Tube Pre-Amp Hum SP-18

Finally got my SP-18 pre-amp back from Roy after he put the final touches on it after I built it.

Hooked it up to my ST-120 and the sounds is absolutely beautiful.

HOWEVER, I do get some hum. Roy mentioned during testing that he did hear a "small amount of hum that I couldn't eliminate".

While listening though, there is a noticeable hum even during silent parts of tracks.

If I turn on the ST-120 and SP-18, and turn the volume above half on either, I get the hum from the speakers. I know it isn't the ST-120 because if I turn the pre-amp off, or use a different pre-amp, no hum.

As Roy couldn't eliminate this with my particular pre-amp, it may just be how it is. But maybe someone here has some tips for me to try to see if I can get rid of it?




Thanks!

I'll bet no one on this board ever heard of an SP-18. Why 2 extra tubes? What is it?

The SP-18 is, from the looks of it, an SP-14 and a phono stage on one chassis. It is not listed in the tubes4hi-fi website. And it looks like it may be modeled after the Don Sachs version.
When I purchased my SP-14, I did immediately notice a slight hum with my ST-120. After much fiddling and diagnosis to eliminate interconnect, poor physical placement, potential ground loops and all the usual suspects, I discovered that somewhat counter-intuitively the solution was in adjusting levels. I set the attenuator on the ST-120 to a level (around 12 o'clock) so that I could run the levels on the preamp as high as possible - both L&R all the way up and the master volume between 2 and 4 o'clock. No more hum. At all. Zero.
It may also be related to using a largely wooden chassis. You may simply have insufficient shielding. I'm sure that someone must have some remedies for this - some imaginative way to line the box, e.g.
One question - does the problem hum occur equally in all inputs, or is it worse in the phono input?

Kentley, correct it is a custom build with the phono stage built in. I didn't want to have seperate phono pre.

Roy also thought must be the wood chassis. But just doesn't seem right.

I did mess a bit with attenuator levels and can get it down, but the hum is still there... And the music is then very low!

You're probably stuck with the hum, sorry to say.
You might could try a metal shield (Pb) between the phono and line stages inside the box. And, if the bottom of that wood chassis is open, perhaps install a metal ground plane.
These are issues TMadden on this board is quite competent with; I'd zap him a note.

deepee99 wrote:You're probably stuck with the hum, sorry to say.
You might could try a metal shield (Pb) between the phono and line stages inside the box. And, if the bottom of that wood chassis is open, perhaps install a metal ground plane.
These are issues TMadden on this board is quite competent with; I'd zap him a note.

Already has a nice metal plate on the bottom. MEssed with the attenuators and gain some more, have a buzz no matter how low they go. Even with the volume and both gains all the way down, going through a SS amp, get hum.

TMadden any possible help?

Dr. Z,
Glad you got connected with Troy. He can fix your flat tire, Merle. Another consideration, after looking at your photo again, is component placement. You might consider building a shelf for the turntable above the tube components to get it out of the radiation zones of all those tubes and transformers. Phono cartridges and tone arms are very adept at picking up stray electronic noise. Inversely, the windings in a turntable motor generate a bit of hash as well which nearby tubes can pick up.
Mounting your turntable on a wall stud will also eliminate any rumble issues you might encounter, because the wall vibrates at a different frequency than the component stand on the floor, so there's no feedback issue. Just use a hunk of MDF and a couple of shelf brackets.

Alternately, put the power amp on the floor, so you can separate the record player from the pre amp,by a foot or more. Just to test the separation theory before making holes. I'm all for putting turn tables on proper stands, though, and good solid wall stands can be had for pretty cheap.

You might try "lifting the ground" on the pre amp. There are 3 into 2 female to male plugs for,this purpose, which cost only a couple bucks.

here's something you may try...line the inside of the wooden chassis with self adhesive copper sheeting, the kind you can get from hobby stores. Do the whole chassis inside, then ground that copper to chassis ground....perfect shield!
I do that to the preamps I build, made a big difference.
You can also use mumetal, self adhesive stuff, although I found that for this purpose that thin adhesive copper sheeting works just fine!

MontanaWay wrote:here's something you may try...line the inside of the wooden chassis with self adhesive copper sheeting, the kind you can get from hobby stores. Do the whole chassis inside, then ground that copper to chassis ground....perfect shield!
I do that to the preamps I build, made a big difference.
You can also use mumetal, self adhesive stuff, although I found that for this purpose that thin adhesive copper sheeting works just fine!

