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4S Tube Preamp with 12AU7 Cathode-Follower Output Stage
http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=5186
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Author:  mwhouston [ 02 Sep 2019, 19:15 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Finally got my version of this schematic in a finished preamp. Too much gain and some thermal noise - hiss. Excellent sound though, very fast and snappy with more bass weight than my 6SN7 preamps of late.

Any thoughts on how to drop the gain and remove more of the hiss would be appreciated. Currently Mullard re-issue 12AX7s in but tried it with Psvane 12AU7s which dropped gain slightly and some of the hiss. Very impressed with the sound. Better than my very expensive 6SN7 builds. My output cap and interstage caps are Russian Military PIO. Output cap is 1uf into Class D amps.

Images to follow.

Author:  gofar99 [ 02 Sep 2019, 20:58 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Hi Mark, sent you a PM

Good listening
Bruce

Author:  mwhouston [ 03 Sep 2019, 06:01 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Attachment:
AF5FACA8-DBFB-4A08-888F-66420A6CA12C.jpeg


Bruce gave me a clue on how to reduce the gain. There was just too much hiss from the preamp due to excessive gain. So I dropped the plate resistor Rp to 30K from100K and the cathode resistor Rc to 820ohms. These values I have used before and found they have always worked well.

Still a lot of gain but the thermal noise is very low. At good volumes I have to put my ear right into my Altec horns to hear a slight hiss. Applying the 1m rule, I hear nothing. Happy man.

I could swap in the Psvane 12AU7 “T” series MKIIs for even less gain and a quieter preamp but I really want to use these Mullard re-issue 12AX7s. The preamp really performs very well glad I persisted. The problems I had in another preamp with the this brd. Must have also been due to the gain. Now not a problem.

Here is an image as promised. Waiting tube rings and name plate but doesn’t detract from a great sound. The finish is Hammer tone blue on a Hammond wood cheek chassis. External PS which uses a 24VAC wall wart.

Author:  ILoveHiFi [ 04 Sep 2019, 02:08 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

You can use a resistor divider at input or output to reduce gain

Author:  laurie54 [ 04 Sep 2019, 16:27 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

Mark. Are these two res the only changes made to the original sch from Matt?

Putting a res divider net on the input is only going to reduce the signal available to be amplified. It does nothing to the noise level (hiss etc) due to excessive gain in the first stage. Putting one on the output of the amp doesn't work either as the noise level will always be present in the same proportion to the signal being amplified.

Author:  mwhouston [ 04 Sep 2019, 18:15 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

laurie54 wrote:
Mark. Are these two res the only changes made to the original sch from Matt?

Putting a res divider net on the input is only going to reduce the signal available to be amplified. It does nothing to the noise level (hiss etc) due to excessive gain in the first stage. Putting one on the output of the amp doesn't work either as the noise level will always be present in the same proportion to the signal being amplified.

Yes only two resistors changed. Hiss is now very low and my speaker are better than 90db sensitive.

Bruce’s recommendation was 15k for Rp and an Rk of 330. This may be the best way to go.This will reduce gain more. I can only just turn the 250K pot up. It would be better if I could operate it more into the middle.

Author:  ILoveHiFi [ 04 Sep 2019, 19:28 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

laurie54 wrote:
Mark. Are these two res the only changes made to the original sch from Matt?

Putting a res divider net on the input is only going to reduce the signal available to be amplified. It does nothing to the noise level (hiss etc) due to excessive gain in the first stage. Putting one on the output of the amp doesn't work either as the noise level will always be present in the same proportion to the signal being amplified.

When you have music playing normally you can't hear the noise anyways and its negleteble losses to sound quality.
I only belive in the pot on output of pre amp, so the noise is zero when you have the pot on 0 volume.

Author:  mwhouston [ 04 Sep 2019, 22:10 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

ILoveHiFi wrote:
laurie54 wrote:
Mark. Are these two res the only changes made to the original sch from Matt?

Putting a res divider net on the input is only going to reduce the signal available to be amplified. It does nothing to the noise level (hiss etc) due to excessive gain in the first stage. Putting one on the output of the amp doesn't work either as the noise level will always be present in the same proportion to the signal being amplified.

When you have music playing normally you can't hear the noise anyways and its negleteble losses to sound quality.
I only belive in the pot on output of pre amp, so the noise is zero when you have the pot on 0 volume.

Reduced Rp to 15k and Rk to 390. There is only 0.9V across Rk resulting 2mA idle current flow. HT has dropped to 264V and filament voltage 12.1VDC. All within acceptable range. Hiss is slightly lower but gain is still high. But not making any more changes.

Author:  laurie54 [ 05 Sep 2019, 23:57 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

""But not making any more changes.""

Well ok but you know this means you still need
one with the bias changing sw. You do build with SS
and Tube. It would seam like a natural addition.

The paint job turned out great. Hammer coat is a nice finish.

Author:  kmakdaddy [ 25 Sep 2019, 04:37 ]
Post subject:  Re: 4S Tube Preamp with 12AU7 Cathode-Follower Output Stage

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

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