Roger, I think you provided this information somewhere, but I can't remember where, and can't find it. So let me ask you: what is a good output impedance number to shoot for in a power amp? How low does it need to be to prevent frequency response peaks and dips due to the speaker impedance/amplifier output impedance interaction? I know the figure will be different for a speaker with a wild impedance curve (the original Quad ESL) than for one with a fairly even curve.
Does anyone care to ask an amplifier designer a technical question? My door is open.
I closed the cable and fuse thread because the trolls were making a mess of things. I hope they dont find me here.
I design Tube and Solid State power amps and preamps for Music Reference. I have a degree in Electrical Engineering, have trained my ears keenly to hear frequency response differences, distortion and pretty good at guessing SPL. Ive spent 40 years doing that as a tech, store owner, and designer.
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Perhaps someone would like to ask a question about how one designs a successfull amplifier? What determines damping factor and what damping factor does besides damping the woofer. There is an entirely different, I feel better way to look at damping and call it Regulation , which is 1/damping.
I like to tell true stories of my experience with others in this industry.
I have started a school which you can visit at http://berkeleyhifischool.com/ There you can see some of my presentations.
On YouTube go to the Music Reference channel to see how to design and build your own tube linestage. The series has over 200,000 views. You have to hit the video tab to see all.
I am not here to advertise for MR. Soon I will be making and posting more videos on YouTube. I don’t make any money off the videos, I just want to share knowledge and I hope others will share knowledge. Asking a good question is actually a display of your knowledge because you know enough to formulate a decent question.
Starting in January I plan to make these videos and post them on the HiFi school site and hosted on a new YouTube channel belonging to the school.
I design Tube and Solid State power amps and preamps for Music Reference. I have a degree in Electrical Engineering, have trained my ears keenly to hear frequency response differences, distortion and pretty good at guessing SPL. Ive spent 40 years doing that as a tech, store owner, and designer.
.
Perhaps someone would like to ask a question about how one designs a successfull amplifier? What determines damping factor and what damping factor does besides damping the woofer. There is an entirely different, I feel better way to look at damping and call it Regulation , which is 1/damping.
I like to tell true stories of my experience with others in this industry.
I have started a school which you can visit at http://berkeleyhifischool.com/ There you can see some of my presentations.
On YouTube go to the Music Reference channel to see how to design and build your own tube linestage. The series has over 200,000 views. You have to hit the video tab to see all.
I am not here to advertise for MR. Soon I will be making and posting more videos on YouTube. I don’t make any money off the videos, I just want to share knowledge and I hope others will share knowledge. Asking a good question is actually a display of your knowledge because you know enough to formulate a decent question.
Starting in January I plan to make these videos and post them on the HiFi school site and hosted on a new YouTube channel belonging to the school.
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- 845 posts total
Roger, You posted the following "As I tell everyone. Light loading can be done on any tube amplifier. If you dont play above 80 dB go to the lowest tap. If you play 90 go one higher, if you play more than 90 you may have to use the high tap to get the volume you desire. This change should be far more apparent than a cable change. 7 ohms on the 8 ohm tap is no problem for the amp but lower taps always perform better technically though how it sounds to you is more important. "4 ohms on the 4 ohm tap produces 100 watts. 8 ohms on the 4 ohm tap around 60 watts. As you go down in taps you spend more time in the class A region. The distortion and damping are much improved. "Would be nice if you had a way to measure the peak voltage you require, then much better advice can be given. You can also get at that with a SPL meter at one meter at listening level. Then with speaker sensitivity we can compute it "Everyone needs an SPL meter. For $50 its an execellent investment. People spend more than that on a poorly designed high end fuse. Please do not buy premium fuses, TuningFuses are the worst and the others I have not dissected but the people who sell them should be dissected." ******* Just an anecdotal experience. I own an ARC Ref 150SE. It has 3 output taps: 4, 8 and 16. I settled on using the 4 ohm tap for many of the reasons you mentioned above. John Atkinson reviewed an earlier version of the a few years ago and made the following comments: "As expected, the Ref150's output impedance varied according to the transformer tap selected. The 16 ohm tap measured 1.4 ohms at low and middle frequencies, rising to 1.9 ohms at the top of the audioband. The figures for the 8 ohm tap were 1 and 1.4 ohms; for the 4 ohm tap, they were 0.55 and 0.87 ohm. All three taps offer quite a low source impedance for a transformer-coupled design; as a result, the modulation of the amplifier's frequency response, due to the Ohm's Law action between that impedance and that of our standard simulated loudspeaker, was relatively mild." See Atkinson report here: https://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements So, in my experience, the Ref 150 SE, on balance, sounds best when light loaded off the 4 ohm tap. At times, I thought the 8 ohm taps sounded better, but after a while, I found the sound bright, probably because there is a 20++ ohm peak at the midrange cross over point. And as you predicted, bass is tighter when played off the 4 ohm taps, although I think speaker impedance in the bass region bounces around quite a bit but has a 3.8 ohm saddle in the 50 to 200 Hz range. As far as loudness is concerned, my speakers are rated at a 92db sensitivity and the amp has plenty of power to drive the music to uncomfortable loudness levels, albeit with little perceive distortion or signal scrambling. So the bottom line is that there seems to be something to be said for light loading. Bruce .
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Thank you for posting this. I have a question specific to a tube tester I own and use. It is a Weston 981 Type 3; it incorporates a line check adjustment where the line voltage is adjusted to a mark on the meter. When I test 6550 or KT88 tubes with the tester is in Gm test mode and the line check is done the voltage drops. The voltage holds steady when testing less powerful output tubes. Should I compensate by readjusting the line voltage or use the reading with the lower line voltage? Or do I have a problem with the tube tester? |
- 845 posts total