Lions and Tigers and Grounding Problems, Oh My! DoZ Grounding...

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See if you can't help out a Kansas boy...:witch:

I built Rod's DoZ a couple of years ago, and was fairly pleased with the results after working out some bugs with the help of ilimzn. But it has always hummed...not loudly, and it can only barely be heard with most speakers. I've measured the hum as being something like -70db under a 1W level. This is all but inaudible with most speakers, but I currently have the amp connected to a high-efficiency set of speakers (101db), and the hum is quite audible.

Here's the schematic, and my colored notations on the grounding, which I know is screwy (the grounding, that is...the colored lines may be debatable).

An externally hosted image should be here but it was not working when we last tested it.


The red lines represent the common ground plane on the PC board, which is returned to the star ground on the chassis via the violet lines. The one green line represents the signal return, which comes in through the input RCA jack, runs to the ground lug of the 100K volume pot, and then is returned to the chassis star ground (violet line). The signal return does not run to the PC board at all, except through the star ground.

I have a vague idea that I need to separate the signal return from the ground plane on the PC board via a 100 ohm resistor, but I also suspect that it will not be as simple as adding the resistor to the star ground line running from the 100K volume pot to the chassis ground. But then, maybe it is... Before I tear into it, I'd eagerly welcome any input that the people here might have...people who have encountered and solved these issues many times.

Thanks for your thoughts...
 
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Echowars, I think if you connect your input pot as shown with your coloured lines you will get a ground loop.

I would return the ground side of your pot to the input ground on the connector and then connect this via a 10Ohm resistor to the input ground connection (not back to central ground).

Also, make sure your input connector ground is NOT in contact with your chassis - it must be insulated from and case metalwork.
 
Bonsai said:
Echowars, I think if you connect your input pot as shown with your coloured lines you will get a ground loop.
It IS connected as shown...and the result is a very low-level hum.
I would return the ground side of your pot to the input ground on the connector
It is...the green line shows that the ground of the volume pot is directly connected to the 'ground' (signal return) of the rear RCA jack.
and then connect this via a 10Ohm resistor to the input ground connection (not back to central ground).
OK...here's where you lost me... The 'input ground' is the signal return side of the input RCA jack, or at least that's what I always called it. The 'ground' runs from there to the ground side of the 100K volume pot, and then to the star ground (common ground). I'm not sure where this 10 ohm resistor is supposed to go...?
Also, make sure your input connector ground is NOT in contact with your chassis - it must be insulated from and case metalwork.
Roger that...input RCA connector is insulated from the case.
 
Oh Dear!!

separate the signal and NFB and very low level return circuits from the higher current circuit returns and separate both of these from the dirty returns including the decoupling and Zobel.

Starting at the left.
The chassis cap may work?
The next 7 returns should all connect to the left most purple wire.
The next two returns R5 & Q5 go to the middle purple wire.
The last two returns C7 & C5 goto the right most purple wire.
The final return R704 goes to a fourth purple return wire.
That leaves just two, C702 could probably be combined with the dirty return.
I am unsure of the best place to return the Zener. My feeling is that the return could be combined with the clean (left most purple) or a separate trace to star ground.
Any comments?
 
Wow...I'm not sure if I can hack up the PC board to accommodate that many separate returns... If it were flawed that badly, I'd have assumed that a lot less people would have bought Rod's board and built the amp.

Your first suggestion though -
separate the signal and NFB and very low level return circuits from the higher current circuit returns
- was closer to my own line of thinking. Join the returns for C2, R3, C6, VR1, R702, and VR701 to a common return path back to the star ground, perhaps connecting it with a 10 ~ 100 ohm resistor. It's just that I don't have a whole lot of experiance with grounding schemes, and it will be a lot of work to get it done, and really difficult to redo if it doesn't do the job.

Any other thoughts as to whether this is the way to go...joining the returns for C2, R3, C6, VR1, R702, and VR701 to a common return path back to the star ground?
 
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Try this first:-

Remove purple wire going to VR701

Join input connectr GND to VR701 GND and R702 GND

Connect this group via a 10 Ohm resistor to VR1 and C6 GND

This should cure most of your problem.

After this, if you feel it neceassry, you can tackle the remaining ground issues.

Its important that your small signal ground connections are cascaded correctly to avoid ground loops.
 
What you described is what I almost did before I started seeing input grounding schemes that included pretty much the whole front end of the amp...much as AndrewT suggested.