Very interesting idea. Do you have an amazon link to a product you know is good?

DrZ123 wrote:

MontanaWay wrote:here's something you may try...line the inside of the wooden chassis with self adhesive copper sheeting, the kind you can get from hobby stores. Do the whole chassis inside, then ground that copper to chassis ground....perfect shield!
I do that to the preamps I build, made a big difference.
You can also use mumetal, self adhesive stuff, although I found that for this purpose that thin adhesive copper sheeting works just fine!

Very interesting idea. Do you have an amazon link to a product you know is good?

not really, I just buy them in bulk as I need them, sorry. Just do a search on self adhesive copper sheeting. Its very thin stuff, quite flexible and usually come in 12x12 or 10x10 sheets

MontanaWay wrote:

DrZ123 wrote:

MontanaWay wrote:here's something you may try...line the inside of the wooden chassis with self adhesive copper sheeting, the kind you can get from hobby stores. Do the whole chassis inside, then ground that copper to chassis ground....perfect shield!
I do that to the preamps I build, made a big difference.
You can also use mumetal, self adhesive stuff, although I found that for this purpose that thin adhesive copper sheeting works just fine!

Very interesting idea. Do you have an amazon link to a product you know is good?

not really, I just buy them in bulk as I need them, sorry. Just do a search on self adhesive copper sheeting. Its very thin stuff, quite flexible and usually come in 12x12 or 10x10 sheets

This looks promising!

http://www.amazon.com/Tapes-Master-10ft-Copper-Foil/dp/B00Z8MCJW2/ref=sr_1_3?ie=UTF8&qid=1438183661&sr=8-3&keywords=self+adhesive+copper+foil+tape+3%22

DrZ123 wrote:

MontanaWay wrote:

DrZ123 wrote:

MontanaWay wrote:here's something you may try...line the inside of the wooden chassis with self adhesive copper sheeting, the kind you can get from hobby stores. Do the whole chassis inside, then ground that copper to chassis ground....perfect shield!
I do that to the preamps I build, made a big difference.
You can also use mumetal, self adhesive stuff, although I found that for this purpose that thin adhesive copper sheeting works just fine!

Very interesting idea. Do you have an amazon link to a product you know is good?

not really, I just buy them in bulk as I need them, sorry. Just do a search on self adhesive copper sheeting. Its very thin stuff, quite flexible and usually come in 12x12 or 10x10 sheets

This looks promising!

http://www.amazon.com/Tapes-Master-10ft-Copper-Foil/dp/B00Z8MCJW2/ref=sr_1_3?ie=UTF8&qid=1438183661&sr=8-3&keywords=self+adhesive+copper+foil+tape+3%22

yup!....poifect!!!

I bought some of this last week and should have it this week
http://www.amazon.com/gp/product/B00NMS67XQ?
1/10th the price of other mu-metal I've seen

UPDATE - got it, it's crap, no way it's 10mil thick!! I'll go back to using this item from ebay . . .
http://www.ebay.com/itm/MuMetal-Ultraperm-Permalloy-Alloy-Shielding-Sheet-Mu-Metal-Audio-Shield-80-/160613440162?

you need either this or some copper shielding between your power transformers and the PCB, and connected to earth ground
good luck getting it in there without shorting anything out - could use cardboard on both sides as an insulator.
Email sent also about insulating the top of the power transformers, possibly with some clear silicon (RTV)

tubes4hifi wrote:I bought some of this last week and should have it this week
http://www.amazon.com/gp/product/B00NMS67XQ?
1/10th the price of other mu-metal I've seen

UPDATE - got it, it's crap, no way it's 10mil thick!!  I'll go back to using this item from ebay . . .
http://www.ebay.com/itm/MuMetal-Ultraperm-Permalloy-Alloy-Shielding-Sheet-Mu-Metal-Audio-Shield-80-/160613440162?

you need either this or some copper shielding between your power transformers and the PCB, and connected to earth ground
good luck getting it in there without shorting anything out - could use cardboard on both sides as an insulator.
Email sent also about insulating the top of the power transformers, possibly with some clear silicon (RTV)

in my experience, MU is great for shielding transformers. I in fact line the inside of metal toroid enclosures, like the kind available from Antek, then run a ground wire soldered to the MU into the chassis and connect it to chassis ground. As for the inside of a chassis, I use plain ole self adhesive copper sheet....its proven, in my gear anyway, to make a difference.