PLEASE match Advice Pass Labs X350.5 (20k) to Hovland HP-100 (2500ohms) Miss-match ???

Hi Analogluvr,

I believe you are referring to the following statement by the OP in the last of his posts dated 10-7-2017:

My buddy told wanting tubes try and find a tube amp that can both get the desired output impedance and preferably one without a cathode follower. Those tend to cause different impedance swings verse a pre uses tubes in paralell [sic] to lower impedance.

You are quite correct that cathode followers are commonly used in the output stages of tube-based preamps and other tube-based components which provide line-level outputs, because cathode followers can provide lower output impedances and better drive capabilities than other circuit configurations employing the same or similar tube types.

Usually the impedance swings he refers to are mainly the result of the coupling capacitor that is used in the majority of such designs between the output of the cathode follower circuit and the output of the device. The impedance presented by a capacitor rises as frequency decreases, so as you probably are aware the output impedance of a tube-based component, or even a solid state component if it employs a capacitor in series with its output (and some do), can be far higher at deep bass frequencies than its specified output impedance. Output impedance specs usually being based on mid-range frequencies such as 1 kHz. That variation in impedance can cause deep bass rolloff and unwanted phase shifts if the input impedance of the connected component is not high enough.

A capacitor having a relatively large value (i.e., a large number of uf, meaning microfarads) will result in that effect being much less significant than with a capacitor having a smaller value. However choosing a capacitor having a relatively large value can often bring tradeoffs into play involving the size, cost, and sonic quality of the capacitor.

The bottom line is that if the power amp has a high enough input impedance to be a suitable match for the output impedance of the preamp at 20 Hz all of this will be a non-issue, and there will be no reason to avoid preamps which use cathode follower output stages. And if the input impedance of the power amp is ten or more times greater than the 20 Hz output impedance of the preamp the match will be suitable. If that ratio is less than 10x the match may or may not be suitable, depending on how the output impedance of the preamp varies over the frequency range.


Best regards,
-- Al

Btw I heard HP100 many yrs. ago. I remember it sounded beautiful non offensive and had a see through quality to it and did all the nice soundstage tricks but lacked that prat and tranisent bite!

This post is PLEASE match Advice Pass Labs X350.5 (20k) to Hovland HP-100 (2500ohms) Miss-match ???

The 10 to 1 match is not covered here but it is close enough to be able to try (maybe) if the 2500ohm doesn't rise real high at other frequencies or better yet goes lower.

The problem is that it does rise and a lot so in this case the Pass Labs X350.5 (20k) and the  Hovland HP-100 (2500ohms) is not only a mismatch but a poor match choice.

The Stereophile measurements below show the Hovland HP-100 (2500ohms) rising to all the way to 4.3k ohms (43,000ohm) at 20Hz.

They recommend at amp with an input impedance of 47k.

The Pass X350.5 has an input Impedance of only 20k.

If this preamp( Hovland HP-100) is used with (Pass Labs X350.5, 20k input impedance) you will lose bass according to the Stereophile's measurements (quote and link is below) 

You will see this play out many times with tube preamps and SS amps for no other reason than tube preamps typically have higher output impedance and SS amps have lower input impedance.

The reason many of these tube preamps have cathode followers is because if they didn't their output impedance would be so high they could match with very few ss or even tube amps.

Some people look at a cathode follower as a band aid for a poor design with way too high of an output impedance. Sort of like how some SS designers use negative feedback to lower distortion. Sure it lowers distortion but at what price? That said there are some amazing sounding amps that use negative feedback just like there are some amazing sounding preamps that use cathode followers. I subscribe to either school. 

For me it comes down to how it sounds and does it match well? If it matches well and sounds great then I am happy! 

Just get to the 10 to 1 across the board and you are good.     

Quote froom Steroephile 
" Its input impedance was a usefully high 100k ohms in the midrange and bass, with just a small reduction to a still-high 86k ohms at 20kHz. Despite the claim that the HP-100 uses a cathode follower output stage, its source impedance was high at 2.4k ohms, rising to 4.3k ohms at 20Hz. The partnering power amplifier would best have an input impedance of at least 47k ohms if the bass is not to sound lea".
Read more at https://www.stereophile.com/content/hovland-hp-100-preamplifier-measurements#Kw6gd5Q68B4s6o33.99

Thanks Al. Is it not possible to do a direct coupled cathode follower?

@Analogluvr, while there are of course ways of designing a tube-based line-level output stage to provide low impedance without using coupling capacitors (for example transformer coupling, or Atma-Sphere’s patented differential output circuit which uses a servo-based method of nulling out DC offsets), I’m not sure how or if a simple conventionally designed single-ended cathode follower circuit might be designed to be direct coupled. That might be a good question for Roger or Ralph in Roger’s "ask an amplifier designer" thread.

By definition a cathode follower has a resistor (or resistors) connected between the cathode and (at least usually) the ground of the circuit. The bias current flowing through the tube and consequently through that resistor will cause a significant DC voltage to be present at the cathode, which must be prevented from being sent into the power amp or other connected device. A capacitor is of course a simple and likely the least expensive way of doing that.

The DC voltage that is developed at the cathode, btw, will be positive and typically serves the purpose of biasing the tube. In effect it makes the grid negative with respect to the cathode, and thereby biases the tube at a desirable operating point. There are other means of developing bias, of course, by directly applying a negative voltage to the grid, but that kind of approach is less commonly employed as it adds complexity, has tradeoffs of its own, and a cathode resistor would still be required anyway if the circuit is to function as a cathode follower.

Best regards,
-- Al