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EAR834p clone with hum. Would appreciate some help

Hi everyone!

I have built an EAR 834p clone but have a hum problem. I have followed the grounding scheme (see picture) from the Lenco Heaven forum (and also implemented the mods) but is seems like the amp is picking up noise from the inputs. I am a novice and I am starting to feel that I perhaps took on a project to complicated for me. It is a good sounding phono stage and also some money invested so I would be thankful if someone more knowledgeable would like to help me sort this out so that I can keep this.

I have a Rega P6 turntable with a MC cartridge. I bought it second hand but from a reputable HIFI dealer that had checked it. I have not tested it with anything else than my DIY 834p build. I have not owned a TT since the eighties so I know very little about TT grounding.

After a lot of testing I have been able to pin down a few things regarding the hum. I have built an input switch for MM and MC so that I can easily switch to MM in the future if I like. I have hum even if nothing in connected to either input so I know the hum is from the 834p.

If I switch to MM input, and then ground the switch with a wire, hum is pretty much gone. It disappears completely if I put my hand over the MM inputs (from the back of the amp), I do not even need to touch them, just cover them. Same thing if I hold a piece of metal in front of them. So it seem like the inputs and the switch act like an antenna picking up hum from somewhere. I used an app on my phone and measured the hum. I have a loud 50hz peak, and also a 100hz peak thas is not as loud.

When I plug in my TT to the MM input, there is a faint hum, and it no longer makes any difference if I ground the switch with a wire or not. It's a Rega P6 so it has no separate ground cable. Ground in Rega TTs is in the left channel cable. Don't know if that has anything to do with it.

If I switch to MC, with nothing connected to the input, which goes from inputs trough a pair of Lundahl LL1931 SUTs and the to the switch, I still have hum even with the switch grounded. A tiny bit little less when grounded, but not much. I have tried different grounding schemes for the Lundahls and this is the most quiet one, but still way too loud. I only have a MC cartridge, so if I want to listen to vinyl, hum will be present if I do not solve this.

In my build I have made it so that the switch switches both + and -. Is that a bad idea?

I have changed all the unshielded input wires on the picture to shielded, no difference.

Also there is a problem with hum with the Rega P6. I get a really loud hum when I touch the tonearm and sometimes it stays after I let go of it, sometimes not. If I move around the input cables from the Rega this loud hum can also start. When I get this hum, I can cancel it by putting a wire from Right input - (shield) to amp chassis (not the Left channel that supposedly has the TT ground), but if I leave this wire permanently in place I still get hum if I touch the tonearm and to cancel the hum I now need to remove the wire.

All of this is so weird to me.

So, what can I do now as the next step? Do I need to ground my inputs in another manner perhaps? I have seen that some people connect all input - to each other and some don't. I am at loss at this point.

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Wow, very impressive build. Here are tricks that helped in my similar tube preamp + Lundahl build:
The magnetic shield around the Lundahls is not sufficient in itself. I applied permalloy shield around them and placed in a separate box with RCA inputs and outputs. Evem so I have to rotate them, they pick up hum from the mains cables in the wall.
Connecting the signal GND to the chassis GND through a 10R resistor helps. Do it separately for both channels. Check Ground Loop Breaker, some solutions use a 35A rectifier bridge+resistor+capacitor.
Make sure there is no oscillation of the input tubes. It can be prevented by soldering a 100R to 1k resistor directly on each grid (series).
If nothing helps, use the Input Ground Loop Breaker. Lift the point where the input circuit meets the signal GND. Insert a 10R resistor in between. The signal neg of the input cable should go to the top of this resistor, instead of directly to the signal ground.
The goal is to have no hum when you insert a shorting plug to each inputs. A shorting plug is very handy tool. When there is no hum with the shorting plugs, connect the input RCA outer signal neg together with a screwdriver or with a piece of wire.
Repeat the above without the shorting plugs. You should hear hiss but no hum.
If all the above tests pass, let's deal with the turntable.
 
