Pass BOSOZ Balanced Preamp

Can’t wait to see photos and read your thoughts about how it sounds. Pretty amazing.

Thanks Joe,

It was a fun build and more adventurous than building the Pass B1 which sounds great by the way but it has no gain and it is not balanced. The BOSOZ has a lot of gain and is supposed to sound better then the original non balanced BOZ preamp. I could have done some of the wiring but not all of it. Plus I could not have put it on a scope and tested it. My EE friend already found that one of the MOSFET's were bad so he replaced them all with matching pairs.

We used a lot of higher end boutique caps and resistors and they do have better sound quality than most cheaper parts. The question is, will it sound better than the sum of the parts. You never know. When we built the B1 I used a relatively expensive TKD remote pot and it was too sibilant and bright but had great detail. I swapped in a much cheaper remote Blue Alps pot and the sibilance and brightness disappeared with just a slight sacrifice of detail.

Anyone want to buy a TKD 2511 MC remote volume pot?

Lol, the brightness and sibilants would kill me but I’m so happy you found a remedy. Just like a great recipe, a balance of flavor is the key. Well done!

I think if all goes well with this build, we may try and build the Pass First Watt F-5 turbo amp (50 watt at 8 ohm version or possibly the 100wpc mono blocks) some time next year. I will have to run it by my EE friend as I will need some help with that one. It would be a great winter project.

It’s great you found a project to work on with your son. I think the time together with him is worth just as much or more than the preamp.

Very cool indeed.
Interested in your opinion on this piece after some time with it.
Yes, we tend to gravitate to the "exotic" parts for our builds hoping they will give us an edge. And sometimes they do.
What is most interesting with Nelson's stuff is that the parts he uses are rather generic.
He seems to concentrate more on circuit design than magic parts.
But I'm like you, got to put in those premium parts..
Keep us posted on your "family" project.

jadedavid said:

Very cool indeed.
Interested in your opinion on this piece after some time with it.
Yes, we tend to gravitate to the "exotic" parts for our builds hoping they will give us an edge. And sometimes they do.
What is most interesting with Nelson's stuff is that the parts he uses are rather generic.
He seems to concentrate more on circuit design than magic parts.
But I'm like you, got to put in those premium parts..
Keep us posted on your "family" project.

Click to expand...

Will do.

Nelson Pass made all his kits with off the shelf cheap parts. He wanted to keep it cheap for people. There are thousands of pages on the DIY forum on all his kits, many seeing improvement in better quality parts. We used high end caps and resistors in the Pass B1 build with success. Caps made the most difference but resistors helped tune the tone and transparency. Naked Z foil resistors have great clarity and impart more air in the input and output stages. Carbon resistors add a little distortion perceived as warmth. Shinkoh and Audio Note Tantalums are more organic sounding. Middle and top of the line Clarity caps have tight powerful bass and no brightness. They are better than the obliggato golds but more expensive.(the golds have a huge almost 3D sound stage and warmth but bass is a little lighter and looser and they are relatively inexpensive for a boutique cap) We would have used Duelund or Jupiter caps but the chassis would have needed to be much bigger.

By the way, my EE who also repairs broken audio gear says the design of the BOSOZ is a little funky and there are simple things that could have been done to do make it more user friendly putting it together without sacrificing the sound. The design though is over 20 years old however.

Whoops, lost track of my point. A friend of mine built a B1 with different high end parts and his sounds great as well but different at the same time. So you can clearly tune your gear with different caps and resistors.

Tomorrow we finally get the finished preamp. My EE friend tested it and adjusted it. He says that it is very detailed and transparent with a large sound stage. I will post a review in a few days once it starts breaking in. It will take at least 100 hours for the caps to settle in.

I finally heard the finished product last night. I was shocked at how good it sounds. I was not expecting it to sound this good. It had great detail, air and transparency with space between instruments. Bass is punchy and fairly tight. Piano sounds natural and has excellent decay. Vocals are textured. The top end is smooth with an airey quality. It is neutral sounding and it has a huge, almost holographic soundstage with depth. My son commented that he was hearing things in recordings that he had not heard before. Once it breaks in some more, I will put it in my system to compare it to my BAT preamp. I would bet that it sounds as good if not better.

However, there are some issues that need to be fixed. There is a low hiss coming from both speakers that is not related to the volume level. There is also a hum when the phono section is playing and there appears to be a ground loop issue.

One other thing about the preamp, it draws .5 amps and after you turn off the preamp, music will still play for about 4-5 minutes do to the energy storage.

Great project and it's wonderful you get to share time on this working with your son !

Any Pass product is worthy of serious consideration IMHO. He's brilliantly innovative. I also greatly admire his willingness to share his circuit designs and provide technical assistance to educate the audio community over on diyAudio.

Waiting for your next project....