An interesting experiment

I have recently been experimenting with different preamps, as I found a while ago that this seems to be the component that influences the sound of my system the most. Being an EE (and a physicist by training)I was trying to figure out the source of the different sounds that tube and SS give me. I am a firm believer that good designs with modern electronics all sound the same (at least to my ears). As tubes are new to me I studied their specs a little, and recently realized that one big difference vs. SS is their output impedance. To be clear, I am talking about minimalist tube design, where the output stage is driven directly by the tube. As it turns out, tube designs have much higher output impedance. For example a 6SN7/6922 based preamp probably has an output impedance on the order of 8K/3K ohms respectively. While a typical SS would be less than 500 ohm. So as an experiment I put together a passive preamp with a series potentiometer that allows me to change the output impedance of the source. I connected my CD output through this "preamp" directly to the power amp. At this stage after listening for awhile I believe that my new setup sounds a lot like my tube setup (which has an audible illusions M3A pre, and magnepan 20.1s). The curious thing is that I had to dial the output impedance to around 30K ohm, which I believe is significantly higher than the tube pre. I know this setup by all accounts should be bad....

Regardless, to my ears this strange setup sounds excellent. I dare even say, better than with the preamp in the loop.
Yes, very interesting. Thanks for sharing. I am curious now.
Very interesting experiment but how do you explain tube preamps that use a cathode follower as the last stage? Their output impedance is comparable to SS but still have, IMO, all the best attributes of tube's.
Oferi hi, did you try just a 10kohm passive pot in place of any active pre, or your source/s direct to the poweramp if it/they have volume control facility?
And what are the output impedances of your sources and the input impedance of your power amp?

Cheers George
Yes, I am presently using a 10k passive pot for volume control (Incidentally I am using a Sescom mkp-25, which is only 50$). I also have at my disposal an ARC LS2 and the AI module 3a. But the pot arrangement sounds excellent to me. More like the AI than the ARC.

My power amp an Icepower based on the 2 x 125asx2 modules (I also have a CJ MV60SE). The ice amps has a rated 10k input impedance, but based on my experimentation it seem to be higher than that. I plan to try this carefully with the CJ next.

My guess is that the series resistor with the pot is resulting in an overall equalization that is closer to the tube amp setup. But clearly it is not simply because of matching output impedance to that of the tube amp (as noted above regarding the cathode follower arrangement).

Anyway, it is just a fun thing to try to get different (and in my case good) sound that is much cheaper than rolling new equipment.
A 30K output impedance into a 10K input impedance is certainly a little different than normal recommendations. 10K is pretty low. It would be interesting to try an amp with 100K input impedance. It would be interesting to look at the frequency output of the amp.

Interesting experiment.
So what sounded most like a tube amp and what sounded most like an SS amp??
No doubt impedance alone can account for a lot of what one hears different between SS and tube amplification gear.
to be clear, the I have a 10K POT for volume control followed by a series resistor (also a pot so I can adjust it). After thinking about it I realized that I made a mistake when saying that the amp is rated for 10K. Since each module operates in fully balanced mode there is a single ended to balanced input buffer. So likely the actual input impedance of the amp I am using is higher than 10K.

The setup with the 10K volume pot and 20K series resistor sounded a lot like the AI preamp. But I wouldn't assume it would be the same with other power amps.

Many SS amps designed specifically for good performance with all pre-amps, including tube pre-amps, have 60Kohm or higher input impedance to compensate for higher general output impedance of tube pre-amps. That helps keep distortion minimal and retain good dynamics with most all pre-amps out there, SS or tube.
Oferi your Ice Power amp/s with 10kohm input impedance is not a good match for any passive pre or even some tube preamps.
However your CJ MV60SE is 100kohms input impedance and this is a great match for a 10kohm passive preamp.

Cheers George
This kind of experimenting with various impedance matches is very interesting. Looking forward to whatever general findings or patterns might be detected.
Georgelofi - not a good match by traditional thinking, except it sounds good to him.
"not a good match by traditional thinking"

It's more than just traditional thinking, it's "Ohms Law", every stage of amplification design is based on on it and Kirchhoff's Law, without them you'd have a mess.
And yes if it sounds good to him he may have stumbled on something that has addressed a problem elsewhere.
Typical example is the Linn Isobarik speaker of yesteryear which was a highly overdamped design (not the ideal .707Q), which worked well with amps that had very mediocre low damping factor such as the Naim 250 at lower than 20 damping factor (highish output impedance). If you drove them with an amp that had a reputation of great controlled powerful bass that had high damping factor (low output impedance) those same Isobarik's had no bass, even though the amp was regarded for it's bass performance.

Cheers George
A point that should be kept in mind is that the commonly stated 10x guideline for optimal impedance matching of line-level interfaces is commonly mistated and misinterpreted, as I see it.

That guideline says that for line-level interfaces ideally the load impedance should be 10 or more times greater than the source impedance, at the frequency for which that ratio is lowest.

That is commonly misinterpreted to mean that if the ratio is less than a factor of 10 sonics will necessarily be compromised. Which is not correct. Meeting the 10x guideline will pretty much assure that there won't be an impedance compatibility problem. But not meeting that guideline does not necessarily mean there will be a problem. It depends mainly (although not entirely) on how the two impedances VARY as a function of frequency.

