Hi !
For the past 10 months, I’ve been working to build a tube amp…After a lot of reading, I’ve decided to go with a separated preamp and two KT88 monoblocks. I’m almost done with the soldering now but I’m not really sure how I should solder the female RCA connectors…
The central pin is the signal pin, which is positive. How about the shield of the connector ? Maybe you can help me to understand more about this :
1- Should I connect the negative pin (shield) of the RCA connectors to the negative bus of the preamp / amp ? At first though, I would connect it to the earthing pin (third pin).
However, on many schematics, the negative pin of the AC input connector seems to be connected to the negative bus (the same bus to which we connect the negative pins of the power supply filter capacitors and the center tap wire of the power transformer). What would happen if we only connect the positive pin of the AC connector ? Would it still work ?
And what if we connect the negative pin of the RCA connector to the negative bus, for both the amp and the preamp, and that, for some reason, the negative buses in the preamp is not at the exact same voltage level of the negative buses of the amps, would there be a current flow in the through the negative wire of the RCA cable ?
As you can see, I’m not quite sure to understand this, even if at first look, it might seems quite simple
Finally, should I connect the metal frame of the preamp and amps to the earthing pin of the AC socket ? I’ve been told to not do this because there could be ground loops resulting from this. Since tube deals with pretty high voltage, I think that it’s still the best decision to ground the metallic parts of the chassis.
Hope to get some clarifications from you guys.
thanks !
Mathieu
For the past 10 months, I’ve been working to build a tube amp…After a lot of reading, I’ve decided to go with a separated preamp and two KT88 monoblocks. I’m almost done with the soldering now but I’m not really sure how I should solder the female RCA connectors…
The central pin is the signal pin, which is positive. How about the shield of the connector ? Maybe you can help me to understand more about this :
1- Should I connect the negative pin (shield) of the RCA connectors to the negative bus of the preamp / amp ? At first though, I would connect it to the earthing pin (third pin).
However, on many schematics, the negative pin of the AC input connector seems to be connected to the negative bus (the same bus to which we connect the negative pins of the power supply filter capacitors and the center tap wire of the power transformer). What would happen if we only connect the positive pin of the AC connector ? Would it still work ?
And what if we connect the negative pin of the RCA connector to the negative bus, for both the amp and the preamp, and that, for some reason, the negative buses in the preamp is not at the exact same voltage level of the negative buses of the amps, would there be a current flow in the through the negative wire of the RCA cable ?
As you can see, I’m not quite sure to understand this, even if at first look, it might seems quite simple
Finally, should I connect the metal frame of the preamp and amps to the earthing pin of the AC socket ? I’ve been told to not do this because there could be ground loops resulting from this. Since tube deals with pretty high voltage, I think that it’s still the best decision to ground the metallic parts of the chassis.
Hope to get some clarifications from you guys.
thanks !
Mathieu
Have you read this article?
http://www.diyaudio.com/forums/diya...udio-component-grounding-interconnection.html
http://www.diyaudio.com/forums/diya...udio-component-grounding-interconnection.html
Schematics usually don't show the details, just the topology, especially for "ground" connections. It's not just where things eventually connect that's important, it's also HOW they connect.
For the RCA signal inputs, which should have both parts isolated from the metal case, connect a wire to each part (center and outer). Those wires go to your circuit input point, which is often the two ends of a resistor that's connected from a tube's grid input to ground. The wires should stay as close together as possible, so there is an absolute minimum of "loop area" formed between them, which would enable them to have currents induced in the loop by AC fields in the air. So twist them tightly together, the whole way. Better yet would be to have them both be inside a shielded cable together, with the shield grounded to the case at the input end, only. (The shield would not carry either the signal or its ground. The cable would have to have two wires inside plus an outer shield.)
You don't want to connect the RCA signal connector's ground through any other path, since that would separate it from the center signal conductor and they would form a big loop that would have AC currents induced in it, which would induce a hum voltage across the grid input resistor as well as across the source impedance. (See "Faraday's Law", or, for the general case, "Maxwell's Equations".)
The "minimal loop area" thing works for minimizing transmitted as well as received interference. So all other wire pairs should also have minimal loop area between them. So twist tightly together any lengths of AC input pairs, AC secondary pairs, heater AC pair, any DC +/- or +/gnd pairs, output pair, etc. And keep the small signal wiring away from all AC and output or large signal stuff.
Yes, you MUST ground the chassis to the safety ground wire or to earth ground. And it must be connected to the chassis either with a bolt or by welding, i.e. not by soldering, since that could melt and disconnect before your fuse or circuit breaker blew. Without the safety ground connection, it would be much easier for your equipment to kill you, or, even worse, someone else.
You will also want to read about "star grounding". That involves having one central ground point and having separate ground return conductors that all go to that point. The reason that is necessary is that wires and metal in general are not perfect conductors. They have distributed inductance and resistance, at the least. So ground-return currents flowing through a wire will induce a voltage across the wire, back at the non-ground end. And a time-varying current will induce disproportionately-large time-varying voltages. So, for example, you don't want your grid input resistor's ground reference point to share a ground return conductor with a large OR dynamic current from somewhere else, because then your input's ground reference will be a bouncing voltage and that bouncing ground voltage will be arithmetically summed with your input signal.
