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Chris A

Transconductance amplifiers?

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No apologies needed...I'm excited that you took the ball and advanced it.  In this case, building a First Watt F1.  I'd be interested in knowing how it sounds relative to a typical Class A.  I've lately been head down on other stuff (MEH, etc.) so I've not been focused on this subject.  I would expect that there might be a bit of a market for F1Js and F2Js if the theory (by NP and others) turns out to be workable in practice.

 

The theory says that it should sound very clean, but will likely require a re-EQ on the bass bin, since current feedback ignores the increase voltage levels closer to the driver's loaded Fc--as you pointed out with full-range drivers (e.g., Fostex, etc.).  Nelson Pass did a pretty good report on the differences as applied to quite a few full range drivers that he got his hands on.

 

Chris

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On 12/1/2016 at 8:43 AM, Chris A said:

William,

 

The First Watt F4 is notable in that it can be used with an existing integrated amplifier that provides the preamp duties, while the F4 provides power (with no voltage gain) to the loudspeakers.  It's an unusual idea, too. Thanks for the description of your SE F4 experience. 

 

I was really looking for experiences integrating current-source (transconductance) amplifiers with horn-loaded loudspeakers.  Nelson Pass apparently used his F1 and F1J designs with full range drivers (Lowther, et al.).  While that's an interesting application and they are high-efficiency drivers, it's not quite the same as hooking a transconductance amplifier to a pair of La Scalas, Belles, Klipschorns, or K-402s with compression drivers. 

 

In particular, I'm looking for experiences with the cleanness of sound--over and above that of any other amplifier topology.

 

Chris

I used to be friends with the chief engineer at VSP Labs in Ann Arbor, Michigan. He designed a Trans MOS transconductance amp that was very robust, well reviewed in the early 80's. It had was designed to handle 2 ohm loads and behaved much more powerfully than other designs of the day. It was rated at 150 watts into 8 ohms. I owned 2 of them, which I got at dealer net price, but other people wanted them and made me good offers.

 

There were no speaker fuses. The amps were burned in at the factory (which I visited many times) by shorting out the outputs and letting the thermal cutouts cycle in and out. A very rugged design that could pound the heck out of any speaker that was hard to drive because or reactance and impedance issues. They were the first company to do this as I recall.

 

BTW, because of the "golden eared" reviewers that believed that an all discrete front end of a power amp was better than one using Integrated Circuits, the designer came out with a "Gold" version that cost more, but didn't perform any better. He said he had a very tough time trying to equal the performance of the regular version using discretes parts and considered the less expensive "IC" version to be superior. The power "back end" was identical on both. The difference of the extra 50 Watts on the Gold version was just not being as conservative of a power rating as the original. IOW, it was based on marketing, not science.

 

http://www.ebay.com/itm/Vintage-VSP-Labs-Trans-MOS-Power-Amplifier-POWERFUL-Works-WATCH-VIDEO-/192243405850?hash=item2cc299941a:g:hbwAAOSwWxNYzMj2

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On 12/6/2016 at 2:31 PM, Chris A said:

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.

The VSP Labs Transconductance amplifier used only nested feedback loops, as far as I can recall.

 

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On 12/08/2016 at 1:00 PM, Ski Bum said:

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.

 

Yes and no... If you start mixing in a digital format, more bits is better. Every dsp action adds bits, which is why most DAWs or digital mixers use 32, 40 or higher resolution, and if done correctly, only dithered on the final. Otherwise, you will lose infomation.

 

Still, all the analogue stuff, i.e., performance, acoustics, microphone, preamp, until it hits the adc is what makes it good/great.

 

Bruce

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Bruce,

 

It's been a little while since you mentioned the subject of audible effects of bit depth.  I've since thought about this subject at some length and have come up with an explanation of the listening differences using greater bit depth recording.  This probably should go into its own thread, but I figure that this explanation here will suffice.  See the following figure from Toole's book, noting the bit-depth scales to the right of the Fletcher-Munson (equal loudness) curves:

 

59e63510924a0_Effectivebitdepthtohearing.PNG.042f1a4a358892cc1a0d23bd1f2bdc44.PNG

 

