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Transconductance amplifiers?


Chris A
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Curious sidebar, here are a couple of solid-state Marshall power sections from the '90s.  current feedback/higher output impedance was a feature.  One of my to-do list exercises is to install a switch to modify or eliminate the current feedback and hear its effects.
I have thought these might make decent stereo power stages, as well.


Now, back to the program.

Marshall-VS65R-65W-Schematic.pdf Marshall-VS100R-100W-Schematic.pdf

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5 hours ago, 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?

 

4 hours ago, Chris A said:

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

I have 4 D75s that I refurb'd a few years back. I have lots of time into them already and think this maybe worth the effort. After all this is a hobby right?

No offense to you John, I know this is more than a hobby to you. You raise an interesting point which Mr. Merilainen acknowledges but argues against.

I think it will be fun learning experience at the least and who knows it may be a lot more.

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First amp up on bench. Mechanically this is going to be a tight fit. Need to add a 4PDT switch and a SPDT or repurpose the existing SPDT bridge switch to a DPDT. The 4PDT is right in the output circuit so needs to be able to carry lots of juice. The others are low level signal. Parts on order awaiting delivery. Fortunately the voltage reg. boards I fabricated during my mods a few years back have enough space to accommodate  the feedback diff amp for bridge mode.

BTW @Chris A which FirstWatt do you use?

If I post pics of what I am doing do you want me to start a new thread?

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27 minutes ago, babadono said:

BTW @Chris A which FirstWatt do you use?

I use an F3.  As far as I'm concerned, any progress toward using current source amplifiers is welcome in this thread. 

 

30 minutes ago, babadono said:

If I post pics of what I am doing do you want me to start a new thread?

I may not be contributing strongly due to the very apparent "growing pains" of putting out this subject for public consumption (and the battle scars that apparently go with that), but anyone that is actually trying something on this subject has highest priority to post here--in order to make this thread more useful.

 

Chris

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The Lansing guys frequently do some good work.

 

"While that results in very good sonics there was still room for improvement. Nelson Pass’s first First Watt design was the very tranceconductant F1; the second was the somewhat less purely transconductant F2. He later came up with a power JFET modification for each; he offered to refit the F1s himself and built a handful (three to five, not sure) F2Js and help out with modifying the one hundred existing F2s. All but three of the one hundred F1s built were eventually modified to F1Js. I have one of those and one of the “built as” F2Js. What these do is solve several of the single driver issues.

A transconductance amplifier supplies a current, rather than a voltage, that closely follows the input signal.

 

If the driven speaker has a tight gap and a suitable magnet structure, the control over the cone is much more precise and more, shall I say, firm, than a voltage driven arrangement. While most transducers are designed to deal with the sloppy nature of voltage drive as best they can, most single driver speakers are not. They respond very, very well to current drive.

 

The result is twofold. Frequency reproduction inaccuracies are greatly lessened and bass response, besides having excellent linearity, is extended downward. In the case of the Super 12 it still does not qualify as “bass impact in your gut”, but it is so improved it is startling. I did eventually install a subwoofer but I never use it anymore, it is simply not needed.

I did mention that the system was designed with a crossover and I did build them. After a period of adjustment (of my listening and expectations, not of the system), I acquired an F1J and replace my F2J with it. The results were so outstanding I decided to try it without the crossovers and the Zobel and instead inserted single 2mfd caps in the lines to the tweeters. FYI, in tranconductance circuits the crossovers or caps are wired in series and as close to the amp, not the speaker, as possible.

The result was again very pleasing and I have stayed with it. The system efficiency gained 6dB, to 101-103 and with the ten watt F1J a quarter watt will drive me from the room.

So… My system is technically a single driver augmented, has mucho bass, very good frequency response accuracy and still “very magical and give you a clear window into the music”. Everyone who has heard it is absolutely fuccing amazed. "

 

http://www.audioheritage.org/vbulletin/showthread.php?41886-Single-Driver-Done-Right

 

 

The author is still active, you might want to reach out to him on lansing.....

