DIY Audio Projects Forum

I think i mentioned it before but i have a cell ph array close and tonight i heard a little data stream going out but only after i turned the amp off and caps where draining about 1/2 empty. When ON, nothing is heard, TG. The fan (50mm) located in the PS is dong as it should maintaining temp. The smaller 30mm fan i put in the amp however after an hour was producing high pitch wine in the sig stream and in room so out it came. After 1 1/2 hrs of listening, both cabs are well within acceptable heat levels and just warm to the touch. A larger top surface would not be so lucky due to transmitted IR from the tubes themselves, necessitating the need for venting immediately around the tube bases. The 30mm fan has a sleeve bushing and as such not very effective blowing strait up. Also not meant to be mounted horizontally i am sure.. I would think a bearing type would be much better.

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Matt, just to clarify, for use in a different project and apologies if i messed up the quote attribution, is this at the chassis safety ground? How best to physically attach signal and chassis ground? Is the paralleled X2 cap and resistor recommended in Bruce’s article on grounding also required? Also how should the potentiometer shell be treated.
Much appreciated.

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John

John;

All my projects carry a separate signal/power return ground and chassis ground. First some clarifications on ground types:

The chassis ground is just that; the chassis. This ties in all the exposed metal on the amp (chassis, exposed screws, switches, potentiometer shafts/shells, transformer shells, etc.) and ties directly to the mains ground pin on the IEC C14 connector. This is the safety ground an all my projects. This arrangement ensures that any fault to the chassis will not result in someone getting shocked by touching bare metal on the amp.

The signal ground (or "signal/power-return" ground) is that ground used by all the internals to the amplifier. It "starts" (if you can call it that) with the negative side of the power rail and connects all the ground reference points in the amplifier circuitry. It is in this ground system where you should never have any loops. The ground can "tree out" but there should never be a point where a loop is formed.

Now, you really always want your signal ground to be close to (or at) the same potential as your chassis ground. This is not just good practice, but it is also a key part of achieving a good quiet setup. You should look at this link for a detailed write-up on system level grounding (http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4628&start=71).

So now the question becomes how to make the tie in the amplifier between the two grounds. Because I ALWAYS use a three wire mains connection, I always tie the two grounds together at a SINGLE point with a direct connection. Typical Engineering standard of this tie is a total resistance < 2mΩ. I have never had this approach give anything but a dead quiet amplifier. If the component is not a main amplifier (such as a preamp, RIAA stage, mixer, etc) I always include a ground lift switch for the reasons identified in the linked post.

However, there are many that advocate the older method of parallel cap and resistor. In the old two wire systems you can see where this makes sense. Because you really can't tell what the chassis reference will be between pieces of equipment when power plugs could be inserted in either direction (b.t.w. this is why some older guitar amps have line reversing power switches, i.e. it keep guitar players from taking 120v through the lips if they bump the mic) it makes sense to provide some resistance between the two grounds in the event that there is a ground side closed circuit between two pieces of equipment. So it depends on your power system and how the equipment is going to be used.

If you only use three wire (hot, neutral, ground) mains systems, you should use the direct tie method. If you are using two wire (hot, neutral or hot, hot) mains systems, or a mix of the two, you should use the cap/resistor isolation method.

Does this answer your question?

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

6.3v dc sounds right, 6.3*√2 = 8.909V DC
8.909 - 2*vd
= 8.909 - 1.4v (diode voltage drop 0.7 per diode and 2x for full bridge rectifier)
=7.5v DC at output ignoring the effect of voltage ripple



In some strict countires like New Zealand, anything that is exposed metal and user servicible must be connected to mains ground with resistance less than 0.5 ohms.
Connecting to ground via resistor and parallel cap is illegal because resistance greater than 0.5 ohms.

From my exprience, its normally hard to get chassis to same ground potential as earth,
Most amplifiers have RCA jacks and outputs isolated from chassis.
Meaning signal ground will be fully isolated from chassis unless you connect the two by a wire or using ground from mains.

This means as long as the case meets saftey standards by directly grounding to mains.
You can then also directly throw a wire to mains ground for your signal ground.
This worked best for my projects and I perfer it this way.

Sometimes you may need to ground to mains via two seperate low resistance cables if your amplifier has a poorly constructed mains ground for left and right channel.
This means a direct ground for left channel ground then another for right channel.
Sometimes maybe need to use one cable comming out of mains ground, then solder on two extra wires that then connects the left and right grounds together to it.

In case of amplifiers with feedback, the feedback resistor that connects to ground needs to be directly grounded to mains.
(Most OCL amps have feedback resistor connected to ground via capacitor, this means the grounded end of capacitor needs to go to ground directly.
The input ground of amplifier and feedback resistor ground should have low resistance to mains.

The gain of feedback amplifiers are fully set by feedback resistor and input voltage, it virtually ignores most or all of the noise from supply voltage.
Thats why getting the ground of input and feedback resistor right is crictical.

Yes thanks Matt I'll give that a go.

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John

ThanNk you also Kochiya.

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John

Thanks again Matt. After a few reads it’s sinking in. There’s a lot to it. The 2m ohms you mentioned—where would I measure that on the chassis/components. Regards

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John

The 2mΩ number is just the normally used chassis bonding requirement for metal cased equipment. Don't worry that much about the actual number, just make sure that the tie between signal ground and chassis ground is of minimal resistance (like a good solder joint).

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog