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Old 5th June 2012, 05:29 PM   #1
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Default Amplifier grounding


I'm wondering why almost any guitar amplifier in the market has a very simple grounding layout: All ground leads are connected to the chassis lug next to the respective component, while the chassis itselves is connected to power ground via the 3-pole cable and prong.

As I'm planning to build a low-noise bass guitar amp, I'm considering to apply a proved manner I've build every audio amplifier to this date: Consequent star grounding with insulation of signal gnd and chassis. The only linkage between both will be a pair of anti-paralleled diodes and a cap of, let's say 100 nF, across them. The chassis will be grounded by the usual way (3rd lead of the power cable/prong...).

Thus I could minimize hum (I hope so, at least...) and, by the same time, avoid ground loops, especially if a grounded FX device or a 2nd amp, slave etc. is connected to the amp.

Any complaints?

Best regards!
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Old 5th June 2012, 05:43 PM   #2
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Quick guide on Grounding
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Old 5th June 2012, 07:02 PM   #3
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Many guitar amps have poor grounding schemes. Especially tube amps that try to emulate vintage amplifiers. Even if they have low hum levels, there is a common mode hum on the ground that shows up when you connect to another piece of equipment with a single ended audio connection. The anti-parallel diode circuit helps eminensely, but does not guarantee a hum free system unless employed on all equipment in the system.

The soultion often used in the US is a "cheater plug" that defeats the safety ground. THIS IS DANGEROUS AND SHOULD NEVER BE USED!!!
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Old 5th June 2012, 07:35 PM   #4
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I have a problem with an amplifier that every time I connected the earth ground the amplifier pickup a hum. The only way that I was able to get it off was by using the back to back diodes with a 10 ohms resistor and a .1 uf in parallel with the diodes. Since that I been recommended the used of the diodes. One thing it is that make sure that the earth ground and the diodes side to earth ground are connected to the same screw. Do not connect the earth ground and the audio ground in separate screws in the chassis. This will pickup a low but very low hum.
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Old 5th June 2012, 07:55 PM   #5
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Yes. As I wrote before, I've always had good experience with "soft grounding". Nevertheless, my main question is: Do there arise any safety issues in music instrument amplifiers?

Best rgeards!

Last edited by Kay Pirinha; 5th June 2012 at 07:56 PM. Reason: orthographic corr.
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Old 5th June 2012, 09:24 PM   #6
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Star grounding can have noise too, what i tend to find out what works best for me is to just separate high currents from low current. Preamp tube cathode grounded to input ground along with tone/volume controls near input jack. Power tube grounds tie to secondary center tap of output transformer.
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Old 6th June 2012, 04:44 AM   #7
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I know I'm probably on my own here but I always use a bussed 0V line connected to the chassis in just one place, the bottom of Rg1 on the input tube. Then use isolated input jack(s) and shielded cable to the input tube with shield connected to this same point.
Connection to the 0V buss follows schematic, that is, input tube Rg1, then input tube cathode resistor 0V side, input tube B+ bypass cap 0V side, etc. till right at the other end of the buss you have the main filter cap 0V connection. This keeps the large circulating currents away from the sensitive input stages.

If you have 2 channels then I have NOT experienced any ground loop problem by splitting the 0V bus to follow each channel and then joining them again at the mixer point.

I have a safety earth connection to the chassis as well and I try to put this on opposite side of the chassis (as far away from the 0V buss chassis connection as I can manage).

One more thing the heater centre tap or balancing resistors centre, I do not wire to the 0V buss but directly to chassis - unless I'm floating the heater on a DC stand off, in which case I divide off the output tube screen supply which references it at the "far end" of the 0V buss.

This is just what has always worked for me for both Guitar and HiFi Amps. I've tried star earths but did'nt get the same result even though the theory says it should be better.

Oh! and the Common side of the output tranny secondary I tie to the 0V buss at the point along the buss where feedback would normally be applied if used (e.g. the phase inverter cathode resistor 0V connection point).

The 0V buss can be connected to chassis via 10 Ohms or back to back diodes or a combination of these BUT the safety ground from the mains earth wire MUST be hardwired to the chassis directly and never removed. In fact electrical wiring standards in most coutries specify that this connection must be made via it's own dedicated bolt and that a locking device (split washer/star washer/nyloc nut etc) must also be used.
Don't forget to scrape of the paint to ensure a hard connection to chassis at this bolt. If you are worried about rust/corrosion etc then a lot of the electronic component suppliers carry an "alodine" pen, use that to treat the bare metal. Alodine is the surface conversion chemical process which is used for chassis in a lot of aircraft and military electronic chassis - why because it is conductive whereas paint and anodizing etc is not.

If you want to use the back to back diode method then I recommend making it "bullet proof" by using a Chassis Mount 25 Amp bridge rectifier block, connect + and - together for one connection and connect the 2 AC inputs together for the other connection (doesn't matter which goes to the 0V buss and which goes to chassis).

A bit of a "rave" - hope there is something useful in there.


Last edited by gingertube; 6th June 2012 at 04:47 AM.
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