Yes I have extremely happy Lightspeed customers that are driving it into 22k poweramps with tube riaa phono stages as the source.
As for driving interconnects the only time this comes into factor is when the interconnects is poor quality and has vast amounts of capacitance (>200pf per foot), then it can roll of the highs, but the same cables will also roll of the highs with high output impedance tube preamps, tube phono stages and tube cdp's of which there are quite a few around.
Cheers George |
George, I agree with you. Wow, I think this is a first! My tube preamp has an output impedance much like Almarg outlined. Can go as high as 4k etc.....
I have run it with all kinds of amps with as low as 22k input impedances with no issues at all. Seems this whole thing of a 10 to 1 rule is really not hard and fast. My experience indicates it is not really that helpful. My last solid state amp had a 22k input impedance and my 4k output impedance preamp sounded fantastic with it with no roll off at all. Also, the output cap's value was a low .47uf again with no issue or bass loss.
Just my actual experiences. Others own the same preamp and with amps that have 20-40k input impedances with no issue. Pass Labs and Nuforce amps....
This question comes up often and the answers with the ratios have puzzled me of late simply based on my and others experience. |
Regarding tube preamps would an output trandformer in place of the capacitor eliminate or reduce this potential fluctuating impedance mismatch ? The issue with an output transformer is loading. If loaded at an impedance too low, it will roll the highs. In addition, the tube driving the transformer will see a lower impedance and so will make more distortion. If the load is too high the transformer can ring. This will add brightness. In some cases where the loading is really way too high, the transformer will begin to express the inter-winding capacitance as part of its output (rather than the turns ratio), causing a loss at the frequency extremes and what response there is will not be flat. Amplifier input impedances can vary by 10:1 so it is helpful to know what the transformer is set up for. The manufacturer of the preamp can supply that information. |
Thank you all very much. To further clarify why I am asking I will list my equipment;
Audible illusions 2d pre
Cary 308 cdp
Emotiva xpa 2 gen2 amp
Gallo reference 3.5 speakers with gallo sub amp
Power cords are tributaries silver and grace note custom
Interconnects are homegrown audio super silver bw all components
I am waiting on a set of aes six pac tube amps to arrive as well.
I ask about the impedance mismatch bc I was recommended to have at least 50k ohm input impedance on the amplifier side. Sometime next year I am looking to purchase an emotive sira or custom emotive amp and Fred volts too says that impedance matching for a 23k ohm amp is difficult.
I have a slightly bright and forward sound right now that I am hoping to cure with the six pacs. But, I really like the dynamics of the emotiva amp. I picked it up as a fluke and am actually really enjoying it. It sounds very krell like to me. That being said, I never bought krell gear because while they have always sounded good, I always found them bright.
I have been wondering of late if the impedance mismatch could have been causing some of the brightness I have been hearing. It wasn't there with my old b&k ex442. But, based on the comments above it sounds as if the bottom end is where I would be hearing anomalies. |
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Hi Charles,
Yes. In general I'd expect output impedance variation as a function of frequency to be much smaller in the case of a tube preamp having a transformer coupled output than in the case of a tube preamp having a capacitor coupled output. Assuming reasonably good design of the transformer and the output stage, of course.
Best regards,
-- Al |
Hi Al,
Regarding tube preamps would an output trandformer in place of the capacitor eliminate or reduce this potential fluctuating impedance mismatch ?
Charles, |
Thanks, Georgelofi. As I said earlier, not meeting the 10x guideline (properly defined as described in my post) does not necessarily mean that there will be a problem. But meeting that guideline assures that there won't be a problem.
And as I'm sure you realize, the specific value of the coupling capacitor (which affects the amount of variation of output impedance as a function of frequency), and the deep bass extension of the speakers, and the deep bass content of the source material, are all relevant variables. So I would suggest caution to anyone who might be tempted to apply the results of your experiments in a universal manner.
For preamps that have been reviewed by Stereophile, the measurements and associated comments that John Atkinson usually provides in conjunction with the reviews are a valuable resource in assessing this issue.