If you think again about the vertical scale (SPL or loudness), note that most people do not turn up the gain on their preamplifiers such that the output is at the original concert levels in their listening room.  Instead, they listen with the top of the loudness scale considerably below the maximum loudness shown here.  In effect, the home listener is "squeezing" the loudness scale into one which has lower ultimate loudness for the music they listen to.  In effect, if they have more bits of loudness left over from 20 or 24 bit mastering (i.e., less overall compression is used), then the size of the loudness quanta is smaller, in effect squeezing down on the intervening loudness step sizes in the time domain.  Since human ears are extremely sensitive on music decays (as it has come to be known also for "room reverb" of medium and high output impedance amplifiers), the human hearing system hears these smaller loudness levels as "more solid" sounding (using my words for what I hear).  At least, that's what I experience when listening to higher bit depth recordings done using less compression.

 

In addition, when you think about the recording engineers and what they're doing, they typically turn up the microphone gains until the peak level of the recording is as close as they can get it to 0 dBSPL.  So in this case, the effective size of the least significant bits in terms of delta-SPL is maximized, assuming full-range compression is not employed during recording, i.e., only the peaks are compressed.  The sound that I hear is again "more solid" than shallower bit depth recordings.

 

Chris

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On 30-11-2016 at 3:11 PM, Chris A said:

Anyone here use transconductance amplifiers with their Klipsch loudspeakers...such as First Watt F1, F1J...or ABACUS amplifiers?

 

Hi Chris,

 

I think that (Klipsch) horn speakers benefit from transconductance amplifiers.

 

But, I think that amplifiers with an output impedance of about 40 Ω are not realy transconductance amplifiers (or current-drive amplifiers). Those are more power-drive amplifiers, with an output impedance around the impedance of the loudspeaker (within a factor of 10). So, amplifiers without feedback and valve amplifiers are all (more or less) power-driven amplifiers.

 

A real transconductance or current-drive amplifier has an output impendance of 400, 4k, 40k or more Ω.

 

You read and mentioned already the paper of Esa Meriläinen and learnt that in the small-signal domain of drivers there is a huge benefit of current-drive. I don't know whether the Klipsch drivers have non-conducting voice coil formers (bobbins). If they have, the benefit is even greater (as you could read in his paper).

 

I think in the large-signal domain around resonance a horn system is probably inherently better than other systems. But because of the elimination of electrical damping (one of the non-linear distorting effects of voltage-drive) you have to cope with the rising response around resonance (with -real- transconductance Qts equals Qms of the driver!). EQ does the job. You also need EQ for the rising response on the high-end due to the elimination of inductance effects (damping and non-linearities).

 

But if a horn is used (far) below resonance there is a drawback. Around resonance a driver is sensitive for second harmonics of frequencies half of the resonance and for third harmonics of frequencies a third of the resonance (so, with driver resonance at 60 Hz, the driver is sensitive for second harmonics of 30 Hz signals and for third harmonics of 20 Hz signals). This goes for every driver, but with a closed system you can realize enough acoustic damping of the resonance and of the sensitivity. I don't see how you could do that with a horn system. On the other hand, I do wonder whether a horn is ever used so far below resonance...

 

So, I think that every driver thats work with an electromotor (F = Bl · I) benefits from transconductance / current-drive in terms of distortion, bandwidth and stability (no destabilizing effects of heating voice coils). (I don't know how piezo-drivers, electrostats and other exotics react to current-drive.)

 

But... But traditional crossover don't work with transconductance! They need a redesign or elimination. Esa explains how to build passive crossovers for current-drive. Even better is to use an active configuration. You can use DSP for driver correction, time alignment of drivers, crossover, time alignment of loudspeakers and for room correction.

 

Leaves the problem of availability of (real) current-drive or transconductance amplifiers. I build one myself (it is no rocket science, just changing the feedback, and the book of Esa helps), build my own crossoverless 2-way system (sorry, no horns) and bought a 2x4 channel DSP system with DAC. The sound is crystal clear, soundstage is great and stable and an active system is very efficient. I have 4x 80 W available, but I wonder if I every used 1 Watt per channel...

 

Send me some Klipsch-horns and I will tell you how they sound. :-)

 

How do you go on your quest for lower distortion and better sound?

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On 12/3/2016 at 4:11 AM, Chris A said:

There's a very interesting book on the subject:  https://smile.amazon.com/gp/product/1450544002

 

51vzOuc6A1L.jpg

Reading on this Gents website, he shows how one can turn a regular voltage source amp into current source amp. Looks pretty darn easy. I think I'll try it with one of my Crown d75s. 

