Jump to content

Transconductance amplifiers?


Chris A

Recommended Posts

As has been alluded to above, a buffer amplifier with no output feedback when combined with a voltage-controlled current source (VCCS) input stage (assuming two-stage amplification...which NP has said that the First Watt F1/F1J is not) might approximate a transconductance amplifier (TA).  The problem is, a buffer amplifier by itself isn't a TA.  You've got perhaps a piece that might could be incorporated into TA.

 

The way that Mr. Pass describes them, a perfect power TA would appear to have infinite open-circuit output voltage, and near zero closed circuit voltage with a known load--such as a resistor of value in the range of typical loudspeakers: 8 ohms.   But the amplifier wouldn't care if the load had reactances that attempted to oppose or add to the current passing through it: the output current would remain unchanged.

 

 

OutputImpedanceMeasure.gif

 

Z source = RLoad*(VOpen Circuit / VClosed Circuit - 1)

 

Amplifier output impedance is determined by the above formula (typically at 1 kHz output frequency).

 

Chris

Link to comment
Share on other sites

I believe that you'd still need an amplifier that exhibits the characteristic that I alluded to: open-circuit voltage is much higher than closed-circuit voltage.  This is an unusual amplifier characteristic nowadays. 

 

How you achieve that characteristic is implementation-specific, as there are probably many ways to do it.  That's of some interest here, but not the prime interest.  What I'd like to know is what horn-loaded loudspeakers (suitably EQed for this type of amplifier) sound like.  I assume that the sound will be exceedingly clean, transparent, and faithful to the source recording...i.e., "hi-fi".

 

However, calling "a tail a leg"...doesn't make it a leg.  Parts of amplifiers that "could be transconductance amplifiers"...aren't transconductance amplifiers.  I'm not interested in integrating my own presently...as my plate is presently full with demastering and building full-range MEH loudspeakers. When this activity begins to wind down (which may take many months or perhaps years), maybe there will be time and resources for making transconductance amplifiers--or perhaps other designs that seek to offset the effects of loudspeaker back-EMF sources (voice coil-magnet flux modulation, solenoid forces, under-hung voice coils, nonlinear driver compliances, etc.). 

 

I'm currently looking for a better amplifier for MEH loudspeakers that can show off the increased fidelity of single-point source full-range horns without resorting to using full-range drivers with their attendant issues. 

 

Chris

  • Like 1
Link to comment
Share on other sites

3 hours ago, Chris A said:

I'm currently looking for a better amplifier for MEH loudspeakers that can show off the increased fidelity of single-point source full-range horns without resorting to using full-range drivers with their attendant issues.

The amps you're wondering about seem to work well with over-damped drivers, which the one's in your MEH are, if I'm not mistaken.  And this is for a fully active rig, no?  How passive networks fit into the equation I'm not sure (I suppose that's partly why you asked in the first place, if anyone has tried).

 

But it seems that if your (presumably passive) MEH are designed with the intent to use a high source impedance, whatever specifics that entails, that it would work provided it was driven with such an amp, as would designing to the more prevailing engineering standards would on the other side of the coin.  I dunno, I'm just a dabbling diy-er with more imagination than wisdom from hands on experience.

 

Link to comment
Share on other sites

Yes, active crossover and direct coupled drivers.  I've found that even if not using passive crossovers, there will be a frequency response shift using direct coupled drivers and a transconductance amplifier.  I hope to be able to show this in the next couple of days.

 

Since we're all used to voltage-source amplifiers, we're not thinking in terms of current-source EQ instead of voltage-source EQ of drivers.  The EQ does change when you encounter changes in input impedance of the drivers.

 

Chris

  • Like 1
Link to comment
Share on other sites

29 minutes ago, Ski Bum said:

But it seems that if your (presumably passive) MEH are designed with the intent to use a high source impedance, whatever specifics that entails, that it would work provided it was driven with such an amp, as would designing to the more prevailing engineering standards would on the other side of the coin.

