Pre-Flight Check - Funny Ohm-it-out

for the DIY ST35, the Dynakit and every other PP EL84

Re: Pre-Flight Check - Funny Ohm-it-out

Postby 20to20 » Wed Nov 19, 2014 7:34 pm

Fishstix wrote:
20, so what is your bead on grounding transformer bells? What would that accomplish? It was a night and day difference in my last amp, but I know grounding schemes are not all the same.

(And again, thanks for the help guys. I *really* appreciate it.)


The PT might need it but the OPTs shouldn't require it. I have a PT with an internal shield and ground lead that really made a difference in hum level when I connected it to the chassis ground. But that is kind of expected from a PT. Grounding the bells on the PT could act as a shielding ground, too. Most of my other amps are bare metal chassis so the trannys get grounded by default.

NICE BOX! Simple but artistic. Kind of Japanese looking.

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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Thu Nov 20, 2014 5:16 pm

Thanks, 20!

The last amp I built was a behemoth; took up a silly-large chunk of the AV cabinet. I wanted this to be as lithe and light looking as possible. The caps actually stick well below the wood on the underside, but the amp doesn't sit at eye-level, so nobody can see them. The legs were just the best I could do with what I had on hand. The proportions were okay, I guess, it still doesn't sit too tall. I was *really* happy to see that Front Panel Express could infill with the same RAL color as the Edcor transformers. Matching the wood to the plate to the transformers would have been hard enough. I had a little purpleheart in the scrap pile, just enough to build the surround, and I think it works well enough with Edcor 'Gentian Blue.'

I am going to have some free time next week, I'll probably sit down and tear things apart so I can ground the PT. I want to clean a few things up on the underside anyways, it will be a good time for it. With some more reading this morning, it seems a fair few people have reported capacitive hum between tranny and chassis. Also that connecting the two can't do any harm, and if nothing else makes the tranny safer if god forbid a wire should come loose. Thanks for the feedback, I'll report my findings...

Cheers!
-Fishstix
Technics Sl-Q30 / RuneAudio -> Kenwood Basic C2 -> Parks Audio ST-35 -> KLF 10
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Shannon Parks » Fri Nov 21, 2014 9:56 am

OK, I like your safety ground and PCB ground pic.

1) Does the hum go away with a cheater plug?
2) Does the hum go away with shorted inputs?
3) What if you disconnect the input cables and just put some jumper wires at J6 & J7?

Your PT lams are aligned with your output lams which is bad, but they are so far apart there should be no issues with induced hum from fields. They'd have to be probably just 2" or less from experiments I've done.

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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Wed Nov 26, 2014 10:40 pm

Hey Shannon,

1) Cheater plug doesn't help; no difference between that and a grounded cord.
2) The hum..... *does* go away with shorted RCAs. Dead quiet then. No music of course, but I assume we are getting somewhere? I haven't pulled the board to get to the input jumpers (they're between the PCB and chassis) but I assume that we are on the right track? I did some Googling for answers and I can't seem to find consensus on Hum/Shorted RCAs topic. Or I don't understand that there is consensus. It could be that.

I built an S-5 for a friend and spent some time playing with the transformers; left the leads long and just shoved them around while the amp was on. I had to get them pretty close to get anything to buzz. Glad to hear you are okay (ish) with the PT placement on this rig. I worried a little bit when I ordered the top plate from FPE, and again when I put the rig together and got hum. I tried putting bread tins over the top of each, and it didn't do anything to squelch the hum. Clearly, though, I am not a purist. At this point it's a battle between the best my ears can appreciate, and what I am looking at on the shelf. My ears aren't amazing, but neither are my eyes, so it's a lazy sort of a battle. As with everything, if I were to build for a friend, I would be shooting for perfection.

Cheers!
-Fishstix
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby 20to20 » Thu Nov 27, 2014 9:39 am

Stix,

Have you tested this away from your work bench, in a "quiet" (electronically) place? This seems to be a case of high sensitivity (input impedance) and environmental noise, if not from the PT being close to your J-6, 7 connectors which might be getting a high radiation dose, from above. Since it's quiet with the shorted jacks says it's input line/circuit related.

Then if you still have the same or less hum, try jumpering another 470K resistor in parallel with the existing input grid resistor to lower the total to 220K and see if that's enough to kill the hum.

I think it's all related to your input line shielding and J-6, 7, connection and the high gain of the 12ax7's and maybe your PT that radiates a hefty bit. Mechanical connectors like little jacks are notorious for not having perfect connections, too.

Have you eliminated the possibility of microphonics? Loosen the PT mounting bolts to see if it changes the hum level.

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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Tue Dec 02, 2014 12:48 am

FIXED!

Spot on, guys. I diverted the input jumpers out the front of the chassis on twisted wires (rather than instrument cable) and then connected it round the back with longer RCA cords. The hum completely dissapeared, I mean completely dead quiet even with every buzzy awful thing in the house plugged in. In fact, I connected all the power back via two-pin in the back of my pre-amp and it still sounds *great* with an almost inaudible hum. So it looks like nearly all of the hum was induced from the PT. I still need to figure out how to reconnect the RCA jacks to the jumpers, maybe run twisted wire directly away from the PT, then out beneath the output transformer jumper. Or even all the way around the margin of the chassis tucked up near the plate. Should be a balance between shortest circuit path and keeping it away from radiation. Maybe a non-grounded conductive jacket around that as well?

