Schade Common Gate (SCG) Preamp

Vunce

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2014-01-26 8:20 pm
LI, NY
Time to listen ;)
 

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Vunce

Member
Paid Member
2014-01-26 8:20 pm
LI, NY
Modded SCG has been driving my Big Mofo’s going on 10hrs now. I’ve been carrying on my day as normal with random stops for listening. I would say this mod is an improvement, although not a drastic one in my system. The music has more air to it, my living room seems bigger and the speakers were harder to place. This was more evident listening to genres like smooth Jazz, orchestral, rock and modern country. I didn’t notice a difference listening to alternative and heavy metal. For you fellas in possession of this pcb version SCG, the modification is so easy to do, try both ways. I will leave mine set up with the Mu-Follower mod in place!
Rahul, what VFET amplifier circuit are you using? I’m interested in trying one of Ben’s follower amps. ;)
 
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Thanks Vunce! That is pretty much what I heard as well, though it was more dramatic for me. The speakers were disappearing earlier, but now the walls have completely melted. Such an ease and naturalness to the sound. Even more so than before. Now I am fully 'there.'

Sonic properties aside, I don't notice a huge difference in the harmonics at 1 kHz, but I haven't yet looked at THD v output. I suspect THD remains rather flat with level and even at high output giving it the "ease."

Vunce, my circuit for the VFET amp is similar to Ben's. I am using the 2SJ28 and instead of the CCS in the lottery amp, I use the Hammond 193T choke. Bias is at 1.5A. No buffer or transformer, so directly driving the gate with the SCG. I can post a schematic if you are interested, but it is similar to Ben's.

Also getting the Dorati from Abrax. Looking forward to the NOS DAC sound.
 
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Alfi

Member
2004-10-13 9:53 pm
Rahul,
I built a headphone amp using your Preamp PCB where the output is connected to an output transformer(Jensen JT-10K61-1M).
It's driving my Sennheiser HD600 headphone.
I really like what I'm hearing, it's transparent and nature sounding with good imaging.
Thank you for the design and for providing the PCB and gain devices.

Regards,
Alfi

IMG_8216_3.jpg
 
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Alfi

Member
2004-10-13 9:53 pm
That is awesome, Alfi! Another great build. Congratulations! :cheers:

How did you wire the Jensen's?

Looks very neat!
Rahul,
I omitted R109/R209. I wired the primary of the jensen across the output of the PCB.
I wired the jensen's two secondaries in series for a 4:1 step down.
I believe this results in an overall gain of 2.5 for the headphone amp.

Regards.
 
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A little off topic, but thought George (@Tubelab_com) might be able to help me. I experimented with a power amp using the Schade method. Here's the schematic.
1657390637159.png


I am getting very poor bandwidth at the top end. Probably not even making it to 10 kHz and quite a bit less than the sim. Getting good bit of gain. The device is an SCT10N120 with nice looking curves. It biases up exactly as shown. This is not an SCG topology, but it is identical to a Zen amplifier just with an SiC device. I even added 1000 ohms of source resistance to the input voltage generator, but the sim still shows very wide bandwidth.

The same device but with the inductor on the source and the output taken from the source pin sounds quite good. Okay, it is no VFET, but it sound like a decent class A amplifier. I pushed it to 1.5 amps and it was doing fine.

I made R1 into a 100k trimmer pot for easily changing the bias. It works nicely. The bias does wander a bit, but mostly stays close to where you set it. Certainly could be improved.

The inductor is the Hammond 193T. The chassis and setup was my choke-loaded VFET amp, but I pulled the VFETs for the OS2 amp and now using the setup for experimentation :hphones:
 
A little off topic, but thought George (@Tubelab_com) might be able to help me. I experimented with a power amp using the Schade method. Here's the schematic.
View attachment 1070781

I am getting very poor bandwidth at the top end. Probably not even making it to 10 kHz and quite a bit less than the sim. Getting good bit of gain. The device is an SCT10N120 with nice looking curves. It biases up exactly as shown. This is not an SCG topology, but it is identical to a Zen amplifier just with an SiC device. I even added 1000 ohms of source resistance to the input voltage generator, but the sim still shows very wide bandwidth.

The same device but with the inductor on the source and the output taken from the source pin sounds quite good. Okay, it is no VFET, but it sound like a decent class A amplifier. I pushed it to 1.5 amps and it was doing fine.

I made R1 into a 100k trimmer pot for easily changing the bias. It works nicely. The bias does wander a bit, but mostly stays close to where you set it. Certainly could be improved.

The inductor is the Hammond 193T. The chassis and setup was my choke-loaded VFET amp, but I pulled the VFETs for the OS2 amp and now using the setup for experimentation :hphones:
Try sticking a 30 pF cap across R2 in your sim and see what it does to your HF response. The spec sheet shows 9 pF with 400 volts across the part, but the graph shows about 30 pf with 10 volts across the part. It also varies a lot with the signal voltage in the range you will be operating it at.
 
Back to the SCG. I am getting some buzz on both channels with a 50k Alps pot at the 12-3 pm positions. It goes away below 12 pm and above 3 pm. What's the experience of others (@Alfi, @Vunce )who are using pots before the SCG? Getting any hum/buzz pickup at various pot positions? It could be the specific pot, but I want to ask before I dissemble it.