Toroid question

Ok, I have the diodes, which read around 0.54VF. I'll have to check that they are ok to use on the old prototype one, i'm not certain the current only flows in one direction?

Using them, under quiescent load and with a bridge should give +/-21v, and so long as the mains voltage doesn't fluctuate then that should be ok. Have to say though the light bulbs definately dim when out hoover (Dyson thing) is first started and this is a modern house. That represents a voltage drop anyhow, so long as it doesn't go up for any reason I should be ok.

Argh, still not finished. I'm getting +/-21.8 unloaded with the diodes, bit close eh? Anyhow, that can only go down with load so I shan't worry for now...

Anyhow, at the moment I have another concern. Are toroidal transformers by nature double insulation safety rated? I thought they were, but am now thinking otherwise. This is to do with "will I need to earth the secondary ground or can I leave it floating"? Does anyone know what UL478 is as a rating, I honestly can't find it and it's the last possible rating that could tell me whether is it is double insulation or not. I would like to not have to ground the secondary because of the increased possibility of ground hums. So whether I do depends on if the transformer is double insulated.

You're confusing yourself with the term "double insulated", this applies to the entire unit, and NOT to individual components.

The secondary side needs a path to ground 'somewhere', either through the other units connected to it, or a discharge path back through the mains (which places the secondary side at half mains potential, at a low current).

You've got a nice isolation transformer, ground the secondary and use a three core lead - to prevent any earth-loop problems, stick a ground lift switch on the input socket that your preamp plugs in to!.

Ok, I included the provision to earth the secondary ground, so I will join it up. The metal chassis is of course also earthed. I'm not sure how the earth lift could work, this amp will probably be connected straight to a PC soundcard output. Earth loops are rare though, and can be avoided? Is it good to have it plugged into a multiway outlet with it's connected devices, at least they share the same earth then?

This is nearly done now, and although I have tested the amplifiers and power supply seperately, i'm betting something doesn't work once it is all joined up. Ah, well i'll let you know, and pic later.

Dr.EM said:

Ok, I included the provision to earth the secondary ground, so I will join it up. The metal chassis is of course also earthed. I'm not sure how the earth lift could work, this amp will probably be connected straight to a PC soundcard output. Earth loops are rare though, and can be avoided? Is it good to have it plugged into a multiway outlet with it's connected devices, at least they share the same earth then?

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No, you're still using multiple earths, and this is what causes earth loops.

What's hard about an 'ground lift' switch? - the input socket on your power amp has three connections, right, left, and ground - connect the ground one via a small toggle switch, so you can disconnect it!. A common trick for PA use, if you don't have a ground-lift switch, is to use a cable with the screen disconnected at one end! (which is the same thing).

That seems very unusual. I think I know what the idea is. The ground, instead of travelling through the shield on the interconnect cable, travels through the earth connections on the equiptment, avoiding a possible conflict between the two? The phono jacks I have have the grounds connected by solder rings/tags, I will just unscrew them if I have a problem them, thanks.

This is confusing me too.

Normally I would use a centre tapped transformer for this type of thing and ground the 0V rail. I take it that the purpose of this ground lift switch is to disconnect it from ground and use the ground of the external equipment when it's connected, is this true?

Dr.EM said:

That seems very unusual. I think I know what the idea is. The ground, instead of travelling through the shield on the interconnect cable, travels through the earth connections on the equiptment, avoiding a possible conflict between the two? The phono jacks I have have the grounds connected by solder rings/tags, I will just unscrew them if I have a problem them, thanks.

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Yes, the ground connection is then through the mains leads of the two items. Your phono sockets should NOT be fastened directly to a matel chassis, they should be isolated from the chassis, with a grounding lead attached (usually to a central point, this is good design) - this grounding lead is where you place the switch!.

Hero999 said:

Normally I would use a centre tapped transformer for this type of thing and ground the 0V rail. I take it that the purpose of this ground lift switch is to disconnect it from ground and use the ground of the external equipment when it's connected, is this true?

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Partially, the 'ground lift' switch lifts the ground on the INPUT only - both power amp and preamp are still earthed via their mains leads.

It's important that there's only ONE ground connection to each piece of equipment, the screened lead between preamp and power amp adds a second ground connection, this means you can get mains currents circulating through the ground connections, this can induce mains hum into the audio.

