Help with S.E.X 2.1

[Marcin]

Hi, I just completed assembling Single Ended eXperimenter's Kit 2.1. Amplifier seems to be producing ~70Hz humming noise coming out of headphones. I can hear the hum in both channels and the noise doesn't change with the volume. I was wondering if someone can help me with resolving this issue.

- Resistance and Voltage check is showing:-

1, ~, 198V
2, 0, 0V
3, 0, 0V
4, ~, 396V
5, ~, 197V
6, ~, 375V
7, 0, 0V
8, 0, 0V
9, 248K, 0V
10, ~, 394V
11, ~, 359V
12, 0, 0V
13, 0, 0V
14, -, 0V
15, 896, 17V
16, ~, 374V
17, 0, 0V
18, 0, 0V
19, 248K, 0V
20, ~, 393V
21, ~, 358V
22, 0, 0V
23, 0, 0V
24, -, 0
25, 897, 17V

A1, 248K, 0V
B1, 248K, 0V

A2, ~, 352V
B2, ~, 350V

A3, 685, 16V
B3, 686, 17V

A4, -, 0V
B4, -, 0V

A5, ~, 73V
B5, ~, 67V

A6, 1.19K, 2.1V
B6, 1.19K, 2.2V

A7, no response, -0.1V (value varies)
B7, no response,  0.4V (value varies)

A8, no response,  3.9V
B8, no response,  3.5V

C1, no response, -0.1V
C2, no response,  3.3V
C3, no response,  0V
C4, no response, -0.1V
C5, no response,  3.4V


- Summary of actions taken:-

  - I went through the forum and documentation searching for any similar issues - no results.
  - Replaced heater cables.
  - Replaced 10000uF capacitor with a new one.
  - Replaced volt meter batteries.
  - Heated and re-soldered all the connections at least twice.
  - Reviewed setup and direction of all the Schottky diodes.
  - Made sure that all the terminals strips and other components are correctly grounded.

  - After closely inspecting the tubes, one tube seems to be missing top vertical plate. Would it be possible that the tube was damaged during the shipping or not properly tested?

I'm running out of ideas what to check next. Any help would be greatly appreciated.

[Paul Joppa]

The tube voltages are all correct, so the circuit is operating correctly and the tubes are fine.

First, let's make sure it's from inside the SEX amp. If you hear it with nothing else connected (no input device) and at zero volume, then it's probably inside the SEX amp. If removing the external source cures it, then you could have some ground current (commonly called a "ground loop").

My other thought is - I hate to say this - you may have highly sensitive, low impedance headphones. What are you using, and do you have specs for impedance and for sensitivity?

[Marcin]

I can hear it with volume all the way down, and nothing else connected. I only tested it with two different pairs of cheap headphones from Apple. I didn't want to risk with any other headphones ..

[brianf7408]

I had a similar problem problem with the Crack.  All voltages checked out fine but I had a low hum in one of the channels, even present when no source was connected.  The tube checked out fine on a tube tester but when I replaced the tube the hum went away.  This was a new tube by the way.  Just food for thought.

[Grainger49]

Hum is the bane of tube aficionados.  When I built my Seduction I had hum at startup.  Then I remembered that I had kept the tubes upright on the table and knocked them over a couple of times.  This can damage the tiniest of the tube's elements.  I had some to roll in that were used ebay finds.  They were fine, as for hum, but the USSR Surplus tubes were the ones that really light my fire.

Hum can come from so many places.  Everyone should have a backup set of tubes.  But that will only cure hum in some cases. 

Ultimately my Seduction had a tiny bit of hum.  You could barely hear it when everything was on and no music playing.  But even an acoustic guitar intro would bury it.  The Eros is absolutely dead quiet.

[Marcin]

The noise that I'm hearing is just way too loud - it's also also present in both channels. Besides ebay can you recommend any other place where I can buy matched pair of tubes?

[Doc B.]

70Hz would be a rather uncommon frequency for noise. It is most likely 60Hz or 120Hz. Since it is in both channels it is unlikely to be a bad tube. If it is 60Hz, which is typically a softer hum, it is probably due to magnetic coupling of a nearby power transformer. Moving the amp might change the magnitude of the hum if this is the case. If it is 120Hz, typically characterized by a buzzy component, it is more likely to be a problem in the amp itself - bad ground, mis-wire, backwards filter cap, etc. Since it is in both channels it may be in the power supply circuit or a ground connection common to both channels. It might also be a ground loop having to do with the circuit wiring of your house. A cheater plug might help to rule that out.

[Marcin]

Few more things tested:-
   - Cheater plug didn't help.
   - New tubes didn't help.
   - Replaced 100uF and 10000uF capacitors, no difference.
   - I agree that it might be not 70 or 60Hz. Actual noise,

[Marcin]

Few more comments, observations and questions:-
   - Moving the amp doesn't make a difference.
   - All terminal strips seems to be grounded correctly touching the aluminium plate.
   - Everything is wired for 8 ohms.
   - The PC-3 black wire is not used?
   - 0.1uF 630V and 1.5uF 630V capacitors are not directional?
   - Accidentally melted the power switch - any pointers where I can buy a new one?

[Alonzo]

Best of luck, I know that sound very well, been searching for the source of it in my (non SEX) amp for months.  A component of mine was vibration, I could hear the buzz when I put my ear near the power supply transformer.  New mounting screws and grommets helped some.  My next try at a fix will be to rebuild the power supply with FREDS, maybe better diodes would help you.  I didn't see a pic from you, can you post one of the power supply area and of your wire routing?  Maybe one of the other helpful gents on here can see if your picking up interference in your wiring.

[Marcin]

Thanks for the suggestion! Sharing few pictures:-
   - https://plus.google.com/photos/117011424417543401185/albums/5837281958597853329?authkey=CKPsv8iOweXzNg

[Doc B.]

The sound is 120Hz buzz, indicating a ground issue somewhere. I notice that the chassis plate is painted or powder coated. Did you make sure that the safety ground tab makes electrical continuity with the chassis through the finish? Also, I don't think that changing out components to different ones, like adding FREDs, is a logical approach. The amp is capable of -90dB S/N with the stock parts and should be sorted out using them. This kind of thing is most often fixed by reflowing solder joints at ground connections throughout the amp. A close inspection with a magnifier might turn up a solder joint that isn't quite conducting.

Eileen will be happy to send you another power switch. Contact her at queen at bottlehead dot com.

[Jim R.]

Marcin,

Also check that your input RCA connectors are completely isolated ffrom the chassis plate -- you get no continuity between the outer  conductors of the rcas and the chassis plate.  Als make sure they have the plastic isolation washers in place and tightened down correctly.

-- Jim

[Paul Joppa]

Can you measure the hum? Grainger's most recent post here describes it for the Paramour:

http://www.bottlehead.com/smf/index.php/topic,2476.msg33528.html#msg33528

I always like to see numbers, it's a habit left over from when I was an engineer  :^)

[Grainger49]

Don't let Paul fool you, he is still an engineer and designs much of the Bottlehead equipment.