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SET Headphone amp

108.193.1.40
Posted on August 1, 2016 at 22:14:32
OldNuke
Audiophile

Posts: 141
Location: Silicon Valley
Joined: November 18, 2014
 In the spirit of not bothering Mike on every little problem, I'm posting here for help. I've built a 45 SET parafeed amp for headphones/speakers (Oppo PM3's at 28 ohms mostly). I can null the 60 Hz hum but have a constant 120 Hz hum (or background since its on AC heaters) that I'd like to damp or eliminate completely. Also I have only a 12th turn of volume pot now (with a 75K/33K Lpad on the 100K pot). Mike has been advising because the amp is with MQ plate chokes and output transformers.
My question is to damping the amp without losing the great qualities it now has. My next move is to pad the input more to hopefully lower the background hum to the hairy edge with series resistors on the input and keep the magic the 45's give.
I'm looking for ideas and options before resorting to DC heaters, I'm open to rebuilds (already in the plan to improve the wiring layout) and adding parts.
Thanks in advance. This amp is sweet on my HD650's but I need closed back phones for my office cube.

 

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If you don't succeed..and go to DC heaters..., posted on August 3, 2016 at 05:37:55
Bas Horneman
Audiophile

Posts: 4083
Joined: March 28, 2001
 use Rod Colemans http://lyrima.co.uk/dhtreg/dhtRegIntro.html or the low noise Tentlanbs heater thingies.

 

RE: SET Headphone amp , posted on August 3, 2016 at 10:12:49
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 The hum pot can only cancel the 60Hz, as you have said. The 120Hz remains untouched. If the 120Hz is in fact coming from the filament, then either DC or one of the more exotic high-frequency AC will be needed.

But there are other possible sources of 120Hz hum. Inadequate power supply filtering is one example. If the hum has a buzzy character it is likely to be capacitive coupling between power transformer windings, or ground loops.

 

RE: SET Headphone amp , posted on August 3, 2016 at 11:14:08
OldNuke
Audiophile

Posts: 141
Location: Silicon Valley
Joined: November 18, 2014
 I do have a buzz that is apparent on the lower settings on the volume pot, disappears as I turn further up the pot then reappears and gets louder as I turn up the pot. The sweet spot is about 30% - 50% on a 100k pot. I assumed it was interference I could clean up with a better potentiometer and/or more padding.
The power supply is a CLCRC setup; 100uF/10H/100uF/360ohms/2.2uf. Thinking back maybe the last cap was the smallest oiler I had on hand. I'll experiment with a 1K resistor and 70 - 100 uF cap for the last filter and see what that does.

 

RE: SET Headphone amp , posted on August 3, 2016 at 14:33:47
Gordon Rankin
Manufacturer

Posts: 2928
Joined: June 9, 2000
 ON,

I have built a number of these for customers using the TL404, Dowdy Choke and tube rectifier. I use AC heaters with the 45 and use a 10 or 20 turn hum pot and was able to make the AC noise less than 0.25mV at 60Hz. I don't use any DC regulation but do use a LCLC power supply and usually a 5Y3 rectifier and the LCLC is more like LC---split LC for each channel.

You really have to think about grounding here and the layout.

Thanks,
Gordon
J. Gordon Rankin

 

RE: SET Headphone amp , posted on August 4, 2016 at 11:46:03
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 The total impedance to ground of the first stage grid is highest at your 30%-50%. That could mean the buzz is picked up by capacitive coupling from some nearby source. I agree with Gordon, grounding and layout are the most likely routes to success.

 

RE: SET Headphone amp , posted on August 7, 2016 at 16:21:50
OldNuke
Audiophile

Posts: 141
Location: Silicon Valley
Joined: November 18, 2014
 Thanks for the encouragement! I'll start with some cleanup and move to improving my layout.
Starting at 6.6mV and 10mV on Right and Left channels, shooting for 0.5mV each.

 

RE: SET Headphone amp , posted on August 8, 2016 at 01:39:34
OldNuke
Audiophile

Posts: 141
Location: Silicon Valley
Joined: November 18, 2014
 Latest steps: increased last power supply capacitor to 82uF (all I had in the junk box). Changed out the pot and padded the input. Moved TL404, headphone jack and RCA grounds to the same point. Moved speaker connection ground to PS safety ground. Bypassed last PS cap with a 1.0 uF Obligatto gold. Moved BCP-15's around to get the lowest hum heard over headphones. Still at around 10 mV AC on the speaker outputs.
Listening to Gary Clark Jr's album, sounds sweet but the same problems. Hum in the quiet points and just below the music and buzz that can be nulled at 30 -50% of volume pot and increases as pot increases.
I'll tube roll the 12at7 and 45's to get a quiet set while I wait for a BCP-15+ from Mike L, will re route the input wiring away from everything.

 

RE: SET Headphone amp , posted on August 14, 2016 at 23:22:20
OldNuke
Audiophile

Posts: 141
Location: Silicon Valley
Joined: November 18, 2014
 A little bit closer to being done.
Cured the buzz by using a .47uf ceramic cap to ground on the filament ground of the input tube.
Added BCP-15+ for plate chokes, at this point I'm at 6mV and 7.2mV hum. Took the padding off the volume pot and put 33 ohms on the input of the TRS jack (all I had in the garage). Still have a little hum in the quiet passages and when input is paused. Ordering more resistors to find the right listening level.

 

RE: SET Headphone amp , posted on August 22, 2016 at 08:09:44
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 Is the input wiring shielded?



 

RE: SET Headphone amp , posted on August 22, 2016 at 08:57:45
OldNuke
Audiophile

Posts: 141
Location: Silicon Valley
Joined: November 18, 2014
 Yes,
The input wire is a shielded twisted pair with the shield wire connecting RCA inputs ground on one end and star ground on the other.

 

RE: SET Headphone amp , posted on August 24, 2016 at 10:32:25
OldNuke
Audiophile

Posts: 141
Location: Silicon Valley
Joined: November 18, 2014
 Well, I think I fixed the issue but I didn't solve the issue. I added a -8db pad onto the TRS jack (38K in series, 49K to ground) that cured all the hum problems. Completely black background on my Oppo PM3's, no hum anywhere.

The pad is a little too much, my first 5 clicks on the volume pot are useless and the max volume is just below the "rocking out" level I want so I'm going to try a -1 or -3db when the grid chokes I ordered come in. I know this is another source of possible hum but I never thought this would be easy :)

Going to try something I saw on the Bottlehead big headphone amp and route the inputs thru grounded copper pipes and relocate my setting switches as far away from the power transformer and the 404's as possible, shouldn't hurt and will make the amp pictures more interesting.

 

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