Whichever way I go, it will be a difficult modification. What might be the pluses and minuses of going one way of the other (besides the fact that you're last suggestion will be by far the easiest)?
 
It would be helpful if you could describe the way the power supply is built (single for both channels, dual...). Also, if you have separate ground lines for the left and right channel, before they connect on the (common?) central ground point.
Finally, does it hum when the inputs are not conencted to anything?
 
Hums regardless of whether anything is connected at the input or not. I used a shorting plug at the input RCA for testing. Anyway, very low-level hum from the woofer (the high-efficiency speakers are off for re-foaming, and all I have for test purposes are some Dynaco A-25's).

Here's the power supply:
An externally hosted image should be here but it was not working when we last tested it.


And here's the view of the underside of the amp (a mess, I know):
An externally hosted image should be here but it was not working when we last tested it.


The big chunk of copper is the return point for all the grounds.
 
OK, there are a couple of things I can see that could cause the problem. Having a look at the hum on a scope should also help the diagnosis some, as you can check if it is a 60Hz sine, a 60Hz pulse, or a 120hz pulse.
Each suggests different mechanisms of hum injection. 60Hz sine would point at earthing issues or ground-earth loops (usually therough the mains power lead and interconencts), or inductive/capacitive feedthrough from the mains.
60Hz pulses mean either stray field (possibly due to transformer core saturation) induction, or induction via current loops formed by non-braided wires on either the primary or secondary of the trransformer.
120Hz pulses (all going 'one way' whereas with 60Hz you get one positive and one negative going pulse each 60Hz cycle), suggest filter cap charging current being shared with some part of the power/signal grond of the amp circuit, or the same current passing through unbraided wire that forms a loop sufficiently large to induce the same waveform in other circuits adjecent to it.

From what i can see in the picture of the underside, you have two problems:

1) Your rectifier lines to the main filter cap are not braided topgether, but instead form a rather large loop. Keep in mind that there are narrow pulse currents of many amperes here, so the inductance of this loop actually is quite signifficant, and due to the pulse edge rate, you get a rather unexpectedly high magnetic induction effect. ALWAYS look at these problems by drawing out the actual loop the current has to traverse. It is your goal to minimise the area of this loop as much as you can so that it does not end up being an AC magnetic field source. The wires from the rectifier to the main filter cap MUSt be braided or at least tied together in parallel. Conbtrary to what one would expect, it is not always prudent to keep the length of this wire very short or make it very thick - in fact some series resistance helps dampen the ripple current pulses, a little goes a long way. You get less noise and less strain on the transformer, rectifier and cap.

2) Never wire your ground so that the fil;ter cap charge current and any signal related current share a common path. Even fractions of a milliohm of resistance of a common path is enough to get a mV or so generated by the cap charging currents, intot he signal circuitry, where it risks being amplified. When you refer the result as dB vs 1W output, you'll be surprised how easy it is to lose 20-30dB of signal to noise ratio just on account of this.
If you look at your grounding bar, the rectifier ground attaches to it as well as the filter cap ground, but there is another ground attachment between it, as well as more grounds on the laft and righmost end of the bar. Hence, some part of a ground path is shared between signal and power supply.
The right way to do this is to keep the rectifier ripple currnt loop as much within the filter cap as possible - in other worde, the rectifier has to conenct directly to the cap pins. Since the cap has single pins, the only remaining shared path are the cap contact studs and internals, which canot be avoided anyway in this case.
The ground wire from the main cap then has to go to the two extra filters, and it has to be connected as a sort of T-formed wire link, similar to what you did on the positive side. The horisontal line of the 'T' should connect the two ground pins of the left and right final filter cap. Re-orient them so that the actual length of this wire is minimized (again, trying to minimize common paths of charge and audio currents). The vertical part of the T should then connect the mid point of the left/right final filter cap - pins to he main cap - pin. The central ground point will then be where the vertical and horizontal parts opf the T conenct. Ideally, all other grounds should connect to this point. However, connecting this point to the ground bar and them connecting the other grounds to the ground bar is also acceptable. In this case, the wire run from the central ground point to the ground bar is a shared signal current path for the left and right channel, which means that any resistance it adds reduces the left and right channel separation - but this is FAR less critical than injecting ripple current into the signal!

