VTA SP12 and PH12 Build Thread

Alright. I just let the smoke out of the diodes on my rectifier board. I think it's time to call it a night.

I wanted to read the current going through it...but I didn't do that correctly.

jjones - explain with a bit more detail where you took that measurement? I'm trying to translate that to a SP-8 that uses pretty much the same DC supply.

And I agree jd, just helped my 16year old change his timing belt. Wow cars are easy! And you can see everything without your cheater glasses on!!!

tubes4hifi wrote:thanks much for the info John, so you and I have both built this preamp and it turned out "dead quiet"

Below is what it looks like on the scope - channel 1 is right off the 12x4 (5v/div) and channel 2 is after the CLC filter (50mv/div). Around 6Vpp and 33mVpp respectively. This is the CLC(SP12)LC(PH12) setup. And yes, the SP12 is dead quiet and the PH12 is dead quiet until step 23 on the volume where I get a little broadband noise.



If you need any other measurements let me know as (if my parts come in) I'll be changing the unit to use a shunt regulator on the B+ this weekend. When I do that I'll change the filter back to CRC and post the measurements of that configuration before I put the shunt in.

Tom wrote:jjones - explain with a bit more detail where you took that measurement? I'm trying to translate that to a SP-8 that uses pretty much the same DC supply.

I'm just measuring the B+ before and after the filter. The SP8 B+ supply is actually quite different than this version of the SP12 as the SP8 has regulated B+. The current version of the SP12 uses regulated B+, but the version Ed and I have does not.

Here's a copy of what I just sent to Ed, because it's all good general info . . . .

John's got better test equipment than I do, I'm jealous!!

When he first emailed me about hum, it was just slight hum in SP12 and major hum in PH12. So he measured the SP12 by itself (just remove the tubes from PH12, don't need to disconnect anything). He had 1.4v p-p AC ripple (that's about 500mv on an RMS meter).

I told him I would have guessed (I still haven't measured) that it should be about 20-40 mv.

He wound up doing some rewiring, and he had a problem with florescent light fixtures, and he cut it to under 100mv nearly right away. With the extra load of the PH12 I had recommended a choke between the SP12 and the PH12.

I don't know if you have any choke at all, but it would help. He wound up getting two of them, one to replace R17 on the SP12 and another between the SP12 and PH12, plus adding a couple more 33uF 400v caps.

On my newer version with regulation, the regulator does all that hard work. Regulators eat up about 60db (that's 1/1000) of ripple, so I suspect it's nearly unmeasureable at all, but certainly no more than about 5-10mv. John says he measures 33mv P-P which is about 10mv, so that's really great not using regulation. The chokes are about $15 each and the caps about $2-3 each, a regulator circuit is about $5.

John had posted a great photo on post 79 (page 6) of this topic,
here's a photo of my latest SP12 that I finished up about a month ago, perfectly quiet the very first time,
no problems, nice and clean inside, no chokes, just the same simple regulator circuit that I use on my SP8 and SP9 circuits.
latest SP12 photo

Just to keep anyone who might be interested posted, I am still slowly working on the debugging of this preamp.

Roy has been a great help, and I am learning more than I imagined from this experience. My frustration level hasn't really risen yet, so I am going to continue reading and consulting friends until I either figure it out or reach my wit's end.

I have always enjoyed hobbies like this. I've built PCs, model airplanes, websites and attacked other odd things that were definitely over my head at the onset, but eventually broadened my understanding of "how stuff works." For me, the challenges of debugging are always the most growth promoting.

I am confident that this will get figured out and that I will be all the better for it. If I start losing sleep over it I'll be shipping it to Roy and asking him to check it out. Until then I've still got two PAS-2 preamps that are serving me very well.

Am I eager to finish this project? Hell yes. If I send it away to get fixed now, there are some things that I won't learn for the next project...and I ain't in tears yet.

So I'll post to this thread as I have my experiments, failures and epiphanies, and I'll post my final review of this unit when it is debugged.
(In other words, I'm still being stubborn. This is DIY, after all.)

Peace.
Ed

tubes4hifi wrote:John had posted a great photo on post 79 (page 6) of this topic,
here's a photo of my latest SP12 that I finished up about a month ago, perfectly quiet the very first time,
no problems, nice and clean inside, no chokes, just the same simple regulator circuit that I use on my SP8 and SP9 circuits.
latest SP12 photo

Thanks, Roy. Have you built or do you have a picture of an Aikido SP12 with no chokes or other modifications?

