Cube Audio Nenuphar Single Driver Speaker (10 inch) TQWT Enclosure

Chris ( @cal3713 ) thanks for quoting my post from another thread in this one, where it is also relevant as you indicated.

@riaa_award_collectors_on_facebook, keep in mind that the majority of tube-based components having line-level outputs employ coupling capacitors at their outputs, which often cause their output impedance at deep bass frequencies to rise to **much** higher levels than the specified output impedance (which is usually based on a mid-range frequency such as 1 kHz). So in those cases the 10x ratio should be applied to the 20 Hz output impedance, if it is known. (If the component has been reviewed by Stereophile JA’s measurements will indicate that number).

If the 20 Hz output impedance is not known, and there is reason to believe that the tube-based component employs an output coupling capacitor as most of them do, I suggest applying a ratio of 50x to 75x to the specified output impedance. Some here have even suggested 100x.

Also, the minimum recommended load impedance ARC almost invariably recommends for their line stages and preamps is 20K, including for the Ref 10, even though its specified output impedance is 600 ohms balanced and 300 ohms single-ended. And for a few of their designs I’ve seen recommendations of "60K recommended, 20K minimum."

And just to cite one further example, the Herron phono stage which I use has a specified output impedance of 400 ohms, but its manual recommends a minimum load impedance of 50K "for optimum performance."  Although Keith Herron has indicated that 20K would usually be acceptable, in most systems.

Perusing JA’s past measurements of tube-based line-level components will reveal numerous other such examples.

Regards,
-- Al

Thanks Al.  I should have paid more attention in school :)  Somebody gave me that formula and told me the REF 10 and Bakoon would be fine. Thanks for stopping me from blowing up my gear!! LOL

Although I know RIAA's above comment is in jest, just to be clear for future readers... If you don't follow the 1/10th ratio rule, you will not get a flat amplification of the preamp signal with frequency aberrations in the bass response.

Also, it may be possible to modify the amplifier to address this issue (depending on amplifier design). My first watt f4s (simple class A without feedback) have a 47.5k input impedance, but can be modified to have a higher I.I. simply by changing out the resistor on the input.  I was informed that I could use anything up to 250k without changing the operation/sound of the amp. Doing so enabled me to use much smaller output caps on my Don Sachs 6sn7 preamp, and allowed me to afford some top end duelund's that greatly improved the sound. No idea if this is possible on the bakoon... but hopefully useful information regardless.

Having an outstanding tube line pre driving a 10k load is possible. I fact I had Don Sachs configure one output to drive a 10K load and one to drive the normal 100k load. Sounds fantastic!  Just takes a big ass high quality cap.