Question about passiv preamp

[hanser]

I heard that for passiv preamps the impedance of source and amp must fit somehow. Could somebody please tell me, what should be observed?
My CD player has an output impedance of 120 Ohms, my power amp an input impedance of 27 kOhms. It also has input sensitivity of 1 Volt. Will a passiv preamp like the Creek OBH12 work?

Thanks in advance,

Hartwig Hanser

[David Meek]

Hi Hartwig! Welcome to the forum. :T There is a very good article on passive pre-amps in the latest issue of Listener. I'm not technically adept enough to repeat it from here at the office, but I'll look at it this evening and post more info then.

Stay around - there are good, knowledgable and friendly folk here.




David - HTGuide flunky
Our "Theater"
Our DVDs on DVD Tracker

[Andrew Pratt]

Welcome to the forum! I'm sure Jon or Thomas will chime in here soon with an answer for you

[John Holmes]

Welcome to the Guide Hanser!




"I came here, to chew bubble gum and kickass. And I'm all out of bubble gum!!!" My DVD's

[Lex]

Unfortunately, it appears that Jon may be unavailable for a few days. Now it is possible, that he could happen by and see the post, but I dont' think he will get the email I sent him sking him to take a look here, until the 15th.

Welcome to the forum, and a great first question!
Lex

[brucek]

Hartwig,

The short answer to your question is yes, this passive will work fine in your system. I suspect you want the long answer also, so I'll give you that too - feel free to go to sleep.

I'll give you a simple take on input and output impedance's first.

The typical interface between a preamp and power amp (or between a source like a CD player and a preamp) is a line level, high impedance connection, where the output impedance of the preamp is usually in the order of about 50 ohms to 300 ohms and the input impedance of a power amp is in the order of 10Kohms to 50Kohms.

This is known as a voltage bridge connection where the preamp is acting as a voltage source and almost no current is drawn. In fact, a perfect voltage source would have an output impedance of zero ohms at all frequencies. This would result in all the output voltage from the preamp being dropped across its load (the power amp) with no voltage being lost to the output impedance of the pre amplifier.

Output impedance is basically the internal resistance of an amplifier seen at its output. The value can be slightly frequency dependant because of reactance caused by inductance and capacitance, but either way, a low output impedance is desirable.

In its simplest terms you can view the internal resistance of a preamplifier and its load (the power amp) as a voltage divider. The higher the output impedance of a preamplifier, the more voltage will drop across "it" instead of its load (the power amp). This in effect means less voltage will be received at the power amp.

If the input impedance of a load device is not significantly higher than the sources impedance, the signal will be reduced or "loaded down" and its signal to noise ratio and frequency response will suffer. Certainly the load can become too much for the source to supply adequately.

Generally, a high output impedance requires close attention to cable lengths. The concern is that the high reactance (frequency dependant resistance caused by capacitance) of the longer cable, combined with a high output impedance of the preamp, creates a low pass filter which adversely affects bandwidth. This distortion of the higher frequencies increases with higher output impedance's.

So, here's the rub. Unlike speaker cables where we don't give a darn about capacitance, with an interconnect it becomes important no matter what the output impedance, although the situation is exacerbated with a higher output impedance. The interconnects capacitance results in a reactance (frequency dependant resistance) that will roll of the higher frequencies. It becomes more and more a factor, the higher the output impedance becomes. This interface is actually the same as the connection between a source like a CD and the input impedance of a preamp, so the same rules apply there too.

Anyway, there's an old rule of thumb that says, the input vs output impedance of a voltage bridge interface should be a minimum of 10:1. That's minimum. It should be greater in a high end system to ensure high frequency preservation. So, if I fed a power amp with 10Kohm input impedance I would just be OK with 1000 ohm output impedance, but I certainly wouldn't want to split that signal.

