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

So the the point with a tube preamp with a cathode follower is they will typically (nothing is always) have a higher output impedance or one that rises quickly and/or both typically and this will cause a poor impedance match with a said matching ss amp.

So
with a preamp the output impedance rule of thumb is the lower, the better.

The simplified reasoning for this is the minimum of a 10 to 1 impedance match you want for your preamp to your amp.

There are many technical reasons for this but to make it easy to understand just know as a rule of thumb the minimum is 10 to 1 but to have more is better.

Why is this? Mainly to keep from having your bass roll off or other roll off issues.

So if the output impedance of a solid state preamp is 100ohm (SS preamps are typically lower than tubes) and the source (input) impedance of a solid state amp is 30k then you know you are good.

Why because 10 to 1 or 10 x 100ohm = 1,000ohm. 1,000ohm is much lower than amps input impedance of 30,000ohm or 30kohm. 

In this event you will be covered and good to go.

You will have your 10 to 1 covered via (10 x 100ohm) being 1,000 which is well under the 30kohm (30,000ohm). Remember the more head room the better. Here your amp max is 30,000ohm and you are only at 1,000 so you are in great shape.  

Now the idea is that a tube preamp with a cathode follower will have a much higher output impedance. Also, the impedance is not only much higher to start with but also rises much quicker also so its double trouble.

Lets say for this example this example the tube preamp with cathode follower output  impedance is 3,000ohm ohm. The Solid State preamp was only 100ohm. These are common numbers of tube and ss.

Big difference here. My SS preamp has an output  impedance of .5ohm which is nice and super low.

As a rule this is why SS preamps and SS amps work well together. Same said for tube preamps and tube amps. The output  impedance of tube preamps is high but many times the tube amps source impedance or input impedance is equally high to form a good match as such as 80k or 100k so it can handle the tube preamps higher output impedance.

This is why people say that tube preamp and especially tube preamp with cathode followers can be tough match for ss amps. Because tube preamps with cathode followers typically have higher output impedance and many (not all) solid state amps do not have very high source impedance to cover the 10 to 1 delta.    

So back to our tube preamp with cathode follower example that has an output impedance of 3,000ohm. Take the 3000ohm x 10 and that is 30,000ohm or 30k.

Your ss amp has a source impedance 30k and this puts you right up against the ss amps source impedance of 30k.

This you might be able to squeeze by with but not  ideal. The problem is like I stated before tube preamp with cathode follower not only starts high but also rises. At some frequencies it could rise to 5,000 or 6,000 and now you do not have the 10 to 1 covered and you can get bass roll off and/or top end roll off or both and now for no other reason than a improper  impedance match your rig will not provide anywhere near its potential.  Never mind how great each preamp or amp may sound if they don't match electronically your rig will be flat and rolled off and has all kinds of problems.

I hope this hacked version helped clear some of this up.

Thanks  

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?