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Impedance Curves


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I have a small pp tube amp that I often use with RF-7s. I use the 4 Ohm transformer tap and place an 8 Ohm 5W resistor across the amp output in parallel with the speaker. This presents a lower impedance to the speaker and halves the resistance variation % to the amp. Also, it gives the amp something to do while a fraction of a Watt is consumed by the speakers.

I'm coming very close to springing for a SET amp pair. I think it's time to try. I can't squeeze any more accuracy out of the KT66s. I may replace them with 300Bs or 2A3s or 6B4s to see if a real triode can do better.

Anyway, with a Watt or two to spare, the resistor trick is an idea for the pp solution.


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I don't doubt for an instant that you said these very same things from the beginning. But the "beginning" was a long time ago, and I'm just now starting to get a decent grasp on some of these concepts. I wanted to come back to it for further clarification. I also wanted to write things out the way I understood them, so that if I was wrong -- those of you with more background could correct me. I didn't mean to give the impression I was sharing some new revelatory material.


Congratulations. You just fried my last brain cell.

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And lagging behind as usual....

OK - speaker impedance.

I seem to remember seeing that the Heresy 2, in common with the RB5 it seems, has a large peak of impedance going up to something like 38 ohms. As one who has never fully grasped the implications of what that actually means I took it to be the reason behind the following observation:

Initially I ran my Heresy's from an SS integrated amp, the Accuphase E211 - 90 wpc into an 8 ohm load. Later on I bought a pair of pp monoblocks eventually rolling the tubes for KT88's. This lead to an output of something around 45-50 wpc again into an 8 ohm load.

Aside from the sonic benfits (in terms of quality - whatever that means to my ears) there was a major difference in the performance of the 2 systems in that:

The Accuphase would drive the Heresy's down to 63 Hz at normal listening levels but no lower.

The tubes drive the Heresy's down to 47 Hz with a major peak at 50Hz.

I have assumed that the impedance loads would be responsible for this. This would imply that that SS amp coped less well with the increased peak than the tubes - which flies in the face of what has been written here.

The question, therefore - finally, is that if I am wrong and it wasnt the impedance that causes this effect - what was it?

A further observation (based on nothing scientific - simply my hearing) is that having switched the Heresy's out for a pair of aging Sansui speakers I notice that there is, simply, a lot more music being played. It is almost as it the Heresy was selecting parts of the music and omitting others. The end result was very pleasant but it was not a complete portrayal. This has only become apparent now that I have switched.

Impedance issue?

Result of a sealed unit design in a relatively small box?

Something else?

I am fairly sure it is not an issue of amplification. I have heard the Heresy's on around 5 amps and 3 receivers. None portrayed a greater proportion of the actual music, although some did portray it nicer than others.

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