thinking of a 1W Marshall next, but confused by this schematic

[markmalin]

Hey all,


I'm thinking of trying a 1-Watt Marshall JTM style amp. There are several threads on this on the internet, most notably by a guy called Matec from a Brazilian forum.  I don't claim to completely understand this, but I'm confused by the diodes and capacitor (and the resistor/cap) shown near the ground lug of the power plug.  I keep staring at this and these components appear to be between chassis ground and signal ground...but why, and what are they doing?  Or more likely, what is it that I am not understanding about this schematic?  (I admit I'm a hack at best)

Thanks everyone


Humbly
Mark

[kagliostro]

It is a way to avoid humm loop due to earth connection, it is used on some commercial amp


Franco

[markmalin]

It is a way to avoid humm due to earth connection, it is used on some commercial amp

Franco

Grazie, Franco.  That's the only thing that made sense to me every time I looked at this schematic!


Mark

[kagliostro]

I'm not able to find the original Elliot Sound Products article, but here you can find some explanation

Elliott Sound Products article Earthing Your Hi-Fi – Tricks and Techniques article give some construction details how A High Current Safety Loop Breaker Circuit (pretty similar to one believed to be inside HumX) could be built:




This circuit example has the ground isolator between the mains power ground and audio part zero voltage line. Here the circuit the current loop breaking the loop is done with the 10 Ohm resistor, the current is now less than 200mA, and the voltage across the interconnect will be very much smaller, reducing the hum to the point where it should no longer be audible. This is how the circuits work when the potential difference over the 10 ohm resistor is lower than the voltage drop of the diodes on the rectifier bridge (around 1.2-1.4V). The capacitor will pass high frequencies (RFI protection).
In case there are is some serious ground leakage the diodes will start to conduct and pass through enough current needed to burn the mains fuse if needed without too much vooltage drop (no dangerous voltages over the isolator circuit). In the event of a major fault, one (or more) of the diodes in the bridge will possibly fail. Semiconductors (nearly) always fail as short circuit, and only become open circuited if the fault current continues and ‘blows’ the interconnecting wires. High current bridge rectifiers have very solid conductors throughout, and open circuit diodes are very rare. Use of the bridge means that there are two diodes in parallel for fault current of either polarity, so the likelihood of failure (to protect) is very small indeed.

Here a schematic easier to be understand (no bridge, diodes)





BTW, the version on the schematic you posted seems wrong, there is a cap in series with the resistor



Ciao


Franco

[markmalin]

Wow, that's pretty fascinating.  Grazie per la spiegazione!  Now to decide whether to use this or not...I wonder if it's really needed?


Mark

[shooter]

I wonder if it's really needed?

If it hums add it, if it sings leave it

I used it in 2 early builds, my rat nest phase, it did help, once I cleaned up my builds haven't had to go there.  With a 1W, your signal will not get super big, relative to possible noise, might be something to consider.

[markmalin]

I wonder if it's really needed?

If it hums add it, if it sings leave it

I used it in 2 early builds, my rat nest phase, it did help, once I cleaned up my builds haven't had to go there.  With a 1W, your signal will not get super big, relative to possible noise, might be something to consider.

Thanks, shooter.  That makes total sense.  (just being a cheapskate...It's something my wife tells me I need to work on, but that's another story...)

[markmalin]

BTW, the version on the schematic you posted seems wrong, there is a cap in series with the resistor

well...that's interesting.  I actually have a copy of one of the Marshall 1w schematics, but different model.  This one doesn't show that resistor.

[kagliostro]

Here you can read an explanation about the presence of the resistor (pag 275-276 on the pdf)

http://www.valvewizard.co.uk/Grounding.pdf

Franco