A quick google finds loads of hits, here's a site with some useful information, including a nice graphic https://www.epanorama.net/documents/groundloop/basics.html

Ok, well I did finish this yesterday, and am pleased to report it works

Connected to my CD player which is unearthed it produces very good quality sound (through my B&W speakers). There is virtually no background hiss or humming with this (you'd need your ear a couple of inches from the speaker). Comparing it to my proper Hi-Fi amplifier it actually comes quite close (especially seeing as how this amp is £35 or so and the Hi-Fi one is £250). It is quite bright, but so is my proper amp, I think I just prefer less treble as I find most amps to be bright without using the tone controls.

Anyhow, the amp was designed to be connected to a PC, which i've also tried. As the PC is earthed there is very small amount of hum when they are connected due to the ground loop (it is very quiet though, inaudable at a normal listening position, but I will try the ground lift sometime anyhow)

I have watched a film with the sound through it and it handles well with that too, presenting the low explosion sounds with no audible distortion and a good stereo image. I haven't tried putting up to full, but it seems it will go loud enough for bedroom use at least with these speakers.

Other than the ground loop issue (which can hopefully be eliminated) there is a minor issue with the volume control which is sort of expected. It is a cheap dual gang and they don't track very accurately, causing the right channel to be quieter on low (under quarter or so) volume settings. This is overcome by lowering the PC output level and raing the dial position though.

Here are some pics, seems no matter how neatly I plan a project, it always looks a mess inside, must be the wires. The heatsinking is taken care of by the case and half of an old CPU heasink, it barely gets warm usually. With respect to the original question, the top plate is covered in a thin plasic sheet and insulation tape so the bolt cannot touch the metal at all.

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Hello again. Well, although that ground buzz is quiet, it is still annoying. I can't get this ground lift to work properly. I have tried disconnecting the shields at the input of the amplifier. By doing this I can get it so there is no buzz at all with the volume on 0, but it increases with advancing volume, presumably because there is effectively no shielding for the 1.5m cable length? It is louder than the ground buzz over most the range.

Does it need to be disconnected at the PC end instead? I tried this by putting selotape over the ground band on the 3.5mm plug and seemed to get similar results, but mabye that wasn't too ideal

So, if that doesn't work, mabye this would be a solution:

Use Of Loop Breaker Circuits

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I tried disconnecting the earth on the circuit 0v line and it eliminated the buzz, so this should work, but is it legal here?

Just a while ago I tried connecting through a couple of LT44 audio transformers and these do totally eliminate the buzzing problem, but i'd prefer not to use transformers. They sounded suprisingly good actually seeing as the bandwidth wasn't supposed to be much over 4khz, I could hear all the cymbals and highs quite clearly tbh. A pair of more suitable transformers costs £20, and that's too much really.

Dr.EM said:

Hello again. Well, although that ground buzz is quiet, it is still annoying. I can't get this ground lift to work properly. I have tried disconnecting the shields at the input of the amplifier. By doing this I can get it so there is no buzz at all with the volume on 0, but it increases with advancing volume, presumably because there is effectively no shielding for the 1.5m cable length? It is louder than the ground buzz over most the range.

Does it need to be disconnected at the PC end instead? I tried this by putting selotape over the ground band on the 3.5mm plug and seemed to get similar results, but mabye that wasn't too ideal

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The screen has to be connected at ONE end, this provides the screening for the full length of the cable - what causes an earth loop is the screen been connected at BOTH ends, and BOTH units been earthed. You can either break the ground on the screened cable, or break the earth connection to ONE of the units (but this has safety implications).

This reminds me of when I connected my radio's headphone socket to the soundcard in my PC. I used two cables, one for the left and the other for the right, no earth connection.

I have tried disconnecting the screen at each end, not both at the same time, but still get the buzzing. Disconnecting the safety earth eliminates it though, but has the safety issues, hence is it wise to use the loop breaker? Is it legal here, presumably it is in australia.

Dr.EM said:

I have tried disconnecting the screen at each end, not both at the same time, but still get the buzzing. Disconnecting the safety earth eliminates it though, but has the safety issues, hence is it wise to use the loop breaker? Is it legal here, presumably it is in australia.

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I can only presume you've still got an earth path somewhere?, but I see no problem with the 'loop breaker' from Rod, in fact I was using it in a bass guitar amp I was building (which is currently shelved because I bought a Behringer combo).