Source impedance - passive linestage/preamps

Just started knocking a basic linestage up. But I've just been reading unsettling things about source impedance. I need a low impedance for this?
Example: the power amp (Quad 306, or 2x bridged Rotel RB971) is connected to the linestage (alps blue pot and rotary selector) and sources are cdp, phonostage, computer soundcard. Will this work?
Can specs be got telling me output impedance for these sources and input impedance for the amps? Am I worrying needlessly?
Thanks
Harry

 

no you don't worry needlessly as what is driving your amps is output imp of your sources. that's why i believe that the passives are better option for a single source system with a fixed source impedance than having 3 different sources with 3 different impedances that might produce different sounds. if they are rather similar than of course it is harmless but in the case they are not, you may hear serious differences.
the lower output impedance the better for a passive preamp, that seems to be a rule.

 

ok, that makes sense. But how do I know what a sources impedance is? Can I measure it (guessing not as it conatins a complex number doesn't it?), is it ever stated?

 

You can't really measure it yourself but it should be in the specs of the kit, which you might also be able to google. The general rule is that the upstream item (e.g., CD player) should have a low output impedance relative to the downstream item (e.g., preamp). This applies to everything really, but it potentially pops up more often as a problem with potentiometer-based passive pres, whose input and output impedance can vary according to the volume position. There are rules of thumb such as minimum 1:10 or even 1:100, so if the source is 500 ohm and a preamp is 50KOhm or more then that would be fine.

 

this is an interesting question ......

I was looking in two essentially the same valve power amps yesterday.

the both with 100k resistors from signal to ground on the input to set the impeadance [I may be wrong!] the odd thing one of the power amps had a 1m res in signal in put I can't work out if this was an error or to act as an attenuator. Both were wad kits by the way .... The power amp with the 1m required the volume control to be at 2pm for a decent volume where as the other produced the same volume at 9pm.


My initial thoughts were to to just desolder the 1m to make both units identical .......as the owner is thinking of bi amping hence the two WAD amps.

 

ok, I'll apply these rules of thumb. But first I need to keep trauling and bring up some impedance values. Finding them a bit tricky right now. And I'm not sure I could do Zanesh's trick and pull the cover off and identify what's setting the impedance... Maybe I'll just go and try, nothing's going to break the bank right now

 

I have just found that (amazingly or not?) Wikipedia explains the principles quite nicely, e.g., output impedance, but in practice I'd be inclined to look at reviews of passive pre's to see the issues involved, and try it, as you say, if the numbers didn't look too wrong.

I had to go through this to check in a similar situation recently. My most important sources were down in the low 100s output impedances, the amps never dropped below about 50 K throughout their impedance curve. Even so, the amp's review said that tonal balance was likely to be affected with source impedances more than 1.5 KOhms. So I chose a type of passive preamp where the worst case output impedance was just a few hundred ohms and at other volume settings was much lower, and the impedance presented to the source is typically very high (250 K)

As to how you actually modify or solder kit to better reach your requirements, I haven't a clue

 

It's necessary to hold the grid of the input valve at 0v to establish the operating point of the amp, the resistor to ground does this. The grid must be negative of the cathode for the valve to work, the cathode resistors set this according to the standing current you want through the valve.

More or less.

So yes the 100k sets the input impedance, but it's also necessary to the operation of the amp. I guess it's value is somewhat arbitrary.

Paul


Paul