Unbeatable hum problem in new build

[defaced]

Schematics of the SLO are freely available online, including on the SLO Clone forum dai mentioned. I'd recommend looking at the Avenger schematic on that site so you don't have to figure our the channel switching on the SLO. Comparing the SLO/2203 schematics, it looks like the SLO added a fourth stage, after the first stage in the 2203, and changed alot of values. The SLO is a pretty straight forward high gain channel. Cascade four stages, make one of those stages the cold clipping stage (stage 3), CF drive the tone stack, LTPI, 6L6 power amp with NFB.

[John Fisher]

It works great now! A new question with a new twist

Well it all works great now and very quiet. Thanks to all. The ground wire in question was only about 1 half inch away from were it was suppost to be and it made all the difference in the world.
The amp is going to be a stereo amp or 2 guitar amps in one. each amp will have 2 channels and an effects loop on each amp.

My main question now is when I build the second amp will I need to do all the star grounding on the same star ground as the other one or can I make another star ground.

It will have it's own rectifier and filter circuit but the power tranny will be the same for both amps. Thanks for your valuble comments

[Wilder Amplification]

Well it all works great now and very quiet. Thanks to all. The ground wire in question was only about 1 half inch away from were it was suppost to be and it made all the difference in the world.
The amp is going to be a stereo amp or 2 guitar amps in one. each amp will have 2 channels and an effects loop on each amp.

My main question now is when I build the second amp will I need to do all the star grounding on the same star ground as the other one or can I make another star ground.

It will have it's own rectifier and filter circuit but the power tranny will be the same for both amps. Thanks for your valuble comments

Since you're using a common power supply for both amp circuits I would also do a common star ground arrangement.

[KevinOConnor]

Hi Guys

Star grounding in a guitar amp is generally a bad idea. This brings currents together that should never see each other and you end up with more hum than a randomly-grounded amp.

The Galactic Ground scheme shown in TUT3 eliminates all ground noise and supply-related noise. The grounding that you used with collected _related_ points tied together before going to any other bus or star is close to Galactic Grounding.

With multiple channels, look at the signal flow through each path and it will be obvious that stages in different paths can share supply nodes - BUT sharing of supply nodes within a path is problematic.

Have fun
Kevin O'Connor

[Wilder Amplification]

Star grounding in a guitar amp is generally a bad idea. This brings currents together that should never see each other and you end up with more hum than a randomly-grounded amp.

I have to disagree with this completely. I've built SEVERAL high gain channel switchers with a star grounded arrangement and have had absolutely zero noise issues. Moreover, it's generally a bad idea to use the chassis as "one big phat wire". It should be used as merely a shield...no current should flow through the chassis whatsoever. If you lay out your ground scheme properly you will only have 2 connections to the chassis...amp circuit ground and the AC mains safety ground. With this arrangement the chassis only serves two purposes...(1) acts as an RFI/EMI shield and (2) acts as a "mains fuse blower" should the hot side of the AC mains come in contact with the chassis.

I have to say Merlin Blencowe wrote one of the best articles on proper grounding schemes and star grounding. Check it out...it's freely available on his site.

[KevinOConnor]

Hi Guys

Thank you for completely misinterpreting what I wrote. Please read it again.

I never condoned random grounding, and in fact have spoken against that for decades.

Galactic Grounding is NOT random grounding, nor is it star grounding. What you describe follows the principles of Galactic Grounding: one safety ground connection to chassis and one circuit ground to chassis, the latter NOT being anywhere near the input jack.

Have fun
Kevin O'Connor

[guitarmike2107]

I take it "galactic grounding" is a term you have coined? it mostly brings up star wars references in google
I assume it is multi point star grounding system with each filter cap as a node?

[KevinOConnor]

Hi Guys

Yes, Galactic Ground is my term, popularized in TUT3 which explains how to wire guitar amps for lowest noise, but introduced long before that when this forum was still called Ampage. I discussed this on other forums, as well, since 1995.

It did not surprise me when the original poster on this thread said he had "star grounded everything" but still had hum. Many hobbyists wasted hours rewiring their Fender amps with a star ground because the "star" is the only organized grounding scheme they understood. because it is the only organized ground scheme _most_ people understand, star grounding was promoted by many as the best for all purposes. Unfortunately that could not be further from the truth.

