VTA SP12 and PH12 Build Thread

Well, I messed around with the SP12 a little this afternoon but no fix was found. Despite the hum, the unit plays music very well and if you crank up some Van Halen or something loud, you can drown out the hum and everything sounds great!!!! Just don't turn down the volume and it's all good This is also a good time in this post for me to state that I'm powering my speakers with my ST-70 kit that I built 3 years ago using Bob Latino's kit with all upgrades and am using the Gold Lion KT66 tubes. I have been using a passive preamp and this combo is DEAD QUIET. Before the passive preamp, I was using a Bottlehead Foreplay Extended kit that I had built. This also had no hum and was fairly quiet. The ST-70 has also been paired with an NAD C340 integrated amp, obviously just the preamp section with the jumpers removed to make way for interconnects to the ST-70. Also Dead Quiet.

I'm 150% sure that my ST-70 and other components are not at fault here. However, the ST-70 does not have a grounded power cord. It is 2 prong. Perhaps I should try upgrading the ST-70 to IEC connector and power cord. I've seen it done by others but I don't really want to mess with it and I also don't think it would solve the problems at hand. Somebody correct me if I am wrong in that thinking.

Before I messed with the SP12 today, I had disconnected some things to try to find where the hum was coming from. I disconnected the PH12 board inputs and the B+ and ground and removed it from the risers. No change had resulted from that. Today I reconnected the PH12, although I am waiting for a pair of resistors that are currently missing because I damaged up 2 of the original ones trying to remove them (had them in the wrong spot!).

I had the HV connected from the transformer to the Rectifier board in parallel for 250 V without center tap and thus was using the pair of resistors and pair of diodes on the rectifier board. Today I switched it to wired in Series for 500V with center tap and removed the resistors and diodes from the rectifier board as per Roy. STILL HUMS! However, it seems a little different in that when I play music through it the volume is less than it was before. It is not possible to make the music loud enough to drown out the hum

I replaced the metal risers below the PH12 with plastic ones that I had in my parts box. NO CHANGE that I can tell. The board should be grounded through the ground wire that connects to the Aikido board but I may find later that I need to ground it to the chassis as well. I'm not sure.

I played around with the grounding jumper on the back corner of the Aikido. I forget the number right now but it's the one that connects ground to one corner where the screw holds the board to the riser post. This made the Hum Quieter but still present.

I previously had disconnected the Mute switch. I had also wired it incorrectly the first time. It did not seem to affect the hum before I disconnected it in either switch position. Today I wired it back in correctly and the hum COMPLETELY disappears when I have the switch on Mute, even with the power on. On or off, it doesn't matter. Mute switch on Mute - NO HUM. Mute Switch on Operate - HUM with power on and power off.

So, it seems that there is definitely something allowing AC to get into the signal path on the Aikido board somewhere, but the dang thing plays music so I have no idea where to begin. So I won't. Not today anyway. I have reviewed all the connections on the Aikido but I am afraid I'm going to have to pull it and check it all again.

I'll probably go back to 250V HV without the center tap and put the resistors and diodes back into the rectifier board. I think it worked better that way.

I was not my usual methodical and scientific self by only changing one thing at a time like I should have but since nothing has really changed, I guess I'm no worse off. If I had managed to fix it, then I'd really not know which solution had worked. Alas, none of them have worked thus far.

What will I try next? I'll have to ponder that one. I did NOT see any improvement by placing my hand between the transformer and the PCBs. There was no change. I hope I don't bust any more resistors!! I made my second order with Tube Depot (AWESOME STORE) and I always feel bad for having a 50cent order so I end up shopping the site. Today I added a sweet T-Shirt:
Last time it was upgraded tubes for the PH12. I hope there isn't a next time! Who knows what I'd blow extra money on

Until next time...Best Wishes from Oregon.
Joshua

Not sure why I was posting as a guest. Anyway, one last thing I just thought of: A question.

Can anyone share some common mistakes that often lead to HUM even with power off or even when music will play well?

Ed, very much enjoying your build. I'm working on an SP-8 but your thread has been very helpful. Thank you. Please let us know how you solve the hum.

Ed and J,
do you guys have your signal ground buss tied to chassis AC ground?
The only way you will get hum with the power turned off is a ground loop between the amp and the preamp

Unless AC is getting through the right channel output ground wire from the PCB, mine is lifted completely. I'll take off the other ground and see if there is any change.

ed

And as I suspected, that unleashed all the demons of hell. Don't try that one at home.

