the pot. I’ve found that works best.

Play with earthing to remove hum.

Author:  ILoveHiFi [ 02 Jan 2020, 03:22 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Change the earthing to remove hum

Author:  lowhifi [ 02 Jan 2020, 09:25 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Forgot to say, the ground lift switch change nothing also my two Lacewood have not their signal ground earted maybe i should add a ground lift switch to them also, The filament voltage is a little high at 6.9v, i read somewhere i can add a thermistor and maybe my Mullard 6201 will not flash when cold starting. Regards

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