The ratio is a minimum. Ideally you want a minimum of 10:1. Anything over that is fine.
Best,
Erik
Best,
Erik
Impedance matching with tube amp
I have a Mccormack DNA 0.5 which has an input impedance of 100kohm. I'd like to get the Schiit Saga preamp, which has an output impedance of 180 ohms. Now, I've read about the 10:1-100:1 ratio of input:output impedance for power amps and preamps, so this wouldn't quite fit that range of ratios. Would this still be a good option, and if not, what other (<$500) tube preamp options do I have?
Thanks!
Thanks!
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"this wouldn't quite fit that range of ratios" suggests a misunderstanding of the ratio "rule". The pre-amp's output impedance doesn't have to fall within 1/10th and 1/100th of the power amp's input impedance, it's that the ratio of power amp input impedance to pre-amp output impedance should be 10 to 1 in the opinion of some, 100 to 1 in the opinion of others. But that 180 ohm figure for the Saga is a nominal one, not it's complete one. Even so, 180 ohms times 100 is still only 18k, far less than 100k! |
To clarify, If you go too low in the ratio, say 5:1 you run into the real possibility that the mismatch will cause significantly non-flat frequency response. Sometimes the volume control settings can radically alter the frequency response in minimalist tube preamps with the wrong amp. You can see the effect in many Stereophile reviews like this one: https://www.stereophile.com/content/conrad-johnson-premier-17ls-line-stage-preamplifier-measurements |
@bdp24 So you're saying that the nominal impedance is a measurement of output impedance at some chosen reference frequency? Is it typical for there to be large enough fluctuations in impedance across the frequency spectrum so that problems occur? From what others have said, I feel like the large input impedance coefficient of the power amp provides me with a lot of headroom to tolerate preamp output variations, but maybe I'm wrong. |
Exactly muffinhead123. Not that problems WILL occur because of the input impedance varying with frequency, but that they can. For instance, a tube pre-amp may have an output impedance of 600 ohms at 1kHz---which is probably fine for an amp with an input impedance of, say, 20k ohms. But the pre’s output may jump up to 2000 ohms at either a higher or lower frequency than the 1kHz figure---which would be marginally acceptable for the amp’s 20k ohm input impedance, causing a noticeable roll-off in bass. But 100k is about as high an input impedance in a power amp as I’ve seen. By the way, when I said the ratio "should be" 10 to 1 or 100 to 1, depending on whom you ask, I inadvertently left out the words "at least"---should be at least 10 to 1 or 100 to 1. In a lot of amps, the designer’s chosen input impedance, which could be within a range of values, is a balance chosen between a couple of competing factors---a couple of dB of noise being sacrificed to get a higher input impedance, for example. I think it was Nelson Pass who said that a lower input impedance (10k to 20k) makes possible a better amp, but that a figure that low results in greater demands being placed on the drive capability of the pre feeding the amp. Perhaps Audiogoners atmasphere or Al, experts in these technical matters, will provide more complete info. |
Anyone designing a preamp with a 100K ohms output impedance is in the wrong business...And none exist, except for passives. and 600 ohms for the input of the Sphinx is, if correct, by far the lowest I have ever seen in a power amp.We make a tube preamp that can drive 32 ohm headphones... 600 ohms is the old school defacto standard for balanced inputs. Our MA-2 amplifier input impedance is switchable (600ohms and 200Kohms, balanced), since the low impedance impedance (if the preamp can drive it) is helpful for eliminating any 'sound' from the interconnect cables. |