BOSOZ for a beginner?

[KatieandDad]

Originally Posted by mannish boy

Hi everyone,

I'd really appreciate some advice please!

I'm trying to get my parts order together, but as I am hoping to get everything I need from RS Components here in the UK rather than importing from Digikey I need to make some parts substitutions.

1. Can I use a 2x55V transformer for the power supply? I think this should be enough for the 60VDC output but I may have missed something.

2. Are wire wound resistors suitable for R1 to R6?

3. Can I use 221ohm 0.6W Vishay MRS metal alloy resistors instead of the 0.25W metal film on the parts list? The 0.25W they sell are 10x the price of these Vishays.

4. I already have some 10uF film capacitors that may be suitable for the blocking caps C1 and C2, but they are huge (90x40mm) Obbligato film and oil types with the leads at one end. Assuming I can fit them in the case, would these be a worthwhile improvement over standard MKT/MKP types?

I'm going to wire everything point to point, so PCB size isn't an issue...

Cheers,
Patrick

Us DIYers can even make the big Obbligatos fit.

Here is the Pass B1 with Obbligato PIOs.

[mannish boy]

I'm pretty sure the info I've given is correct Andrew, does it make sense? The recommended pot value is 5Kohm and the cap value is 10uF.

[AndrewT]

fig2 & fig3 show what you need.
Yes, +-60Vdc for the BalZen
That +-60Vdc comes from the regulators. They need ~60Vac+60Vac from the transformer. Your 55Vac+55Vac will almost certainly do the job.

Attenuating pots on the inputs and outputs will ruin the balanced impedance of this BalZen.
Balanced impedance connections require very tight tolerances of impedances. Both resistors and capacitors must be very accurately matched.

[mannish boy]

I see, you were on about the transformer, I thought you were answering my question about the coupling caps! Thanks for that all the same.

It seems the more I try to learn, the more I'm confused !

[AndrewT]

re fig2 & fig3.
What are your specific questions?

10uF and a load of 5k//100k//receiver impedance will roll off the very low bass very slightly.

If I were doing this I would follow Rane's (or maybe it's Jensen) advice: A far too big non-polarised Panasonic electro//good film cap, to ensure that the whole audio signal can get out. You still need to match these component values between the Hot and Cold (what Pins 2 & 3 see).

[KatieandDad]

Most of the Pass Pre-Amps use a 1uF input cap, the B1 certainly does. Increasing its value simply decreases the low frequency roll-off point.

10uF is the normally specifed output cap. Again, you can increase this at extra cost.

It is the quality of the caps that really matter in this instance.

WIMA MKPs are good for the input cap and whatever you can afford for the output. 10uF MKPs will do but there are much better caps around for a good deal.

If you look at my post #11. The original had MKS 1uF at the input and Polydrops at the output. The final version had Siemens MKP's at the input and Obbligato PIOs at the ouput.

The FINAL, FINAL version had a Salas Shunt Reg PSU which did make a big difference.

[KatieandDad]

PassDiy do the B1 PCB at a good price. Its also easy to DIY.

The B1 will work up to about +/- 15V or a single 30V supply.

This is well outside of your traffo. Although it doesn't need it, it does work well with a regulated PSU.

[mannish boy]

Originally Posted by AndrewT

Attenuating pots on the inputs and outputs will ruin the balanced impedance of this BalZen.
Balanced impedance connections require very tight tolerances of impedances. Both resistors and capacitors must be very accurately matched.

I have been giving this some thought. One alternative, switched attenuators look ideal, but are expensive and look tricky to make. Another option I have been trying to get my head around, but not having much luck, is a balanced "shunt" type (U-pad) volume control, such as: Passive balanced volume control using a single attenuator? Possible? or Aleph P with balanced shunt attenuator. Looks cheap and easy to make, but will it work with the BOSOZ and if so, what value resistors and pots would I need (or better still where can I find this info for myself)?