Dallas Rangemaster clone: Various questions

[Cozybuilder]

That looks better.

I'm no fan of the protection diode wired to either ruin the battery or the circuit if it goes first. IMHO its far better to place a 1N5817 Schottky in series with the supply source and accept the 0.2V drop. If power is connected backwards its blocked, no fried battery, no fried circuit components.

[mac]

I'm no fan of the protection diode wired to either ruin the battery or the circuit if it goes first. IMHO its far better to place a 1N5817 Schottky in series with the supply source and accept the 0.2V drop. If power is connected backwards its blocked, no fried battery, no fried circuit components.

I like a series Schottky too,
and since it has a small on resistance, I add a big cap after it to have a sort of polarity protection RC filter.

There is a mosfet protection at Geo, iirc.

mac

[R.G.]

There is a mosfet protection at Geo, iirc.

Yes, there is: http://www.geofex.com/article_folders/mosswitch/mosswitch.htm
and:
http://www.geofex.com/article_folders/mosswitch/mosswitch.htm

The economics have changed a little since 1999 when the first protection article was written. The P-channel TO-92 MOSFETs now are about $0.40 to $0.70, so they're a bit cheaper, but N-channels are dramatically cheaper, down around $0.10.

Both the MOSFET and transistor solutions can have significantly less voltage drop than even a Schottky, but not as low as a sacrificial reverse biased diode.

[Cozybuilder]

I use the BS-250 P-channel MOSFET protection on a lot of builds- its just a few mV of supply drop, well worth it.

[pedlar]

(Thanks for the above tips on protection!)

No real questions this time, just thought I may as well keep this thread updated until I complete the project.  So, here are some pics of the circuit laid out on a breadboard.




I have yet to put a guitar signal through it.  So far I have just been checking what voltage I am getting from the collector (to ground) with each transistor (Ideally they should be getting approx -7V.)
It looks like the NKT275 will be the one most likely to give the original sound.  I can get it to -7v without too much adjusting of the two biasing variable resistors.  I got the OC44 to about -7.2, but both resistors are pretty near their limits of what is ideal for this circuit.  I do have the option of using a higher value boost pot (25k instead of 10K) to get the OC44 working, so I will try that too.  Again, all my information is from here: http://www.geofex.com/article_folders/rangemaster/atboost.pdf