"MK84" or KM84 in an Oktava MK-012 body

[McIrish]

quick listen with speakers on my desk, they sound similar. I will have to listen in the studio on some good monitors.

 

[OneRoomStudio]

quick listen with speakers on my desk, they sound similar. I will have to listen in the studio on some good monitors.

Yeah, once they're level-matched, the differences are subtle. This is only a change in head amp circuit after all, so I expected them to be similar. I do hear differences though, and a null-test confirms that there are differences.

 

[ostrol]

second versions have noticeably less low bass. That the stock?

 

[OneRoomStudio]

second versions have noticeably less low bass. That the stock?

Since no one else is chiming in. Yes - you are correct. The stock/original Oktava circuit has noticeably less low bass, plus it's a little scooped in the 3-7K band. The "MK84" extends much lower, has a slight lift in the upper-mids, and has a higher output overall.

 

[kingkorg]

plus it's a little scooped in the 3-7K band

has a slight lift in the upper-mids

These two parameters can not come from the circuit itself. If they are there, they might come from slight mic placement difference and/or slight capsule variation.

 

[OneRoomStudio]

These two parameters can not come from the circuit itself. If they are there, they might come from slight mic placement difference and/or slight capsule variation.

I switched placement (up/down relative to source) and capsules between takes, and the lift followed the circuit. Not sure how to explain it, but it’s there.

Could be some reactive loading by the transformer on the fet maybe?

 

[kingkorg]

Could be some reactive loading by the transformer on the tube maybe?

Can't be that range specific. But when it comes to pure frequency response and placement interaction, literally millimeters of difference can make quite large deviations. That's why shootouts like this are very tricky. Not to take away from your effort, to me both sound great, but they definitely have different flavors. Great job!

 

[k brown]

FWIW, I modded two pairs of MK012, one to an adaptation of the 'First Watt' B1 Buffer (2SK170BL with constanat-current load), and the other pair to the Royer 5840 cathode follower (both pairs with transformerless out). Both sounded noticeably better than the stock circuit (which were bought 20 years ago) but I could never tell the difference between the B1 buffer pair and the CF pair.

https://www.firstwatt.com/b1.html

 

[mrgrooves666]

What about noise on your 84-012? That is one of my gripes vs the 012 preamp, particularly the older ones. They can get noisey sometimes...?

 

[k brown]

Just another update. The transformer has arrived and fits perfectly. Plus, the circuit works as intended and sounds very good upon first power-up (before even messing with the fet bias!). I want to do another board revision though, to optimize the high-z connections and make it a little easier to assemble. More to come...


...And when I am satisfied with the final version, I will record some A/B comparisons with the original circuit.

Have you measured the polarization voltage the capsule is getting?

Dave Royer has said that the'012 capsules really don't like anything over about 40v.

Compared the overload levels with stock?

 

[Emmathom]

Nice project indeed !
I have got a pair of Russel Technology boards built up but not yet installed nor tested...
They have J113 + BC556 according to Russel but could maybe be upgraded with another Jfet (?)

@OneRoomStudio : which jfet do you use on your board ?