Impedance Question?
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Quick and dirty answer: you want the amp to have an input impedance at least 10x the output impedance of the pre. Here's a solid thread: https://forum.audiogon.com/discussions/how-to-match-preamp-and-power-amp |
Others have had issues regarding impedance. If the amplifier input impedance is 10k, it is really only suitable to match to a solid-state preamp. With a tube preamp, it can work, but you may start to see weak bass/midbass (among other issues). Look for a higher impedance amplifier. For example, the Parasound A21 has input impedance of 33k for RCA and 66k for balanced. I have seen other amps with input impedance of even 100k or 200k. |
The amps input impedance should be 10 times the output impedance of the preamp. This is considered the minimum and since the preamps output impedance varies with frequency, it's better for the amp input impedance to be 15 or 20 times that of the preamp. So if your amp is 10k, then the preamp should absolutely be no more than 1k, and to be safer the preamp should be in the range of 500. |
toddverrone554 posts09-29-2017 11:50amQuick and dirty answer: you want the amp to have an input impedance at least 10x the output impedance of the pre. This is a good simple answer. Cheers George |
Others have had issues regarding impedance. If the amplifier input impedance is 10k, it is really only suitable to match to a solid-state preamp. With a tube preamp, it can work, but you may start to see weak bass/midbass (among other issues). Look for a higher impedance amplifier. For example, the Parasound A21 has input impedance of 33k for RCA and 66k for balanced. I have seen other amps with input impedance of even 100k or 200k.So I had this 10k (Symphonic Line RG1) hooked to a Cary (SLP-03) 500ohm and yea I lost some bass/midbass for sure and couldn’t figure it out. However I’ve heard the SL hooked to a SLP-98 (800ohm) and it sounded way better, So I’m trying to match a pre with this amp and need all the help I can get. Would a pre that has 600ohm -<1Kohm be safe to match? |
Not all preamps that have equal output impedance specs, say 600 ohms, have the same impedance across the frequency band, and often much higher at the lower (bass) frequencies. The result would be a loss of mid/low bass with those having this condition when paired with an amp having low input impedance. To mate a preamp with a 10K input impedance amp one would need to know the output impedance of the preamp across the frequency range. |
It is so confusing isn't it?! I'm currently looking at room measurement mics so I can stop going completely by 'feel'. Good luck! I use a dared mc7-p preamp (class a tube) with my class d monos and it sounds beautiful. I think it only has an output impedance of 100 ohms and there is certainly no roll off in the bass zone. |
OK so I got an email back from Rolf at SL and he suggest staying 100-300ohm. I would agree with this recommendation. Many say the general rule is 10:1, Amp input impedance:preamp output impedance. However, in many cases this is not enough. Many manufacturers, like Audio Research, recommend a 20:1 or 30:1 minimum ratio. The main reason is because most stated preamp output impedance is not a ruler flat measurement, but rises quite a bit in the lower bass regions. For example, my ARC Ref 5 preamp has an output impedance rating of 600 ohms (balanced). ARC, the preamp manufacturer, recommends an amp load of a minimum 20K input impedance (33:1 ratio). The reason being that measurements show the output impedance of the Ref 5 rises to 1447 ohms at 20 HZ. This is common with many tube preamps, the stated output impedance can more than double in the lowest octave. Another example is the Cary SLP-98 that you mention. The spec sheet states a 800 ohms output impedance. According to Stereophile measurements, " The output impedance in the midrange and treble was usefully low, at 370 ohms, but this rose at 20Hz to 6.8k ohms, due to the limited size of the output coupling capacitors." In a nutshell, your manufacturers recommendation of a preamp with an output impedance of 100-300 is a good one for an amp with a 10K input impedance. This will eliminate most tube preamps, but not all. Cheers, John |
jmcgrogan2 has the best answer +1 Our tube preamps don't have an output coupling cap- so they can drive 600 ohms let along 10,000 ohms with ease. The output impedance is the same at 5Hz as it is at 1KHz or 30KHz. Generally speaking though, most tube preamps are likely not to play bass right driving 10K, simply because of the output coupling cap being too small. |
The Dared apparently has a low input impedance.Yes as you stated 100ohms in your previous post, which will drive any poweramp. But being a tube it will be capacitor coupled, and if that cap is not big enough in uF, that 100ohms output impedance can start to rise in the bass and increase that 100ohms, and if it rises enough, into a 10kohm amp you could get bass rolloff. The cap needs to be at least 5uF into 10kohm, to be rolling off the bass -3db at 3hz which just starting to rolloff at 20hz This is why direct coupling is so good, no cap to **** things up and to colour the sound. Best cap is no cap. Cheers George |
Why would they not use a higher value capacitor if that improves the sound and provides more pairing options? Cost can't be the reason since the price differences is not much. There must be other reasons or design tradeoffs.You are correct. The bigger the cap, the more coloration it will impart, regardless of the dielectric. As a result, you want the smallest cap to do the job right, if low coloration is your goal. A 0.47uf cap sounds noticeably better than a 5.0uf cap; capacitors are wound on a machine so they have not only capacitive qualities but also inductance and resistive qualities as well. The dielectric (Teflon, paper, polypropylene, etc) affects the sound as does the length and diameter of the part. This is the main reason that some tube preamps don't play nice with some solid state amps. The coupling cap is too small, and one big enough imparts more coloration than the manufacturer is willing to accept. So the manufacturer solves the problem by declaration (example: ARC limits their preamps to power amps with 30K or more input impedance). We solved it by direct-coupling; others use output transformers (which have a different set of issues). |
I’m curious to know what these issues are with direct-coupling are since I may go this path with the SLP-98.The Cary employs direct-coupling but not at its output- there it uses a conventional coupling capacitor. But to answer the question, direct-coupling issues are 1) the unit must not make DC that could damage an amp or speaker 2) the DC must be able to stabilize during warmup without damage to amp or speaker 3) This requires a servo control; the servo circuit must be stable at all input signal conditions and not impart any sound of its own The tricky bit is actually 2) above, in many solutions a protection relay is used and the relay itself can contribute (or rob from; depends on how you look at it) to the sound. We patented a circuit that is reliable enough that a protection relay is not needed. I may have overshot here- in case you were only asking about the direct-coupling used prior to the output of the Cary. If that is the case, the big issue is power supply stability, as the circuit, except for the output, has bandwidth to DC. Power supplies, even though they are a source of DC, have a timing constant that is something just shy of actual DC like a battery has. If the circuit can go lower than the power supply, low frequency instability can result. 'Motorboating' which is a problem in amps and preamps with failing power supplies, is an example of this phenomena. |
Maybe this was mentioned and I missed it but is there a simple mathematical relationship between the value of the coupling capacitors and the output impedance or other variables are in play?There is- Zout+C, where C is the impedance of the cap at the specified frequency, and Zout is the output impedance of the circuit driving the cap. At 1000Hz, typically the cap has no influence on the output impedance. But as the frequency is decreased, the output coupling cap begins to have a greater and greater influence as the cutoff (-3db point) frequency is approached, such that it is the major component of the output impedance rather than the circuit driving it. Keep in mind that this is a bit oversimplified as there can be complicating aspects that do affect output impedance, such as the application of loop feedback which will reduce some of the effects of the output coupling cap. |
