Schade Common Gate (SCG) Preamp

Jump to Latest
#1,527
So no need to adjust R103? That will make things much simpler.

Is there a voltage recommendation for the test rig? Any advantage to use a voltage similar to what they will see? (You expected that question from me, I feel sure)

Will have boards on order momentarily. You can get 20 for just a bit more than 10 so if anyone wants one at my cost - $6.40 plus the postage - let me know. I am getting 2 mm thick - 2 ounces copper - 2u ENIG finish.

Board measures 93.8 x 71.8 mm - 3.69 x 2.82 inches. Cap space is 50.2 x 14.2 mm w/ lead spacing - 47.6 mm resistor lead spacing 7.66 mm - using the tool at JLCPCB.
 
#1,532
Here is the PCB with two channels on the same PCB but without switching or volume control. The mounting holes match the original SCG PCB. I have not tested this one yet. I am working on updating the full version with switching and volume control and will then test both in the next month. For the included BOM, the quantities listed are per channel and you would need double the quantity of each part for two channels. The fab.zip file is the one to upload to the PCB house.
 

Attachments

  • SCG_JFET_PRE_Schematic.PNG
    SCG_JFET_PRE_Schematic.PNG
    174.2 KB · Views: 114
  • SCG_JFET_PRE_Stereo_PCB.zip
    SCG_JFET_PRE_Stereo_PCB.zip
    208.8 KB · Views: 37
  • Like
Reactions: ElArte
#1,538
Hey Rahul,

Very excited to try this new version of the SCG preamp. I appreciate all the work you've put into this! I'm planning to build one and use it as an integrated gain stage in front of a version of the Total Domination VFET I am planning to build with the 2SK180 SITs I bought at the Burning Amp auction in 2022. I heard both the TDV and the earlier version of the SCG preamp attached to your speakers that year and I was impressed with both!

While I await the PCBs I ordered, I purchased the JFETs. I had no trouble selecting JFETs for Q105, Q106, and Q103 that fell within your specified range of currents.

But I noticed from the data sheets that the Vp / VGS (Off) values vary pretty widely for the J111 (Vp 3V - 10V) and J175 (Vp -3V - -6V) JFETs.
  • From a batch of 20 J111's I found the lowest Vp were around 7V.
  • From a batch of 25 J175's the highest Vp (I assume when you say "choose the highest" you mean the most negative or the highest absolute value, rather than the closest to zero) were around -3.5V.
Do you think these values will be acceptable for use in the circuit, or should I roll the dice again and order another batch of J111's and J175's?

I don't fully understand the implication of these values in the circuit but I am (wildly?) guessing that the Vp of Q101 will influence how large of an input signal can be fed in before clipping? I am not sure the reason for the low Vp target for Q102.

Thanks in advance for your thoughts,
Francis
 
  • Like
Reactions: ra7
#1,539
metaphile said:
Do you think these values will be acceptable for use in the circuit, or should I roll the dice again and order another batch of J111's and J175's?
Click to expand...
Yes, those look fine.
metaphile said:
I don't fully understand the implication of these values in the circuit but I am (wildly?) guessing that the Vp of Q101 will influence how large of an input signal can be fed in before clipping? I am not sure the reason for the low Vp target for Q102.
Click to expand...
True based on my observation but not in theory. Q101 operates in follower mode, which means we aren’t limited by Vp, but to me the wider Vp devices sounded a bit better.

For J111, I didn’t find a huge spread, especially at the operating point. So, it doesn’t matter too much which one you use within the batch. But within the batch, the smaller Vp might provide higher gain, which is better here. There is still more than enough headroom, thanks to J111.
 
  • Like
Reactions: metaphile
You must log in or register to reply here.
Top Bottom