[gkinberg]

My question is related to the compatibility of my tube preamp  and my subwoofer with respect to impedance matching. I have a sonic frontiers line 2 tube preamp that is fed through balanced inputs from my DAC. The pre has a balanced output impedance of 270 ohms and a single ended output impedance of 135 ohms. I run single ended cables from the preamp to my rythmiks subwoofer that has an input impedance of 30k ohms.  My system already has some rolled off character on the high end or maybe a slightly dark presentation. I am considering adding a second rythmik subwoofer to my system and cabling it in the same fashion. Is there a “large enough” difference between my pre’s output impedance and the two subwoofer’s input impedance such that I should not hear any detrimental effects?

I know that there are no absolutes or “perfect” numbers when it comes to impedance matching scenarios but I wanted to get some educated opinions regarding compatibility.

Thanks, Garth

[Steve]

My question is related to the compatibility of my tube preamp  and my subwoofer with respect to impedance matching. I have a sonic frontiers line 2 tube preamp that is fed through balanced inputs from my DAC. The pre has a balanced output impedance of 270 ohms and a single ended output impedance of 135 ohms. I run single ended cables from the preamp to my rythmiks subwoofer that has an input impedance of 30k ohms.  My system already has some rolled off character on the high end or maybe a slightly dark presentation. I am considering adding a second rythmik subwoofer to my system and cabling it in the same fashion. Is there a “large enough” difference between my pre’s output impedance and the two subwoofer’s input impedance such that I should not hear any detrimental effects?

I know that there are no absolutes or “perfect” numbers when it comes to impedance matching scenarios but I wanted to get some educated opinions regarding compatibility.

Thanks, Garth

Hi Garth,

I don't know the output circuitry, whether it is transformer output or other. The output impedance (Z) is only 135 ohms, so the IC capacitance would have virtually no affect on the highs (see below for an exception). From a white paper I wrote:

How about frequency response changes.

This section deals with the high frequency response of our active preamplifier with and without a buffer stage. We will use a 50pf IC vs 250pf IC. The output impedance with buffer stage is 100 ohms. Without is 2,000 (2k) ohms.

"udb" is micro db, or millionths of one db, "mdb" is milli db, or thousandths of one db. Z is impedance.

First, the high capacitance 250pf interconnect cable and the buffer stage, 100 ohms. The high frequency response drops approximately 44udb at 20 khz. With output Z of 2khz, the drop is 17mdb at 20khz. Not much is it.

Now we use the 50pf interconnect cable 100 ohm output impedance. The result is 1.8udb at 20 khz. With output Z of 2khz, the drop is 0,6mdb at 20khz. Again, not much different. (Rarely, a longer IC with higher capacitance is neccessary and a buffer output stage is necessary)

As one can see, the added buffer stage not only does not lower the distortion, but also does not appreciably extend the high frequency response.

As one can see, Garth, even with 2000 ohm preamplifier output impedance (Z), the high frequency response is virtually not affected. With 30k or even 15k to 135 is nothing. RCA Radiotron Designers Handbook, written by 26 engineers recommends a minimum of 5:1, amplifier input Z to preamplifier output Z. I recommend 10:1 to be safe. In your situation, the ratio is 30k to 135 is 222:1, so probably no worries with adding another IC Garth but......

Now if the preamplifier is output transformer coupled, then the extra capacitance could cause a resonance with the OPT of the preamplifier in the audio band. But it won't hurt anything even if that occurred. It would brighten the highs to some X extent.

Hope this helps Garth and good luck.

Steve

[G Georgopoulos]

i dont know what's impendance matching
preamps driving powr amps are volts driven
the most volts the better the drive
o/p impendance should be low
i/p impendance should be high
a factor of 1:10 is enough
 

I know that there are no absolutes or “perfect” numbers when it comes to impedance matching scenarios but I wanted to get some educated opinions regarding compatibility.

wrong
matching impendance here is not of any benefit
you need to dumb as much as voltage as possible
this means low preamp z into high amp z
this will enhance audio and dynamics

cheers
George

[Steve]

i dont know what's impendance matching
preamps driving powr amps are volts driven
the most volts the better the drive
o/p impendance should be low
i/p impendance should be high
a factor of 1:10 is enough
 wrong
matching impendance here is not of any benefit
you need to dumb as much as voltage as possible
this means low preamp z into high amp z
this will enhance audio and dynamics

cheers
George

Well, it is not really true, but being a man of good faith, like most, you and many others learn from other forums, diy, or even books, with many individuals claiming "expert" status based upon opinion, faulty understanding, and even marketing, yes, marketing, or some or all of the above etc. Low output impedance stages are necessary only in high capacitance outputs caused by long IC runs, or high capacitance ICs. And they better be capable of descent output signal current or that 10khz musical signal wave will become a triangle wave. Just because it is a low output Z stage does not mean it will work properly.

