X-BOSOZ first tests

[wuffwaff]

Hi all,

I finally (almost) finished my X-BOSOZ/ONO preamp. It seems to work fine (on the scope) but there are a few unexpected (by me) things that make things a bit complicated.

I´m using a balanced stepped volume control wich works very well when I measure the balanced output (with an unbalanced input and -in connected to ground).
Problems start when I only measure the +out to ground (unbalanced out). Here the volume control has a very weird influence on the output signal. It does nothing at very high attenuation settings, then in the midrange reduces the output signal and then finally seems to be working ok.

I can change this behaviour by connecting -out to ground through one of the volume controls 1k2 resistors. Then it works as it is supposed to be.

Can someone explain this behaviour? Can it be cured by using current sources instead of resistors?

The complicated part I was talking of before involves comparisson with my old (unbalanced) preamp. I would have liked to do this unbalanced and balanced so I could test it with my Aleph and with my Crescendo amps. It would also be nice to use the same cables and see if a Kimber PBJ (balanced) beats a Kimber select (unbalanced).
When it stays like it is now I will be changing quite a few things when changing preamps and can only use the Aleph.

Here are two pictures of the (almost) finished X-BOSOZ/ONO.
The first is the power supply with Pquadrats boards for the ONO and 4 x 60VA transformers (2 x 2x30V and 2 x 2x60V).
The second is the preamp again with pquadrats Ono boards and a veroboard X-BOSOZ. The cases are not finished yet and no led is working........

william

[wuffwaff]

and the amp itself...

[Raka]

Magnifique!!

Do they sound?

[wuffwaff]

Hi Raka,

thank you. Yes the first half sounds. I´m just listening to a more or less crippled X-BOSOZ (unbalanced in, unbalanced out) on my upstairs equipment.
Even in this state the sound is a lot better than expected , verry detailed, good room information width and dept. Before I listen downstairs I have to activate the ONO part and the source selector relais.

The only problem I have is the smell of the resistors.....it smells like someone is ironing shirts with a very hot iron. I hope this´ll go away after a while.

william

[amo]

Very nice! What brand transformers are those? They look grat!

[wuffwaff]

Amo,

the transformers are from a local manufacturer www.tauscher-transformatoren.de. They are not really cheap but very good quality. I also ordered the tranformers for my Aleph-X from them. Their 600VA is just as big and heavy as a 900VA from another supplier.
It seems that you can specify the VA rating in a lot of ways.....

william

[Netlist]

Very nice William.
Looks like you didn't cut on costs and good parts for this project.
Hope you can solve your attenuator problem.
I have no idea in which direction to lead you.

/Hugo

[wuffwaff]

Hi Hugo,

well it is kind of solved. Balanced operation is ok and for unbalanced I must connect the negative output resistor to ground.

My problem was more in understanding why the volume control has such a funny effect when using only one phase and if this behaviour can be changed by using current sources instead of resistors.

william

[wuffwaff]

well,

I´ve finally wired up everything (see picture below) and although it looks a bit messy for my taste everything seems to be working ok. Source selection is improvised with an external switch cause the final selectors will be in the top of the case wich is not ready yet. I´ve installed a blue led inside the case to keep everyone happy.......

I hooked it up to my main system yesterday and had a bit of a listen wich I repeated today. Most listening was done with a balanced connection (Kimber PBJ) to my Aleph5. Comparissons are always relative to my previous preamp (a tuned elektor "the preamp" with OPA627/637, Elma ladder, Pan. FC, big power supply etc.)

First impressions are not that bad.

There´s absolutely no humm or noise with cd or phono sources.
The midrange is very nice and natural sounding with a good detail (enhances the effct the Aleph had).
Soundstage is ok but not really special. Performers are not as 3D as I´m know them.
The bass also sound very natural with good detail but is a bit soft and seems to be a bit down in level (here I must check if my balanced attenuator in combination with the 10muF output caps don´t make a nice highpass).
The highs are also very detailed and fast but can be a bit grainy sometimes.

Listening to the ONO there´s a further loss of bass and the highs are a bit too much (like too much VTA) but I suspect that it needs a few more hours before it´s fully burned in so I´ll comment on that a bit later.

I think I will listen a few more days before I will start finishing the cases (the tops need lots of holes for cooling) and fronts.

Will keep you informed

[wuffwaff]

as I feared my choise of output caps wasn´t the luckiest.
The 10 muF together with the 1k2 output resistors give a nice highpass with a -3dB point of 30Hz and -1dB at 100Hz. So first I will up the resistors to 2k2 and the caps to 20muF.
This still will fit on the veroboard and give -1dB at 30Hz and -3dB at 9Hz. If this still doesn´t work I will have to use something like BG N 47muF like in Henriks schematic (wich fits even better )

william