Recall that I stated the necessity of using an active crossover with the K-402-MEH.  I wouldn't try to use a passive crossover without some sort of upstream digital EQ somewhere that has the same capabilities as the Xilica crossover (even though that upstream PEQ doesn't have to be after the crossover split of high from low frequencies--all the PEQs can be upstream of the bi-amp split). 

 

It's easy to include voltage-source EQ in the active crossover settings, in addition to current-source amplifier PEQs, as well as settings for MEH positioned at mid-wall elevated above the floor, mid-wall/on the floor, in a corner but elevated above floor level, and corner but on floor--all for the MEH loudspeaker(s).  It only takes a few seconds to type the settings in and save them for future reference/use--to be available for use in perpetuity.

 

Here's what I've recently found using the Xilica XP crossover I acquired recently: going from ± 3 dB frequency response flatness to ± 1.5 dB on both the Jubs and the center MEH...and also cranking down on the frequency response variations from the surround Cornwalls, the perceived sound quality of the entire setup just about doubled

 

I know that there are many people here that don't want to believe that, but my ears, my wife's, and her sister's ears all have attested to that level of increase in perceived fidelity.  That information is bit disturbing I know...but that's what we found, nevertheless.

 

Chris

Link to comment
Share on other sites

This is a very interesting and thought provoking thread. Thanks Chris. I am still trying to wrap my head around some of the technical points. I will post more if I feel I have something to add. I get how it would be advantageous to put the loudspeaker in the feedback loop of an amp if it were purely resistive but the counter EMF that a loudspeaker generates, where does it go in this type of topology?

Link to comment
Share on other sites

14 minutes ago, babadono said:

but the counter EMF that a loudspeaker generates, where does it go in this type of topology?

A true transconductance amplifier output isn't sensitive to voltage changes on its output.  It only seeks to control current output that's commanded on its voltage-controlled input. 

 

If you also remove the output current feedback of the amplifier (i.e., feedback-less, or at least no output-loop feedback), then any load reactances are simply ignored by the amplifier--the output voltage goes wherever it goes, but the current remains the same as the commanded input.  That's the point in fact of doing it this way.

Link to comment
Share on other sites

When you think about it, it's simply the opposite of what we all have now driving our loudspeakers - current control instead of voltage control.

Link to comment
Share on other sites

4 minutes ago, Chris A said:

any load reactances are simply ignored by the amplifier

...and you just have to account for the undamped response by different eq than you otherwise would, right?  

 

Seems merely like different ways to skin a cat, if the ultimate goal includes the freq response, impulse response, over all system Q for the preferred bass response/character, low overall distortion, and the other stuff that really matters.      

 

I think I need to slink back into the shadows, munch some popcorn, and think about it some more.  I have no problem believing that a well calibrated, fully active rig sounds better than a slightly less well calibrated, fully active rig, just not sold on the transconductance approach as being fully responsible for the claimed audible improvements.

Link to comment
Share on other sites

23 minutes ago, Ski Bum said:

...and you just have to account for the undamped response by different eq than you otherwise would, right?

Yes.

 

23 minutes ago, Ski Bum said:

Seems merely like different ways to skin a cat, if the ultimate goal includes the freq response, impulse response, over all system Q for the preferred bass response/character, low overall distortion, and the other stuff that really matters. 

It's subtly different.  The culprit that's driving all of this attention to detail are the various sources of back-EMF and other reactances that exist in the load (the drivers/horns pushing acoustic waves into air...and the reverse microphonics).  When you realize that the human hearing system is basically a log-log scaled system (frequency, SPL are perceived both on logarithmic scales), then you might see that it's the details of decays and dynamics that are affecting what we hear--very low level stuff in the decays.

 

Think about "hi-rez digital" recordings...they shouldn't have any audible difference in their presentation than 44.1 kHz/16 bit recordings, but I can tell you that I hear a definite difference (we're talking original recordings done in 24/96 vs. 16/44.1, not "up-rezed recordings")--and it's in the decays.  Same thing for the reactances in loudspeaker drivers and all the little nonlinearities of the magnetic circuits and mechanical compliances inside of them.

