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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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PostPosted: 28 Dec 2012, 19:58 
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Joined: 11 Dec 2012, 05:13
Posts: 7
HT Performance wrote:
Hi!

I still believe that the transistors shown on this pcb are reversed.

The 50ohm resistor is conected to drain and the 3.16k to source.

This is not what you see in the schematic, neither on the 2sk170 datasheet.

Please look carefully...



Yes, I think its obvious it is reversed, except... if there are 2 versions of 2SK170 with different D and S. Bad Chinese copy of SK? :)

Image


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PostPosted: 15 Feb 2013, 21:19 
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Joined: 14 Feb 2010, 13:13
Posts: 620
Location: Lisbon, Portugal
By request...

The modifications / improvements to this lovely pre-amplifier.

- Slightly gain increase
- Nearest RIAA curve
- Exceptional soundstage

Check theory and measurents on this thread (about 5 pages behind).


Happy listening!

Miguel


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PostPosted: 16 Feb 2013, 18:36 
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Joined: 06 Jun 2008, 18:23
Posts: 3804
Location: Australia
HT Performance: I just received the BHL MC stage kit. I hope to have have it finished in the next few weeks. Currently I'm finishing another piezo preamp this time with OPAMPs. Once completed I'll tackle the BHL MC stage. I can compare it with my Le Pacific and Jean Hiraga bi-polar MC stage (I like this one the most but others like the Le Pacific).

As an aside I now have two good schematics for two stage tube phono preamps. The one I built for a friend I thought was exceptional and (one day) hope to build one for myself soon.

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Projects: "Parley" - Tube preamp with Pass B1 buffer | "VoXConcentriC" - Beyma high efficiency 12" speakers | "Black Lace" - all Ozzie tube 6V6 Power amp - old school
Website: [ retro-thermionic ]


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PostPosted: 14 May 2013, 11:31 
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Joined: 12 Apr 2013, 05:28
Posts: 15
Location: Boise, Idaho, USA
I finished my Boozhound Labs MC pre and Phono pre tandem and used advised changes for R 5, 10, 12 and add C of 470 uF to R3. . Sounds really good, but I still have some hum-noise when my volume is high and no music played. How do you guys connects signal grounds and how do you connect PSU and chassis grounds? BTW, I use Nemic-Lambda EWS-15-15 PSU (+15V and -15V) which adjusted to 14V DC. Do you shorting output RCA returns? Please advice.


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PostPosted: 14 May 2013, 20:42 
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Joined: 10 Feb 2011, 14:01
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I have had similar hum problems, but in the end they've all been traced back to the turntable. While I can't advise about what to do, clips and wire are your friend. Clip to various places on the TT and phono pre and see what happens. Pay close attention to the tonearm and headshell. Clip together the RCA jacks at the inlet, and also at the tonearm wires for each channel. If no hum, the issue is probably related to the cartridge not being shielded and you may need to do some TT tweaks (mu-metal platter mat?).

My preamp is grounded at one place (star grounded), in my case at the power supply inlet (I have a linear supply I made that I use a typical DC jack to connect to the phono pre). The TT ground lug on the phono pre (which not all of my TTs have) is isolated from the case and connected to the ground plane, and then to the case/power supply ground. Not a true star ground, but it seems to work really well. Alternatively, ground the case at the TT ground lug and isolate the PS inlet from the case. If the case is grounded at several places you could end up with ground loops.

Finally, are you sure it isn't coming from the PS?


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PostPosted: 15 May 2013, 02:18 
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Joined: 12 Apr 2013, 05:28
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Location: Boise, Idaho, USA
Thank you for your feedback. I have that hum even when my TT in not connected to pre amp. So, you do advised to short inputs. Right? I'll try to attach a drawing of my setup. Please take a close look and maybe you can advice more obvious direction.

Attachment:
Drawing of my structure.png


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PostPosted: 15 May 2013, 06:52 
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Joined: 10 Feb 2011, 14:01
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I'd first try shorting the inputs, but my guess is that won't change anything. Next, if they aren't yet, twist all incoming AC lines right up to the PSU. Test after each step.

Your grounding looks mostly OK but a little odd. I'd prefer a star at the power inlet and all downstream components tied directly to it without going through any other components. Right now your grounds go to a star, that feeds the PSU and then that connects to the mains ground. I don't know the PSU design but perhaps there's an issue running grounds through it.

I'd change the wiring to send mains ground directly to the star, then all other grounds (including PSU) to that star. I know the mains ground and PSU ground are probably tied directly, but you might still be getting ground loop from the PSU-MC-Phono. I'm pretty sure the signal negatives are connected to the ground plane so it's hard to figure out where the loops might be.

Also, maybe the PSU has some issues? Can you disconnect the PSU and wire up a pair of 9v batteries in series (or similar for 18v) to the supply lines from the phono amps (9v alone is at the limit of the amps' ability to operate)? Jason says 24v is fine so the 18v won't be a problem. If you still get hum running on batteries, there's a ground loop for sure.

Lastly, can you tell if there might be a ground loop forming with the downstream components? You might connect to a different amp or pre-amp downstream. I suspect the grounding within the phono stages and PSU more than that though.

Good luck.


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PostPosted: 15 May 2013, 11:48 
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Joined: 14 Feb 2010, 13:13
Posts: 620
Location: Lisbon, Portugal
In my opinion your problem is having AC inside the chassis.

On phono-preamp I generally use external power supplies, like a wall-transformer and a DC jack.

It really helps all hum problems, in fact, there is no hum at all!

So what about getting a cheap 24vdc wall type PSU? They are very cheap at eBay ;)

One doubt...

This JFET works with +24v and 0v. I see your PSU is +15v and -15v which is a symetrical PSU. Are you OK here?


Cheers,

Miguel


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PostPosted: 15 May 2013, 12:53 
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Joined: 10 Feb 2011, 14:01
Posts: 30
I haven't had problem with AC in the chassis for my MM stage (Boozhound) but haven't worked with the MC step-up, and I could see that being a problem. On the other hand, with just the MM phono stage and a wall wart, I had terrible hum. Jason at Boozhound suggests some work and some don't. I'd imagine that wouldn't be a good approach with the MC step-up without serious filtering in the case before the board.

The JFETs and this design apparently provide terrible ripple rejection. My own MM stage uses an external supply as you suggest and hum is very low except when certain turntables are attached (and then the hum can get quite noticeable. IMLE the real gremlin is ground loops, and they can be a bear to find. Induced AC hum, especially with twisted wires and perpendicular crossing of DC lines where that has to happen, just isn't a big deal. Again, in my experience.


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PostPosted: 15 May 2013, 17:56 
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Joined: 06 Jun 2008, 18:23
Posts: 3804
Location: Australia
HT Performance wrote:
In my opinion your problem is having AC inside the chassis.

I agree here. Get the AC out of the case. I even have an inline box with rec. bridge and some basic filtering before it gets into the case. Then I have large storage caps inside the case. This way only DC gets into the case.

_________________
Projects: "Parley" - Tube preamp with Pass B1 buffer | "VoXConcentriC" - Beyma high efficiency 12" speakers | "Black Lace" - all Ozzie tube 6V6 Power amp - old school
Website: [ retro-thermionic ]


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