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6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A)
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=1528
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Author:  Beftus [ 02 Mar 2010, 01:55 ]
Post subject:  Re: 6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A)

matt1234 wrote:
Regarding the Caps (you mean the 220 uF right?)
Yes I do.

matt1234 wrote:
there should be no problem for 12.5 mm, maybe with a bit of leg-bending but should fit quite ok on the 272A in my opinion.
The largest output cap you can mount flat on the old kit's PCB is 12.5mm in diameter. Larger than that and it will require some creativity.

The new kit looks like it has the output caps very close together, closer than on the old kit.

matt1234 wrote:
So my question would be if Silmics would give another improvement
Since they're in the signal path you'd better use the best you can afford. Elna Silmic caps are highly regarded. I'm very fond of them.

Author:  Gio [ 02 Mar 2010, 13:06 ]
Post subject:  Re: 6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A)

Beftus wrote:
I've read that higher value resistors are inherently more noisy than lower values. The opamp in a stock build is run at unity gain using two 1 Meg resistors per channel. Would changing both the resistors to the same but a (much) lower value have an effect on the noise? Any side effects when that's done?

Yes, lower resistor values have less thermal noise. Could be worth trying. FWIW, the application note in the datasheet for the PT2308 headphone chip uses two 3k9 resistors.

Also, here is a good article on resistor noise: http://www.aikenamps.com/ResistorNoise.htm

One could also try a resistor (10R to 100R) inline with the output. This may help reduce noise, but will also reduce the output level. You will want to use the lowest value you can get away with. Subjectively, I prefer not to use a resistor here.

matt1234 wrote:
So my question would be if Silmics would give another improvement
Beftus wrote:
Since they're in the signal path you'd better use the best you can afford. Elna Silmic caps are highly regarded. I'm very fond of them.
Film caps in parallel with the output capacitors should also help. If you can find 1uF 50V they are not that big and should easily fit on the underside of the board.

Author:  Beftus [ 02 Mar 2010, 13:30 ]
Post subject:  Re: 6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A)

Gio wrote:
Film caps in parallel with the output capacitors should also help. If you can find 1uF 50V they are not that big and should easily fit on the underside of the board.

I already have 0.1µF MKP1837's in parallel with the output capacitors, and they fit easily on the underside of the board. :mrgreen:

Author:  deniss [ 03 Mar 2010, 00:25 ]
Post subject:  Re: 6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A)

Yay - Branko replied to me and told me they will ship K272A, even though I ordered through their website and not eBay.

I have to start keeping track of all the mods suggested here! For me, this will be a pre-amp. In that capability, are these tubes nice, or are better tubes advised?

Author:  mwhouston [ 03 Mar 2010, 00:36 ]
Post subject:  Re: 6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A)

deniss wrote:
Yay - Branko replied to me and told me they will ship K272A, even though I ordered through their website and not eBay.

I have to start keeping track of all the mods suggested here! For me, this will be a pre-amp. In that capability, are these tubes nice, or are better tubes advised?

Stick with the tubes but upgrade the passive components and move to an OPA2134 for the buffer/output chip.

Author:  KrammeAcoustics [ 03 Mar 2010, 08:03 ]
Post subject:  Re: 6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A)

mwhouston wrote:
Stick with the tubes but upgrade the passive components and move to an OPA2134 for the buffer/output chip.

Could it be worth upgrading the PT2308 -> OPA2604 ? or are the differenses in soundquality between OPA2134 and OPA2604 to small to mention, so it doesn't matter ? :trumpet:

In other forums, i have seen people prefering the 2604.

Ebay prices: 2134 and 2604 = 5 - 10 US $. 8-)

Author:  Soundbrigade [ 03 Mar 2010, 08:22 ]
Post subject:  Re: 6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A)

Where to find the PT2308? It could be fun to have a few for experiments.

Author:  Beftus [ 03 Mar 2010, 10:52 ]
Post subject:  Re: 6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A)

KrammeAcoustics wrote:
Could it be worth upgrading the PT2308 -> OPA2604 ? or are the differences in sound quality between OPA2134 and OPA2604 to small to mention, so it doesn't matter ? :trumpet:
In other forums, i have seen people preferring the 2604.

Which opamp you prefer is a matter of taste. You can try all kinds of dual opamps in this preamp. Just make sure they share the same pinouts. I've tried it with an AD8066 too. Sounds great!
Attachment:
DSC_0421.jpg

Something to read about opamps:
http://audioreviews.blogspot.com/2005/1 ... -amps.html
http://tangentsoft.net/audio/opamps.html

Author:  deniss [ 03 Mar 2010, 10:59 ]
Post subject:  Re: 6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A)

Wowwie! What is that 2-story sandwich-like contraption and why cant it be mounted directly on the board?

Author:  Beftus [ 03 Mar 2010, 11:33 ]
Post subject:  Re: 6418 Tube / Opamp Headphone Amp (Oatley Electronics K272A)

deniss wrote:
Wowwie! What is that 2-story sandwich-like contraption and why cant it be mounted directly on the board?

That's a SO-8 to DIP-8 converter for opamps that aren't available in DIP-8 form factor. That mini PCB is 10x10mm.

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