John Curl's Blowtorch preamplifier part II

[godfrey]

Originally Posted by john curl

Cost is not so important, minimum number of series relays is important.

Hi John

It is a pity that it is impractical to use two separate pots (for high range and low range). That would allow only one relay contact to be in circuit at a given time. It would also be easier to use with only 2 switch positions.

I understand the difficulty of having special pot/motor combinations custom-built, but would it not be possible to use two completely separate motorized pots?

Regards - Godfrey

[john curl]

Too much hassle, and space, I'm afraid.

[godfrey]

Originally Posted by simon7000

In the case of a 1K pot with an end resistor the resistors should be 988 ohms, 1.91k, 3.75K, 7.39K, 14.3k, 28K and 54.9K.

Hi Simon

The problem with using this kind of approach with a 1k pot is that you end up with about 100 different switch settings.

Just sweeping e.g. from 50k to 60k will involve going through 10 combinations of series resistor. There will be much clattering of relays - not ideal while one is trying to tune by ear!

Cheers - Godfrey

[simon7000]

Originally Posted by godfrey

Hi Simon

The problem with using this kind of approach with a 1k pot is that you end up with about 100 different switch settings.

Just sweeping e.g. from 50k to 60k will involve going through 10 combinations of series resistor. There will be much clattering of relays - not ideal while one is trying to tune by ear!

Cheers - Godfrey

Depending on the complexity of the controller more relays can be used to minimize the number of switches being used as shunts. I don't think the miniature relays being used are as noisy as you expect. Inside a solid case or two they should be well below the problem level.

Somewhere around 15 relays will limit you to mostly 2 switches sometime 3.

[SY]

Originally Posted by godfrey

Hi Simon

There will be much clattering of relays - not ideal while one is trying to tune by ear!

That's why God invented the reed relay.

[john curl]

I am blocked once more, lost another one, that would clarify the situation.

[john curl]

Well thanks everyone, for your inputs. It might be better to move onward to some other topic that is more general.

[simon7000]

John,

I am still measuring resistors and an interesting artifact has shown up in measuring the Vishay bulk metal film resistors. From my preliminary measurements I would characterize these as being a "Warm" sounding resistor.

Have you or any one else have experience with them?

[john curl]

Interesting

[john curl]

Simon, we will probably use naked Vishays for our project. I don't mind using original Roderstein resistors or Corning Grey resistors, either.