I had *no idea* the PT could induce *so much* voltage into the cables. It was completely unlistenable at it's worst, and I understand why Dynaco put their input jacks at the front in their models. Next time I do one of these I'll be putting the RCA jacks on the very outside (thought still rear) of the chassis with the banana jacks towards center. And likely pivoting the PT 90 degrees for good measure (thanks for the reminder, Shannon! (b) ) At least I have a benchmark for what this should sound like now, if my mods don't get enough mileage off rerouting the input leads, I can pretty easily and quickly slap together a new wooden surround with better interconnect placement. It's built like I do picture frames, not challenging.

And it sounds GOOOOOOD. [:) Bass is much tighter than I am used to, and I am no longer using the Pre-amp equalizer to compensate; it's lower and louder. Just *Brilliant.* Everything is much cleaner/crisper than I've heard before. I know what people mean when they say 'muddy' now. The top end rings less, which is great since KLFs are already tinny in the upper end. Grand improvement there. My Kenwood Basic C2 plays very well with it; I can get three quarters out of the volume pot before the room becomes too small for the decibels, which is just fine with me. Also, the speakers start doing funny things much beyond that. So, good balance all around. Tonight everything else is getting turned off so I can spin vinyl and melt into the sofa. I'll go back for a more permanent on the input leads sometime next week, just so things are cleaned up.

Since getting into this hobby a few years back I've given considerable research and thought into what I wanted my 'permanent' amp to be, and spiraled closer and closer to Dynaclones, especially the "Baby" Model. It's perfectly sized for my needs, there is lots of room to fiddle and learn and it has a great track record for just sounding *great.* There are so many options out there; rehabbing old ones, buying rehabbed old ones $) doing point to point, and the variety of available PCBs out there. This is a SaWeeeeet board you've put together, Shannon. It's made both the price point and technical aspects accessible to relatively new hobbyists like me. Thanks for helping knocking an item off the bucket-list. It's a short list, but I place real importance to these things.

Thank you *both* very much for your patience and help. The body of knowledge out there is impressive, but I am always more taken with how generous people are with offering their knowledge and time. With your help we've got another gorgeous sounding ST-35 off the ground.

All the Very Best,
-Fishstix
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby 20to20 » Tue Dec 02, 2014 8:43 am

High 5!

PT can radiate quite far. I had a hum come in from a MM phono cartridge well shielded, grounded etc., on a TT about a 1 1/2' from an amp. When I shifted the position of the TT it went away. But a cartridge circuit is VERY sensitive so it makes sense. A normal line in isn't quite so... but as you found it can be a gremlin and line/driver circuits should be planned far away from the PT as possible, as well as your phono pre and TT. (b) (b)

All part of the "fun." (sick) (sick) But it's Sweeeeeet, when a problem gets solved. (wine)

This board needs a dancing bannana!

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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Shannon Parks » Wed Dec 03, 2014 9:54 am

Dancing banana, huh? I agree. This board needs a smilie update for the New Year.

Great that the problem was fixed, and enjoy! What cables did you use? Were they shielded at all? In the pictures they looked pretty stout. I just use single conductor shielded wire and use the shield as the ground path - never have a problem.

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Re: Pre-Flight Check - Funny Ohm-it-out

Postby 20to20 » Wed Dec 03, 2014 12:28 pm

Shannon Parks wrote:Dancing banana, huh? I agree. This board needs a smilie update for the New Year.

Shannon



Wooohooo!
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Shannon Parks » Thu Dec 04, 2014 9:16 am

20to20 wrote:
Shannon Parks wrote:Dancing banana, huh? I agree. This board needs a smilie update for the New Year.

Shannon



Wooohooo!


(banana) (banana) (banana) (banana) (banana)
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby Fishstix » Fri Dec 05, 2014 2:06 pm

(lol) That bank of smilies, and one lone dancing banana. That's *great!* Here's to 2015. (d)

The previous cables were Mogami W2524, 90+% shielding, as I recall. Nice stuff if you are ever looking to put together RCA cables. VERY impressed with how the shielding lays, it's super well packed. Whatever the copper grade is, it wetted out really nicely when soldering. Great joints very easily. I guess having them too close to the PT and running over the top of the PCB was enough, though. In retrospect, the shielding is part of the circuit, maybe I should have have expected something like this.

20, that's just wild! Foot and a half! I think I am going to buy some MuMetal and build a jockstrap for when I need to use the dials. I could, of course, just build the MuMetal onto the amp, but that would be way less fashionable. ;)

It'll be a week or two before I get the chance to do a more permanent job of swapping out the input cables, I'll check back with you guys after I've figured something out. And photos to follow, of course.

Again, thanks for your help!
-Fishstix
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Re: Pre-Flight Check - Funny Ohm-it-out

Postby 20to20 » Fri Dec 05, 2014 5:01 pm

Fishstix wrote:
20, that's just wild! Foot and a half!

-Fishstix


It seemed to be cartridge specific with a vintage Technics TT I was using at the time. I have a bunch of headshells with different carts. so I can easily swap them in and out. I discovered the affect with it hooked up to a circa. 1975 SS Technics amp. One cart. was an AT and another was a Grado. Both on original Technics headshells. They were good cartridges, maybe too good, high output. And then I've had other TT/cartridge combinations that didn't care how close they were to the amp. Dead quiet... better internal shielding, perhaps.

Later! (banana)

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