It's BUILD Time!

a DIY, modified Mullard 5-20 monoblock design

You might not believe it, but...

Postby nineno » Thu May 11, 2006 5:41 pm

...I actually finished my pair of monoblocks!

Pictures to follow, but I do have a question for those of you that have completed these amps already.

Earlier in the "It's Build Time!" thread I asked a few questions about reducing the 60Hz humm back when the amplifiers were electronically complete, but without a chassis. I found that I hadn't properly grounded the transformers to the safety and signal ground. Once I did that the amp was silent at idle...

However, now that the amps are done the slight 60Hz humm is back. The metal top panel, and the transformers are grounded to the signal/safety ground. I've found that directly connecting the line-level input shield/ground directly to the amplified signal ground reduces the humm by about 50% at idle (on my chassis that requires about a 2-inch jumper wire, which short circuits the long leads that go to the shield/ground terminals on the PC board.

The other element that might be limiting me is that my input shield (that is connected to one of the ground plane terminals on the 11-position terminal strip) lays right on top of two of the input capacitors...could that my my 'humm antenna?'

I'll move the input shield lead away from the caps and report back.

To be fair, the humm is very minimal and only noticable at idle...but I'd prefer if it wasn't there at all. Yellow_Light_Colorz_PDT_17

Thanks!
drew*
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Postby WA4SWJ » Thu May 11, 2006 6:19 pm

Drew,

Congrats! Aren't they awesome?! Pictures would be great.

Shannon published a change to a few of the components a while back part of which reduced the overall gain some by increasing negative feedback. That might help with the hum a little by reducing the gain. Mine still hum a bit too but I have not made Shannon's suggested changes yet. I hope to do that soon. Like yours mine aren't too objectionable. I did notice that moving the external input cables around helped a bit too. I have just been enjoying them like they are.

Probably your best bet is making those component changes if you haven't already. The shielded cable would likely help too but it's hard to know.

Enjoy them!!!

Regards,
Ed Long
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Eiclone Hum

Postby Shannon Parks » Sun May 14, 2006 7:44 am

Hi drew* and Ed,

We should be able to get your Ikes pretty quiet. I'll throw one of my Ikes on the test bench this afternoon.

The important mod is step #1 here:
http://www.diytube.com/phpBB2/viewtopic.php?t=836

The mods here aren't as important. They just lower the gain to a more 'standard' gain level. So the noise floor isn't amplified as much.
http://www.diytube.com/phpBB2/viewtopic.php?t=891

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Re: Eiclone Hum

Postby Shannon Parks » Sun May 14, 2006 11:13 am

separks wrote:The important mod is step #1 here:
http://www.diytube.com/phpBB2/viewtopic.php?t=836


OK, I think I've got the magic fix - but I'm still testing. Instead of re-routing the input ground to the above mod (which does drop the noise in half), I've re-routed the input ground to the base of the LTP. Noise drops below 500uV at that point and the SNR measurements look great. Right now I'm just experimenting with inulated/non-insulated RCA jacks and ground straps for the definitive fix.

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Postby WA4SWJ » Sun May 14, 2006 12:39 pm

Shannon,

Cool! I have not put any mods into my Eiclones yet except I did do the input cable ground connection change. Helped, but not enough. I'll work on them when I get moved.

I'm anxious to see your results.
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Postby nineno » Sun May 14, 2006 1:03 pm

Thanks for the replies guys.

Shannon, I'm anxious to see what the results are with the various mods. I'll probably hold off on the component swaps until I have the noise reduced as much as possible with out un-soldering, and then I'll go that route. After all, I just go these guys together...The thought of taking them apart is daunting. Ha-Ha.

Just to verify: The LTP that you'll hooking your input sheild up to is the long tail pair, right? Where would I tie into that?

Here are some links to pictures:
Twins http://i66.photobucket.com/albums/h255/nineno/IMG_0017.jpg
Single http://i66.photobucket.com/albums/h255/nineno/IMG_0021.jpg
Aluminum back panel http://i66.photobucket.com/albums/h255/nineno/IMG_0018.jpg
Neutrik jacks plugged in http://i66.photobucket.com/albums/h255/nineno/CompletedIkes001.jpg
Wiring routes (Notice LOOOONG input cable...oops) http://i66.photobucket.com/albums/h255/nineno/CompletedIkes003.jpg

I had some really big plans for the wooden chassis along the way but none of them really panned out. Eventually I just made a simple wooden frame and screwed the top panels down.

The power buttons are not hooked up yet because I'm still trying to sort out and appropriate latch circuit for them (turning a 5V momentary push button into a toggle switch for 120VAC). The buttons are really cool though. One lights up red and one lights up blue. Their made by Bulgin. Picture here: http://www.bulgin.co.uk/Products/Switches/switches_images/MPI1002_400px.jpg
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Postby Shannon Parks » Sun May 14, 2006 3:46 pm

nineno wrote:Just to verify: The LTP that you'll hooking your input sheild up to is the long tail pair, right? Where would I tie into that?


Hi drew*,

You can simply remove your input ground from J4-3 and swing it over to the ground of R7. You can solder at the unused R52. For reference, the ground side of those resistor spots is towards the center of the PCB.

Seems my best noise performance comes with an insulated RCA, safety ground to the chassis, and no ground jumper from the PCB to the chassis. Adding the ground jumper or uninsulated RCA doubles the noise floor to 1mV.

Awesome photos, by the way. I'm very jealous! Yellow_Light_Colorz_PDT_21

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