Hello Ed et al,

Ed, I'm not sure what you mean by "...mine is lifted."

Only one of the output signals from the Aikido to the output RCA jacks should have ground connected at both ends. One should NOT be connected at the RCA outs.

As per Roy's post earlier today, I added a ground wire from the Ground hole in the middle of the Aikido Board to the chassis. This reduced the hum quite a bit, but it is definitely still there.

The diagram I have from Roy shows a potentiometer with Balance knob. Ed and I both have the left and right stepped attenuators for balance with a stereo 24 stepped volume attenuator. The diagram from roy shows only one of the input wires TO the Aikido as being grounded at both ends (The Aikido and the Potentiometer). However, all of the pictures I've seen of the dual stepped attenuator balance with stepped attenuator volume show both left and right inputs to the Aikido as being grounded at both ends, i.e. black wire is connected to Aikido and to the volume attenuator which in turn has ground wires to each of the balance attenuators. However, the ground wires end before the tape/source switch (to which they are not connected.

So, sorry if that is confusing!

I'm glad my hum is less now but I'm not sure why it is still present. I will have more time to mess with it on Wednesday or the weekend.

Josh

Josh...I was saying that the buss is isolated. The only ground attached to it is the ground wire to the right output from the PCB.

I don't see anything from Roy suggesting running a ground from the PCB to chassis, but I'll see if that helps my cause. I will aslo be disconnecting the PH12 to add a voltage dropping resistor tonight. I'll see if the problem is helped when the phono stage is out of the circuit.

ed

Ed/Josh this link may help:

http://www.diyaudio.com/forums/tubes-valves/94206-aikido-hum-has-me-stumped.html

Regards, Mike

Great Link! It will take me a while to understand and digest it but I feel there is a solution in there somewhere.

Josh

Ed,

Here is a quote from Roy found a few posts back:

"Ed and J,
do you guys have your signal ground buss tied to chassis AC ground?
The only way you will get hum with the power turned off is a ground loop between the amp and the preamp"

I assume this means that there should be a connection from the signal ground to the chassis. When I added this, the hum got quieter, but I may be misinterpreting Roy's advice. I'm still trying to figure out what the solution was on the DIYAUDIO forum link that was posted by Mike. Not sure what the author means by PSU. I get what PCB is obviously, but he used a lot of acronyms not all of which are familiar to me.
Josh

Josh,

I believe that Roy was making sure that we DON'T have our signal busses grounded to the chassis. He is suggesting that as a possible path of least resistance for a ground loop with the power switched off.

I am also just about positive that the hum is at 60Hz. I can't do this any more tonight...I'm hearing that pitch everywhere now. Fans turned on, the fridge running......

Just an update...

I've taken a couple of days off so that I can go back at it this weekend with a calm mind. I played with all of the grounds and found that the more I shield the PT from the line stage, the quieter it gets.

Without anything attached to my VTA ST-70 there is the tiniest of hums. It is so quiet that you have to stick your ear into the speaker cone to hear it. Add the preamp with power off but plugged in, then the same 60 cycle hum gets louder. Switch on and that pitch intensifies more.

I'll attack it in earnest this weekend.

Hope everyone is having a good week.
Ed

Here is a thread from audiocircle which may spark some thoughts
http://www.audiocircle.com/index.php?topic=91996.0

That was a great article when I finally got to the end to see what worked. I tried it but to no avail on my SP12/PH12/

Like Ed, I use a VTA ST70 as my amp, which has a 2 conductor power cord with no earth ground. It is plugged into an outlet strip along with all of my other components.

My hum is LOUD, although quieter with the power off.

Switching to mute silences the hum (both with power on and power off) as well as the music.

As I said before, the preamp will play nice music along with the hum...except that I hadn't tried the phono stage. So I tried it today and the music that came through was quiet and sounded terrible. It also added more hum! I removed the PH12 all together but the SP12 still hums as it did before. I tried the things that Ed tried but didn't get the same improvement. Increasing the distance between the PT and the PCB or adding shielding between had NO effect.

I will now have to put it away until I regain my sanity and patience, then dive back in from step one to go over every single resistor, capacitor, diode, and whatnot. There must be a short or improper ground somewhere!