This reasoning is why they insist when you use a "passive" preamp you use super short, super low capacitance cables, because they have fairly high output impedance's. This impedance usually varies too, because the output is connected directly to a potentiometer that alters the output impedance as the dial is turned - pretty undesireble. People weigh this off against the benefit of low noise of the passive preamp.
Tubes preamps also have high output frequency dependant impedance. This is what gives them that "warm" sound you hear about, actually it's distortion - oh well, each to their own. Cables are very important to valve lovers for a reason.......Now you know why.

Anyway, there are several types of passive preamps. The Creek is a typical potentiometer type in a simple configuration I believe. This means when the volume is turned up high (least attenuation), then the higher the output impedance becomes.
With the Creek, I believe at full volume its output impedance is around 10K. This is sourcing your power amp with an input impedance of 27K. You can see you've upset the 10:1 minimum rule - and combined with long (high capacitance) cables you would be altering the response of your system somewhat. But, you probably won't be operating at full volume, although you have to realize that the volume control is really just an attenuator and it will likely be at least dialed to half volume to obtain a good listening level. You have a typical CD player and its output should tolerate the insertion loss of the passive without any level concerns.
At half volume I believe the output impedance is a fairly reasonable 1.5K. This is not too bad. Your amp possesses a decent input impedance and this situation should work fine. Be sure to keep those interconnect cables short though at any rate.

There are other passives that use various means to lower the output impedance as the attenuation is reduced, but in the end this Creek OBH-12 is as good as any and it's kind of cool having a remote with it...............

brucek

[Andrew Pratt]

Thanks Bruce 8O

[hanser]

Thanks all for the welcome, and especially Bruce for the extensive answer. I think I will need some time to digest it, since I am not very knowlegdable in those electronic things.

perhaps I should add some background info:
the CD player is the NAD 514, the amp is the Rotel RB 956AX 6channel amp, bridged to 3 channel.
I started with an audio only system (then with an HK integrated amp), and then added more and more home theater components to it. I am now looking for a Home theater prepro or alternativly a good receiver to use as prepro. Many people seem to think, that running stereo music through a separate preamp instead the HT prepro might be a good idea, therefore my idea is to use something like the Creek for CD and feeding the output from the prepro or reciever preout into the second input of the Creek. The output of the creek would go to the Rotel which fires my 3 front channels.
If you want to have a good laugh, here my actual signal chain:
Harman Kardon VXI 645 for channel selection, then via the external processor output (before volume!) to Yamaha DSP E580 Dolby Surround Decoder (for volume control), to Technics SHC 500 DD/DTS decoder to the Rotel. So you see, why I now want to clean up things a bit! By the way, Stereo sound very good in spite of the 2 pass throughs. Loudspeakers are B&W CDM7NT.

Hartwig

[David Meek]

Hi Hanser,

It took a little longer than I anticipated to get the info from my issue of Listener. Bruce nailed it anyway! :T

For what it's worth, here's the three points for a properly matched source/pre-amp/amp, in the article by Herb Reichert in Listener - Volume 8, Number 2:

1) The output impedance of the source is very low - ideally a few hundred Ohms or lower - and the input impedance of the amplifier is sufficiently high - at least 20 kOhms.

2) The combined gain of the source and amp is sufficient to drive the loudspeakers to a satisfying level. And,

3) The length of the interconnect that the preamp drives is sufficiently short to avoid rolling off the high frequencies.

There is a lot more to the article than that if you are interested. Hope this helps.




David - HTGuide flunky
Our "Theater"
Our DVDs on DVD Tracker

[Lex]

Wow, Bruce, great answer. Jon? Who needs him, haha. Just kidding Jon, if you chime in late. I am sure your perspective is welcome too.

Thanks Dave for the short answer too.

Great thread guys. :45: I don't give 5 stars easily, but I can be bribed, lol.

