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EHHA Rev A - Interest Thread - Page 104

post #1546 of 1733
Well, that certainly is not expected.

What does your bias measure on each side?
post #1547 of 1733

I also had a channel on a Rev.A that had high offset. It turned out to be a bad solder joint on a resistor, which I believe was R12, so you might recheck

all of your joints. The bad one on mine looked perfectly fine until I pushed down on the top of the resistor, when I discovered that the solder didn't adhere

to the board foil. Sneaky little bugger.

post #1548 of 1733

The problem is coming from the tube itself - swapping the tubes

brings the high offset to the other board and vice versa.

 

Bias is ~200mV each side after warming up.

 

 

Alex says that this is within the normal behavior.

The servo has to be that slow.

 

 

Well, seems that I will have to get an e12 and stuff it into the

already packed case :-( (There's room left, but wiring is tricky)

 

What brings me to another question - why do you use two e12s for the Ehha?

The relay can delay up to four channels - so one board should be sufficient?

 

Nonetheless - I'm happy that the amp lives now :-D

post #1549 of 1733

I'm back with another problem.

 

I have now wired the inputs, the pot and the headphone jack.

 

 

It plays music without any flaw - but I have a very loud hum one the left

and a more quiet one on the right channel.

 

I used wiring method one and all the signal carrying wires all fully shielded.

 

 

I'll post a picture of the inside of the amp tomorrow.

post #1550 of 1733
You do not have to use an e12. You could just let the amp warm up and the bias settle before plugging in the headphones. I would advise you do that anyway, even with an e12.

You need two e12 as the EHHA rev A boards have independent power supplies. One e12 for each supply. You could get by with one, but, it is not an optimal solution. There is a complete discussion on this topic earlier in the thread. I would suggest you search it out and read the various opinions.
post #1551 of 1733

Hi Sathimas,

 

the hum problem you are describing most possibly is a grounding issue... like a ground loop because you have two ground points (probably not) or something else.

In one of my builds I had hum because I attached the shield of the input wiring on both sides... duh. Took me forever to find it (glad it's years ago ;)).

 

Anyway good to hear that your EHHA is playing music now!

post #1552 of 1733

Concerning the offset - of course I could always disconnect phones.

I'll do that anyway.

 

But I also use the EHHA as PreAmp for my active speakers 

AND I am not the only one using it, my wife has a bad influance

on technical gear ;-)

 

So it's just to be sure - and to have the delay.

I also understand why I should use to of them after some reading,

but as you see, the case is more than packed and two would not

really fit. (The small sheet of paper is about the size of an e12)

 

 

 

Concerning the hum:

 

First, a picture:

 

 

I have one star ground, all ground connections got to that point

and from there another wire goes to the ground connector on the AC input.

(Yellow lines ;-)  )

 

If I disconnect the wire from SG to the AC-inlet, the hum is still there.

 

 

Input ground is isolated from the chassis (using neutrik jacks) and just

conncted to the pot (and SG on the boards from there.)

 

The output wiring is just temporary.

 

 

Any idea where to start from?


Edited by Sathimas - 12/11/13 at 1:49am
post #1553 of 1733

Hi;

I have 4 possible suggestions.  One of your RCA's is very close to one of your transformers.  You mention that one channel has a louder hum than the other.  Is that the channel?  Your on/off switch wiring runs very close to one of the amp boards.   Again, is that the channel with the loudest hum?  On mine, I used an IEC connector with a switch built in to avoid running AC wiring in what is a rather confined enclosure.  I can't really tell from your photo, but you also might try running a ground wire from one of the screws on your volume pot to ground.  Seems that a number of people had issues with the pot causing hum. Lastly, I ran all my circuit board grounds through a ground loop breaker and then to chassis ground.  Mains ground is attached directly to the chassis.

 

My build had a very faint hum on both channels with one being slightly louder than the other.  Turned out to be a ground loop problem. 

Hope this helps and good luck!

Jim

post #1554 of 1733
Muskyhuntr beat me to it with his suggestions... wink.gif
I had the same thought about the proximity of the RCA and especially the wire from the IEC inlet to the switch...
try carefully wiggeling that wire around and see whether something changes.

My EHHA RevA is still on the breadboard but exhibits zero hum... and my wiring is less than perfect on a breadboard.

BTW... this is going to be a beauty!
post #1555 of 1733
I have 4 possible suggestions. One of your RCA's is very close to one of your transformers. You mention that one channel has a louder hum than the other. Is that the channel?

- The two left RCAs are not connected at the moment, they're supposed to be the preamp outputs.
- Can't be the cause of the hum at the moment

Your on/off switch wiring runs very close to one of the amp boards. Again, is that the channel with the loudest hum?