A galaxy is a collection of stars. Each node of the ground is a collection of _related_ elements and is a star, but these stars are not tied independently to the chassis. rather, they tie to a buss in an organized fashion that follows the loops of circulating currents. The entire ground system is _wired_ and related portions are twisted with supply feeds that are themselves stars on the supply feed buss. The buss has only one connection to the chassis and can be lifted if required.

The layout drawings in TUT3 and TUT5 clearly illustrate how this wiring is laid out and clearly explains why it is so effective at noise abatement. It is a ground scheme that is equally applied to other technologies and even to PCB layout.

Have fun
Kevin O'Connor

[dai h.]

any scheme that has due consideration given to what is happening in the ground returns should work (at least that's my understanding). You can use stars, planes, daisy chains. I'm not understanding why the frame ground (circuit to chassis) connection should not be anywhere near the input jack. It's probably not a good idea to use the actual input jack (because it can come loose) but at that point, connecting to chassis would seem okay to me (and I've seen it done by someone who is apparently an engineer in one of his amps IIRC) since there isn't much going on current wise (which fits in with the idea of using the chassis as a shield and connection for safety ground only).

[guitarmike2107]

Thanks for clarifying, thats what I thought you were talking about.

[KevinOConnor]

Hi Guys

The chassis is just for shielding but the currents within it should be considered as "error" currents, and thus noise.

Although one can tie any part of the Galactic Ground buss to the chassis, neither end is a good choice. The input carries the smallest and most sensitive currents, so can be RF grounded but not hard grounded. The rectifier end carries the noisiest currents and makes an unsuitable reference connection. This leaves the middle of the path where signals are large enough to compete with noise and circulating currents are still small enough not to induce problematic errors into the chassis.

Engineers are people and people can make mistakes.

Have fun
Kevin O'Connor

[Steve Conner]

At least it's easier to bury our mistakes than if we were surgeons or pilots.

I don't know what Galactic Grounding is and don't intend to buy the TUT books to find out. But I've never had trouble making amps completely hum free. As long as you only ground one end of the preamp ground bus, there will be no circulating currents in it. You have to connect the ring of the input jack to the end of the bus where the first preamp tube is, but you don't have to ground it to the chassis there.

Lately I have been grounding the preamp bus to the chassis at the input jack ring, because that helps with RF, and using a separate power amp ground bus. The preamp bus includes the PI, so the change of ground reference between the two buses is tackled with a big hefty signal, that's balanced to boot.

[loudthud]

In TUT5 Kevin installs a ground lift switch. It isolates the circuit ground from the chassis although there is a parallel RC between the two. Does this pass muster with safety agencies? Don't they test your product by some crude means like shorting the hot side of the power line to the guitar strings? Have you noticed that Fender guitars have the CE mark?

You can have a perfectly quiet amp until you try to connect to another piece of grounded equipment such as a Fender Reverb unit. Although a ground lift switch might reduce or eliminate the hum, how much of the hum is from a small difference between the safety ground and the signal ground?

[TransLucid]

The whole trick is to have a safe AND quiet design. loudthud's comment about hum caused by interconnection of equipment is apt. You can have a well-designed grounding scheme in your amp be ruined through connection with an improperly designed effect, mixer, etc. The most common problem is the use of pcb-mounted (and grounded) jacks, which can cause a ground loop with the AC-grounded chasses.

Anyway, there has been a lot of discussion on this topic in the Audio Engineering Society (AES). Neil Muncy has done a lot of work in this area.

Watch out for ground lifters, expecially the ones that isolate the AC ground wire...

Bob

[KevinOConnor]

Hi Guys

Hey Steve, you don't have to buy TUT3 to know what Galactic Grounding is if you simply read the previous post more carefully.

Safety and quiet go hand in hand. Muncy et al have done great things to define the issues and provide certain solutions. Ground loops within and between equipment is all covered in TUT3.

If you look at schematics of modern gear, you'll see a cap in parallel with anti-parallel diodes in parallel with a resistor as the link between the circuit ground and chassis. The AC third-wire is hard-connected to the chassis as always. There is an assumption that ground voltage differences will be in the millivolt range, but this is not always the case. Ground faults can be as large as the mains voltage - which often indicates some basic wiring flaw or defect in the equipment. In the "assumed" world, the diodes never turn on and the RC provide an RF ground.

There are ways to eliminate ground loops through effects, etc, the most obvious being the traditional transformer. This provides galvanic isolation and an opportunity for balanced connections if one desires. We use this method in our amps. You can also use ground-canceling circuits that are limited in scope to millivolt errors.

Have fun
Kevin O'Connor