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SET 300B Amp Hum(both amps)

68.57.194.152
Posted on October 25, 2020 at 05:04:57
Cfarrow
Audiophile

Posts: 17
Location: st louis
Joined: April 18, 2007
 I recently picked up a pair of Silver Circle Audio 300b monobloc amplifiers and both have a really loud hum from speakers. Tubes are new in both which are KR-300b, Mullard CV-378 and RCA 5692. I use a Border Patrol Preamp, SW1X Audio Dac, Audioquest Niagara 1200 and Audience Adpet aR2p power conditioners, all Audience Au cabling and Cardas Golden Reference Power chords, lastly Von Schweikert DB-100 speakers.


I can switch back in my Audio Tekne IT-2 amp And Marantz 8B tube amps with zero Hum....ideas on the fix? I am wanting to get back into SET amps.

For reference I have owned 2 pair of Wavelength 300b amps and Sophia Audio 300b amps with no hum issues in the same system.

Thx for any suggestions

 

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RE: SET 300B Amp Hum(both amps), posted on October 25, 2020 at 10:02:55
Cut-Throat
Audiophile

Posts: 18286
Location: Minneapolis - St.Paul Area
Joined: September 2, 2000
Contributor
  Since: May 16, 2021
 You could try lifting the Ground of the Amps temporarily and see if that removes the hum. If it does, you'll know have a ground loop problem. Easy to diagnose, a bitch to eliminate however.

I assume you've adjusted the Hum Pots?




 

RE: SET 300B Amp Hum(both amps), posted on October 25, 2020 at 17:56:21
rodge827
Audiophile

Posts: 354
Location: South Jersey
Joined: October 1, 2004
 A friend had the same issue recently he switched to shielded interconnects and the hun went away. I would start with lifting the ground on the amps as previously mentioned.

 

RE: SET 300B Amp Hum(both amps), posted on October 26, 2020 at 18:24:02
Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since: December 3, 2015
 All interconnect cables should be shielded, IMHO. Unshielded is an invitation for problems, hum being the least of them. I would worry about HF oscillations.

 

RE: SET 300B Amp Hum(both amps), posted on November 1, 2020 at 09:40:07
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 What if you put a shorting plug in the RCA jacks?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: SET 300B Amp Hum(both amps), posted on November 1, 2020 at 10:03:11
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 I get a ground loop hum if I have more than one unit in the chain earth grounded. So my preamp is the only thing that is grounded.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: SET 300B Amp Hum(both amps), posted on November 1, 2020 at 11:07:48
Cut-Throat
Audiophile

Posts: 18286
Location: Minneapolis - St.Paul Area
Joined: September 2, 2000
Contributor
  Since: May 16, 2021
 Since I have 115db Horns, I am very susceptible to any Hum at all. What I finally did was put all of my Stereo Gear on 1 Circuit, then put a GFCI on the Panel Breaker for that 1 circuit... And then (the horror), I lifted all the Grounds...
Which is a much safer installation than keeping the grounds and having no GFCI on the circuit.. This is per the NEC (National Electrical Code). Remember most older homes have no ground at all.. Just 2 wires. The GFCI will make them much safer too.



 

RE: SET 300B Amp Hum(both amps), posted on November 2, 2020 at 12:48:31
SteveBrown
Audiophile

Posts: 2454
Location: Portland, OR
Joined: November 14, 2002
 Never easy to find hum issues. One key is to determine if it is 60hz hum or 120 hz hum. The 60 is bound to be ground loop, but the 120 comes from a power supply someplace. My preamp (as Tre' also does) is the only place where the earth and signal grounds both meet. In the power amps the chassis are grounded but the signal grounds are isolated with the doubled up parallel diodes (you can find it on line). All devices connect to a common outlet which is isolated back the breaker. My system is dead quiet even running a low output MC cartridge through a SUT and then a tube based Phono stage. Only bit of advice is what I learned in High School chemistry class - never change more than one variable at a time.

 

A better way to do that..., posted on November 3, 2020 at 11:40:30
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 Its good safety practice to ground the chassis. But it allows ground loops if the audio circuit is chassis-grounded.

The solution is simple- don't ground the audio circuit to the chassis. Instead, put a resistor or a pair of diodes in parallel (going in opposite directions) between chassis ground and audio ground. The resistor does not have to be a very large to prevent significant ground loop currents. In this way the audio circuit will float at ground potential.

If there is noise in the chassis, it will no longer be part of the audio ground. But the chassis will provide shielding. In this way I think you'll find it quieter than the circuit was before.

 

RE: A better way to do that..., posted on November 4, 2020 at 09:37:34
Jim D.
Audiophile

Posts: 854
Location: FL
Joined: October 26, 2000
 Are you saying you connect all the power supply grounds and audio circuit grounds to earth ground with the resistor or parallel diodes? I'm thinking of a tube rectified DHT amp with center tap, PS capacitor grounds, cathode grounds, RCA grounds, etc.

I've had good success with returning all grounds to a central chassis point using a simple star ground.

Jim


 

Not saying that., posted on November 4, 2020 at 11:44:53
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 The circuit is normal in every regard except that its grounds are simply not tied to chassis. So insulating rings are used on the RCA input, ground connections on tube sockets not used and the chassis isn't used for ground. All the grounds are tied to a common point in the power supply. I'm not going to get into whether star grounding or a ground plane is being used.

The chassis is tied to the green wire of the power cord directly.

The the resistor (or diode array) is connected from the circuit ground (I often do it at the RCA input) to the chassis.

That's it- pretty simple and it works. The only thing is that if an amp or preamp is grounded in this manner it will still be possible to short out this grounding scheme by another component in the system. But if that aspect is sorted, no ground loops- yay! If the preamp is grounded through its chassis, still no worries - yay!

One upside of this is you have greater ability to control noise in the ground, and this leads to quieter circuitry with lower IMD. So if this is an amp we're talking about, it will be immediately quieter, smoother sounding and more detailed.

 

Thank you Ralph! (nt), posted on November 4, 2020 at 13:21:30
Jim D.
Audiophile

Posts: 854
Location: FL
Joined: October 26, 2000
 .

 

RE: A better way to do that..., posted on November 10, 2020 at 18:07:44
JimBop
Audiophile

Posts: 1286
Location: Colorado Springs, CO
Joined: October 7, 2002
 I'd like to try this with my 2A3 SET amps. What do I need to look for in the diodes?

 

For a 2A3 SET, a pair of 1N4007s will do the job., posted on November 11, 2020 at 11:28:45
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 The diodes won't be making noise.

 

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