[Tone Deaf]

Hi Jim,

My Clarinet got put together over the weekend in a makeshift metal enclosure.  Very, very easy to build!

However, I have a low hum!  Volume/balance controls do not alter it.  All voltages check out ok, & I have re-soldered all the joints, to no avail.  Resistor values all tested & ok.  Do you have any suggestions on how to track down the problem?  

It's entirely standard, no substituted parts.  Only deviation is I have used 1/2 inch standoffs rather than 1 inch - forgot to order these!

Apart from the hum, sounds great so far.  It seems to have a lot more gain than my previous line stage (Audio Innovations L1).  In fact I wondered if the hum is due to the input sensitivity of my power amp being too high.  It needs around 2v for full output.

Anyway, any suggestions on where to start looking for trouble would be greatly appreciated.

Thanks
Glyn

[hagtech]

ToneDeaf,

There shouldn't be any hum.  The Clarinet is totally dead quiet.  My guess is that your chassis might be floating?  Make sure there is a ground connect (Earth terminal) between circuit board and the chassis and the mains ground.  

Is it hum or buzz?  If volume control doesn't do it. maybe there is an interconnect loop problem with the power amplifier.  Maybe some interaction between power transformer fields?  Is this without any signal source connected?

jh  

[Tone Deaf]

Hi Jim,

I've tried with floating ground and with chassis grounded, with no change in hum.   The chassis is grounded by a short wire from the pcb earth lug to the chassis by the input jacks.  While floating, the chassis became  grounded when connected to the power amp and I did wonder about earth/ground loops.  Temporarily disconnecting the Clarinet from Earth made no difference though.

It's a definate hum, not a buzz, & is present with or without a signal source connected.  As it runs on for a good while after power-off, I quickly pulled the rectifier out & switched back on.  No hum then, just nice clean valve hiss - but not sure if I've proved anything!

I'll give it a try with a different power amp, see what happens.  Otherwise I'm stumped on what to try!  

Cheers.
Glyn

[hagtech]

Ok, great debugging.  Excellent experiment.  I would first try a different rectifier tube.  Then try to track down supply ripple on the B+ lines.  Maybe a bad cap or resistor.  There is a ton of filtering in there.  

But I still can't figure out why it's there after power is shutoff.  That should remove any signal from the transformer secondaries.  Also removes magnetic flux.  

Oh wait!  The metal shells on the control pots and select switch are floating.  I did not connect to ground plane to eliminate a loop with the chassis.  The front panel metal plate must get grounded through the rest of the chassis pieces.  Maybe they are picking up something from the rect tube.  Ok, that's a longshot.

jh

[Tone Deaf]

I've tried two rectifiers - no joy.

With a different power amp, hum still there, but quieter.  Possibly just a less sensitive amp, so also tried a battery powered headphone amp.  Felt like using a stethoscope on the thing!  The hum is still present, and about as loud as the background hiss.

Sorry, I badly phrased yesterday's post.  Hum stops instantly when power is turned off.  I meant there was still a signal present for a while afterwards.  (Actually there was a buzz when power was off & signal faded, but this was because my bottomless enclosure was sat on a pile of other equipment & cables.  Moved away & it disappeared.)

Metal bushes on pots are connected to grounded front panel, so that's alright.  I'll see if I can trace the ripple, as you suggest, & maybe swap the caps for good measure.  Also I'll try moving the transformer away from the chassis, see what that does.

Thanks
Glyn

[hagtech]

Tone, can you post a photo?  That's the easiest way to see what's goign on.  Especially if you didn't use the stock chassis.

jh

[Tone Deaf]

Hi Jim,

Here's some pictures.





Since last post I have swapped the standoffs for 1 inch ones & probed about a bit with my DVM.  I got some odd readings from the meter along the way, but AC measures only 5mV after that second electrolytic.  Seems pretty low to me.

In use the hum is barely noticeable.  Perhaps I'm being too critical.  Wife & I have listened to the Clarinet quite a bit over the last week or so, & it's knocking spots off the old Audio Innovations one.  Clarity in particular is much better.  Could do with reducing the gain though - can I just increase the value of R311?

Cheers
Glyn

[hagtech]

Well, everything certainly seems to be built ok.  Nothing obvious.  Have you tried shorting one of the inputs?  Is hum in both channels?  What if you only connect one channel?

No way to lower gain except using the volume control.

Everything plugged into same outlet?

jh

[Tone Deaf]

Hi Jim.  Beginning to think I'm not going to get to the bottom of this..!

Hum is on both channels.  Shorting the inputs has no effect, nor does shorting input to ground.  Connecting just one channel has no effect.  Hum is there when the Clarinet is the only thing plugged in driving a battery-powered headphone amp.

I have now replaced all the capacitors as well, so those are in the clear.  All resistors measure ok when cold.  Are there any in particular you might suspect to be hummers??

Also, I forgot to order 330k resistors at the start, so I used some I had already in stock.  Wattage is okay, but I don't know what composition they are.  Might be metal oxide but could also be just carbon Could that make a difference?

Also the RCA sockets at the input are common ground - 3 pins per pair &  I notice the board has 4 holes.  Does it matter?

I'm about to star-ground the heater circuit caps, as in the Cornet, & see what this does.

Cheers
Glyn

[hagtech]

Hmmm.  I don't get this one.  The heater caps are already star grounded.  No resistors are suspect.  I predict a ground loop somewhere, but can't find it.

My Clarinet is so quiet.  It has less than zero hum, practically.  So I know the design is ok.  It's not a matter of tweaking values.

All I can suggest at this point is to send it in.  I'll take a look.

jh

[Tone Deaf]

Jim,

Really, that's very much appreciated.  Thanks.  I'll give it one last thorough look over in case of   & get it ready to send.

Regards
Glyn