I will review my entire build and then reread all of this forum thread and then try a couple of things. I may then do what John did with chokes and such. They're not very expensive.

"Just to keep anyone who might be interested posted, I am still slowly working on the debugging of this preamp."
Are you kidding? I check in here every day, just to see if you've figured it out.

"My frustration level hasn't really risen yet, so I am going to continue reading and consulting friends until I either figure it out or reach my wit's end."
Go outside and kick a box around ( cardboard only, wooden box just makes you angrier)!

I don't think this would be your problem, but on my Mk4s I had a terrible noise coming out of one speaker. Turned out the RCA ground lug was touching the chassis. Well in the interest of full disclosure, I mounted it UNDER the nylon washer by mistake It needed to make contact with the board ground only. FWIW
Gordy

JD, funny, you are asking if I have a photo of an SP12 with no choke or mods, but you're including the link to it !
Or maybe you meant an original old style AIKIDO, well, I've have to go back thru 2-3 years worth of old photos, but certainly yes !
Most of the good stuff was on an older computer I don't use anymore, I'll check it.
Ken McGuire has the best photos of an AIKIDO he built from my kit about a year ago, I added blue LEDs under all the tubes.
Here's a couple old photos, there's not much to see, it's really a simple circuit . . .

I undid the damage that I did to the rectifier board....Turns out that I blew up the resistors so they have been replaced.

My next step was to replace all the HV caps, because it is acting as if they aren't even there. Of course I was stymied by the inventory at the local Radio Shack. Everyone at the Radio Shack was very eager to sell me a new cell phone, however. Next stop, Fry's.

If the capacitor replacement does not make any difference, I'll be adding chokes. You can marvel as my knowledge of Henries exponentially increases.

My B+ is dirtier than your B+,
Ed

Caps replaced. No difference made. I've got one of those cool USB oscilloscopes on its way. Then I can learn some more.

Can I ask which type of USB scope you are buying? I am interested in buying a scope and am just educating myself on the merits of the various types....

Thanks...

Any further updates on this thread? Perspiring minds want to know. That build looked so impressive.

Sorry for the radio silence, but for a couple weeks there I was almost certain that I was picking up my family and moving to another state. Real estate agents and new paint everywhere...but then it all fell through, so never mind. He we are, safe and sound, back in good 'ol 1955.

SO.................

Yesterday I spent about an hour with a friend of mine who is a bench tech with a lot of experience with guitar and bass tube gear. We fired this baby up, and as expected the hum was the same in his shop.

We hooked it up to his scope and first checked the output signals. Signal generator in and the output signals on both channels were perfectly symmetrical and identical. They lined up perfectly with the attenuators doing exactly what they were supposed to do.

We then looked at the B+. On the scope, the AC in the power was negligible; we had to really zoom in to see it at all. So we checked it with his real DMM and at the center B+ tap, there was only about 45 mVAC in there. Which is where it should be.

These tests pretty much showed us that the circuit is working the way that it should and that the channels are each equal. Also it says that my B+ is CLEAN! I suspected my auto-ranging Radio Shack DMM was freaking out on the AC readings, and putting it on a meter that cost 10X as much seemed more reliable.

So what am I thinking now??? Although those toroid transformers have been great in the past, I am wondering if I got the defective one. Even if there has never been one that has caused EMI, you can count on me to find the one that does. So from here, I’ll buy a new $35 PT. If I have the same problem it is hereby eliminated from the suspect list.

What really makes me think is that when I make an aluminum foil shield and place it between the transformer and the line stage PCB, the hum is substantially reduced. Antek also sells a shield that might fit into the equation after the trafo experiment.

After messing with those things, I think that I’m out of ideas. If I am still stumped after all that, I’m going to enlist the help of the master and ship this thing to the lovely Pacific Northwest for professional therapy.

So that’s where things are as of Memorial Day weekend.

Best,
Ed

PS MERLOT - I was looking at a Hantek USB Scope. (eBay item 150444755131) Roy warned me about some possible limits on the level of DC power that it can handle, so I aborted the mission pending more research.

Sorry it has been so long since my last update, but there has been nothing to report. The summer is the busiest time for my business and I got a little distracted this month.

I have however bitten the bullet, admitted some defeat and sent this thing off to Roy. I am excited and nervous to see what exactly is wrong with the thing.

