The A.N.T. Audio Kora 3T Phono Stage circuit published.

[tiefbassuebertr]

Originally Posted by x-pro

As I've recently stopped production of the "Special Edition" version of the Kora 3T phono stage (the "Limited Edition" version is still available), I've uploaded a short history of its development and full circuit diagrams for both MM and MC versions at the "Special Edition" level. The Limited Edition MC version was awarded the "Phono stage of the year" title by Hi-Fi World magazine in 2009 and it is still the phono stage to beat, in my opinion. I will be happy to answer possible questions here.

Enjoy! Alex

Thank you very much for publishing the schematics so as the circuit descriptions.
I wonder why that does not make the most companies of audio devices. I mean, the only reason why many write "strictly confidential - not for release" at their schematics, because they are ashamed to publish this cause bad engineering.

[UV101]

Ok, so I'm in receipt of the bits and pieces Alex kindly agreed to supply me.

I'm looking forward to building the phono stage for my Expert stylus co modded MC DL-103 sat on my SL-1200 (modded of course!!)
I will use premium parts through the build including cerefine electrolytics, silvered mica in the filter and RC55Y resistors throughout. I'm also thinking about mundorf supreme as the output cap as I've used them with great success in the output of several CD players I've messed with.

Alex also supplied me with a std psu to get me going. I intend to build my own using one of my low noise regs with Large mundorf smoothing and compare the 2 supplies.

I'm really looking forward to hearing this phono stage

[UV101]

Finally got round to ordering some bits today!!!

Mostly RC55Y 0.1% resistors, PPS 100nF, and mica and styrene in the RIAA filter. PSU caps will be mundorf and cerefine. I'm also going to build as MM use a couple of partridge SUT's from my cart. Nice little Xmas project.

[rjm]

Read over the thread. Thanks, Alex, for publishing your circuit design.

I'm surprised no one has asked the obvious question, so I will:

What are the advantages of using N-channel, depletion mode MOSFETs in a phono stage?

Of course "Why not?" and "Because is sounds good." are both acceptable answers in this game, but from the standpoint of the electronics what are the specific reasons / relative merits to consider?

[x-pro]

Originally Posted by rjm

Read over the thread. Thanks, Alex, for publishing your circuit design.

I'm surprised no one has asked the obvious question, so I will:

What are the advantages of using N-channel, depletion mode MOSFETs in a phono stage?

Of course "Why not?" and "Because is sounds good." are both acceptable answers in this game, but from the standpoint of the electronics what are the specific reasons / relative merits to consider?

Because it sounds very good, that is the most important bit. However from the purely electronics point of view these FETs have one of the highest voltage gains I know in a FET - over 10000, which, combined with high input impedance and fairly low noise, makes some very simple and well performing circuits possible. On top of it the BSS139 has a very good consistency in a batch, usual spread less than 5%, so the Kora 3T SE could be made without pre-selection or matching the FETs which is for a local NFB FET only circuit is quite unusual. I supply FETs for the Kora 3T kits just from a reel, in a factory packaging and the DC voltage difference between channels on the output FETs is usually below 50mV (out of 12V !). And the voltage gain is almost completely defined by passive components so if capacitors and resistors are 1% tolerance the gain difference between channels is usually below 1% (0.1dB).

Cheers

Alex

[UV101]

Hi Alex, how do you feel about increasing capacitance on the psu? Generally I've found it of benefit. I notice on the PCB there is space for additional caps. How about swapping the 470uF for 1000uF? Also I was going to use a very high value mundorf on both the external psu and inside after the 1st diode?

[x-pro]

Originally Posted by UV101

Hi Alex, how do you feel about increasing capacitance on the psu? Generally I've found it of benefit. I notice on the PCB there is space for additional caps. How about swapping the 470uF for 1000uF? Also I was going to use a very high value mundorf on both the external psu and inside after the 1st diode?

The original Kora 3T SE version has 2200uF + 470uF. It is possible to increase the capacitance further, I've used 3 x 1000uF Panasonic FM on some units. I would advise against increasing the value of the first capacitor (C15) though, as it would increase the peak current from the power supply on switch-on or on connection.

Cheers

Alex

[schneebly]

I ordered a special edition stage from Alex awhile ago and he made it for me and sent to US. I experimented with battery powering it but currently use with an Elpac Linear power supply (which I had gotten as an upgrade for the original Creek OBH 11 headphone amp). This stage sounds very nice to me, it is pretty much like the Hi-fi world review mentions. If you want a ton of snap and detail it might not be for you, it is more easy going and has a liquid "tube like" quality about the sound. The less than ideal noise measurements on paper never intrudes in practical use- the enjoyment of playing records. I might have to contact Alex and get a hold of one of the kits.

[h_a]

I just noticed this thread - thanks for posting! Definitely interesting material

[Ken Newton]

I too, have just stumbled across this thread. Very few commercial vendors seem keen to share their product circuit details, for understandable reasons. Those few who are comfortable sharing, such as Nelson Pass and yourself, are to be applauded.