Lex

[Robbie]

hanser

Your setup sounds a little like what I was trying to do when I came up with a brainstorm (of sorts). When I started most pre/pros were $2000 plus. Way above my budget. Any ways what I did to accomplish my setup was to feed my Adcom amp with the Technics 500 and feed my Adcom Preamp into the technics bypass inputs (L/R only). All the analog stuff is connected to the Adcom Pre while the DVD and Dish are connected to the Technics digital in. Its a clean albeit a little clumsy and I don't get DPL using the VCR but thats OK since I don't watch movies on the VCR. It works really well for me anyway.

Good listening

Robbie

[hanser]

Thanks David for the additional info. I have indeed another question. You and Bruce say the interconnects have to be short. How short? is 60 cm short enough?

Robbie,
thanks for the tip. One of the main reasons that I am thinking of getting a preamp or receiver is that I need a 6 channel input, since I plan to get into DVDAudio and SACD. The 6 channel input of the Technics is not volume regulated, so I need something else. Otherwise it sound like a good idea.
Hartwig

[brucek]

Hartwig,

I think your idea is a good one, but it would be nicer if you could find some solution to eliminate the "passive" from influencing the HT portion of your setup.
Your idea works great for CD - through the Creek #1 input and directly to the amp. But when you select #2 input, you're placing it between your new processors main channels out and the power amps input. This means if you crank the Creek up full, then you're suffering its 10K output impedance, and if you dial the Creek down low to reduce the impedance, then there may be too much insertion loss and you'll have to crank your processor too high..
You say you want to get into SACD. That's a fairly pristine signal that you would be passing through the Creek when using your processor.

This would suggest some modification to your plan, although it will still work as you've laid it out.
Others may be able to help here. My suggestions would be:

1. Modify the Creek so that when #2 input is selected, the internal potentiometer is taken out of the circuit and you have a "bypass" for your HT processor. (Why didn't Creek do this).. I would ensure if I did this modification that it only affected #2 input, because you wouldn't want to inadvertently switch to your CD when the Creek was in bypass - yikes.

2. Build or buy an audio A/B switch that either selects the Creek or your HT processor mains. Yeah, I know it adds a device in the mains chain, but it's passive and you can make it fairly transparent.

3. Purchase a processor with a decent true analog bypass and don't get the Creek. This is obviously a poor suggestion, because processors with decent bypass are quite expensive. Your solution allows very good two-channel, and then purchasing an average digital processor for HT duties.

Oh yeah, 60cm (23 inches) is fine.....

brucek

[David Meek]

is 60 cm short enough?

Again, Bruce is way above me on the knowledge totem pole, but I'd venture to say 1 meter is a good stopping point for interconnect lengths.




David - HTGuide flunky
Our "Theater"
Our DVDs on DVD Tracker

[Robbie]

Hartwig

What I'm saying is turn it around. Instead of running the processor into the Creek, run the Creek into the processor. The bypass appears to be true bypass (no processing or volume control)to me anyway. I don't think it will degrade the signal much if at all (some experiments may be in order). That will eliminate all your impedence problems.

Robbie

[hanser]

Thanks for the added comments. Bruce, I am pondering those alternatives. Perhaps I just wait until I can afford a good prepro or until prices go down.
Robbie, I understand you, but I would still have no volume adjustable 6 channel input on the system.

Hartwig

an afterthougt: Bruce, could I just use the third input of the Creek for the L/R channels of SACD?

[brucek]

Hartwig,

Yeah, I think the third input is the tape loop, which would work fine for the SACD....

brucek

[JonMarsh]

A day late and a dollar short....


Most relevant points have been covered well, but just to add my 0.02;

Creek's specs *could* be a little clearer, but it seems pretty clear they're using a 10K gain pot; what isn't completely clear (at all!) from their specs is how the balance control interacts in this picture.

Thomas W just got one of their "bigger" units; the remote works nicely, and is quite conventient, but I miss the "thunk-thunk-thunk" of the Shallco rotary conrols in the Marchand PR41. It's a bit light weight in comparison.