I'm guessing he rides a ducatti bike

http://www.audioheritage.org/vbulletin/search.php?searchid=5997361

 

I did a quick search for f1j kits, zero will check e-bay later....

 

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F2 review, naked pic of amp layout

https://www.stereophile.com/artdudleylistening/1205listening

 

Compared with the F1, Nelson Pass went for a lower output impedance this time. By putting a trio of 47 ohm resistors in parallel with the F2's output section, he endowed it with a nominal impedance of just over 15 ohms (compared with the F1's 80 ohms), for a bit more electrical damping with low-mass, full-range drivers. If you don't think you need the damping, simply remove the resistors and you'll be left with a source impedance of about 700 ohms. But don't be surprised if you're also left with more bass overhang than you'd care to hear.

1205listen1.jpg

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Current Drive Demystified

Mills and Hawksford coined the term current drive in 1989, in two essential papers published in the Journal of the Audio Engineering Society, dealing with Current Drive Technology.

 

To understand the beauty of current drive, one needs to first understand how a conventional voltage-driven, moving-coil, loudspeaker operates. The current into the speaker is limited primarily by the voice coil resistance and inductance. The voice coil current interacts with the magnetic field generating a motor driving force. As soon as cone motion occurs, an opposing motional impedance is generated, which further limits current flow. From the amplifier's perspective, it is doing its job, since it strives to maintains a constant voltage at the loudspeaker terminals for a given volume level. It should be obvious that linearity between the driver's cone velocity and the applied signal is dependent on the impedance of the various electrical elements in series with the voice coil. And that distortion would therefore result if the signal itself happened to induce an impedance change.

 

A major culprit turns out to be the voice coil resistance, which is not a constant, but actually varies with the audio signal.

 

 

How is that possible you ask? It is well known that most of the power delivered to a speaker is dissipated as heat in the voice coil. Speakers are poor transducers of electrical energy into acoustical energy. A typical conversion efficiency for a domestic speaker is about one to two percent. That means that up to 99 percent of the signal is spent heating up the voice coil. Voice coil hearing has been estimated as being on the order of 0.4 percent per degree centigrade for copper wire. It is not uncommon for voice coil temperatures to spike up to 200 degrees centigrade, at which point a 6-ohm nominal voice coil resistance increases to 10.3 ohms. As the voice coil heats up, the power delivered by a voltage-controlled amplifier decreases. This leads to loss of sensitivity with increasing drive signal, also known as power compression. Since the loss of sensitivity would be most prominent in regions of minimum impedance, shifts in frequency response would also occur with drive signal level.  For a woofer, the impedance minimum corresponds to the lower midrange, which implies tonal balance shifts as the vice coil heats up and cools down. In addition, the increased resistance reduces driver damping and leads to misalignment of crossover networks.

 

Current drive offers a major alternative to the speaker-amplifier interface. A transconductance amplifier is operated as a current source with a high source impedance relative to that of the speaker. The output impedance must also be linear and frequency independent. An ideal current source would possess an output impedance so large as to be considered infinite relative to that of the speaker. The F1's 80-ohm impedance is certainly adequate in this regard. As a result, as Nelson puts it: "the  amplifier delivers a precise current to the voice coil of the loudspeaker driver, ignoring the series impedance elements in the circuit, including the wire, the inductance of the voice coil, the resistance of the voice coil versus temperature – all that stuff."

 

Current drive offers the potential for dramatic reduction in moving-coil speaker distortion due to power compression and other voice-coil non-linearities. However, there are two basic practical problems to overcome. On the amplifier design side, it turns out that it is difficult to design a high-power current source. The high-power designs investigated by Mills and Hawksford are rather complex. The simplest approach lends itself to low-power designs, which fortunately fits nicely into the high-efficiency loudspeaker scene.

 

On the speaker design side, things are also different. For example, the ploy of reducing a driver's rated impedance from 8 ohms to 4 ohms in order to gain 3dB in sensitivity does not work with a current source amplifier. Instead of doubling the current draw, as would be the case with a voltage-controlled amplifier, the current draw remains the same, which means that sensitivity is actually lost. On the other hand, increasing impedance leads to increased sensitivity with current drive. Consider that a woofer's resonance region is no longer current limited and sinks the same current as the frequency band above resonance. That makes for a huge boost in bass response around the resonance region. The bottom line is that using drivers of differing nominal impedance but identical sensitivity ratings will result in output mismatches.