3) Tis I am not certain of as i cannot see it well enough, but it is a common problem with all amplifiers where the left and right channels are dislocated to one side of the enclosure, but the power supply is common. This invariably implies that at some point the ground connections for the two channels separate from the common star or bus ground. The problem that occurs here is that normally the grounding scheme is such that signal ground is derived from power ground. Typically, a power ground line goes from the central point to the output of the amp (point of maximum curent swing) and then from that point downstream towards the input stages, 'becoming' the signal ground. Because you have two channels phisically distant, you actually in the end get two signal grounds. When a signal ground wire is then routed from each channel to the input connectors, if you drw out the actual ground paths from one input ground to the other, you get a sort of a C shape, enclosing a whole lot of bits and pieces generating various nasty magnetic fields - very often the most obvious enclosed thing is the power transformer. Now immagine what happens when the C is closed into a full loop (O) by conencting the two grounds on the inputs together, which happens when you conenct an interconenct cable, which tends to have the grounds conencted on the 'other end', at the source device. This loop becomes a 'secondary' for the transformer stray field, and indeed for any current loop formed by unbraided wires carrying signifficant current.
There are two general solutions to this problem (excepting the obvious one, having separate isolated power supplies for each channel), one is to make the ground look like a sort of 'fishbone' structure, where one end is the attachment to the central ground (power ground), the other is conencted to BOTH input conenctor grounds. The end of the 'ribs' connect to various ground points in the amp circuits. The other possibility is to run the grounds in parallel to each other even though the lengths end up different, so that there is no chance of a loop being formed (or ends of a 'C' formed ground wire bundle becoming connected and forming a loop).
 
Regarding signal and power grounds in the circuit:

Strictly speaking, the input ground, ground end of VR701, R702, C6, C2 connected together would form the signal ground, and this has to return to the ground pin of the speaker connector. Now if you think about this in terms of point 3) in my above post, you will notice that all of these should be the one and the same point for both channels, quite hard to do if each channel is on it's own side of the chasis! You could get away with, say, connectong both input jack grounds, pot ground ends and R702 ground ends to a single wire, then routing this to the ground bar. Also, you could conenct the ground ends of C6 and C2 together and route a single ground wire from either channel to the ground bar. Finally, you could connect all the output jack grounds together and route them to the ground bar as well. The tricky part is, that all of these wires have to share a common path - in other words, they have to be run bundled together, with 'offshoots' from the bundle going to appropriate places.

Placement of every one of these ground connections along the ground bar is not arbitrary, either. In theory, they should all conenct in one point. In practise, when there are so many wires, the 'point' becomes far to large to still be considered a point, so the various grounds have to be spread along the ground bar:
The power supply ground (power ground return) goes to one end of the bar, then, going along the bar, connect the 'power' grounds of the circuits (wire routing is not going to be as critical but if possible braid or twist these grounds with the appropriate power line), then comes the conenction to the output conenctor grounds (load return), then all the rest of them. In other words, one end of the ground bar is the power ground, the other is the signal ground. The ground bar is the only place where these shopuld ever conenct.
In practise (see 1) and 2) in the post above), the power supply ripple current loop is contained between the transformer and the filter cap(s) and never reaches the ground bar. Load currents (including parts that flow within the power part of the amplifier circuit) only flow along the ground bar between the power supply attachment and the load return. Anything on the ;other side' of the laod return attachment point on the ground bar is low current, so placement of various other signal related grounds on the ground bar is not as critical, as their currents will generate negligible voltage differences on the ground bar.

Finally, the ground bar itself should be insulated from the chasis - so that a well defined chasis ground conenction point can be made. In general, this conencts to the same place where the load return attaches to the ground bar. C701, which conencts the ground and earth, should actually be placed between the ground bar and the chasis, should also connect to this point with it's ground side. If the power supply cord is of the earthed type, then it's earth line must NOT connect directly to the chasis, even though this is what is usually recomended. The reason for this is that other parts of the system might also connect earth to ground on their chasis, and these in turn are conencted to the circuit ground. When the parts are interconencted, the chais then conenct via two paths, through the earth lines in the power cords, and through the ground wires or shields in the interconenct cables - and thus a ground loop is formed. To avoid it, an earth loop breaker is needed, the simplest form are two antiparallel diodes conencted between earth and chasis ground. The diodes have to survive enough current to burn the mains fuse should a ground fault develop.
 
So basically you seem to feel that the grounding of the signal return, as shown in the schematic, may not be the cause of all the trouble...? Anyway, I thought for sure that the hum was 120Hz in nature, but I'll look at it one more time to confirm.

I shall digest your suggestions and form a plan of action. Certainly, looking at it now, I have no idea why I returned the ground of the rectifier to the ground buss rather than the cap itself. I shall 'rectify' :) this, and see what other changes I can make to conform to your suggestions.

