Bass amp grounding/hum issues

[Gregg]

Hi,

I built a 300W 6xKT88 bass amp and everything works but I have some ground/hum issues with the preamp.
The power section is a copy of SVT2 Pro. The preamp is a mix of several other bass preamps. The amp looks like an SVT2 Pro - same transformer and tube placement but I have only 3 tubes in the preamp section.
The power amp by itslef is dead quiet. When the preamp is connected to it a hum appears which is not loud but is audible (if you play at loud volumes you won't notice it). Maybe as loud as fan's noise.
What's is strange (for me at least) is when I move around preamp's ground wire inside the chassis the hum level increases or decreases depending on the direction to which it is moved. There's only one path when the hum level picked up by the ground wire is at minimum but still audible.
If I short preamp's output to ground the hum is still there and as I said depends on wire position. Even if I short power amp's input to ground connecting the ground wire to PI ground introduces the hum in question. Master control (where the preamp "ends") doesn't affect hum level.
Initially I ran preamp's ground wire to it's corresponding cap ground on the PSU PCB but the hum there is much louder. Also tried different ground points only to confirm that minimum hum level is achieved if preamp's ground connects to PI ground and then only one path gives me the mimimum hum level.
What is strange also is hum level increases when I boost the 300Hz EQ band.

I would appreciate if anyone can comment on this issue.

[loudthud]

Congratulations on your build so far. On a project this size, grounding becomes exponentially more compliciated than your average guitar amp. I wasn't able to find a schematic of the power supply online. From your description, it seems to me that whatever grounding scheme was used is working fairly well. The experiment I would run would be to ground the preamp at the PI but move the ground for the preamp filter cap to the preamp.

A 120Hz buzz comes from grounds in the power supply and rectifier circuits but a 60 hum can come from the heater supply and lead dress. Is there a hum balance control. Is the heater winding CT grounded or hooked to a positive voltage?

[Gregg]

Congratulations on your build so far. On a project this size, grounding becomes exponentially more compliciated than your average guitar amp. I wasn't able to find a schematic of the power supply online. From your description, it seems to me that whatever grounding scheme was used is working fairly well. The experiment I would run would be to ground the preamp at the PI but move the ground for the preamp filter cap to the preamp.

A 120Hz buzz comes from grounds in the power supply and rectifier circuits but a 60 hum can come from the heater supply and lead dress. Is there a hum balance control. Is the heater winding CT grounded or hooked to a positive voltage?

Thanks for the tip about moving the cap to the preamp. I'll try that tomorrow.
By the way there are two more decoupling caps on preamp board.
The power supply and driver section are as per SVT2 Pro schematic (toghether with the bias monitoring/fault circuit).
Power tubes and driver section tubes share the same AC heater winding, two 100 Ohm resistors to ground. As I already mentioned only the power amp is dead quiet.
As seen from the PSU PCB preamp's heaters are fed 12VDC regulated, not elevated, negative end connects to ground. I'm always using this in my amps and even high gain amps are totally quiet.

Here's one more link to SVT schematics including SVT2 Pro:

http://www.esnips.com/doc/11768640-0...2db250cc7e/svt

[Gregg]

Tried the cap suggestion but it didn't work - hum level is the same.
Maybe it's just the preamp tubes being close to the transformers and picking it up from them?
By the way my transformers are not standup but lay on the chassis one side of the coil into the chassis. I have shields around the openings but maybe it's not enough.

[Gregg]

I measured (DMM) 12mV hum level at speakers output (8Ohm). It doesn't look line much but it's audible. Or I just have sensitive ears.

[loudthud]

The formula for signal to noise is 20*Log(V/V). (Thats Log base 10) 300W and 8 ohms is about 49V. So I calculate 72.2dB. Not too bad really. Sounds loud in your bedroom but on a stage you might not be able to tell the amp is on.

Here's where a true RMS meter makes a big difference. Hiss has a low RMS "factor" so that 12mV could be only 6mVRMS which would change the calculation 6dB.

[Steve Conner]

Sounds like stray flux from the power transformer inducing hum into the preamp.

You should find that the ground wire routing that gives minimum hum, is the one that minimizes the loop area between the signal conductor to the PI, and the ground conductor.

You can minimize this loop area still further by using a piece of screened cable, and grounding the preamp to the PI through the screen.

However, if you ground the preamp with a wire connected to some part of its ground bus, then you must isolate the input jack from the chassis to avoid a ground loop, that will pick up still more hum from the transformer. (Unless the PI is floating, as this breaks the loop just as well.)

I don't like isolated jacks because I believe they make the amp more vulnerable to RF pickup, so I like to have the preamp grounded to the chassis through its input jack only. Basically, you want to avoid any closed loop in your ground wiring, and if you have to have one, make sure it has minimum area, is far away from the PT, and doesn't pass through anything sensitive. (So for instance, P2P amps that have a preamp ground bus will only connect one end of the bus to the chassis.)