 

23 minutes ago, Ski Bum said:

I think I need to slink back into the shadows, munch some popcorn, and think about it some more.  I have no problem believing that a well calibrated, fully active rig sounds better than a slightly less well calibrated, fully active rig, just not sold on the transconductance approach as being fully responsible for the claimed audible improvements.

:ph34r2::emotion-46::emotion-29:...:unsure:

 

Chris

Link to comment
Share on other sites

Oh, Chris.  It's all in the production quality, not the bits.  Quality production work in redbook format preserves the decay of notes as well as any hi res format can.  But that's neither here nor there.

 

Here is a paper that dives into all of this (transconductance amps, not the hi res debate) that others may find interesting:

https://hal.archives-ouvertes.fr/hal-01103598/document 

and another:

http://sound.whsites.net/z-effects.htm

  • Like 1
Link to comment
Share on other sites

I note that the second article that you posted is testing various output impedances using a direct radiating subwoofer ("4 Ohm 300mm Cerwin Vega subwoofer in a 28 litre sealed box").  I would expect sloppy response using a high-output-Z amplifier with this type of loudspeaker (12" diameter woofer with response down to 15 Hz). 

 

It is interesting, however, that the changes in output impedance (-3, 0.1, 3, and 32 ohms) did just exactly what one would expect in terms of impulse and damping performance.  The question that I still have is: how would a extremely high-Z amplifier sound using really good quality large-format compression drivers (with Be diaphragms) and fully expanded horns?

 

Chris

 

 

Link to comment
Share on other sites

I finally read through the first reference, above...

 

Quote
The harmonics and the intermodulation products in the current voice coil have been reduced up to 20 dB in the current driving method as expected... The other loudspeakers confirm a substantial reduction of around 10 dB for all harmonics...
 
The current drive is completely free from voice coil variation or back electromotive force.  Therefore it provides a less distorted coil current than the voltage driving method for all loudspeakers under test.
 
As the mechanical force applied to the mobile part [of the driver's diaphragm] is proportional to the coil current (F=Bl×Is), the current mode provides a better linearity to transform the electrical signal to a force.  In this way the global electro-acoustic transformation by the loudspeaker is more linear.

 

This is what is driving my interest in this subject.

 

Chris

Link to comment
Share on other sites

I do see the appeal.  The weak spots in canned music are always the transducers, mics at one end and speakers at the other, so any effort to minimize speaker distortion seem like a worthy goal to me too.  You have the flexibility of adapting your MEH to this sort of amplification easily enough (well, that's easy for me to say), you just need one to try, right? 

 

Seems that the compression drivers would lend themselves to this quite well, with either active correction or zobels so they play nice with the current source.  

 

The woof's Qm may not be quire right for this approach, and they will be operated at their resonance frequencies.  Since you're foregoing electrical damping by choosing the current source route, the woofs would need to make up by having rather heavy mechanical damping, somehow or another.  Maybe if you stuff the crap out of the enclosure it would help?  Maybe a few holes (aperiodic port) would drop the impedance peak at resonance, too, making the rest of the system calibration easier?  It could demand different woofers, or the one's you have may demand the more orthodox voltage source approach.

 

There also seem to be plenty of schematics for transconductance amps you could build that would not come anywhere near the cost of an F1, tube or ss.

  • Like 1
Link to comment
Share on other sites

You're right.  There is design latitude bi-amping with the MEH design: bi-amping and crossing below 500 Hz allows the use of two different topologies of amplifiers--one for the compression driver and another channel for the two 15" woofers. 

 

As you likely know, most impressions of fidelity are formed above 400 Hz. Those loudspeakers where the back-EMF bleed over from the woofer channel gets into the HF driver channels when using passive crossovers and mono-amping are probably most affected by voltage-source amplifiers.

 

Amplifier damping of the MEH's woofers is something that could definitely be tried out using different output impedances and/or amplifier types.  There is also the choice of relying on acoustic suspension approaches to back chamber volume and stuffing percentage to add to their damping, thus reducing the size of the cabinet further (...this is vaguely reminiscent of Villchur and Kloss's design approach at Acoustic Research in the 1950s). 

 

Anyway, I've got plans with regard to this subject.  Specifics to follow...

  • Like 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...