The main problem are the poor designs, bad parts, and layouts used. I have been able to obtain the exact output to input with respect to my preamplifier, this via both measurements and listening tests over years and years (with others). And that is with 2,000 ohm output impedance (Z). Dynamics, sound stage, tonal balance, harmonics, every aspect is identical to the input. There is even a control to adjust for different amplifier input impedances (Z) from 20k and up. Why aren't these other "expert" designers doing such? (I am not manufacturing in any capacity, I am retired.)

The fact that all these "experts" claiming a low output stage is necessary when it is not (as even my white paper clearly demonstrates with respect to frequency response) is evidence of their lack of understanding of science and design. I mean this is first and second year electronics. But then marketing rules over all else.

Sorry if this is a rant.

Cheers
Steve

[G Georgopoulos]

I've been to college,i have designed analog color tv's pal/ntsc/secam, mono tv's made solely from discrete devices,vhs players,speakers,amplifiers,cassette players,vinyl turntables and more audio mixers, am modulator/transmitters,the list goes on...etc

thank you for your kind words Steve

cheers
George

[Steve]

I've been to college,i have designed analog color tv's pal/ntsc/secam, mono tv's made solely from discrete devices,vhs players,speakers,amplifiers,cassette players,vinyl turntables and more audio mixers, am modulator/transmitters,the list goes on...etc

thank you for your kind words Steve

cheers
George

Your welcome George. Most others use postings, books etc. The question I have is why these other "expert" designers have not been able to accomplish such performance? Basically, four reasons off the top of my head.

1) Improper tube selection. If the plate resistance is too high, one needs a low output Z buffer with descent current draw. For instance, if a 12AXX or 6SXX or related tubes are used, a buffer is necessary due to the high plate resistance, hence output impedance of the 12AXX or 6SXX types.
2) Parts quality. The all parts sound the same is nonsense.
3) Part values. Parts values determine more than a simple loadline.
4) Layout is very important.

Simply solving a few equations is not enough to produce an accurate/natural "sounding" component. Much more is involved. That assumes they actually understand what it takes to design and build an accurate/natural product, which is a huge if. But then most companies continue the parade of new models, or new diy projects, as sales is what keeps them in business. And marketers will always tear down the most reputable designers in order to compete in the market.

Cheers
Steve

[gkinberg]

Hi Guys,

Thanks for your input. First off I am not too proud to state that I am no expert in electrical engineering. I remember hearing or reading that as one connects two amps to a pre-amp rather than one, the pre’s output impedance doubles… or the amps input impedance halves or something like that.  Then, if I connect a third amp, would the resistances be effected by a factor of three or four? But based on your input, even in a worse case scenario, with an output impedance of 1080 driving into an input impedance of 30K, there is no cause for concern. In addition, when selecting my interconnects, I selected one with as low of a capacitance that I could find while keeping the gauge at a reasonable level.

Thanks again, Garth

[G Georgopoulos]

Hi Guys,

Thanks for your input. First off I am not too proud to state that I am no expert in electrical engineering. I remember hearing or reading that as one connects two amps to a pre-amp rather than one, the pre’s output impedance doubles… or the amps input impedance halves or something like that.  Then, if I connect a third amp, would the resistances be effected by a factor of three or four? But based on your input, even in a worse case scenario, with an output impedance of 1080 driving into an input impedance of 30K, there is no cause for concern. In addition, when selecting my interconnects, I selected one with as low of a capacitance that I could find while keeping the gauge at a reasonable level.

Thanks again, Garth

Correct about the amp/preamp scenario ,i assume you refer to solid state as tubes have Zin of 470k ~ 1M
when it comes to IC's some make big fuss about capacitance/inductance/resistance etc,90% of audio cables
will work that's my opinion...

all the best

[Steve]

Hi Guys,

Thanks for your input. First off I am not too proud to state that I am no expert in electrical engineering. I remember hearing or reading that as one connects two amps to a pre-amp rather than one, the pre’s output impedance doubles… or the amps input impedance halves or something like that.  Then, if I connect a third amp, would the resistances be effected by a factor of three or four? But based on your input, even in a worse case scenario, with an output impedance of 1080 driving into an input impedance of 30K, there is no cause for concern. In addition, when selecting my interconnects, I selected one with as low of a capacitance that I could find while keeping the gauge at a reasonable level.

Thanks again, Garth

Again, probably no sweat unless basically two conditions occur.

1) The output of the preamplifier is capacitively coupled. In that case, the lower impedance (Z) will reduce the low frequency response. The question is how much. It sounds like all is ok in that regard.

2) The other is transformer coupled output. Besides ICs, each amp input has capacitance. One has to consider the total loading capacitance, and resonance, and optimal damping. The optimum is no resonant peak, minimize ringing, yet have a nice rise time (attack time). Again sounds like no problem. It is possible to have reduced bass output, but I doubt it.

If it develops either low bass output, or becomes bright, then it is time to question the setup.

Cheers and good luck Garth.

Steve