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Old 29th January 2008, 03:53 AM   #1
gary h is offline gary h  United States
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Default Another ground question

Hi all,

My first preamp project sounds great. It's a kit I installed in an old Dyna Pat4 chassis over a year ago. My problem is that ever since I built it I get a significant hum. Both low drone of 60Hz through the woofers and the buzzing 120 Hz through my mids. This was confirmed when I hooked the preamp up to my laptop and witnessed the peaks on my Signalscope FFT analyzer. The transformer is quite loud and periodically rattles the PCB especially when warming up.

I get the hum regardless of volume setting, without any source hooked up and, here's what leads me to believe this must be a grounding issue, even when the preamp is turned off. This doesn't happen with my SS preamp and I have tried several different combinations of gear swapping out cables, amps, cd players, outlets etc.

I have spent a lot of time on this site reading about possible fixes. This is what I've done:

1. Checked all connections for cold solder joints and shorts.
2. Rewired all connections.
3. Removed switcher, balance and ganged double in/outputs, all except 1 in, 1 out, 1 new 50ohm vol pot.
4. Replaced filter capacitors.
5. Experimented with numerous grounding schemes.
6. Removed tranny, turned it in every direction, mounted it on rubber, put bricks on it, put a shield around it.
7. Created a simple set up with preamp connected to amp to speakers, all powered off one bathroom GFI, home run to panel with no other appliances, dimmers etc. plugged in.
8. Replaced tubes
9. Sent it to get fixed.
10. A dozen other things that didn't work.

I have read a lot about star grounding but I am under the impression that the ground plane on the PCB is one big loop since it goes through and around the whole PCB and is attached to the chassis via aluminum seperators.

Here are links to 4 pictures and the PS and Line schematics:

http://home.comcast.net/~garyworld/
or individually at:
http://home.comcast.net/~garyworld/preamp1.jpg
http://home.comcast.net/~garyworld/preamp2.jpg
http://home.comcast.net/~garyworld/preamp3.jpg
http://home.comcast.net/~garyworld/preamp4.jpg
http://home.comcast.net/~garyworld/schematic1.jpg
http://home.comcast.net/~garyworld/schematic2.jpg


So this is my last try. If I don't fix this it will be a big defeat and a waste of a very nice sounding preamp.

All help is greatly appreciated.

gary
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Old 29th January 2008, 04:27 AM   #2
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Gary,

I hear your problem, it took me ages to get the hum in my Aikido to acceptable levels. One thing to try is to remount your trafo using rubber washers or even the method used by VoltSecond - that is if your trafo is chassis mounted! - SORRY, just noticed that you've done that already.

Another problem that I encountered with the ground loop occurs with my TV and DVD-player which cause extra hum. I ended-up disconnecting signal GND for these devices. Following the same reasoning, maybe experiment by hooking-up your preamp to power amp with signal GND disconnected - maybe it'll work and you'll get sound. Incidentally, it didn't work for me, but it was a quick thing to try.

You might also try a ground lift in your preamp. I used the method advocated by Rod Elliot (http://sound.westhost.com) - look around the website for it. I only use 20R of ground lift and I got significant reduction in hum.

As for the power supply, there are a number of things that may improve things:

1. check the value of C101 against the maximum cap value for a cap-input filter for the rectifier you are using.

2. The value of C202 could easily be increased to 60uF or higher. This should reduce 120Hz ripple.

3. You could add a choke section, or even rebuild / redesign the PSU using PSUDII available from www.duncanamps.com

4. Replace heater PSU with a regulated heater PSU. There are kits available - I used the kit PSHREG from Welborne Labs. It works, but read postings on Welborne's customer service. This made the BIGGEST difference in my Aikido for the 60Hz hum.