Hi, Do that, had the hum,
-1st thing to do is get your trans into a separate metal box with about a 3 mtre connection lead.
-2nd. The clone puts out about 58dB, so you do not need any step up tans for a MC connection. Just put two loading plugs in the line in with the correct loading for the cart. MC is subjuect to far less noise than MM, I ran a 103 MC cart into my system through my 834, more that enough gain. If you run a 103 use 1000 ohm loading plugs for best sound. Other cars follow the manufacturers recommendation.

Cheers
 
Wow, very impressive build. Here are tricks that helped in my similar tube preamp + Lundahl build:
The magnetic shield around the Lundahls is not sufficient in itself. I applied permalloy shield around them and placed in a separate box with RCA inputs and outputs. Evem so I have to rotate them, they pick up hum from the mains cables in the wall.
Connecting the signal GND to the chassis GND through a 10R resistor helps. Do it separately for both channels. Check Ground Loop Breaker, some solutions use a 35A rectifier bridge+resistor+capacitor.
Make sure there is no oscillation of the input tubes. It can be prevented by soldering a 100R to 1k resistor directly on each grid (series).
If nothing helps, use the Input Ground Loop Breaker. Lift the point where the input circuit meets the signal GND. Insert a 10R resistor in between. The signal neg of the input cable should go to the top of this resistor, instead of directly to the signal ground.
The goal is to have no hum when you insert a shorting plug to each inputs. A shorting plug is very handy tool. When there is no hum with the shorting plugs, connect the input RCA outer signal neg together with a screwdriver or with a piece of wire.
Repeat the above without the shorting plugs. You should hear hiss but no hum.
If all the above tests pass, let's deal with the turntable.
Thanks! Lot's of useful information here (even tough I don't understand all of it yet. Need to do some reading). I think I will do as you said, put the SUT in a separate well shielded box. My idea for putting it in this enclosure was to have one less unit in my rack. Maybe I'll find a way to fit it inside of this one, but more shielded.
 
Hi, Do that, had the hum,
-1st thing to do is get your trans into a separate metal box with about a 3 mtre connection lead.
-2nd. The clone puts out about 58dB, so you do not need any step up tans for a MC connection. Just put two loading plugs in the line in with the correct loading for the cart. MC is subjuect to far less noise than MM, I ran a 103 MC cart into my system through my 834, more that enough gain. If you run a 103 use 1000 ohm loading plugs for best sound. Other cars follow the manufacturers recommendation.

Cheers
I tried using the 834 without a SUT but it was not enough for me. I have a ClearAudio Essence MC, 0.4mV, 11ohm, so the SUT was definitely an improvement (apart from hum). I use Lundahl LL1931 which has 1:8 and 1:16 step up and I even preferred 1:16 over 1:8. But I think you might be right about putting the SUT in a separate metal box.
 
It's possible your tonearm ground wire is not attached to the tonearm metal part internally. The armwand is supposed to act like a shield for the tonearm wires but when not attached to the ground wire it loses its shielding function. Check it with a continuity meter or beeper between tonearm body and ground wire.
 
"If I switch to MM input, and then ground the switch with a wire, hum is pretty much gone. It disappears completely if I put my hand over the MM inputs (from the back of the amp), I do not even need to touch them, just cover them"
This can be oscillation, as well. Are those grid stopper resistors in place? A schema would be helpful.
 
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Nice build!

I do not have any additional suggestions here. My built does not show any nasty hum (but it is not very silent either). However, some ground loop breaker PCBs just landed yesterday on my door. If you want to try them (before I do) I can send you a couple. I am based in Sweden too, so just send me your address via PM.
 

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At least the input tube should be shielded and not sticking out from enclosure . Sockets with shields are normally used for phono stages
If it is from the power transformer you should have some seconds of hum free sound when swiching off , until the power supply caps discharge
Shielding the tubes has no effect on hum pickup whatsoever, in my experience.
The other idea is worth trying: run the heaters from a lead-acid battery and see if the hum changes. If it does, it is related to the heating.
 
