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Power amps - Simple or complex?

Started by teemuk, April 14, 2006, 11:17:16 AM

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joecool85

I think you have a setting that needs to be adjusted or something.  I can see it fine.  I would PM crystallas about it.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

teemuk

Quote from: RDV on April 30, 2006, 08:33:21 AM
It seems that you(I) are not allowed to download or view attachments on this board.

I think there´s some sort of an cookie error troubling this forum system: I get the same message now and then. If you were logged in and got the message (as i suppose you were) do not try to log in again,  it will not work very likely. Instead, clear the web browser´s cache and cookies and restart it. Now login again and everything should work. This works at least with IE.

skey

Teemuk - this is great information and a couple of very interesting surprises!
I was expecting more out of the modern design.


teemuk

As mentioned earlier, the constant voltage gain -circuit can be replaced with a current driven circuit or better yet, with a circuit that mixes both of them. This so called mixed-mode-feedback topology can actually provide very nice results, especially if the gain is matched to a specific speaker impedance. In a mixed-mode amplifier the other speaker node is not directly connected to ground, instead it is in series with a small value "current sensing" resistor. The load current over the resistor is fed back, thus effecting the output impedance of the amplifier. As a result the circuit operates almost like a constant current gain amplifier that provides more power to high impedance loads.

While the previous might have sounded difficult it's actually not and the amount of extra components required is only a handfull - in minimum two resistors. I've seen this topology used in various guitar amplifiers (both chip and discrete) but it's quite rare in PA- and especially bass amplifiers - i guess for obvious reasons. I will not go into further details of how to design a mixed-mode amplifier since everyone can find that out by themselves from the link i that provided earlier, however i will demonstrate how it compares with a transformer coupled circuit.

1. The expensive?, tricky-to-use and very often custom-type transformer can be omitted and still comparable results can be yield from the circuit.
2. The circuit is a combination of constant voltage -and current drive so it's features are compromises of both topologies.
3. It is difficult to match up the "punchy" bass and emphasized highs of a transformer coupled circuit but the mixed-mode amplifier definitely comes a lot closer than a constant voltage gain circuit. Differences between the tone of these circuits is not so big anymore. It's easy to distinct which topology is used only during bass-heavy content.
4. What is important, at least for Hi-Fi, monitor and PA amplifier designers, is that the topology can be used to gain a very linear frequency response.
5. Setting the gain of higher impedances too high will weaken the low frequency response.
6. Mixed-mode circuit's work as well as intented only with speaker loads for which they were designed to.

I included a plot where the "modern" circuit's output impedance is as closely matched to the transformer coupled circuit's as possible without affecting the low frequency response too much. Note that the circuit's frequency response could be tweaked easily to provide a very linear response too. Once again, the plots are dependant of the used speaker's impedance. For comparison the "vintage" plot is identical with the previous one.

skey

I recall reading about a Bob Carver Challenge in that he had found similar results.
Hard to find current information on this challenge.

teemuk

Looks like Vox uses similar feature (mixed-mode or current drive) in the power amp section of Valvetronix line amplifiers:

http://www.korgmidwest.com/vox/tips/Valvetronix_Overview.htm
http://www.voxamps.co.uk/products/valvetronix/vtoverview_inside_story.htm

The following text is excerpt from the page under the first link:

"The Reactive Feedback technology used in the VariAmp Circuit "reads" the impedance curve of the speaker and then reports this reactive information back to the secondary side of the output transformer. This information is fed back to the primary side of the output transformer and therefore changes the loading on the tubes, another important part of the vital role an all-tube power amp plays in the creation of traditional tube tone.

Unlike most solid-state amplifiers used in guitar amps which are Constant Voltage designs, the Valvetronix Constant Current design used in the VariAmp allows the output voltage of Valvetronix amplifiers to vary along with the speaker's changing impedance, just like in a traditional tube amplifier."


The implementing of a transformer coupled, low power tube push-pull stage before the real SS power amp was quite clever but even more clever was the idea to make it sense the speaker impedance, this should give a realistic loading for the tube "power amp" section. My theory is that a huge part of the tube sound lies in the compression of the voltage peaks that occur during a high load resistance. I have been playing around with ideas to use mixed-mode feedback and compress the signal within the power amplifiers gain set/feedback loop to achieve similar results.

The "marketing statements" presented on the article have too many contradictions which makes it very hard to figure out what kind of circuit the Valvetronix power amp really is. I have been looking for the Valvetronix schematics but haven't found any yet - it would be really nice to see what kind of circuit the power amp section really hides inside.

teemuk

I could be utterly wrong but i guess the basic topology is something similar to this: (Note the two feedback paths.)