Impedance mismatch question

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

Thank you Al.

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.

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.

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.