- yes, it is, I'll try if the hum goes away when I put the wires further away from the board.
- but still there's the hum (more quiet) on the other board

On mine, I used an IEC connector with a switch built in to avoid running AC wiring in what is a rather confined enclosure.

- Having the switch on the backside would be very impractical. let's see...

I can't really tell from your photo, but you also might try running a ground wire from one of the screws on your volume pot to ground.
Seems that a number of people had issues with the pot causing hum.

- I'll try that too.

Lastly, I ran all my circuit board grounds through a ground loop breaker and then to chassis ground. Mains ground is attached directly to the chassis.
post #1556 of 1733
News from Germany:

Pot screw connected to (input-) ground:

- no difference

Used another power switch and put the ac wires out of the case,
as far away as possible:

- no difference

There's nothing more I can test today, gotta go to bed.
post #1557 of 1733
Quote:
Originally Posted by Sathimas View Post
 

(edited) It plays music without any flaw - but I have a very loud hum on the left

and a more quiet (hum) on the right channel.

Hate to tell you this but I'd bet good money its the proximity of those transformers to the amp.  Especially with the low-level signal/tube socket extension wires (acting like antennas).

Lots of examples of hum problems happening w/various DIY headamps when transformers are very close to amp channels.  Google & you will find them. 

 

E.g. If the LEFT channel board is the LEFT one in the picture, see how those 2 transformers are "ganging" up on it?  Perhaps their overlapping electromagnetic fields are bathing the left channel & feeding your tube-extensions/antennas.  In less extreme cases, rotating the transformers to try to lessen hum is something to try.

 

Switching to expensive, possibly larger shielded & encapsulated transformers could help some, as could mu metal shields perhaps BUT nothing works like physical distance.  It's why I transitioned to 2-box builds soon after (re-)starting audio DIY years ago.


Edited by cfcubed - 12/11/13 at 5:07pm
post #1558 of 1733
cfcubed,

I absolutely respect your point about the proximity of the transformers to the circuit possibly causing the hum...but I also somehow doubt that it would result in the strong hum Sathimas describes. Those are quality toroid transformers and not a microwave oven generating a massive magnetic field (just taking this as an example wink.gif ).

I have a hard time believing that two toroids would interact with the circuit/wiring that badly... when tons of other builds and commercial gear exhibit the same proximity of the transformer/s. Even in DAC's it is very common to have the toroid right on the board, and there we are much more into RF/HF territory than in a headphone amplifier.
I looked up a few random images of audio equipment with toroidal transformers (including my own EHHA, a Lehmann Black Cube, Stella DAC etc.), and while for some reason I don't get the option to attach the images here at my workstation (Java??) they all show the transformer sitting close to the amplifier section or even digital section! Of course I might be wrong, but I still suspect another cause of the hum problem.

Sathimas, how are your tube heater psu’s grounded?
post #1559 of 1733
Quote:
Originally Posted by stixx View Post

I absolutely respect your point about the proximity of the transformers to the circuit possibly causing the hum...but I also somehow doubt that it would result in the strong hum Sathimas describes. Those are quality toroid transformers and not a microwave oven generating a massive magnetic field (just taking this as an example wink.gif ).

I have a hard time believing that two toroids would interact with the circuit/wiring that badly... when tons of other builds and commercial gear exhibit the same proximity of the transformer/s. Even in DAC's it is very common to have the toroid right on the board, and there we are much more into RF/HF territory than in a headphone amplifier.
I looked up a few random images of audio equipment with toroidal transformers (including my own EHHA, a Lehmann Black Cube, Stella DAC etc.), and while for some reason I don't get the option to attach the images here at my workstation (Java??) they all show the transformer sitting close to the amplifier section or even digital section! Of course I might be wrong, but I still suspect another cause of the hum problem.

Sathimas, how are your tube heater psu’s grounded?

 

OK, I'll bud out.  What you say about commercial gear is spot on.  My post was more anecdotal based on my own experiences (CK²III, β22s & my EHHA) & those of others that have posted fairly frequently, e.g.:

http://www.head-fi.org/t/406258/mistakes-you-made-with-your-22-or-22/15

And I don't see how your suggestions + review of his build picture otherwise explain the large difference in hum heard between his left & right channels.

Also, I've found it fun to listen to hum level differences in my DIY amps on the bench as I move/rotate their (unshielded, non-encapsulated) toroidal transformers about them.  Esp. noticeable w/high-sensitivity cans.

But carry on, I really need to stop posting here:)


Edited by cfcubed - 12/12/13 at 7:47am
post #1560 of 1733
Tube heater ps are wired directly to star ground, as you see in the picture.
(Yellow lines mark ground wires)

I'll try getting the torroids further away with some improvised cable extensions anyway.
Getting a custom wound encapsulated torroid also wouldn't be that expensive.

Main Problem is size...
And having the torroid in an extra place would require a new power inlet in the back.

But - we'll see.
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