[Dion]

I've been told a while back that my tube preamplifier needs to be matched with a high input impedance power amp - the minimum is 45k. my amp has 50k. I recently purchased an aci force and I believe it has input impedance of 10k. will this be a problem if i matched it why my tube preamplifier?

thanks,

[WEEZ]

Hi Dion,

Here is the formula: (Zin of Amp 1 X Zin of Amp 2)  divided by (Zin of Amp 1 + Zin of Amp 2)

In your case, the load the pre-amp would see would be 8.3k.

WEEZ

[Steve Eddy]

Your preamp would still work, but you'd lose some signal gain and some bandwidth.

Then reason your preamp wants to see such a high impedance load is most likely because it has a rather high output impedance, which is fairly common for tube preamps that use little or no negative feedback, or don't use a cathode follower or output transformer on the output.

I'm not familiar with the ACI amp in question, but there's probably no reason it couldn't be modified to give a 50k ohm input impedance which would make it a better match for your preamp (and I don't fault the ACI amp for its 10k input impedance but rather the tube preamp for its high output impedance).

se

[Lkdog]

My tube preamp has a rather high output impedance. I forget exactly what is is-maybe 1.2-1.5K ohms or so.
I use Odysssey Stratos Monoblock Extreme amps with a low input impedance of 11K.
I have dual LFM SE subs which use a Force amp you speak of. I use preouts straight from the preamp.

I have absolutely no issues with either my monoblocks or the Force amps in the subs.
I discussed this with the tube preamp manufacturer and the amp manufacturer ahead of time.
Both indicated there would be no issues. The input impedance is still 10X the output impedance of the preamp.

I also know that the new Odyssey Candela tube preamp has an output impedance even higher of over 2K. No one has had issues with the Odyssey amps relatively low input impedance.

Ask Mike when he gets freed up, but I doubt you will have any issues.

[dion]

Weez - thanks for the calculation.

Steve - thanks for the info. can you be more specific when you say "lose some signal gain and some bandwidth." which bandwidth am I loosing and is the loose of signal gain uniform across whole bandwidth spectrum? Also, what do you need to do to increase impedance input? I'll ask Mike if he doesn't reply here shortly.

Lkdog - I'm glad to hear your story. I'll ask the pre amp company (VTL) and Mike about this. my preamp has 200 ohm of gain (lower than yours) but VTL recommends matching with something over 200x (40,000 - 45,000  ohm). Since you have done your research, perhaps you have some insights to my questions above.

thanks

[Lkdog]

Yeah, I would doublecheck with Mike-maybe he can mod the amp they use for you in the Force if needed.
I don't know the VTL preamp design-but they would know for sure if it presents a problem.
Sounds like you are fine with your mains amp @ 50K.
Also-there may be no issue with impedance matching with a sub amp due to not reproducing signals over 150hz.

[Steve Eddy]

Steve - thanks for the info. can you be more specific when you say "lose some signal gain and some bandwidth." which bandwidth am I loosing and is the loose of signal gain uniform across whole bandwidth spectrum? Also, what do you need to do to increase impedance input? I'll ask Mike if he doesn't reply here shortly.

A good visual example is this frequency response plot of the Cary SLP-05 from the September, 2006 issue of Stereophile:



The plot is of the Cary driving loads of (from bottom to top at 100kHz) 600 ohms, 1k, 10k and 100k.

You may not notice any difference. Just explaining why makers of tube preamps recommend the minimum load impedances that they do.

As for increasing the amp's input impedance, depending on the amp, that could be as simple as replacing a 10k ohm resistor with a higher value resistor. But even if you made the amp's input impedance the same as your other amp, your preamp would still be seeing a load impedance half that of the recommended load impedance. This is because if you're driving both amplifiers (something I didn't know you were wanting to do at first), their input impedances are in parallel with each other as far as the preamp's concerned.

se

[Steve Eddy]

Also-there may be no issue with impedance matching with a sub amp due to not reproducing signals over 150hz.

That wouldn't make any difference. Unless it's a rather unusual sub amp, its input impedance would be the same regardless of frequencies the amplifier itself was reproducing. And even if the input impedance was 10k ohms only below 150Hz, it's not just high frequencies that will be rolled off.

se

[Lkdog]

Interesting info and graph data.
Thanks for sharing.

Guess first place to start is checking with mfr's of the units and getting their input.