Interconnects between SS and valve components

Technically (yes, I know that's a four-letter word here) impedance matching between SS and tube gear can be problematic. SS gear typically has low (100 ohms or less) output impedance and relatively low (~10 k ohms) input impedance (some SS does have much higher input impedance). Tube gear can have output impedance over 1 k ohm (well over if there is not a cathode follower) and input impedances of 100 k ohms'ish. As a result, SS gear usually has no problem driving tube gear, but tube gear can have problems driving SS gear. The latter can result in lower output and higher distortion and noise. This is a problem no matter the interconnect (unless the cable includes some sort of impedance transformation or active buffer).

Higher tube output impedance can also reduce bandwidth but that is usually negligible in most systems. Lower-capacitance interconnects could help but are rarely the dominant capacitance. Higher impedances can also increase noise sensitivity, so better shielding may help in some cases.

Note some tube preamps include SS buffers, often output FETs, to reduce their output impedance.

FWIWFM - Don

bonzo75 said:

What if I was using a valve dac or phono to drive a SS pre, will this arise?

Click to expand...

Thanks for raising this Kedar. I am grappling with exactly this issue. On some tracks, I can hear a bit of distortion ( even at low spl) with the lampizator driving my pre. My pre has an input impedence of 11kohm and I have currently set it as low as -6db. Not sure whether there is signal saturation or what but strangely it only happens only on some tracks.

Would be grateful to get some steer on this.

Cheers

Sujay

DonH50 said:

Technically (yes, I know that's a four-letter word here) impedance matching between SS and tube gear can be problematic. SS gear typically has low (100 ohms or less) output impedance and relatively low (~10 k ohms) input impedance (some SS does have much higher input impedance). Tube gear can have output impedance over 1 k ohm (well over if there is not a cathode follower) and input impedances of 100 k ohms'ish. As a result, SS gear usually has no problem driving tube gear, but tube gear can have problems driving SS gear. The latter can result in lower output and higher distortion and noise. This is a problem no matter the interconnect (unless the cable includes some sort of impedance transformation or active buffer).

Higher tube output impedance can also reduce bandwidth but that is usually negligible in most systems. Lower-capacitance interconnects could help but are rarely the dominant capacitance. Higher impedances can also increase noise sensitivity, so better shielding may help in some cases.

Note some tube preamps include SS buffers, often output FETs, to reduce their output impedance.

FWIWFM - Don

Click to expand...

Thanks, Don, for this informative post. I have an SS DAC (Berkeley Alpha 2) driving tube amps, so no problems here.

bonzo75 said:

What if I was using a valve dac or phono to drive a SS pre, will this arise?

Click to expand...

For low-level components:

• Valve anything driving SS anything is potentially an issue. You have to take it case-by-case and look at the suggested minimum load on the tube (valve) device versus the input impedance of the SS device.
• SS anything driving valve anything is rarely if ever going to be an issue.

Driving speakers, you have to deal with varying output impedance with both SS and tubes, tube amps generally have much higher output impedance and greater load sensitivity. Thus amp/speaker integration can be trickier with tube amps than SS.

IME/IMO - Don

Are there not several ss amps on the market that allow for adjustable input impedance? I know that my Rowland 8T does exactly that. Never had a problem driving it with my tube CAT preamp.

@bonzo75: Interconnect impedance matching is really not an issue for analog audio. The impedance of the cable itself does not matter at audio frequencies unless they include some kind of special circuit inside. I know some cables include internal networks but I'd need a schematic to show me what they do. I would not worry about cable impedance as the source (driving component) and load dominate the impedances. I would look for cables with better shielding when using tubes because impedances are generally higher, but the sound of cables is not something I care to comment upon. Not exactly sure of your intent, I took this thread to target listening impressions, and I don't have any worth providing. I wanted to bring up the technical side because the characteristics of the components is very different and I did not feel they should be overlooked. Sorry for the diversion!

@DaveyF: I do not know but would not be surprised. Raising the input impedance likely decreases the gain and increases the noise, but it depends upon their specific implementation, and may be inaudible in any case. I used the words "potential issue" because it really depends on the source and load. I ran a tube preamp into a SS amp for many years with no problems, but a different preamp and/or a different power amp would not play together. At one point I had a tube preamp without cathode follower outputs driving a 10 k-ohm SS amp and it did not sound or measure well. BW was a little low, improved slightly by switching to a special cable, but the distortion and noise was bad. Ultimately the tube pre simply couldn't handle the load. I first built a buffer circuit then later switched power amps. These days most tube preamps or other components will drive most SS inputs just fine, but there are always exceptions and sometimes even when rated to drive say 10 k ohms a tube preamp's sound (and measurements, natch) may change when presented with a 20 k or 100 k load.

I am afraid I am off-topic if this is strictly about the cables. My bottom line is that I would make sure the source(s) and load(s) are compatible first, then decide what cable works (sounds) best with those components. For me it's a horse before cart thing.

bonzo75 said:

What if I was using a valve dac or phono to drive a SS pre, will this arise?

Click to expand...

sujay said:

Thanks for raising this Kedar. I am grappling with exactly this issue. On some tracks, I can hear a bit of distortion ( even at low spl) with the lampizator driving my pre. My pre has an input impedence of 11kohm and I have currently set it as low as -6db. Not sure whether there is signal saturation or what but strangely it only happens only on some tracks.