 

Edit: Looks pretty darn easy----yea accept for the bridging part:)

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 I have used current source amplifiers for many years. All were variations on the Mauro Penasa My_Ref project. 

  The amplifier was an extension of a Musical Fidelity amp. 

  The output stage has a voltage gain of 1.The current gain is 1000. But the output impedance is not high due to feedback loops, several. It has local and global feedback. 

  It drives my LS II very well. Also HIII’s. My_Ref_FE - Schematic 1.5.pdf

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  It looks too simple. A friend ran this sweep on an AP analyzer. This is with a linear power supply. Notice how low in amplitude the charging spikes are at multiples of the AC line frequency.CB1E3A63-FC5F-473B-9872-EF4BCE3E861B.thumb.jpeg.168054561e748fb9ccd5bc67f3105c61.jpegr

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Well I finished up my "buffer box" to boost the signals into the Xilica....

next up I  will be modifying a Crown d75 for current drive..

first thing I need to get some resistors the author recommends....0.5 ohm 3W minimum

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Good stuff babadono - interested to see how it turns out.

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Modifying for straight 2 channel is pretty straight forward. Keeping ability to bridge the amp is going to be a sticky wicket.

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Whew....💡   I THINK I've got what I need diagrammed to keep 2 channel and bridge mode intact. Well at least in theory.

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Forgot the diff amp for bridge mode...oops....got it incorporated now........parts ordered...let the fun begin.

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On 5/11/2021 at 3:25 PM, babadono said:

Well I finished up my "buffer box" to boost the signals into the Xilica....

next up I  will be modifying a Crown d75 for current drive..

first thing I need to get some resistors the author recommends....0.5 ohm 3W minimum

Dale makes a 0.536 ohm, 5 watt LVR resistor. These work great as an output in a current amplifier, 

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Theory question: if you use DSP and active EQ to fully flatten the frequency curve coming out of your speakers, have you not done better than either constant-voltage or constant-current and delivered constant-power?

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2 hours ago, DirtyErnie said:

Theory question: if you use DSP and active EQ to fully flatten the frequency curve coming out of your speakers, have you not done better than either constant-voltage or constant-current and delivered constant-power?

  That is deep.

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5 hours ago, DirtyErnie said:

Theory question: if you use DSP and active EQ to fully flatten the frequency curve coming out of your speakers, have you not done better than either constant-voltage or constant-current and delivered constant-power?

You can only do that for one point in space (in your listening room, etc.) by measuring and correcting SPL response in-room.  Everywhere else in the room, it's not flat SPL and/or phase response. 

 

When you design for anechoically flat power response over a certain sector of a horn's output, you've done a whole lot more than what you can do using DSP to flatten DSP at one place in the room.  In fact, you can't correct for power response issues (a.k.a., polar coverage) with DSP.

 

Additionally, the reason for using transconductance amplifiers (as least in my view) is not flatten power response in-room, but rather reduce distortion due to room/horn/driver effects on back-EMF from the drivers.  Transconductance amplifiers ignore the back-EMF noise from the drivers in order to produce a cleaner output that more closely matches the input signal to the preamp/amplifiers.

 

Chris

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3 hours ago, Chris A said:

Additionally, the reason for using transconductance amplifiers (as least in my view) is not flatten power response in-room, but rather reduce distortion due to room/horn/driver effects on back-EMF from the drivers.  Transconductance amplifiers ignore the back-EMF noise from the drivers in order to produce a cleaner output that more closely matches the input signal to the preamp/amplifiers.

 

99.999999% of audio amplifiers are voltage sources.  If transconductance amplifiers were a better mouse trap, the world would have marched to them immediately.

 

Why do you think it did not?

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On 5/15/2021 at 6:05 PM, John Warren said:

99.999999% of audio amplifiers are voltage sources.  If transconductance amplifiers were a better mouse trap, the world would have marched to them immediately.

 

Why do you think it did not?

 

You can argue with the author...(i.e., not me):

 

51vzOuc6A1L.jpg

 

My view is to try it to see if it works like he says.  I think that the application is perfect (TAD TD-4002 on a K-402 horn). 

 

Chris

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