Changing from 16-ohm to 8-ohm tap for neg.feedback on ST-70

[mugsy]

Hiya!

Is it possible to change from the 16-ohm to the 8-ohm tap for the negative feedback connection (as was done for the ST70 series 2 design) ? If would I need to change any components to accomodate this change? The series 2 manual briefly mentions the gain resistors need changing?

Many thanks.

- John

[erichayes]

It's not only possible, but desirable to hook the NFB to the tap you're using to power your speakers.

I'm gonna make you think a little, John, about what changes would have to be made. (I don't have a schematic in front of me, so I don't know the actual resistance value of the NFB resistor; let's call it 4KΩ.) At a given output level, there will be X volts present at the 16Ω tap. There would be half that voltage present at the 8Ω tap, and a quarter at the 4Ω tap. If the given output voltage is 1 volt at the 16Ω tap and, because this is a canned problem, there's 1mA flowing through the NFB resistor, according to Ohm's law, what would the resistance have to be changed to using the 8Ω tap in order to maintain that 1mA current flow?

[mugsy]

Use..... my BRAIN?!?!? AGHHHHHHH :o

You task me, Sir!

Haha OK well here goes:

If - as you say - 1v is on the 16v tap, then must be 0.5 volt on the 8-ohm tap.

So am I correct in saying I need to divide 0.5volts by 0.001 amps to get the resistance value? Which would be 500 ohms?

Incidently the NFB value from the 16-ohm tap on the circuit diagram is 1k.

Circuit can be found here:

http://www.quadesl.com/schematics/st70_schematic.gif

Thanks,

- John

[erichayes]

Well, you got the right answer, for your purposes, but your methodology doesn't take resistance into account.

In my example, 1 volt was flowing through a 4KΩ resistor at a rate of .001 amp in the 16Ω configuration. By connecting to the 8Ω tap, the voltage available is halved. In order to maintain the same current flow, the resistance must also be halved--in this case, to 2KΩ.

The reason you got the "right" answer was because your conditions were even more canned than mine; you had 1 volt flowing through 1K at 1mA. The Kilos and millis cancel each other, so half is ½ is 0.5 is 50% of 1000, which is 500Ω

[mugsy]

Ahhh I see!

OK so how to I apply this to my original question please? The existing NFB resistor is 1K on the 16-ohm tap. I'm unsure of the current/voltage running from this point - bit of a newbie here!

BTW I noticed on the circuit for the ST70 series 2, that they use the 8-ohm tap instead for NFB. They use a 680 ohm resistor, along with a 390pF capacitor. I imagine copying those two values would work as the circuit is largely the same as the original ST70?

Thanks,

- John