SOBOZ / APOX Preamp finally finished

[Coulomb]

Well I finally finished a project!!

My PASS DIY BOSOZ Preamp with an APOX 5K Ladder as an output attenuator. I am not using any potentiometers on the inputs, just the 220R input resistors as I am using Zener protected inputs. As the BOSOZ is running at full gain all the time, chose to use P5 to set the overall system gain that gets attenuated at the output. I decide against potentiometers here as well and went for sonic purity. I made several gain modules of fixed resistances to adjust the gain until I found one I like best. I am currently using the 12DB setting achieved with 250R modules which is added to the 150R on the board in this circuit.

I am using the Preamp in the fully balanced mode through the APOX for input selection. I found when I used the APOX in SE mode the output from the BOSOZ was not equal on positive and negative outputs. In fact there was almost 20% less on the negative than the positive. This was easily corrected by grounding the negative signal right at the Preamp input, but rendered the APOX useles as a Balanced switcher.

For now I am running it in fully balanced operation and using a pair of DRV134 circuits to provide a balanced input from my SE devices.

The quality of the output is absolutely amazing, eclipsing my Primare Preamp in performance. The most noticeable aspect of it's effect on the music is the presentation of vocals. Midrange performance is superb, the preamp was a little harsh for the first few hours, but then it really started to mellow. The detail it presents is amazing, not the forward staging, but the sounds eminating from the rear of the stage where crystal clear. Passages that presented what can only be described as background din on my Denon reciever (3801) were actually detailed bits of instrumentals on the BOSOZ. My Primare also excels in this area, but it was $5,000!

As you can see from the photo's I put a lot more effort into the aesthetics than one might normally undertake for an entry level Preamp. I have a few more tweaks in mind for this unit, I am not even sure if it will make it into my main system, currently running off of a Denon TX-DS989. I wonder if I could talk the wife into a music only room.

In the following photo's you can see the Plitron transformer in the front right corner which feeds a pair of HEXFRED regulators. From the regulators I filter the output through a pair of 10,000 uF caps which are shielded and then to the standard BOSOZ power supply. The black cable running from the IEC connector at the back is in fact Tech Flex covering a ground sleeve isolating the AC line in.

Next to the Toroid closer to the center is the 5V supply and filter section for the APOX and Fan. It is also shielded.

On the left side at the back is the APOX input selector which feeds the BOSOZ, from thwere to the Relay/Resistor Ladder and then back to the I/O boards at the back. I use shielded cable throughout of course and any power lines running past the Audio Section is also been shielded. You can see extensive Copper/Brass shielding around the power supply and Preamp section.

I machined the Aluminum front plate (.375" thick) on a vertical mill, the rear plate was ordered from Front panel Express, I used thier software to do the layout. I mispelled BOSOZ, go figure!

The Top cover I bent up on a bender and cut on a brake, the Brass I cut with a bench shear. The bottom plate (.185) I ordered cut to size from Metal Supermarkets. The frame I made from 0.5" square Aluminum rod. The Chassis is painted in a Zinc Chromate for RF Shielding and finish, the top Cover is done with Tremclad Hammertone Black. The front panel was finished by rough sanding 80 grit, then 240, the 400 wet, being carefull to always sand in one direction only in striaght lines, which I made a wooden jig to accomplish.

Many Thanks to Nelson Pass for being the person he is, the guys at DIPCHIP who no longer make the APOX Kits and Sound Colony for the preamp boards.

High Rez Photo's can be found here.

http://www.briangt.com/gallery/coulomb-soboz-apox-pre

Regards

Anthony

[grataku]

Hi Anthony,
looks good.
is that the Apox1, right? I have a problem with that volume control in the sense that the number of relays is too little and the encoder too fine and a small rotation of the volume makes all the relays trigger.
It is weird because I don't remember that problem when I demoed the system.
Did you experience that?
I hope to be able to modify the code and introduce some delay in the action of the relays.
That wasn't the cheapest kit and I am disappointed that it is not in service, yet.

[Coulomb]

Originally posted by grataku
Hi Anthony,
looks good.
is that the Apox1, right? I have a problem with that volume control in the sense that the number of relays is too little and the encoder too fine and a small rotation of the volume makes all the relays trigger.
It is weird because I don't remember that problem when I demoed the system.
Did you experience that?
I hope to be able to modify the code and introduce some delay in the action of the relays.
That wasn't the cheapest kit and I am disappointed that it is not in service, yet.

Hello Paolo, It is the APOX 1 as you say. I have the last Hex file loaded that Craig and Dale made available. The velocity sense sub routine seems to work fine on mine, moving one step at a time when I turn the encoder slowly and rapidly progresses through the Restive ladder when a spin the encoder. I have noticed a different concern than yours and that is the amount of attenuation at the top of the Ladder vs the bottom.

Because the BOSOZ always provides a balanced out in my configuration, I compared the Negative to Positive gain ratio on a dual channel scope. at extremely low setting on the APOX, maximum attenuation, both positive and negative sides of the signal appear in phase. As I increase the volume the signals gradually shift to 180 DEG out of phase, most disconcerting.

Non of this really matters as the 5K ladder on the BOSOZ with 12DB Gain does not produce listening levels of 80DB at 3 Meters until the readout displays 200 on it's scale. Then from there it climbs in volume like a greased pig shot out of a cannon!

Regards

Anthony

[Coulomb]

For those of you too lazy to follow the link to the high Rez photo's here are a few reformatted to fit in the thread.

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Next... and why do my posts keep dissapearing?

Regards

Anthony

[Coulomb]

Wow, so much adilation, I am embarrased from all the attention!

I guess if your not part of the right clique you can even be shunned in this medium.

Oh Well, I started this for my own enjoyment, at least I do not need the feedback for validation.

Bye.