Would be grateful to get some steer on this.

Cheers

Sujay

Click to expand...

Yes, this is an issue... the current trend of DHT output sections including the one used by Lampi have high output impedances and will NOT drive any amp/preamp. The output impedance of the Lampi DHT output section is simply Rp of the tube used, which is likely to be 1000-3000 ohms or so. The general rule of thumb is you want at least 10x more input impedance vs output impedance so if you're running a tube in the Lampi with Rp = 3000 ohms then it will want to see at least 30k ohms input impedance on whatever it's driving. So sujay, obviously you have a mismatch which will cause problems and you need to use a different source or preamp...

Most tube preamps use a cathode follower output and will have an output impedance from 200-600 ohms or so, leaving you with much more flexibility so it's much less likely you'll run into impedance mismatch issues.

This has nothing to do with the role of interconnect cables, unless you're using one of the very few resistive type ic cables made of carbon or some semi-metallic ribbon, but these are just bad designs that should be avoided anyways. Most IC cables have a low enough resistance that it's just not a factor at all. However, subjectively it might make a difference but that's up to the subjective impressions of the user and you can't really say any one thing is going to make an ic cable better than another as a blanket statement.

I was enjoying these complicated impedance matching questions from afar . . . until I realized that I may be going from tube to solid-state to tube and therefore I have a whole impedance matching analysis to undertake!

1) The output impedance of the Io preamp is 350 ohms. The Io might be driving a solid-state amplifier with an input impedance of 10,000 ohms. Is this OK?

2) The solid-state amplifier is built into a bass tower and drives its attached cones. This built-in amplifier has a passive high-pass filter which sends the midrange and treble frequencies to VTL MB-750s. I do not know the output impedance of the amplifier. (It probably is low.) The MB-750 has an input impedance of 130,000 ohms. Is this OK?

bonzo75 said:

What if I was using a valve dac or phono to drive a SS pre, will this arise?

Click to expand...

bonzo75 said:

Guys the issue I am referring to, which Gary brought out was purely to do with ICs mattering more when valve and SS components are mixed. I am aware of normal impedance matching and if that itself was an issue can easily find a proper pre or in case of Lampi get output adjusted by Lukas

Click to expand...

Well, obviously it's an issue with sujay... and likely many others who don't understand the limitations of their fancy new DHT DACs/preamps/2nd order distortion generators...

Also, on a simple DHT output like the Lampi there's nothing that can be done, output impedance = plate resistance. It's as simple as that...

Ron Resnick said:

I was enjoying these complicated impedance matching questions from afar . . . until I realized that I may be going from tube to solid-state to tube and therefore I have a whole impedance matching analysis to undertake!

1) The output impedance of the Io preamp is 350 ohms. The Io might be driving a solid-state amplifier with an input impedance of 10,000 ohms. Is this OK?

2) The solid-state amplifier is built into a bass tower and drives its attached cones. This built-in amplifier has a passive high-pass filter which sends the midrange and treble frequencies to VTL MB-750s. I do not know the output impedance of the amplifier. (It probably is low.) The MB-750 has an input impedance of 130,000 ohms. Is this OK?

Click to expand...

Yes, no problems there... My tube pre has a ~500 ohm output impedance and I've driven two 10k ohm amps with it simultaneously with no problems at all. Honestly, it's more complicated than just impedance, the ability of the driving device to deliver current matters too, and in the case of a tube pre with a follower output you're probably looking at 6-10 mA which is plenty....

bonzo75 said:

What if I was using a valve dac or phono to drive a SS pre, will this arise?

Click to expand...

bonzo75 said:

The fancy DHT dac is the best source in audio, so let's skip what you are clueless about

Click to expand...

Huh, seems to me you're the one that's clueless about how your own DAC works, what it's output impedance is, the fact it can't be "adjusted", and the fact it's incompatible with many amplifiers.

So how about you stfu about anything technical, because you are obviously clueless when it comes to actually understanding how an audio system works.

My comments have nothing to do with how it sounds subjectively either, personally I like it. At the last RMAF my power conditioner and power cable supplied power to a system with the Golden Gate and 211 PP amps... this room won multiple awards and sounded amazing driving the Vapor Perfect Storm speakers. But I'm also honest (not at all clueless) about what it does and it's limitations.

DaveC said:

Yes, no problems there... My tube pre has a ~500 ohm output impedance and I've driven two 10k ohm amps with it simultaneously with no problems at all. Honestly, it's more complicated than just impedance, the ability of the driving device to deliver current matters too, and in the case of a tube pre with a follower output you're probably looking at 6-10 mA which is plenty....

Click to expand...

Thank you very much, Dave!

DaveC said:

Yes, no problems there... My tube pre has a ~500 ohm output impedance and I've driven two 10k ohm amps with it simultaneously with no problems at all. Honestly, it's more complicated than just impedance, the ability of the driving device to deliver current matters too, and in the case of a tube pre with a follower output you're probably looking at 6-10 mA which is plenty....

Click to expand...

+1 to the "it´s more complicated than ..." ! The old x10 rule has more exceptions than applications. Most modern tube preamplifiers have relatively low output impedance but some of them do not like supplying current as you say, although they can do it.

A significant number of solid state amplifiers have input impedance over 50 kohm. And we have reported cases of people using Burmester amplifiers (input impedance around 2K ) with Audio Research tubed preamplifiers with great success, something I could never understand ...