The only thing I see as a potential problem is isolating the ground bar from the chassis ground. Glue to hold it on?

The amp is NOT earthed...it has a two-prong plug.

RE: C701. Many units that I've worked on have small caps of this value connected directly at the rear RCA input between the signal return side of the connector and the chassis. If it might be causing problems, it's easy to clip it out of there.
 
EchoWars said:
So basically you seem to feel that the grounding of the signal return, as shown in the schematic, may not be the cause of all the trouble...? Anyway, I thought for sure that the hum was 120Hz in nature, but I'll look at it one more time to confirm.

I shall digest your suggestions and form a plan of action. Certainly, looking at it now, I have no idea why I returned the ground of the rectifier to the ground buss rather than the cap itself. I shall 'rectify' :) this, and see what other changes I can make to conform to your suggestions.

The only thing I see as a potential problem is isolating the ground bar from the chassis ground. Glue to hold it on?

The amp is NOT earthed...it has a two-prong plug.

RE: C701. Many units that I've worked on have small caps of this value connected directly at the rear RCA input between the signal return side of the connector and the chassis. If it might be causing problems, it's easy to clip it out of there.

C701 is unlikely to be the problem, it's just that you basically need only one of those. Check it out only when you tried the rest of the stuff I mentioned above.

Since the chasis is not earthed, you can't have a earth-ground loop problem, so that's one less woryy.

Regarding the ground bar, the problem with it not being insulated from the chasis is that it stops being a bar, and rather becomes a bar in parallel with the part of the chasis between the fixing points. So, essentially, you have a sort of loop from one end of the bar to the other. The simple matter is, straight lines do not form loops, so this sort of thing should be avoided on principle. You may be able to insulate it by using TO3 mounting hardware (insulation washers).

In any case, it pays to have a look at the hum waveform with a scope. I had a case where I got hum only when the volume pot was at minimum, it turned out to be a common path problem, in this case left and right power ground and power supply cap charging current. The hum waveform was a typical 120Hz rectification pulse train, which was the clue I needed. Just to illustrate how rectification ripple current can be a PITA, the solution to this problem turned out to be moving the conenction point of the power grounds a whole 2mm, after which the amp became totally silent.

I am not sure the power/signal ground scheme is NOT the problem but I suggest that before you can accurately sort that out, you need to address the issues I outlined above, i.e. keep the various hums and ripples within the power supply. The remaining problem will then be returning the power and signal grounds from each channel of the amp located on it's own end of the chasis, in such a manner that there is no loop and no induction.
 
I am not sure the power/signal ground scheme is NOT the problem but I suggest that before you can accurately sort that out, you need to address the issues I outlined above, i.e. keep the various hums and ripples within the power supply. The remaining problem will then be returning the power and signal grounds from each channel of the amp located on it's own end of the chasis, in such a manner that there is no loop and no induction.
That is what I like...a logical and sensible approach to the problem.

I got a Sansui BA-2000 on the bench. Once it's off, the DoZ goes on, and I look at the hum waveform and tackle the power ground first.

Thank you. It'll be a few days before I can report of any progress, but I shall do so as soon as I can.
 
EchoWars said:
Wow...I'm not sure if I can hack up the PC board to accommodate that many separate returns... If it were flawed that badly, I'd have assumed that a lot less people would have bought Rod's board and built the amp.


Wow what a carry on !!!!

I built my 1000watts RMS driver board on veroboard.

There is one zero volts line right across the board.
The input is on the left hand side of the vero and the outputs on the right hand side. I have no hum from ground problems at all.

The only earth I have is from the input jack to the chassis which is earthed to the mains.
 
nigelwright7557 said:
Wow what a carry on !!!!

I built my 1000watts RMS driver board on veroboard.

There is one zero volts line right across the board.
The input is on the left hand side of the vero and the outputs on the right hand side. I have no hum from ground problems at all.

The only earth I have is from the input jack to the chassis which is earthed to the mains.
Some people get lucky... Rarely me. :cannotbe:
 
Don't forget to filter the supply with a big enough electrolytic cap. The filter cap of the transformer might not be big enough (usually they aren't, and most only guarantee a minimum ripple of 20%).

Polyester caps of about 0.1uF near each "amplifier" (as close as possible, between ground and supply) are recommended. In your case, one near the output transistors will be sufficient. Don't use electrolytic caps for this, not even tantalum, because the are not fast enough to cover decouple high frequencies.
 
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