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Input impedance of Welborne Laurel II X 300B mono blocks?

S

Sime1

New Member
Dear Forum members,
I currently own a pair of Welborne Laurel II X 300B mono blocks with which I am utilizing a Solid State Preamp ( NAD C340). I would like to upgrade to a tube preamp, but before I do, I need to know the input impedance (in ohms) of my Welborne mono blocks in order to choose a preamp with the correct output impedance specs. Are there any technicians in the forum that have this information for these Welborne mono blocks that can help me? If there are, I would be very appreciative for your input.
Many thanks,
Sime
 
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Based on a schematic it appears to me it would ~100k, based on the 100k resistor from input to ground.

So, from the 10:1 (or more) rule of thumb, pre out impedance 10k or less. I'd think hard pressed to encounter a preamp with anything approaching 10k...unless maybe a passive pre of some sort. :dunno:
 
Thank you very much for getting back to me so quickly. I will definitely keep your input for future reference and if I get input from other forum contributors we'll see if they share your thoughts. Maybe someone has the actual specs from Welborne Labs.
Thanks again!
 
Sime1 said:
Thank you very much for getting back to me so quickly. I will definitely keep your input for future reference and if I get input from other forum contributors we'll see if they share your thoughts. Maybe someone has the actual specs from Welborne Labs.
Thanks again!
Click to expand...

What could be more telling than the resistor across the input? That is effectively the "spec".
 
Just about any currently-manufactured tube preamp is going to have a low-enough output impedance to drive the Laurels. 100K is a pretty standard input impedance these days. Most modern tube amps use a cathode follower or some other low-impedance output circuit. The Cary SLP-98, for example, has an output impedance of 440 ohms, plenty low enough to drive a Welborne amp. Only vintage tube preamps tend to have high-ish output impedances.
 
If you’ve got an ohm meter just measure it yourself then you will know for sure. Put one meter probe on the center pin of the input RCA jack and the other probe on the outer sleeve. Set it to auto range and measure ohms resistance. This works in most cases—the case where it won’t work is if the amp has a DC blocking capacitor on its input.

Most any modern active preamp, whether tubed or solid state, except certain vintage ones have low output impedance anyway, like in the neighborhood of a few K or less, so it will be easy to drive most any tube amp.
 
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Thanks to all for your input. The preamp that I am most interested in is the PrimaLuna EVO 100, however, it’s output impedance is 2,800 ohms which is pretty high, hence my reason for wanting to know what my Welborne’s input Impedance is.
I would love to have the Cary SLP - 98 but that’s out of my budget. That said, it appears as though I should keep my tube preamp search to those with an output impedance of around 5K - 10K. Any suggestions for a used tube preamp in that range and a budget of $1,200?
 
If your amp does indeed have a 100k input impedance then the Evo should work just fine. It’s not that high of an output impedance for a 100k load. The lower the output impedance the better as far as driving difficult loads or long cables.

See if you can find an Audio Electronics Supply (AES) preamp. AES was Cary’s DIY spinoff. AES is out of business now but their preamps come up for sale every now and again. The AE-3 or AE-3 Signature is one I’d look for.
 
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