For the RCA signal inputs, which should have both parts isolated from the metal case, connect a wire to each part (center and outer). Those wires go to your circuit input point, which is often the two ends of a resistor that's connected from a tube's grid input to ground. The wires should stay as close together as possible, so there is an absolute minimum of "loop area" formed between them, which would enable them to have currents induced in the loop by AC fields in the air. So twist them tightly together, the whole way. Better yet would be to have them both be inside a shielded cable together, with the shield grounded to the case at the input end, only. (The shield would not carry either the signal or its ground. The cable would have to have two wires inside plus an outer shield.)
You don't want to connect the RCA signal connector's ground through any other path, since that would separate it from the center signal conductor and they would form a big loop that would have AC currents induced in it, which would induce a hum voltage across the grid input resistor as well as across the source impedance. (See "Faraday's Law", or, for the general case, "Maxwell's Equations".)
The "minimal loop area" thing works for minimizing transmitted as well as received interference. So all other wire pairs should also have minimal loop area between them. So twist tightly together any lengths of AC input pairs, AC secondary pairs, heater AC pair, any DC +/- or +/gnd pairs, output pair, etc. And keep the small signal wiring away from all AC and output or large signal stuff.
Yes, you MUST ground the chassis to the safety ground wire or to earth ground. And it must be connected to the chassis either with a bolt or by welding, i.e. not by soldering, since that could melt and disconnect before your fuse or circuit breaker blew. Without the safety ground connection, it would be much easier for your equipment to kill you, or, even worse, someone else.
You will also want to read about "star grounding". That involves having one central ground point and having separate ground return conductors that all go to that point. The reason that is necessary is that wires and metal in general are not perfect conductors. They have distributed inductance and resistance, at the least. So ground-return currents flowing through a wire will induce a voltage across the wire, back at the non-ground end. And a time-varying current will induce disproportionately-large time-varying voltages. So, for example, you don't want your grid input resistor's ground reference point to share a ground return conductor with a large OR dynamic current from somewhere else, because then your input's ground reference will be a bouncing voltage and that bouncing ground voltage will be arithmetically summed with your input signal.
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Star grounding
Since I own equipment that was manufactured before lawyers started telling us how to design equipment, I can say that star grounding works better for hi-fi systems compared to each corporation having their own safety ground, protecting them against lawsuits. My preamp, amp, turntable, all have 2 pin unpolarized power plugs. (Western Hemisphere 110VAC one side usually neutral). None of my chassis is connected to either side of the power plug. I get the least hum when ONE of them is grounded to the safety pin of the power strip, not all of them. The others, to maintain star grounding, should be properly (that is use 16 ga wire, screws, and ring terminals firmly clamped on) grounded metal chassis to metal chassis in star fashion with screws to the ONE device that is safety grounded to the power strip.
Of course, since you might sue me, ground everything with a third wire to the power strip with screws on the third wire to the metal case. There, are all you lawyers happy? Better yet, be really safe and don't plug your DIY equipment in at all, buy one at the mega-mart and be satisfied with what the big boys deign to sell you. They have the best lawyers, after all.
Since I own equipment that was manufactured before lawyers started telling us how to design equipment, I can say that star grounding works better for hi-fi systems compared to each corporation having their own safety ground, protecting them against lawsuits. My preamp, amp, turntable, all have 2 pin unpolarized power plugs. (Western Hemisphere 110VAC one side usually neutral). None of my chassis is connected to either side of the power plug. I get the least hum when ONE of them is grounded to the safety pin of the power strip, not all of them. The others, to maintain star grounding, should be properly (that is use 16 ga wire, screws, and ring terminals firmly clamped on) grounded metal chassis to metal chassis in star fashion with screws to the ONE device that is safety grounded to the power strip.
Of course, since you might sue me, ground everything with a third wire to the power strip with screws on the third wire to the metal case. There, are all you lawyers happy? Better yet, be really safe and don't plug your DIY equipment in at all, buy one at the mega-mart and be satisfied with what the big boys deign to sell you. They have the best lawyers, after all.
Yeah, SCREW the lawyers! Especially the trial lawyers! But it was actually AndrewT who convinced me that the safety ground is a must.
Each box containing higher than 24 V with metal you might touch needs a safety ground to blow the power fuse if something shorts to the external metal. However, EACH box having its own safety ground violates the star ground rule for eliminating ground loops, and can cause hum. Hum is induced by the product of magnetic flux, and the area of the ground loop, so separate instruments with BOTH a safety ground AND a signal return cause a great big area. ONE box in the system should have the safety ground. This requires thought and execution by the sound man or equipment owner, and the law assumes the consumer is a stupid slug that deserves a permanent disability stipend as a reward for his stupidity. Thus, every modern instrument in a band has a safety ground except the actual guitar itself, which has metal strings above the pickup. Bands get around this by having DI units or transformers coupling the instruments at the input of the mixer. Another safe way to do it, but good transformers are not cheap. The 24 v comes from the toy train exclusion- experience proved that babies crawling on the floor could be exposed to 24 V ac in toy trains, without death occuring. Cuts in the skin can cause hazard even at that voltage, however.
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