Have you tried it? I did. First try with aluminium cap, then with steel cap, then without cap. There was no hum pickup of the tubes themselves. The only difference is better cooling without a cup. RF might be different, but I think a cap is used there for preventing the radiation of the L.O. outside from the anode, not picking up any RF by the... anode? The grid is deep inside the tube structure, "shielded" by the anode against external influences. Anyway, shielding at audio frequencies is a myth, in my experience.
Furthermore, a spring loaded cap could case mechanical pickup aka ringing.
 
At least the input tube should be shielded and not sticking out from enclosure . Sockets with shields are normally used for phono stages
If it is from the power transformer you should have some seconds of hum free sound when swiching off , until the power supply caps discharge
Good tip! I switched off the amp and hum is there while caps discharges. So I guess I can rule out transformer the?

I know that a lot people have build this clone totally hum free with no tube shields, some with tubes totally out in the open, some like me, only partly, and some inside enclosure. I can say however, that swapping from Brimar CV4004 to JJ ECC83S (which is a low noise tube) lowered the hum a little.
 
It's possible your tonearm ground wire is not attached to the tonearm metal part internally. The armwand is supposed to act like a shield for the tonearm wires but when not attached to the ground wire it loses its shielding function. Check it with a continuity meter or beeper between tonearm body and ground wire.
I thought about this actually. I might add, that sometimes when I get this extra hum from touching the tonearm, right channel goes silent. No music. If I unplug RCA and put back in sound is back.

I do however have hum without TT plugged in so I might be looking at multiple problems here. Or perhaps the amp problem is making the TT act out.
 
"If I switch to MM input, and then ground the switch with a wire, hum is pretty much gone. It disappears completely if I put my hand over the MM inputs (from the back of the amp), I do not even need to touch them, just cover them"
This can be oscillation, as well. Are those grid stopper resistors in place? A schema would be helpful.


The other idea is worth trying: run the heaters from a lead-acid battery and see if the hum changes. If it does, it is related to the heating.

I am a novice so I don't really know what oscillation is (other than that you don't want it), or grid stoppers. If you are thinking that I should not fiddle with this stuff you are probably right. ;) I recently build my own speakers after a Troels Gravensend design and was so impressed of how they turned out and felt inspired to keep on going down the DIY path. I thought if I followed a finished design carefully I would be fine. I was wrong. :p

Here is pics of the board and schematics. I think it is the lower schematics that is for my board. I have however done all the Thorsten and Roberts mods from this thread https://www.lencoheaven.net/forum/index.php?topic=26658.0 but they are tested by others and should work fine.

I might add that I did have a short on the PSU earlier. It was probably caused by breaking apart the PSU board from the amp board, others have experienced this too. You sometimes need to file the edge to prevent a short. The short burned my transistor and I had it replaced. I do however have around 6.0-6.1V on the heaters now and I had 6.3V when I measured before the short, so perhaps there is something more that needs to be replaced. But If I remembered correctly I had hum before the short too.

I tried the tip to turn of the amp and listen for hum while the caps discharge. Hum is still there after amp is turned of while caps discharges.
 

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Shielding the tubes has no effect on hum pickup whatsoever, in my experience.
The other idea is worth trying: run the heaters from a lead-acid battery and see if the hum changes. If it does, it is related to the heating.
The battery idea sounds like a good one. Is it important that it is exactly 6.3V. There are a lot of 6V batteries. Is Ampere relevant when it comes to a battery?
 
If you switched off the preamp and you still got hum it's not a filament/B+ issue either , so batteries are useless
Look then for external causes , like lack of shielding , beside tube shields I see that the back where the RCAs are mounted is just wood
When you work with 2-3mV of signal or less everything must be perfect , including a good quality audio cable

And the extra separate groud wire that turntables usually have is very important for hum free signal .
 

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