Joshua

jdmoninger wrote:My hum is LOUD, although quieter with the power off.
Switching to mute silences the hum (both with power on and power off) as well as the music.Joshua

edgobb wrote:I have tried disconnecting inputs to the Aikido.

Hi, Does this not mean there is a ground loop being completed somewhere between the output side of the mute switch and the input side of the RCA jacks?

Just my dart at the wall,
Frank

I'll say again what I've said a half dozen times before . . .
if you have hum just from connecting the preamp to the amp, and it's not even turned on, you have a ground loop being formed by the interconnect.
The person in the link that was referenced made a great analogy, you're saying the car makes noises when it's not even running.
This is what I always do . . . all electronic components are plugged into one power strip.
Add them one at a time if you must, especially if their plugs are not polarized (try them both ways).
The signal ground buss is tied to AC ground and the chassis is tied to AC ground.
If you still have a hum problem there is likely a problem with the interconnect.
If you can't find a source that doesn't create hum, use a battery operated iPod or MP3 player as a source.

I have the hum with NO sources attached, so the only interconnect is the pair between SP12 and ST70. Then I add my iPod to see if music plays, which it does, and there is no change in Hum.

We thought that you were not supposed to ground the signal bus to the AC ground. Didn't see a place in the wiring to do that but I will add that again. I had gone back and forth and it made little difference. I think I have made a mistake somewhere in the Aikido.

Don't think this will help, but I am reading a book "Getting the Most Out of Vacuum Tubes" published 1960. An easy test is to take a permanent magnet and move it around the tubes. If the hum changes-gets louder or quieter EMI is creating "uncanceled current fields in the heater circuit"

P.S. this book is available as a free download here: http://www.tubebooks.org/technical_books_online.htm I recommend it. For those of you who have expensive OS rectifier tubes, it is a bad idea to tip the amp on its side to check voltages; says the Author.

Good Luck; miss reading about your efforts.
Frank

Thanks Frank. Interesting about the magnets.

I have been trying quite a few things when I get the time.

I bypassed the IEC in case there was something strange going through the back panel.
I removed the front and back panels from the chassis base to see if there was any change.
I lifted the terminal strip with the AC input wires.
I took the primary wires off of the terminal strip so that I could apply power directly to the transformer, bypassing the power switch and the IEC.
I made an aluminum foil shield and placed it in between the toriod and the line stage PCB and it made a marked difference. The hum was greatly reduced, but still present. (That's aluminium for you European types.) Antek sells shield boxes especially for their transformers and that might be something to think about way down the road.

I have a transformer here that I am going to use to build a tube tester (next project). I plan to use it to power the preamp and heater circuit so that I can either eliminate the transformer as the culprit or blame it once and for all.

I do, however have a concern about the extra transformer. I'm not sure what kind of current rating I'll need to power this up and test it. The Antek has two 250V taps @ .2A. They are in wired in parallel to my rectifier tube input. The extra transformer is 250V @ .1A. Is this unit going to draw too much current? (I think that peak plate current of a 12X4 is 245mA, so if I'm just testing it without any source input it should be fine? I'm sure that someone much wiser will school me if I'm confused.)

I am trying to do everything that I can before I start de-soldering. If I get this other PS in there and the hum still exists, I'll start eliminating other components one at a time.

I'm trying not to stress about it and work through it slowly and methodically. The problem can and will be identified...eventually. Cause man, I certainly paid too much money for it not to work properly. The music that comes through it sounds pretty great...except for that hummmmmmmmmmmm.

Another thing is that the phono stage doesn't seem to work properly. The sound (music) coming through it is very quiet, but I'll worry about that after I get this noise taken care of.

Patiently yours,
Ed -- Replacing calamity with serenity.

Hi Ed, Glad to hear you're not hanging from the rafters.

Well, it does seem like an issue with EMI from the transformer. I was looking at your photos and was wondering about that back panel. I was thinking in the opposite direction though. Any possibility there is not a good enough electrical connection to the rest of the chassis? If my SP-12 hummmmmmms,I think I will just put the trans in a separate box and call it good.

I Have a newbie question for you. I don't understand the purpose of running individual grounds to the inputs then bussing them together. It just seems like a bigger antenna then. I guess the twisting eliminates that?