 

Finally, conventional parallel type crossover networks do not work well with current sources. I spent a weekend convincing myself of that. Instead, series type networks are necessary for optimal performance. No wonder then that current source amplifiers require matching loudspeakers, designed for a current drive interface, and capable of taking full advantage of their distortion reducing potential.

 

http://enjoythemusic.com/magazine/equipment/1204/firstwattf1.htm

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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?

 

Beta max was better, but lost to vhs

The competitor to Bluray also died

THX is probably better than the new format, but is getting crushed

- new format requires more speakers and more amp channels,

little wonder the manufacturers are pushing it

 

If the customer installed base is all built around Voltage

and current requires a new kind of amp

a new kind of crossover

a new kind of speaker

 

It becomes a hard sell,  a mountain

that most channels and customers are unwilling to climb

especially since most of the customer base can't or won't hear a difference

Most of the installed base is low end direct radiators that are barely intelligible on a good day.

 

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On 5/15/2021 at 2:30 PM, 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.

 

My two cents....

 

The problem looking for a solution,

is the negative effects from the movement of the voice coil. aka piston.

The greater the movement, the greater the negative effects, linear or non linear.

 

From the discussion, it appears that compression drivers suffer little to no effect from motion.

Leaving voice coils as the bad boy.

LaScalas, for example, minimize the motion of the 15in driver using horn loading

are the negative effects of the LS bass perceptible given the minimal travel ?

 

Given that

- Full range drivers have tolerances more suitable to current drive, empirical

- Authors indicate a different crossover topology for current amps

- Horn loading reduces motion, the bad boy

- Full range drivers, may not have linear performance across all frequencies, and resistance levels

- less crossover points is better

 

Should  a current speaker adhere to; simple is usually better, and declining marginal returns ?

 

For example (not a Danley or Klipsch)

A 2 driver cabinet with identical full range speakers, 4 , 8 , 12, probably 2x12

- non liniarities may require a resistor somewhere or a different ohm rating on one speaker

Passive crossover, slopes tbd

A crossover point somewhere between 300-600 to minimize aggregate travel, empirical data

I believe that most of these appear to be designed for open baffle, and direct radiator, but may be suitable for horn loading in an enclosure equivalent to a LaScala with a horn length of approx 20 inches and you pick the geometry.

I would go for the driver with the best performance for human voice and piano.

 

Alternatively,  you could go for 2 compression drivers if you can find one that drives down in the subwoofer range, or just roll it off at a number like 50hz, and 17K on the top...Probably 2 different drivers.

The assumption being that compression drivers don't generate negative effects from piston motion....

 

For someone with woodworking skills

and all of their fingers

This may be an easy project

and a crossover that allows them to vary the crossover point and slopes

and take some measurements and listening impressions

 

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10 minutes ago, babadono said:

Dang Bubo. So it ain't worth trying?

 

What isn't worth trying ?

 

Running a current amp through some LaScalas to see what happens

sure why not ? The suitability of the voice coils TBD

 

Changing the interface from the current config on the front of the book to the suggested config on the front of the book, sure why not ?

Looks like negative feedback, with a single ground and a resistor removed. .....

Does the mod you are performing on the speaker interface of the Crown qualify it as a current amp, or should it be classified as an experiment in negating the negative effects of piston movement ?

 

The authors I read who have done some work ahead of us, indicate full range drivers, and different crossovers for current.

 

Current Klipsch and Danley are not full range driver products with crossovers as described.

 

As I stated, someone with woodworking skills and some extra money can build an experimental set of speakers or just one speaker to play with to generate some empirical evidence for the beginning of a design. Or at least characterize the behavior of the components and systems. Off the shelf full-range drivers aren't cheap the last time I looked. Suitability for horn loading, I would leave to someone else to determine. If the goal is to reduce piston travel and use a current amp.