Creek P43R

Cable capacitance interacts with output impedance to form a low pass filter; it's best to get the lowest possible capacitance interconnects; 75 ohm cables work better than most standard audio cables. I use some 2 meter homemade Cardas interconnects with Quadlink cable and WBT connectors (not following my own advice); you might check with Lex Luther to see which of his cables have the lowest capacitance per foot. For a quick test, try the new two meter rat shack digital cables (they're orange, easy to spot among their other offerings, and inexpensive, while decently made).

The Marchand PR41 has the minimum output impedance with the volume control up all the way, but it's still not very low. I'm considering adding a NLFB driver stage to eliminate cable interaction.

Integrating a passive preamp into an HT system is tricky. Som processors, like the old Sony EDP9ES, have a bypass mode which works fine with a stereo setup. Most don't, unfortunately.

A good passive preamp is a joy to listen too, mostly because most of them add very little grunge or veiling to a system. It's great for a smaller "purist system" for music at low cost. To match the transparancy in an active preamp, you're usually looking at $3k or more. I especially like the value of the Marchand PR41 kit.

We took over ThomasW's living room last week and setup such a system- a pair of Denver built M8a's, an Ayre V-5, ThomasW's new Creek P43 passive preamp, and my MP-DAC, brought from CA. (his was being built this weekend- should be done today- if some regulator chips arrive via FEDEX as planned), and some of my cables I brought out. Oh, and some really funky DIY speaker stands- more about those later!


Just keep that cable capacitance low, and you'll be OK- using 75 ohm cable, I've gone up to 4 meters with verified -3 dB point above 50 kHz.

Regards,

Jon




Earth First!
_______________________________
We'll screw up the other planets later....

[hanser]

Thanks Jon for adding your comments.

Meanwhile I visited again Creeks homepage. They list the output impedance on the specs sheet as 0-20 K and on the manual as 0-50 K! ???
2 things I still do not understand completely:
1) why is the impedance going up when raising the volume? the higher the volume, the lower the resistance of the potentiometer, I would have thought, and therefore the lower the impedance.
2) can you describe what happens to the sound when I get under the magical 1:10 ratio in impedance? I understand that the high frequencies may be subdued, so does the sound just become a bit less bright? Or does it get distorted, so that it gets less listenable?
Hartwig

[JonMarsh]

The impedance ratio per se is not so important, as long as the source impedance of the driving device is low, and it is "happy" with the load which is connected (not too low an impedance). Most CD players will work OK down to at least a 2 kohm load, so that's rarely an issue.

If the output impedance of the preamp with the volume up full is still a minimum impedance of, say, 10Kohms, then this forms a voltage divider with the input impedance of the power amp, which limits the maximum voltage you'll be able to get at the power amp. For example, assuming 10K output impedance, and 27 K input impedance, the maximum voltage at the power amp will be

CD_Vout X 27K/(10K+27K)

Now, with a max CD out of 2V RMS, that will be about 1.3 volts. This is enough to get many or most power amps to close to full output- but some pro amps, or others with lower sensitivity, may need a little more juice. Remember, though, most of the time you don't need anything near full output.

Power amps with higher input impedance will load the internal resistor networks of the passive preamp less, and the output will be closer to the CD input.

Where the HF roll comes in is the maximum Rout, loaded by the cable capacitance. Here, ratios are the rule, with 10K output impedance, you'd like to keep the cable capacitance no more than 100-200 pF. The capacitance is a shunt element that with increasing frequency has a lower XC, and shunts more of the signal to ground.

The 75 ohm video cables have fairly low capacitance as shielded cables go (look at the big foam cores), so they work better than average audio cables in this mode. Try to check the manufacturer's specs on capacitance/ft.

BTW, I love my Marchand passive preamp because the volume and balance steps are absolutely repeatable, being switched elements using coin silver contacts. On the other hand, no remote, and a more limited number of inputs.

We've been listening to a Creek this weekend, and it sounds quite nice so far, and is quite convenient with the remote control. That's what swayed ThomasW to one.

Best regards,

Jon




Earth First!
_______________________________
We'll screw up the other planets later....