Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

---

[ Follow Ups ] Thread: [ Display  All  Email ] [ Tweakers' Asylum ]

---

[ Alert Moderator ]

---

Re: MOSFETs work fine too...

Posted by Robert Karl Stonjek on December 10, 2001 at 12:21:00

In Reply to: MOSFETs work fine too... posted by Nick on December 10, 2001 at 11:22:19:

It depends on the amount of quiescent current you apply to the output devices. If it is high, then the variation is less. If the speaker load is high (low impedance) then and the quiescent current is only 100 milliamps (normal) then the variation in output can be 3dB or more.

But this depends on the type of load. Electrostatic loudspeakers have an impedance dip at around 10,000Hz where they dip to one or two ohms. That's why their such a difficult load. I've done impedance sweeps on four or five brands of electrostatics.

The impedance of a normal speaker dips when it receives a transient. Consider a woofer moving inwardly with some low frequency signal and then a bass drum thump comes through. The impedance for transients is much lower as the speaker load includes the speakers own momentum which includes the moving air mass etc.

I have rebuilt, repaired and serviced plenty of hybrid amps such as Counterpoint and Forte and I can tell you that where their performance falls down is controlling transients. They tend to be 'softer'. This 'softness' can be an advantage in a system that has a ragged edge on the mids and highs.

You can limit the effect of feedback on tweeters by adding a series resistor (assuming that you can still get the shelving right).

I've designed and built a few amplifiers so I have plenty of experience with feedback. Getting it wrong causes all sorts of horrible sound, so no feedback would be an improvement. But don't forget that any Op amp in the chain will have the same sort of feedback that a power amp usually has. CD players always have feedback devices for instance.

To get the feedback right I use no capacitors in the feedback chain (the amps are DC) and use super-matched transistor pairs as the differential voltage pair (LM394H). This makes the feedback of the same standard as a high quality Op Amp.

It sounds brilliant without being gritty.

Kind Regards,
Robert Karl Stonjek.

This post is made possible by the generous support of people like you and our sponsors:

Parts Connexion

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---