'Frank

"Ten million years from now, when the Sun burns out and the Earth is just a frozen iceball hurtling through space, nobody's going to care whether or not I got this guy out."-Tug McGraw

Ed, is that the Antek AS-IT250? http://www.antekinc.com/details.php?p=671
I've got one also but I'm far from finished building my SP-8.
Will be very intrerested if it is the source of the hum.

My wiring diagram calls for 500V center tap (250v on each side but 1 ground) rather than the parallel 250v. I can't imagine it would make a difference but is yours similar?

Thanks for keeping us up to date on this.
Tom

Tom,

Yes. That is the exact model of my transformer. I'll keep you posted.

Ed

Ed/Joshua,

I fought hum issues on my SP12/PH12 when I built it a few weeks ago. Well, finished it a few weeks ago - battled the hum issues for several weeks. I don't know if the issues I had are similar to yours, but perhaps there'll be tidbits that can help you in what I went through.

The first time I put my unit together I had what sounded like hum at two different frequencies, and another component that was pretty much just noise. I found that I could influence the loudness and sound of the hum/noise just by moving my hand around the chassis - if I put my hand between the trafo and SP12 tubes the hum/noise would lessen, if I touched a tube it would intensify, etc.
I did a lot of tinkering, most of which was probably pointless. In the end, I ended up solder-wicking most of the joints on the SP12 board near clean and redoing them.

While the heavy, thick traces and huge pads on JB's board look nice, I feared that my soldering station (or my soldering skill) wasn't up to the task of heating all that metal properly for a good joint. So I bought one with better temp control and a higher temp range and re-did the board. I had to set the new station at just over 800F to get good wetting without having the iron on the components for more than 2-3 seconds.

As luck would have it, when I put it all back together I still had hum. BUT - the noise was gone and moving my hand around the chassis, touching the tubes, etc. no longer influenced the hum/noise coming from the SP12. So, I chocked that up to some cold joints.

Next, I wasn't sure if I was dealing with a ground loop issue between my amps (Bob's M-125's) or within the preamp. The SP12 can't drive a pair of IEM's (at least it couldn't drive mine), but I found it could drive a pair of cans pretty well, so the rest of my testing I did with a pair of Bose QC2's. Perhaps battery powered computer speakers would work too if you have them - the point was to listen on something that required no power, or could be powered from battery instead of house AC.

I eventually figured out that my hum was not 60Hz; it was 120Hz, which I read was probably power supply ripple. I strongly suggest you search the net for an online signal generator or sound files you can download of 60Hz and 120Hz sine waves to compare to the hum from your preamp - it's very helpful to know what frequency the hum is.

From there the game was improving PSU filtering so I put a choke in the place of R17 (Hammond 155J) which eliminated the hum from the SP12, and significantly reduced it from the PH12. To get rid of the rest of the hum from the PH12 I put another choke (Hammond 156G) in series from where I took the B+ from the center of the AIKIDO board, followed by paralleled (2) .33uF Nichicon caps. That did the trick.

If your hum is 60Hz, then most of this info probably won't help you, and I apologize in taking your time to read it.

BTW, Ed, from your pictures, if you are getting hum from the proximity of the trafo I'd expect it to be stronger in one channel than the other due to the position of the board.

Roy - again, apologies for inundating your inbox through all this, but your help and insight was greatly appreciated.

I'm running RCA clear tops in the SP12 and it sounds marvelous. On the PH12 I'm using Pope 7DJ8's, which sound great as well, but just a tad bright so I'll probably roll those to get something that matches the rest of my system better.

In the M-125's I have EH 12BH7A's (front position tested for low noise and micro-phonics), KT-120's, and Mullard GZ33's. I can't hear a difference between the copper cap and the Mullard, but I prefer the look of the Mullard. I also had the stainless chassis polished to a mirror shine. Excellent amps, Bob.


Best of luck,
JP

Thanks JP,

The hum is stronger in the right channel which is what is leading me to eliminate the transformer suspicion as my next step.

I am also not completely sure that it is at 60Hz. Being a musician, I identified what pitch the hum carries and compared the note to what 60Hz is and it matches. Now that you've made me think about it, 60Hz is a really low tone so maybe it is 120Hz. Doubling a frequency raises the note an octave, so it will still have the same pitch. I'll check that out.

I am hoping to get some time to work on it this weekend. I've got to finish up the taxes. (When us self employed types owe money every year, we wait until the 11th hour to pay it.)

thanks again,
ed