Roy should receive the package tomorrow and we'll see what happens from there.

I hope that everyone is experiencing listening bliss.

Best,
Ed

one of my recent customers posted a review on Audio Asylum, I didn't even know about this until I got an email from someone else
http://www.audioasylum.com/forums/amp/messages/16/166938.html
also photos of another recent SP12 preamp here:
http://www.tubes4hifi.com/DP.htm

OK, Ed's preamp is back and working perfectly! He should have it back in his hands on the 15th (July).
I know lots of people were reading this thread and anticipating the finale.
Ed sent me his preamp last weekend and I took a look at it.
First very obvious thing was that the input/output ground buss was NOT connected to AC ground.
Doing that eliminated 90% of the noise immediately. (see my post #54 in this thread).
There was just a small amount of residual hum left in the right channel only. Well, the right channel is right next to the transformer, and as Ed mentioned before, he had an unshielded transformer. (Most all of these SP12 kits shipped since late February included a shielded transfomer). In addition and maybe more importantly, all of the AC wiring of the transformer was clumped around right where the output jacks are. After considering all the possibilities, the best solution was to re-orient the PCB, that would solve BOTH problems.
After doing that 99% more of the remaining noise was gone. It is now as quiet as any tube preamp.
At FULL volume, with no music playing, I can just barely hear a tiny tiny amount of noise on right channel only
if I am less than 1 foot away from the speaker.
Then onto the phono preamp, which turned out to be very noisy and no gain at all, turns out the JFETs were dead,
my best guess is they got toasted when soldered onto the PCB, as they are very heat sensitive. I replaced those,
and the phono preamp now worked perfectly and was SUPER DEAD QUIET, adding ZERO noise to the overall preamp signal.
Along with the usual service work to get it all working, I replaced the tube rectifier board with a newer upgrade, and installed a power supply choke inline with the phono pre, shorted up most all of the wiring and tightend up the twisted pairs.
So hopefully we will hear back from Ed in a week or two.
Meanwhile, there are photos posted of the rework here:
Ed's preamp service

actually most references to preamp hum problems recommend connecting the signal and power ground to AC earth ground thru a 10 ohm or sometimes 20 ohm resistor rather than directly, there was an earlier post in this thread to a thread on the DIYAudio site, and also a link to this article which is excellent:
http://sound.westhost.com/earthing.htm
Since I've got Ed's preamp until Monday, I'll give that another try, last time I tried it a few months ago it didn't seem to matter whether it was a direct connection or thru a small resistor, but I've got my ears well calibrated for listening to hum issues right now so I'll give it a try.
I think that is where some of the confusion lay back around post #61 in this thread, whether the ground should be isolated or tied to earth.

tubes4hifi wrote:OK, Ed's preamp is back and working perfectly! He should have it back in his hands on the 15th (July).
I know lots of people were reading this thread and anticipating the finale.[/url]

Bravo........ Bravo

Thanks Roy for the excellent update as
I think a bunch of us have been following this one.

Could you elaborate on "tightening up the twisted pairs"?
Are more turns better?

Tom wrote:Thanks Roy for the excellent update as
I think a bunch of us have been following this one.

Could you elaborate on "tightening up the twisted pairs"?
Are more turns better?

Roy, could you also elaborate on the extra choke and 10uf cap? Is the choke there because of no B+ regulation in Ed's preamp? Should I add the 10uf cap to my PH12?

Pete

yes, more turns are better! gives better shielding and less chance of noise pickup, see these photos,
I used to say use twisted pairs, and people would do like the lower pair, so now I say "tightly twisted"
like the upper pair, and this only takes about 10 seconds using an electric drill.

the choke isn't "needed" but a choke tends to take out the very last bit of "crap" (sorry, that's a "technical term" ) in the power supply, which is especially important for the power to the phono board since it's gonna amplify everything another 45db or so. I would definitely recommend adding the extra capacitance, on one of these I did it made a huge difference in the noise level.

Roy, thanks for the PH12 cap info. I will install per your photo. Can you also verify that the choke is installed inline with the B+ line going from the SP9 board to the PH12 board?

Thanks

Pete

yes, I put the choke inline between the line stage B+ and the phono B+, and then add the extra cap right on the phono board, either underneath or on a nearby terminal strip.
Ed should have his preamp back by now, but I haven't heard any word from him yet!