Incidentally, I recently installed 6SL7's in the input positions of my Aikido (replacing 6SN7's). The current draw of the Aikido B+ dropped from around 25mA to around 18mA. As a result, I do not have a little 120Hz hum due to inadequate filtering. Amazingly, it was my wife who noticed it. As with the 60Hz hum, it is inaudible when the ear is moved to greater than 4 inches from the speaker - phew! Incidentally, in true choke input filters (which mine isn't) the choke requires a minimum current draw in order to function correctly. I expect that with a 5uF cap in front of my 10H choke, the PSU is more cap-input than choke-input, but not to the extent that current draw becomes insignificant.

I am sure that other readers will be able to give you more educated help than I. Anyway - Good Luck.

Charlie
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Old 29th January 2008, 05:25 AM   #3
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Hi Gary,
It sounds like you've tried many of the logical fixes without success. However something you said gives a very important clue. And that is that it hums even when it's turned off. This is very curious. But your schematic doesn't depict the on/off circuit.

It would be helpful to know if the hum stays or goes when you pull the AC power plug out. (when it's turned off) If it disappears, the hum is from leakage somewhere in the power supply. If it remains even with the plug pulled, you have extraneous pickup. Perhaps from a faulty ground.

You also say that the power transformer is very loud and sometimes rattles the PC board. This should not be and is another clue. If the circuit is correct and there's no defective parts, (you said it sounds great) I'd question the transformer itself.

I once repaired a tube headphone amp that was always dead quiet. Then, all of a sudden, it developed a low level hum. This hum appeared immediately at turn on, even before the tubes got warm. It turned out to be the power transformer. No amount of reorientation helped. The fix was to remove the transformer altogether and place it in a box with an umbilical to the amp. A replacement transformer wasn't available.

I also once had a case where one diode in a bridge was open. This unbalance caused the transformer to hum and act weird.

Your problem can be corrected, It just takes perseverance and a little thinking out of the box.

Victor
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Old 29th January 2008, 06:24 AM   #4
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It looks like your standoffs are aluminum. If they are then your ground is the chassis. The little thin track around the perimeter of the board is not a sufficient ground by itself.

A question that I have never been able to decide upon is whether to connect the chassis to the earth of the house wiring.

In case you don't know, the "grounded neutral" in modern house wiring has the ground (third pin on wall plug) strapped to the neutral (wide spade on polarized wall plug) at the breaker box. This looks to me like it would have impedance, so it is probably best to actually use the third pin on the wall plug.

It is the impedance of the ground that couples the hum and a star ground is IMHO not as good as a very low impedance ground plane, such as a chassis.


Some equipment grounds are connected to the earth and some aren't. Again, I have never been able to decide which way is best.

You have a power transformer isolating the B+ from the line voltage so I don't think there would be a shock hazard if the chassis is connected to the earth.

I see that you have DC on the filaments so that is ruled out as a source of hum.

My point is that the relationship of the chassis with the power system ground may have something to do with the hum. Hope I haven't been confusing, Mark
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Old 29th January 2008, 09:37 AM   #5
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Hi,

I use a star ground at the minimum impedance point (where the power transformer CT connects) and couple the power ground to earth ground via a 100 ohm resistor.

If you still get a loop, coupling to earth via 2.2 Meg resistor bridged by 0.047uF X2 cap works with all but real leaky transformers.

Cheers!
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Old 29th January 2008, 12:11 PM   #6
SY is offline SY  United States
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Quote:
A question that I have never been able to decide upon is whether to connect the chassis to the earth of the house wiring.
Always! It's a very important safety issue.

Avoiding ground loops set up by the power line is easy- the circuit ground and the chassis ground can be separated by a groundbreaker. But the chassis ground ALWAYS should be connected to the mains earth ground (third pin). Always.
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Old 29th January 2008, 12:35 PM   #7
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You might try disconnecting the red/yellow wire from the PCB and splicing a little more wire to it, then connect that directly to the (-) terminal of the first filter cap. Yes, it's SCHEMATICALLY the same point in the circuit. But you may be re-routing the capacitor ripple current through a path that DOESN'T flow past signal circuits.
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Old 29th January 2008, 02:45 PM   #8
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(1) Have you tried twisting the leads for the high volt? I notice that you twisted all of the others.

