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Bengali 15th August 2007 06:09 PM

Input Impedance
 
Hi,

My preamp specs shows an Input Impedance of 100K.
Does this mean it wants to see 100K or anything below it?

Normally I would connect the cd player to the input.

I'm going to add a remote volume POT between the
output of the cd player and the input of the pre amp.

Is there a sound or electrical issue when using a 10K,25K, 50K or 100K pot?

I'm not sure what is the correct value to use and why one value would work better than another.

My preamp does have it's own volume control.

Thanks for your help.

Bengali 15th August 2007 06:15 PM

Further clarification. It's a primaluna pro3 tube amp and a standard sony dvd player.

I read my previous post and it was suggested to stick with a 10K pot, now I'm sure if this was for a solid state amp.

So I'm still unclear on the correct value for the tube amp.

Thanks!

paulb 15th August 2007 06:40 PM

If the output impedance of the CD player is significantly lower than your preamp's input impedance, you will not have a problem. Adding a pot will increase the output impedance, but not below the value of the pot (at full volume, it doesn't affect it at all).
Even if the impedances are close, all it will probably mean is that you'll get a bit less volume. So you can use any of those pots. I'd choose 10K.

Bengali 15th August 2007 06:49 PM

Hi Paul,

seems 10K is the best choice. is this because it's the lowest
value and you want to keep the impedance low?

I'm in the U.S. I cannot find an online place to order the 10K value(Alps Blue Velvet Motorized).

Also, I'm looking for internal coaxial(RCA) cable. Do you know of any online sources?

Thanks again!

AndrewT 15th August 2007 06:58 PM

Hi,
I'm wondering if you plan to add this CD attenuator to make it's output level match other lower level sources?

The usual rule that works fairly well is source impedance (Rs)<<Input impedance (Zin).
<< can be interpreted as 5times less but 20times is slightly better.

With your Zin=100K this indicates a pot value of 5k or 10k.

Now that you have a pot value apply the rule a second time for DVD. It's Rs <5k/10 or about 200ohms to 500ohms.

If your DVD is less than 500ohms then the passive attenuator between the DVD and pre-amp should be successful. But, you MUST ensure that you keep capacitance down to a minimum, particularly after the attenuator.

Bengali 15th August 2007 07:38 PM

Hi,

that's a great question. to match other low level sources would require some kind of op amp, correct?

I checked the specs on my sony sony dvp-ns90v dvd player.

It says, line out(audio): 2Vrms 10k ohms

So what you are saying is as long as my source output impedance is between 5K-10K, it should be okay.

Any source with an output impedance below 5K requires some kind of buffer amp(?)

just curious, if I did use a 10K pot and hooked up a lower level output impedance source, what happens? lower volume or more noise?

thanks.

AndrewT 15th August 2007 08:51 PM

Quote:

Originally posted by Bengali
that's a great question. to match other low level sources would require some kind of op amp, correct?

I checked the specs on my sony sony dvp-ns90v dvd player.

It says, line out(audio): 2Vrms 10k ohms

So what you are saying is as long as my source output impedance is between 5K-10K, it should be okay.

Any source with an output impedance below 5K requires some kind of buffer amp(?)

just curious, if I did use a 10K pot and hooked up a lower level output impedance source, what happens? lower volume or more noise?

thanks.

no, no and more no.


If the average output level from the CD/DVD player is too high then a simple attenuator is sufficient to reduce the level. One does not need to use an active (opamp) attenuator.

The 10k is the load specification not the source specification.
The spec is telling you to keep the load at 10k or above.
That fixes the pot to a 10k value.
Hopefully, this indicates a source impedance (=Zout) of the DVD <1k0, giving Rs~=Zin/10. That rule again.
With the 10k pot as the next source, the Rs<100k/10 is met. The electronics are happy.
Just use low capacitance cables and keep them short. 30pF to 50pF per metre would be suitable. 1m to 1.5m before the pot and 0.1m to 0.5m after the pot would work well.

If the source impedances are lower, then the voltage delivered to the receiver is higher not lower. The noise rejection of the lower impedances is better than high impedances. This applies particularly to interference. So your lower impedance source will send less noise to the output than a higher source impedance.

Finally, that 2Vrms is the maximum signal that the DVD can put out. The average level from there is likely to be about 10db to 20db below that for louder music recordings. Fortunately movies are not recorded quite so highly and they reserve a bit more for the loud moments using the dynamic range a bit better. I expect the average level to be 20db below maximum. Have you already noticed that CDs sound a bit louder than movie sound tracks?
2Vac @ -10db ~=700mVac, & @ -20db ~=200mVac.

Bengali 15th August 2007 10:24 PM

thanks Andrew for further clearing things up.

yup, the dynamic range of the dvd movies is a bummer especially at night. the voices are always too darn low. I always have to boost my center channel to compensate.

I have been looking for a dedicated CD player but have not found one, even a used one to justify the high price. I've been looking at the Ah Toejb 4000 or the Jolida CD player but even at used prices they are so costly.

I just cannot understand the high costs involved with dedicated cd players from $500 and on upwards. Even an HD or Blu ray player is cheaper these days than a 'audiophile' cd player.

The question then remains, once I get one of these cd players, would I really hear any difference from the sony dvd player I'm currently using.


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