 

My background is developing and selling products to market in data, telecom, PC, mini, mainframes, central office, mil, lots of other applications and environments etc. I'm a marketing econ guy, not an engineer. I never tried to pick the components or board layouts for the engineers working on my products. Something the engineers seem to appreciate, they all wanted to work in our group I also had some great engineering managers to work with that kept the trains running on time, budget and spec.

 

Frequently, when I met with the engineers on a problem or new product, they would get all excited, take lots of notes and rush off to start fleshing out design goal docs etc. I asked one of them one time, if they were rushing off to work on my suggestions, he said no, but you gave us a lot of great ideas to meet the customer's applications.

 

I just wanted winners to sell, and got them over and over again.

 

I understand that part of the fun on here is pure research, why not.

Based on what I have read, I have already thought of a way to bring current based products to market, assuming they really are better and cost effective for the performance gains.

 

If for example each speaker needs 2x12 full range drivers the cost may come in at $2,000 just for the drivers. Not cheap

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2 hours ago, Bubo said:

What isn't worth trying ?

 

 

 

Changing the interface from the current config on the front of the book to the suggested config on the front of the book, sure why not ?

Yes doing what's on the front of book but keeping bridging capability on a Crown D75. Run the mod'ed amp on one speaker and a non mod'ed on the other. See if there is a difference.

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Entitlement in the audio reproduction "chain" isn't in the electronics or even the loudspeaker drivers, it's in the enclosure materials.  And the folks fabricating enclosures out of Aluminum alloy are on to something.  It's a "ffs" moment listening to them.

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Ignoring the cost, which is considerable for high end audio components, driver entitlement becomes a near reality when the enclosure is rigid.

 

Although the images are from the Magico website, Aluminum enclosures machined from plate stock are nothing new.  I heard a pair of JBL L100 clones fabricated from 7/8" Al-plate, milled and assembled using cap screws and Loctite adhesive.  It was heavy but easily managed once placed on a dolly.  Internal bracing resulted in the overall enclosure being somewhat larger than the OEM version.      The difference in the low end performance is hilarious.

   

magico_Cabinet_Main_Structure.jpg

DSC00751.jpg

magico_m7_internals.png

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24 minutes ago, John Warren said:

Ignoring the cost, which is considerable for high end audio components, driver entitlement becomes a near reality when the enclosure is rigid.

 

Although the images are from the Magico website, Aluminum enclosures machined from plate stock are nothing new.  I heard a pair of JBL L100 clones fabricated from 7/8" Al-plate, milled and assembled using cap screws and Loctite adhesive.  It was heavy but easily managed once placed on a dolly.  Internal bracing resulted in the overall enclosure being somewhat larger than the OEM version.      The difference in the low end performance is hilarious.

 

@John Warren,

 

As the OP to this thread, I'm asking you to take this information off to another thread--probably in the Technical/Modifications section, because it appears to be diliberately diluting the topic here--which you've already shown that you really don't wish to talk about.  I'm familiar with the topic you mention, above, but your topic really doesn't belong in this section of solid state amplifiers, I believe (and the moderators can certainly overrule if it isn't a correct assessment). 

 

This thread has a history to it.  I don't wish to add to those issues any further.

 

Apologies for the directness of my request in advance if this causes any issues. 

 

Chris

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Well I received Mr Merilainen's book yesterday. Hopefully I will be able to glean enough info from it to make this D75 mod successful. I apologize to anyone who maybe trying to follow my progress, it will be slow. I am doing it in my "spare" time.

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Some parts of the book are too deep for me(the math). All in all worth the $25

Meanwhile slowly but surely

P1020024.jpg

4PDT added toggle switch on left, 2 DPDT pushbuton switches a little right of center. 4 solder lugs added to tie in the current sensing resistors. Hope to epoxy mount them to the chassis to help with power dissipation

P1020027.jpg

Main board connectorized for voltage reg.

P1020026.jpg

voltage reg connections

P1020025.jpg

voltage reg board  with added diff amp needed for bridge mode

 

EDIT: let the wiring/rewiring begin:)

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