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kennethrl 28th November 2005 04:41 PM

BOSOZ R18 & R19 location
I am making a BOSOZ and I'm having trouble locating where the R18-19 221 ohm 1/4 watt metal film resistor go. They don't appear on the board.

Also the P5 pot. do I need two pots (one for each channel) or can one be used?

hifimaker 28th November 2005 05:15 PM

Who's board are you using?


kennethrl 28th November 2005 07:28 PM

Old Colony board

SteveA 28th November 2005 07:36 PM

They're not on the board. They go with the input pots (if you choose to use them, ie, the pots/resistors). I chose not and the preamp works fine. I'm sure mine lacks the ability to fine tune but, like I said, it sounds fine by me.

Raoul 28th November 2005 07:38 PM

I think R18/R19 are located off board and in line with the positive input on the main board. In other words, between the input pin from the input RCA and the "+" in on the mainboard. I can check the manual when I get home.

Hope this helps.

edit: Looks like SteveA beat me to it.

kennethrl 28th November 2005 09:32 PM

Thanks for the replies....How about the P5 pot. Do I need a dual gang pot or will one do the job.

Raoul 28th November 2005 11:39 PM

A follow-up on to R18/19: You want to use them even if you do not use the input pots, assuming you are using the input protection zeners on the board. This should be covered somewhere in the article, I believe near the end. The idea is that zeners do not like to be presented with a voltage directly.

Also, be careful with the amount of heat you apply to those Old Colony boards. They don't like a lot of heat.

If you use the board single ended only, don't forget to ground the negative input using a small resistor. I think I used a 220 ohm.

I still have some left over transistors and maybe a few other parts I am not using if you want them. You can contact me through the board mailing service if you are interested.

Not sure about P5 off-hand. Sorry.

Raoul 29th November 2005 02:12 PM

About P5:

I think you will need one pot per channel, or a dual gang pot for both channels. Seperate pots may give you more control to accurately match the gain between channels, but I don't think this should be much of a problem. It seems most constructors (myself included) use fixed resistors (R15) to control the gain.

Hope this helps.

kennethrl 29th November 2005 03:09 PM

Thanks for the info.. What value fix resistor value is used for the P5?

Raoul 29th November 2005 03:48 PM

You can use a piece of wire to connect the pads of P5. You can then adjust the amount of gain by adjusting the value of R15. If you leave R15 at 124 ohm as called for on the BOM, you should get about 20dB of gain. Hidden in the very last sentence of the instruction manual, Nelson explains that an R15 value of 430 ohm should yield about 10dB of gain. Less gain (10dB) yields better performance, provided your system can be satisfied with less voltage coming out of your pre-amp. It is easy to swap resistors for experimentation, but again, be careful how much heat you apply to the Old Colony PCBs.

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