Crack with some issues!

[pavlov]

Hey guys,
Just finished building my first tube amp.
It plays fine but I have some troubles with a buzz in the right channel.
The buzz isn't really noticeable at low volume but it gets louder when turning the volume knob. However it disappears the moment the potentiometer is on its loudest setting. I'm new to tube and am not able to resolve the problem myself,nor could I find an answer on the forum. Is it a problem with the potentiometer? Is it a grounding problem? I've checked my joints and they seem fine.

Thanks you for helping me!
Roger

Some pictures: (it was my first attempt and I'm afraid I haven't done such a clean job)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimg37.imageshack.us%2Fimg37%2F1645%2Fdsc1912x.jpg&hash=20c9a6510788633fde3e0cf9bcb2789ae8fbc8b7)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimg18.imageshack.us%2Fimg18%2F9421%2Fnaamloos2zm.jpg&hash=4fcb4498c7caecdbcb0d2ee28040bc1c2fcbe479)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimg696.imageshack.us%2Fimg696%2F1856%2Fdsc1924o.jpg&hash=91facc4f046c7981adbd5a5108bdd315a6e599c1)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimg32.imageshack.us%2Fimg32%2F8514%2Fdsc1935h.jpg&hash=f00048dfc2792bdb0d886480103bbd843a145cea)

[Jim R.]

Roger,

Are the RCA inputs installed correctly and does the ground wire go to both grounds on the RCAs?

Also reflow the solder joints on the RCAS and the volume control.  Sounds like a grounding issue in one channel on the input, so check all the ground connections from the RCAs to the pot.

HTH,

Jim

[pavlov]

Wow nice, I just realized there was some paint left under the screw of tho input tube, which could create a grounding issue.

After unscrewing and some sandpaper, the strange noise in the right channel is gone!  
I'm going to further inspect the grounding since I still hear a small hum in the background on both channels, but this isn't noticeable while playing music so I don't really care that much.

Thanks Jim, I didn't think a grounding issue could appear in that manner and wouldn't have looked it that way without your input.

[Jim R.]

Roger,

No problem.  Maybe do the same check for paint under the main grounding point near te IEC inlet as well.  This amp should be dead silent -- though of course tubes can also be an issue and you may find that a slight hum exists at first and then goes away with some time on the tubes.

Good luck,

Jim

[pavlov]

Thanks, done that but the hum remains.
Is it possible that it has something to do with the headphones?
I'm testing the amp with a AKG K141 (55 ohms).

The crack is however meant for my HD650's, which I don't have here atm. Is it possible it would be dead silent with the Sennheisers?

[Jim R.]

Roger,

That I can't answer for you.  Typically it is just the cathodes forming on te tubes, or something on the power line or nearby that could be inducing hum.  If it's not too objectionable, just give it some time and see if it diminishes.

-- Jim

[Paul Birkeland]

Yes, 55 ohm headphones are well below the threshold of where the Crack will operate properly. 

You could add a feedback loop to the amp to force it to produce more power into your 55 ohm headphones with less noise, but there are some sonic consequences to this modification.

The output stage of the Crack amplifier offers a lot of current gain.  Considering the equation for Power in terms of Current and Resistance: (I^2)*R, when we keep I fixed, the power output drops substantially when R is decreased, requiring all the oomph that the amp has to give, which also raises the noise floor.

I'd recommend staying above 100 ohms if at all possible.

-PB

[pavlov]

Ok great, lots of reasons to wait and see.
Thanks for helping me!

[pavlov]

Just a check-up if someone is interested.

The issue was certainly with the AKG's k141 mkII's.

With my AKG K340 (400 ohm) and Senn HD650 the amp is dead silent!
With the Crack it's btw the first time my K340's sounded good to me. With my former amp (X-Can V3) they sounded very dull and bass-shy, but on Crack these headphones sounds amazing!

Can't wait to try the speedball upgrade

[Jim R.]

Roger,

Glad to hear it -- figured it was that or the newness of the tubes, so glad you digured it out.

Are those AKGs closed back and do they bleed much sound to the outside?

I'm still looking for something nice in a high-impedance closed back can that I can listen to at night without disturbing my wife.  I considered the Beyer DT-770s but too many people told me they allow a decent amount of sound out to the outside world, plus they sound a bit too bass heavy for me.

Now just giive the amp some time to settle in and when you're ready, go for the speedball.

-- Jim

[pavlov]

Well the K340 is a fantastic headphone. It's a closed design and they don't bleed much sound. However it's an old design (I believe from the late seventies) with some perks. They aren't really comfy and are lacking in bass generally.
However they compensate a lot with the mids and highs. Certainly with complex classical music the level of detail in the highs is just incredible and not fatiguing at all.
But I don't know if I can recommend them. Besides the difficult fit there were apparently different versions during the several years of production so they don't all sound the same.

Btw, if you were asking about the K141 mkII, they are semi-open I believe. And IMO to be avoided, they are incredible uncomfy and sound just plain boring. (perhaps too much 'studio' to my taste?)

[Laudanum]

Jim, this is probably a long shot but there may still be some of the MB Quart QP 805's (QP 805HS) around.   I think the seller that had those and the open back Quarts on clearance was sonicelectronix.  I posted about this deal last year, I think you bought a pair of the open backs.   I remember my wife bought me these cans for fathers day 2011.   The 805's arent bad phones and dont leak much sound if any. Fairly well balanced although they do have some bass.  Not bass heavy like the Beyer 770's but they do let you know that there is a drummer and bass player in the band.  A bit accentuated up top for my tastes with some material but I like them for late night or early morning when I need a closed back can and doing casual listening at low to moderate volume.    Not giant killers, but pretty nice sounding phones, especially at the $100 they were selling for.  300 ohms I believe.  Fairly comfy as well although I did replace the ear pads with different/softer pads.   This may be all for nothing as they are probably long sold out by now and discontinued.  But maybe worth a shot.  Maybe used on ebay.   Not too many high Z phones available any more so I though these would be worth a mention. 

[Jim R.]

Hi Desmond,

I've been keeping an eye out for those for a while now and have yet to find a set.  Yes, for $100 the qp-400s are hard to beat and if the 805s sounded anything close, they'd be fine.  I may just end up with a pair of tweaked Denons to use with the s.e.x.

Thanks Roger also for your feedback -- very useful.

-- Jim

[grufti]

There is currently a seller on the German ebay site who has another 7 MB Quart QP 805 HS available for Euro 69.90, about $85.90 at current rates.

The seller takes paypal and will ship inside Europe. You might be able to convince them to ship to the US. Your US ebay logon works on the German ebay site, actually on any ebay site worldwide.

If you are interested you might want to log on to the German ebay site at www.ebay.de and contact the seller. Contact me, if you have no luck, but want to buy a pair anyway. I am somewhat interested myself.

[Jim R.]

Hans,

I tried, but between me not reading German and my screen reader not being setup to handle it, I really can't manage to navigate the site -- I got to the product listing, but can't do much beyond that.  If you want a pair, I'll pay the shipping for both pairs.

Thanks,

Jim