What does this relay and this resistor do?

[Mbira]

Hi folks,
I am continuing my exploration of the G9 circuit, and I'm curious about these two things. 

What is the benefit of the relay?  I have two ideas.  1) So that you can keep the traces short by not having to run the mic inputs to the front of the panel and a switch?  2) Because there is some sort of protection inherent in a relay vs using a switch? 

I'm doing a point-to-point/turret strip hybrid of this circuit, and I'm wondering if the relay is an important thing for the performance. 

What is the purpose of R5?  Better yet-does this resistor have a name so that I can look it up and learn more about the function?  I have a couple guesses: 1) Does this somehow help insure that even an unbalanced signal will work in the circuit?  2)  Does this do something to further balance a signal if for some reason it is  "lopsided"? 

Thank you for your help! 

 

[rock soderstrom]

Hi, this relay switches a PAD (Passive Attenuation Device) in and out. R3-R5 are forming the PAD. This PAD converts the "mic" input to a "line" input. You can do this with an ordinary switch, as well.

https://www.electronics-tutorials.ws/attenuators/attenuator.html

"Circuit description:

The input is taken to Phantom resistors and the line input relay. The line relay - as well as the Phantom power - is controlled by the front panel rotary switch, SW1: (line - mic - mic+p48). When in "line" mode, we set the input attenuation to -26dB, the Zin to~10KOhm - but still Z-matched to the transformer, with an option of either OEP A262-A3E or Lundall 1528."

from http://www.gyraf.dk/gy_pd/g9/g9pd.htm

 

[Mbira]

Hi, this relay switches a PAD (Passive Attenuation Device) in and out. R3-R5 are forming the PAD. This PAD converts the mic input to a line input. You can do this with an ordinary switch, as well.

https://www.electronics-tutorials.ws/attenuators/attenuator.html

Thanks! I had no idea that "pad" was an acronym. :-D

Is there any benefit to using a relay instead of a switch?

 

[Mbira]

Also-thank you about mentioning the  pad! That helped me track down that that one resistor is part of a "U-bridge" attenuator. Cheers!

 

[rock soderstrom]

Thanks! I had no idea that "pad" was an acronym. :-D

Is there any benefit to using a relay instead of a switch?

Not really, I would do it with a switch. IMHO, this relay would make more sense on the secondary side of the input transformer to switch between the "mic" and the "instrument" input. The situation there is pretty high impedance, so it is really important to keep the wires as short as possible. See the Orange86 schematics for reference.

https://www.dropbox.com/sh/g6n57y9ju6hwknz/AABrx_uyGpweCHkqwwMQi6Xja?dl=0&preview=Orang86schematics.pdf

 

[JohnRoberts]

Thanks! I had no idea that "pad" was an acronym. :-D

Is there any benefit to using a relay instead of a switch?

If you have too much money that will help reduce that problem.

Relays make sense for remote control, or keeping audio path short, but generally not cost effective.

JR

 

[Bo Deadly]

Right, the signal at that point is potentially very low so the relay is a little better than running it out to the switch on the panel. In practice, it is fine to use a switch IF you use quality cable with two conductors and a shield with good coverage and don't leave long bits exposed. More specifically, in this case, I would slice open the jacket and pull the inner wires out between the shield just enough to solder to the switch so that the shield is unbroken and as close to the switch as possible.

As you have since discovered, R5 is part of the attenuator (or "pad" as they say).

 

[gyraf]

Agree.

The attenuator presents a medium-size impedance to the line input (ca. 10K Ohm), while at the same time looking like some 450 Ohm to the input transformer.

Absolutely possible to do without relay, but will require careful routing and mechanical thoughtfulness. The relay enables us to remote it from the same front-panel switch we use for the P48.

Jakob E.

 

[Mbira]

Agree.

The attenuator presents a medium-size impedance to the line input (ca. 10K Ohm), while at the same time looking like some 450 Ohm to the input transformer.

Absolutely possible to do without relay, but will require careful routing and mechanical thoughtfulness. The relay enables us to remote it from the same front-panel switch we use for the P48.

Jakob E.

Thanks Jakob-and thank you so much for creating this project in the first place. My two G9s are my go-to tube preamps.

I'm planning on using separate switches for the pad and the +48 anyway, so I'm going to look at losing the relay and seeing how performance is. Cheers!