Hi forum members,

as I'm looking for a SET amp for my Heresy II's (see my other post in the 2-channel section), I'm interested in the impedance curve of the speaker.

Some magazine in Germany noted that these speakers are not easy to drive with SETs due to impedance issues in the mid range. A SET might tend to sound too mid range focused.

Does anyone have details on that? If so, did anyone work on a impedance correction/equalization for this speaker?

Thanks,

  Marc

Mac,

The impedance of the Heresy goes up to nearly 70 Ohms in the midrange. I would NOT use a SET amp with this speaker. Of course I am biased against SET amps anyhow!

AL K.

I have never seen the impedance curve for an H2 published. 

On the other hand there is a plot of impedance of what I believe is a regular old Heresy in the collection of papers called, The Dope From Hope.

You should search the technical section for the word dopefromhope

The specific paper is near the end.

You will see that the paper reports that the high impedance reduces distortion when an amp is driven to near clipping.  Whether this applies to SETs I don't know.  (Given the low power, it seems to me they are always being driven to near clipping.  Smile.)

If I was concerned about impedance magnitude and compensation: 

I note that you can put an 8 ohm resistor across the speaker with impedances of 10 to 70 ohms.  You wind up with parallel impedance ranging from about 4.4 ohm minimum to about 7.2 ohm max (you can and should do the math to check).  I seems to me that the 4 ohm tap on the transformer could drive this load, which probably has less variation than other speakers.

Of course a 10 ohm or 16 ohm could be used too with somewhat similar results. 

Best,

Gil

Gil,

I never thought about a swamping resistor across the entire speaker. That's a good thought. I think I would make the value higher than 8 ohms though. It should be set as an upper limit. Maybe 20 ohms. It is also absorbing power at woofer frequency too you know! As to SET amps, they haven't got much head room even with a Khorn. For a Heresy, they will be running with their tongue hanging out all the way!

SET amps, FOIE!

Al K.

Hi,

thanks so far for all your feedback. The DopeFromHope was very useful and I actually sat down this afternoon and did impedance measurements on my H-II's.

It shows an impedance peak (of almost factor 10 compared the minimum) at 2.5k. I created an impedance correction with L=0.33mH, C=6.8uF+8.2uF and R=10 Ohms. This brings the impedance down to average values.

I will post my chart later, as I need to transfer it to the computer first. It only shows relative values, no Ohm values.

The correction is definitly audible with the EL34 PP, taking away the emphasis on mid-range. As a consequence, the low-end appears stronger now.

I'll try out a SET later this week or earlier next week.

Regards,

  Marc

Fritz,

Yes, my universal would provide a constant load, but the levels to the drivers would be way off. There would be virtually no bass!

Who,

A SET amp has no negative feedback and has a very high source impedance to begin with. That is, a very poor damping factor. This means the output voltage goes up as the impedance goes up. It also allows the drives to overshoot (ring) at every transition. Add this to the rapid increase in distortion as you move away from the linear portion of the single tubes curve (It has to be class A, you know). This happens every time the level goes up. SET  smps are simply crude circuits with a lot of good stuff missing! If you really want to heat up your room with vacuum tubes, get a good Push-Pull amp, like the ones NOS Valves makes (VRD or something like that).

Mark,

A quick session with the calulator says the combination in a R-L-C shunt network resonates at about 2300 Hz. That might even work! BTW: You may know this already, but the ratio of L/C will determine the width of the "notch". Adjusting it will let you make the difference between the impedance curves add to a nice straight line.

Al K.

Dr,

I take it you noticed a slight bias against SET amps!

AL K.

Al, thanks for verifying my calcs.

I actually modified the original LCR to 0,15mH, 27uF and 10 Ohms. This results in a very smooth response now from 1kHz up to 5kHz.

A first listening test with the ASL MG-SI 15 MKII SET results in very smooth mid range response. But it still offers this straight forward sound of the Heresy. So it looks like I'm not destroying the character of the speaker yet

The MG-SI 15 has 6dB negative feedback; so it's not 100% pure

I'll post my charts and my listening results sometime next week, as I'm busy this weekend.

Regards,

  Marc