Regards,
-- Al |
Al sorry I should have mentioned at the end of the day a Cymer tube pre with 2k capacitor coupled output impedance was subsituted for the Lightspeed and similar ratio results were observed. Of course as you say being tube tube and capacitor coupled it's output impedance will change a bit according to frequency. All impedance ratio changes were done on the fly on the input of a Cymer tube poweramp using a 32 position switch resistor loading to ground.
Cheers George |
2out2sea ... is your OP an academic question or an actual concern based on equipment you own or are thinking of buying? If this is an actual concern, based on Al's post, you may be close. Perhaps a call to the preamp manufacturer is in order.
Btw, I've been there and done this already. I bought a custom designed active impedance buffer for a very similar issue, but in my case only involving the sub-woofer. Check my threads. I doubt my solution would be a good fix if you needed an active impedance buffer like mine to drive your amp. I suspect even if it solved an impedance match issue with the pre, overall sonics might be degraded because you would be inserting an active artifact between the pre and the amp. |
Re George's comment, it should be noted that his Lightspeed is a passive resistance-based device. As such it will have minimal if any variation of its output impedance as a function of frequency, especially if it is driven by a source having low or non-varying output impedance.
I would not expect the results of his comparisons to be applicable to most tube preamps.
Regards,
-- Al |
Hi 2out2sea there is no hard rule here.
We've done experiments with very good equipment with 30 odd audiophiles where we could vary on the fly the impedance ratio from 1:100 down to 1:1 (your case is 1:23)
We started at 1:100 with these listening test, and the group could not detect any change till we got down to 1:5 only then only two of the 30 audiophiles thought they detected a very slight compression to the dynamics but weren't sure, and when we got to 1:1 only 50% of the group detected a slight compression.
This was done using quality interconnects of 1.5mts each long that were 100pf per foot of capacitance both sides of the Lightspeed Attenuator passive pre.
So your 1:23 should be fine so long as your interconnects are good quality low capacitance, which most quality ones are anyway.
Cheers George |
Preamp output impedances are most often specified on the basis of a mid-range frequency, such as 1 kHz. They may be quite different from that value at some other frequencies.
For example, it is common for tube preamps to have a coupling capacitor at their outputs, which can cause their output impedance to rise to much higher levels at deep bass frequencies than at higher frequencies.
If the preamp's output impedance varies significantly as a function of frequency, and if the power amp's input impedance is not considerably greater (ideally 10 or more times greater) than the preamp's output impedance AT THE FREQUENCY FOR WHICH THE PREAMP'S OUTPUT IMPEDANCE IS HIGHEST, then frequencies at which the preamp's output impedance is higher MIGHT be significantly attenuated relative to frequencies at which the preamp's output impedance is lower. That does not mean that they will be significantly attenuated if that 10x guideline is not met; it means that meeting that 10x guideline assures that they won't be.
If your preamp is a tube preamp having an output coupling capacitor, it would not be surprising if the 1K nominal output impedance rose to 4K or so at 20 Hz. The interaction between that kind of impedance variation and a 23K load impedance might result in perceptible rolloff of the bottom octave, depending on the deep bass extension of the speakers, and could also result in low frequency phase shifts that might have audible consequences.
Also, I've seen at least a few cases where preamp output impedances can be significantly higher in the upper treble region than the specified midrange value.
So what can particularly cause an impedance interaction problem between components is the combination of a somewhat high nominal (midrange) output impedance and significant VARIATION from that value at other frequencies. Also, high output impedance in itself, even if constant as a function of frequency, can cause an increase in interconnect cable effects, especially if the run length is long and the cable does not have low capacitance per unit length.
Regards,
-- Al |
If your specs are correct you shouldn't have an impedance mismatch. A 10x ratio of output to input is usually recommended. Search the forum for other threads on this topic for a more detailed explanation. |
What are the components? Its possible someone here has used the same amp/preamp combo and can give you some advice. |