If the two impedances are essentially resistive, and therefore do not vary significantly as a function of frequency, and if the load impedance is not so low that it causes a significant degradation of the performance of the output stage of the source, such as a rise in distortion, and if the high source impedance does not result in excessive interaction with cable parameters, especially capacitance, then even a 1:1 ratio would be fine. The only effect would be a small reduction in signal level, which would be compensated for with the volume control if necessary.

Most (but not all) tube preamps have a substantial rise in their output impedance at deep bass frequencies, as a result of the output coupling capacitor most of them use. That will cause a significant rolloff of deep bass response if the load impedance is not substantially greater (ideally 10 or more times greater) than the source impedance at 20 Hz. Raising the output impedance of a solid state source by means of resistors or potentiometers will not cause that impedance variation, and will not have that effect. Although if the output impedance is raised to very high levels, such as the 30K number you mentioned, there will likely be significant rolloff of the upper treble, and consequent softening of high speed transients, resulting from the interaction of that impedance with cable capacitance.

A corollary to all of this is that a high degree of consistency should not be expected between the results of these kinds of experiments among different systems. How the impedances that are involved vary as a function of frequency will be different for different components. Interconnect cable parameters will also differ from system to system, as will the lengths of those cables upon which the parameters are dependent.

-- Al
Almarg for President 2016.
If you use a passive volume control that is 30kohm it's approx highest (worst) output impedance is about 20kohm.
This 20kohm combind with high (poor quality) cable capacitance of 100pF per foot, 300pf total for 1mt will give you a HF roll off of -3db at 27khz (-1.5db at 13.5khz).
As you can see this is cutting well into the audio band. And if the interconnect's capacitance is even higher than 100pf per foot, thing are even worse.

However if you used a 10kohm passive volume control things change for the better, the -3db HF roll off is now at 54khz (-1.5db at 27khz).

And if you use better quality interconnects like 50pf per foot capacitance the figures for a 10kohm passive are -3db at 108khz (-1.5db at 54khz)

Cheers George
I completely agree with you. In fact when I thought of trying this, the effect you mentioned is exactly what I was looking for. clearly if one is looking of a high fidelity setup this is not the way to go. However, sometimes playing with the equalization makes things sound better, and at the end of the day this is what counts. As I mentioned earlier, the ICE amps input is probably not 10K due to the added input buffer (single ended to differential (balanced)).
For fun I also tried a 100K series pot, with the idea of exposing the nature of the amp load and IC cable. With such a high impedance I expect the cable to have significant contribution. With such a high resistance I was basically controlling the volume with the series resistor. The effect was interesting, with rolled off highs.

Anyway, I am not going to make this my permanent setup, but it is a nice arrangement to have in the toolbox.
Cymbop +5 re Al for President. My next comment is not intended to be technical response because I am not a EE.

To the extent Al and Oferi are speaking about input/output impedance matching, I kinda tripped into that issue a while back. Most ARC linestages should not be presented with a combined output load of less than 20K ohms. As I learned by accident, I inadvertently overloaded my linestage.

My amp's input impedance is rated at 300K ohms (XLR input). However, I also loaded a subwoofer off one of the SE Main outputs of my linestage. The sub's input impedance was 20K ohms. OOppss. If you do the math, I think the combined impedance was less than 20K ohms. Plus, as ARC explained, loading the amp off the linestage's XLR output (Main #1) and the sub off the other SE output (Main #2) "asymmetrically" loaded the linestage. ARC said that was a big no-no.

The solution was Tom Tutay who custom designed and built an impedance buffer for me that summed the linestage's L/R channels off the Main #2 XLR output without shorting the Mains AND raised the impedance level of the buffer's input to 330K Ohms. If you do the math, I think the combined input impedance presented to the linestage is 157K ohms.

My linestage is happy and I'm very happy. And Kal at ARC is extremely happy too. We're all happy. ;>')
Cymbop, thank you most kindly! However, I respectfully decline the nomination, as I'd much rather spend time listening to my stereo than dealing with being President :-)

George, thanks. I haven't checked your calculations, but I know from past threads that you are very accurate in these matters. I would just add two points that others should keep in mind.

First, the obvious one that many people will require significantly longer lengths than 1 meter. And cable capacitance will increase in direct proportion with length, which will decrease bandwidth correspondingly.

Second, the bandwidth of the low pass filter that is formed by the interaction of output impedance and cable capacitance arguably needs to be at least several times greater than the 20 kHz nominal bandwidth of our hearing, to eliminate the possibility that phase shifts caused by that filter may have audible consequences.

Oferi, thanks for an interesting thread. Enjoy your experiments!

-- Al
George, after submitting my post just above I did notice something in your calculations that prompted me to look at them further. A minor correction: The attenuation at half the 3db bandwidth frequency won't be 1.5 db.

"-1.5 db" (in three places) should actually be about "-0.97 db," corresponding to:

20log(1/(Square root(1 + (f/f3db)^2)))

f/f3db, the ratio of frequency to 3db bandwidth, being 0.5 in your three corresponding examples.

-- Al
Thanks Bruce (Bifwynne)!

Everything in your post is correct. For two impedances in parallel, assuming they are purely resistive (which is usually a good approximation when it comes to input impedances) their combined impedance is the product (multiplication) of the two numbers divided by their sum.

The answer will always be smaller than each of the two numbers.

-- Al