[nbtone]

Try taking the ground plug going to the wall out of the picture. A two prong plug may be the answer.

[black_labb]

Try taking the ground plug going to the wall out of the picture. A two prong plug may be the answer.

the reason the ground is there is for safety. dont go without it.

[tedmich]

is the pre board floating above true ground? A good practice is a 47FP resistor from pre ground to earth, also try unbolting the transformers and seeing iff moving them quiets it up...at 300w its not ever going to be too quiet

[Gregg]

Below are couple of pictures showing parts placement and ground paths.
As you can see I went for the SVT2 topology. The power amp is SVT2 copy, the preamp is a mix of several other preamps.

http://i7.photobucket.com/albums/y26...BassAmpTop.jpg

http://i7.photobucket.com/albums/y26...BassAmpGND.jpg

Sounds like stray flux from the power transformer inducing hum into the preamp.

It sound like that to me as well.

You can minimize this loop area still further by using a piece of screened cable, and grounding the preamp to the PI through the screen.

Do you mean like groung the preamp to PI through the screen of Preamp Out wire?

However, if you ground the preamp with a wire connected to some part of its ground bus, then you must isolate the input jack from the chassis to avoid a ground loop, that will pick up still more hum from the transformer. (Unless the PI is floating, as this breaks the loop just as well.)

I don't like isolated jacks because I believe they make the amp more vulnerable to RF pickup, so I like to have the preamp grounded to the chassis through its input jack only.

All jacks in this amp are plastic and isolated from chassis but I'll try that.

PSU PCB ground goes to chassis ground via two reverse diodes, cap and resistor combo.

is the pre board floating above true ground? A good practice is a 47FP resistor from pre ground to earth, also try unbolting the transformers and seeing iff moving them quiets it up...at 300w its not ever going to be too quiet

Tried that without result.
Also as I mentioned before Master control doesn't affect the hum level in question.

Try taking the ground plug going to the wall out of the picture. A two prong plug may be the answer.

Uhhhh, no thanks. I'll try to avoid that

[Gregg]

I did some experiments.
Grounding the preamp to the chassis at the input jack resulted in very loud hum and buzz. Actually my input jack is the same as in SVT2 Pro. I'll be really curious how the amp looks inside though.
Grounding the preamp using preamp's out shielded wire however resulted in noticeable improvement. I'm usually avoiding using the screen for grounding but in this case obviously this is the way to go.
There's still some hum left but it's much lower now and I'm only couple of meters from the speakers. Actually when turning the Drive pot up at some point the hiss becomes louder.
Thanks to Steve for the suggestion.
I would appreciate any other comments concernig this build.

[anson]

I have had the exact same problem with my amp, a 4x el34 style guitar amp, for years (about 3). Still haven't been able to get rid of the hum problem. I can make the hum go nearly completely away if I make the 1st preamp stage ground wire long, about a 1/2 meter, and then twist that wire around in various shapes and loops. Can't keep it like that whhen I play though! The best I've done so far is carefully place the ground and grid wires. That lessens the hum abit. As far as I can tell, it seems like magnetic flux from the PT. I've tried many grounding configurations and have changed the heater to DC (preamp) supply. No good. My PT was made custon by a guy from Brisbane. Maby some PT's are made with better shielding than other and mine lacks some of that shielding. I don't know. I am at the point now where I'm considering somehow injecting some of the heater AC signal into the preamp in a negative feed back sort of way, to try to cancel out the hum.
Anson

[tedmich]

tranny shielding is often overlooked:

http://www.vias.org/eltransformers/l...ers_07_12.html


Lundahl trannys ($$$) use a faraday shield:

http://www.lundahl.se/pdfs/papers/groundin.pdf

and see this discussion (a 1/4" Al box?):
http://www.diyhifi.org/forums/viewtopic.php?f=11&t=1074

[Gregg]

I have had the exact same problem with my amp, a 4x el34 style guitar amp, for years (about 3). Still haven't been able to get rid of the hum problem. I can make the hum go nearly completely away if I make the 1st preamp stage ground wire long, about a 1/2 meter, and then twist that wire around in various shapes and loops. Can't keep it like that whhen I play though! The best I've done so far is carefully place the ground and grid wires. That lessens the hum abit. As far as I can tell, it seems like magnetic flux from the PT. I've tried many grounding configurations and have changed the heater to DC (preamp) supply. No good. My PT was made custon by a guy from Brisbane. Maby some PT's are made with better shielding than other and mine lacks some of that shielding. I don't know. I am at the point now where I'm considering somehow injecting some of the heater AC signal into the preamp in a negative feed back sort of way, to try to cancel out the hum.
Anson

I've never had this problem even in high gain guitar amps but parts placement there is totally different. I remember however having similar problems in a 2x50W rack amp I built some time ago which I solved by moving around a ground wire and fixing it.