Schade Common Gate (SCG) Preamp

[Monk55]

I agree. There were some interesting reads as “offshoots” in that thread.

 

[Monk55]

See page 21…interesting read from 1955 on cathode bypass caps. Not to mention great ads!
https://worldradiohistory.com/Archive-All-Audio/Archive-Audio/50s/Audio-1955-Nov.pdf

 

[Marcus Halberstram]

Shades of Sound Practices!

 

[aljordan]

Hi,

I don't know if anyone will be interested in this. It isn't really specific to the SCG preamp, but I figured I would post because it was a finding via the SCG preamp. I was hearing some annoying stridency in certain instruments such as saxophone or violin when listening to my build of the SCG preamp. I took some measurements and saw some ugliness in the noise measurement, so I figured I would try replacing the internal instances where I used unshielded twisted cable with shielded cable instead. I had only used unshielded cable in the run from the volume control to the boards, and from the boards to the RCA outputs. RCA inputs to the selector switch already had shielded cable.

I replaced both runs with shielded cable, and the annoyance disappeared. I don't know if the measurements have anything to do with what might have been creating the stridency, but they do show improvements in both the noise floor and crosstalk figures, so I thought I would post the before and after pictures.

Noise floor with unshielded twisted cable:

Noise floor with shielded cable:

Crosstalk with unshielded twisted cable:

Crosstalk with shielded cable:

Alan

 

[ra7]

Brilliant! Thanks for sharing! Great that you found the source and cured it.

Yeah, there shouldn’t be any stridency at all. It has a nice second harmonic character. Nice but not too nice 😉

What is driving the SCG and what is being driven by it? I have found that wimpy DAC outputs can sound strident.

 

[aljordan]

The SCG is driving a pair of Aleph 2 mono-blocks built using these boards. It is being driven by either a Peter Daniels (Audiosector) DIY DAC, or a Schitt Yggdrasil Less is More DAC, or a Salas UFSP phono stage. The preamp now sounds beautifully smooth through both DACs.

P.S. Here is the distortion spectra of my SCG build at about 1 dB over unity gain:

 

[ra7]

Lovely! Thanks for measuring and sharing!

 

[ElArte]

Thanks, Alan.

How do you handle the shield on your shielded cable? Do you connect it to board ground (and hence to chassis ground) at one end only?

 

[aljordan]

Hi ElArte,

I am running two different cables. The cables from RCA inputs to the selector switch are Mogami W2330 coax. I have the shields attached to the RCA ground buss, which is connected to the single chassis ground point via a 10 Ohm resistor. The shield is not connected on the switch end.

The cables from the board to the RCA outputs are a quad mic cable so I can run the hot and ground together. That shield is connected to the ground at the board channel outputs along with the ground run. The shield is not connected to the RCA ground buss on the other end. The forth part of the quad cable is ignored.

I am using same quad cable from the volume control to the board inputs, but the shield is not attached anywhere. It is a foil shield and the through hole solder pads on the volume control are too narrow to accept the ground run and the shield. I could have soldered the foil shield to the ground run, but the foil doesn't solder well.

You didn't ask, but the overall grounding scheme is:

• Single ground point to the chassis.
• The IEC ground and the board's chassis ground are tied directly to it.
• All RCA jack grounds are tied to an RCA buss ground wire.
• The RCA buss ground wire is tied to the above chassis ground point via a 10 ohm resistor.
• I didn't populate the board's TH Chassis point with a thermistor because I didn't have one with legs thin enough to fit through the holes.

This pic shows some of the sloppy work, but it is very quiet with my ear up to the horns of my speakers.

 

[BRN]

I need to do something like this. I'm getting some humming from my build. The only reason I have not is that it is not that bad. I have to put my ear right up to the speakers to hear it, and I'm slowly working on other projects.

I may try this wiring scheme, or something similar, and see if it makes a difference.

 

[ElArte]

It’s an eternal problem really.

People like Rahul do an awesome job designing great circuits within the confines of a board, but when I place the board within an enclosure, where it connects to the world, there are many opportunities to spoil a good thing.

And the amount of contradictory advice out there is staggering, which leads some to declare “what works in your scenario is what’s best”. (and rain is wet)

So, I am going to consider the problem seriously this time around…

1 reference is attached:

In which you can read:

The author is kinda legendary too.

There a few more "references":

https://www.ranecommercial.com/legacy/note151.html
https://www.hypex.nl/support/faq
https://conradhoffman.com/troubleshoottut.htm

I will post other PDFs I collected over the past few years in the next post.

 

[ElArte]

Additional PDFs...no lack of resources, but a definite lack of clarity, as many of those contradict each other to some degree if not entirely.

 

[aljordan]

So, which one would you follow for a single ended preamp?

 

[Vunce]

The one most quiet

Edit,
I couldn’t resist, hehe.

 

[ElArte]

So, which one would you follow for a single ended preamp?

I am in the process of reading all resources and deciding which one makes the most sense.

I’ll probably go mad.

 

[aljordan]

You need to stay grounded.

Ha ha ha.

 

[Monk55]

I’m in shock, after that grounding!

 

[ElArte]

I'm trying to help and I get grounded. Shame.

I might as well post tidbits I found interesting along the way:

 

[ElArte]

 

[ElArte]

Oh boy