(2) I recall a rule concerning shielded wiring. The shield should be connected to the source. I notice that your shield goes near a tube plate and is then connected at the board. Having it connected at both ends creates a ground loop. I recall that the ground on the input to the shielded wire should be connected to the system ground, usually the chassis ground.

(3) The vibrating transformer is strange. Is this a Chinese transformer? The PA number sounds like a Dynaco part.

(4) The transformer could be setting up an eddy current in the chassis. It may be inducing hum in the ground tracks at the edge of the board. Eddy currents could account for the vibration.

(5) Have you tried taking a clip lead and jumping the board ground to the chassis? Even though the chassis is connected to the board ground tracks (assuming the standoffs are aluminum) there is still the impedance of the tracks before the connect to the chassis.


Be sure and let us know what you find.
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Old 29th January 2008, 04:16 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by SY


Always! It's a very important safety issue.

Avoiding ground loops set up by the power line is easy- the circuit ground and the chassis ground can be separated by a groundbreaker. But the chassis ground ALWAYS should be connected to the mains earth ground (third pin). Always.

Quote:
Originally posted by Tom Bavis
You might try disconnecting the red/yellow wire from the PCB and splicing a little more wire to it, then connect that directly to the (-) terminal of the first filter cap. Yes, it's SCHEMATICALLY the same point in the circuit. But you may be re-routing the capacitor ripple current through a path that DOESN'T flow past signal circuits.
Both above are very important points, another to keep in mind particularly true in the case of steel chassis (eddy currents) is that you want only one single ground connection between the audio circuitry and the chassis.

Make sure that the board is not grounded to the chassis, and that non of the jacks are either (inadvertently or deliberately) both the board ground and the jack grounds should come back to just one common point which may be connected to chassis through a parallel combination of a 20- 33 ohm resistor and a pair of anti-parallel 1N4002 (or similar ) diodes. Sometimes a small ceramic disk cap (say 0.01uF) may be added in parallel as well for lower impedance at RF.

For the 60Hz issue make sure that the transformer is not too close to the board, and for the 120Hz issue check that the filament supply is ripple free. Ripple sawtooth is really high dvdt and couples very effectively through the cathode/filament insulation and radiates (electrostatically) into surrounding circuitry.
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Old 29th January 2008, 05:21 PM   #10
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Quote:
have read a lot about star grounding but I am under the impression that the ground plane on the PCB is one big loop since it goes through and around the whole PCB and is attached to the chassis via aluminum seperators.
I see that you did use aluminum standoffs.

You have two basic design philosophies to choose from. Some want to use the chassis as an effective shield in conjunction with a star ground. A star ground often creates more problems than it solves.....................loops.

It has been my observation that the low impedance ground plane is the best.

You don't have a ground plane on your board, you have a thin high impedance track around the board.

The chassis is thick and is a good ground plane. You are already connected to it, but the eddy currents in the chassis may be setting up potentials at the aluminum spacers.

If you strap the ground track on the board to the chassis in a few places where the components tie to ground this will elimninate the track impedance.

The advice from other posters about the resistors, diodes strapped with the caps, and the star ground may be the way to go, but don't think that the chassis ground is necessarily a mistake.

What you probably have here is several ground problems that are reinforcing each other. The vibrating transformer is the main clue here. It is putting out excessive current due to positive feedback of 60 and 120Hz.

The theory behind the ground plane is that if the ground has near zero impedance then the flaws are nulled.

Your untwisted high volt from the transformer is probably the main source of energy.

I have built solid state with the chassis ground and it is very quiet. But, I have never built a tube circuit this way.

I'm not trying to argue with anyone, only looking for facts and understanding.

It is a lot easier to go ahead and use the chassis as a ground plane first. The star ground is going to be a lot of work.

Strap the grounds at the jacks to the chassis too. If you get everything grounded to the chassis it may become quiet.
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