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SOHA II Builders Thread - Page 106

post #1576 of 1669

Glad that it worked out for you.

 

I have stuffed all resistors that I can stuff last night. That was the greatest accomplishment since I started. biggrin.gif  I think I am going to stop here till I got more parts and some questions answered. If you can, I posted them here:

 

http://www.head-fi.org/t/390553/the-soha-ii-tube-thread/90#post_8231418

 

If you know the theoretical answer to the question, you can help me by measuring the voltage across those resistor. However, I don't want you to do it if you are not totally confident about it or have the right equipment. I said that because I blew out things before while trying to measure a live working amp.

post #1577 of 1669

I can't be certain about this, but:

R6 should be fine. I'd go for 1/4W R4 tho.

Regal's post mentions 2.75mA current through each tube section. Now, if that's the current through each triode, you'd blow R4 because ((2.75mA)^2)*18k=0.136W.

 

If you don't trust my answer, I can check the voltages later with my DMM.

I wouldn't count on the equipment I have at home, but at least I got my SOHA II measured.  That damn multi-meter is a bit moody sometimes. Cheapest money can buy :D

 

EDIT: just noticed the resistor would be 9k. Go for it if you trust me.


Edited by Veiko - 3/17/12 at 1:38pm
post #1578 of 1669

Thanks, I think your assumption made sense.

 

Btw, are you using the kit from Glass Jar Audio? What tubes are you using?

post #1579 of 1669

No, Jeff emailed me the corrected BOM and I got most parts from Mouser. That way I got fast, free shipping and didn't have to deal with customs.

The tubes are from a local collector, got a matched pair for about $12. I think he should have asked more for them biggrin.gif

They're RCA 12AU7 (NOS, made in Mexico).

I'm also going to try 6922 as soon as I can afford a nice pair.

post #1580 of 1669

Is there supposed to be a connection between chassis ground and input ground? If not, my inputs aren't properly insulated.

I tried to listen through my phone with usb connected, and the noise was back. Mostly digital sounding noise, though.

post #1581 of 1669

If you are housing the transformer in the same chassis, the chassis should be grounded to the mains ground for safety. You can not connect the signal ground to the mains ground or you would get ground loop. On the circuit board there is provision for star ground, which is to prevent ground loop while also shielding the circuits.

 

http://www.cavalliaudio.com/diy/soha%20ii/main.php?page=wiring/boardwiring

 

What do you mean by the corrected BOM? Is there an uncorrected ones out there? (just in case I got the wrong BOM)

post #1582 of 1669

The safety ground is connected and I'm using the standoff grounding method, so, that should be ok.

Both ends of my input cable are connected, however. Is this my problem? Should the shield only be connected to the input terminal?

 

Looks like I'm not allowed to attach the file here but where did you get your BOM? The one on Cavalli's website is wrong (according to Jeff from Glass Jar Audio).

post #1583 of 1669

Thanks for letting me know, I have to dig out the BOM from GJA some where.

 

I don't quite understand what you mean on the grounding. Are you using isolated RCA and TRS jacks?

post #1584 of 1669

Or you can PM me your email address and I can send you the file if you want.

 

I'm using isolated RCA jacks (although there's a connection between chassis and RCA ground, no idea if there's supposed to be).

I mean I have both ends of the shielding connected. In guitar amps you'd typically only connect one end. Should I only have the tips of the RCA jacks connected?

 

I think I may have a ground loop because of the input wiring, but there's no hum with a battery powered source.

post #1585 of 1669
With RCA jacks, the outer conductor is the ground connection, so it has to be connected to close the circuit.

With isolated RCA jacks (and TRS too), there is an insulating washer that isolate the ground contact from the chassis. So yeah, I think that is where your problem is. Without the insulating washer the jacks would not be isolated.

I should have the BOM since I ordered from Jeff. Thanks. :-)
post #1586 of 1669

Looks like the hum disappears when I let the amp warm up for a few minutes. I still have some digital noise left when listening through USB, but that's not the amps fault.

 

I also desoldered the RCA jacks and there was still a ground connection, so, guess it was fine after all.

 

Here's some test results if you're interested http://www.sendspace.com/file/epqrmd

I doubt anyone else is going to read this thread biggrin.gif

 

EDIT: for those who can't open my link in the future, I measured a THD+N of about 0.22%, quite steady through the whole audio range (Mostly noise). Amplification @1kHz was 8.2dB, with a 0.5 decibel difference between channels.

Test were run with RCA 12AU7 tubes, not exact pair but close enough.

I could probably get rid of the level difference, but there's really no point since I wouldn't notice a difference below 1dB.


Edited by Veiko - 3/23/12 at 11:36am
post #1587 of 1669

What do you mean by you desoldered the RCA jacks? I assume you are talking about the input RCA jacks?

 

You are probably right that nobody is reading this. biggrin.gif Sadly most of these DIY design seems quickly forgotten. From what I see only the very affordable ones (like CMOY) or the very high end ones remains popular.

 

I was trying to spot the difference between the BOM from Jeff and the one on the website. So far I spotted one difference, I am not sure how significant though. For C6L/R it is "Metalized Polyester/Polypropylene Film Capacitor 1μ 63V" on the website and "Boxed Metal Film Capacitors 63V 1.0μf 5%" on the Jeff's BOM. Does it matter to use "boxed metal film capacitor" over the Wima I have, which is a metalised polypropylene capacitor? It is interesting that on Jeff's Excel sheet it is listed separately from C2L/R, which are the same value as C6L/R but is a metalised polypropylene capacitor (Wima).

 

Edit: In Jeff's Excel sheet, the C6L/R Mouser part number links to a polyester film capacitor, which from what I understand, are similar (if not slightly inferior) to polypropylene film capacitor. I wonder why Jeff do this?


Edited by Navyblue - 3/24/12 at 11:45am
post #1588 of 1669

Yes I meant the input jacks, their insulation was fine.

 

It's sad really that people forget good designs like this. I haven't really compared my SOHA II to many amps, but it sounds excellent to my ears.

I'll most likely build even a better headphone amp after a diy DAC and a self designed headphone amp. And a couple of guitar amps. At least I know where all my extra cash is going this year biggrin.gif

 

Interesting, Jeff told me there are actual errors (plural) on the SOHA II web page. 

I'm quite sure all my non-electrolytics are Wima so you should be fine with them too.

Some of the parts on Jeff's BOM were out of stock at the time I ordered from Mouser and I never updated the BOM myself, but I think I can find out what I'm using if you want to know something specific.

post #1589 of 1669

Hey All,

 

I finally (after a couple of years) have put together my Soha II. Powered up and getting the same voltages as on the site. Running into a snag though in the buffer section of setup. Everything worked great on the left side, but I'm getting 0VDC when I strap R10R. I've replaced Q8R and Q7R and nothing changed. P2R seems to work fine, so I'm kinda not sure what the problem is - anyone have some ideas?

 

Thanks in advance

post #1590 of 1669

Hi Pixeljedi,

can you tell me the voltage on the collector of Q7? Also, can you somehow verify that your servo opamp is the correct one and working properly?

Have you checked for short circuits and cold solder joints?

I barely understand the circuit, but i don't think anyone else is going to try to help you here.

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