Passive Pre Noob Question

[Hewie]

I am building a passive preamp to temporarily control a non-buffered IGC (Lm3875). I am using a 100k Noble pot with the wiper connected to the output, one leg connected to input, and one leg to ground. Many of the schematics I see posted around use a 10k resistor on the input. Why? I haven't included it, am I mistaken?

Thanks,
Dave

 

[officeboy]

It's the amp's input resistance. If you keep using that pot as a volume control you'll be ok. But if you hook something up directly there will be very little input impedance, and you risk blowing up the output of your source, or sending too much current into your amp's input.

 

[Hewie]

Sorry, I must not have been clear. The 10k resistor I am questioning is on the input to the pot, not in the GC circuit itself.

Thanks,
Dave

 

[Nuuk]

Many of the schematics I see posted around use a 10k resistor on the input.

Are you getting confused with the 10K input resistor on a stepped attenuator?

 

[Hewie]

This is the schematic I am questioning.

Thanks,
Dave
PS I hope I attached the image right.

 

[Nuuk]

This is the schematic I am questioning

To me, it looks as though they are using a pot as part of a shunt attenuator. |In which case, the 10K resitor is necessary. But if that's the case, why haven't they specified the vvalue of the pot?

I suggest that you use your 100K pot in the 'usual' way and not worry about the 10K resistor!

 

[Hewie]

Thanks Nuuk.

And also, thanks for the great website and GC instructions. I have a prototype up and working that was completed almost entirely from the directions on your site. I'm working on a case now. Once I finish the case, I'll post pictures and the results. But even using a portable CD player as a source and rubish speakers, I am amazed at how this little thing sings.

 

[Nuuk]

Once I finish the case, I'll post pictures and the results. But even using a portable CD player as a source and rubish speakers, I am amazed at how this little thing sings.

Yes, I don't understand how they do it, but GC's seem to breath new life into just about anything they are used with!

Looking forward to the pictures!

 

[mcs]

Remember to keep the cable between your passive preamp and power amp as short as possible. With a 100k pot the output impedance will be very high. I would normally use a 50k max. pot for a passive preamp.

Best regards,

Mikkel C. Simonsen

 

[hihopes]

If you really want to get the most out of your passive pre, you should seriously think about upgrading to a stepped or even a ladder attenuator. The difference in clarity between that and even the best pot is like day and night. (provided, of course. your system is reasonably revealing already.

 

[Nuuk]

If you really want to get the most out of your passive pre, you should seriously think about upgrading to a stepped or even a ladder attenuator. The difference in clarity between that and even the best pot is like day and night. (provided, of course. your system is reasonably revealing already.

I would say Amen to that but he did say that this is only a temporary solution!

 

[hihopes]

Right NUUK. It was late at night. I must have been half asleep.