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Old 9th January 2006, 03:46 PM   #1
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Default Class T - maximizing damping factor, any ideas?

Hi again,
I am still much interested in this Tripath stuff. Few days ago, as I have been talking to a friend of mine, who is doing professional sound reinforcement services for live acts, clubs ...and I told him about class-T advantages and stuff, that he mostly did not know. But as we were talking, he asked me about typical class-T damping factor. So I told him, that as far as I know, these amps have DF about 300, maybe 400 (@ 8 ohm)(I have seen some test about it some time ago). He smiled... and told me that for him, any amp with DF below 800 is useless.

I think he needs high DF for driving those many-kW rated sets of subs, with as much precision as possible.

So now, I wanna kick his ***
I would love to build a class T amplifier with power about 2x800W (so, most likely based on TDA2500) which would have DF of 800 or more, at 8 ohm load.

I have done a small research and found out, that DF is very much related to the amound of negative feedback inside the amplifier. So, do I have to maximize the amount of negative feedback? How can I do this in class-T amplifier, where only feednack is that switching one, from the input of LC demodulating filter? Could I employ any other type of negative feedback, perhaps just like it is done in class AB, that means, feed the output signal from behind LC demodulator back to the input stage, thru some resistive network and filtering?


Any ideas and efforts to help are greatly appredicated!
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Old 9th January 2006, 06:19 PM   #2
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Just a sidebar question:

What is the physical distance between the amp and the subwoofers? Even if you had an excellent damping factor, that can all go away with the resistance of the cable length.
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Old 9th January 2006, 06:35 PM   #3
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No, it won't.

These guys always try to have a shortest possible distance between the amplifier and speaker. In practical applications like live events, they have cases with amplifiers right behind the speakerbox sets, one for left channel, one for right. This way, they have these cables as short as 1 or 2 m. And, of course, they use thickest available cables, mostly 2x4qmm, sometimes even thicker.

So this cable has very, very small DF degradation effect.
Thanks for a good question, but this is not our problem right now...
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Old 9th January 2006, 06:54 PM   #4
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A small suggestion.......

http://www.hypex.nl/docs/UcD700_datasheet.pdf

The dampingfactor will be >400 till 2000Hz. The UcD modules are known for a strong and solid bass response. Not only for High End application but also for PA application. This year some Professional Audio Companies will choose our Class-D modules because of the clean and deep bass response, instead of other Class-D brands.....

Regards,

Jan-Peter
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Old 9th January 2006, 07:01 PM   #5
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Thanks for an interesting suggestion.

However, although your class-D amplifier could have good bass response, i believe they can never beat class-T in full-range applications. And I do not want to design a subwoofer amp. I want to design an amplifier who will perform best in either bass and full-range amplification.

However, I have to achieve DF above 800, not 400, as you stated.
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Old 9th January 2006, 07:17 PM   #6
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Quote:
However, although your class-D amplifier could have good bass response, i believe they can never beat class-T in full-range applications. And I do not want to design a subwoofer amp. I want to design an amplifier who will perform best in either bass and full-range amplification.

However, I have to achieve DF above 800, not 400, as you stated.
Sorry I don't wants to be rude, but an UcD can never beat Class-T in full range??? Please read first the Datasheet of the both amps, and put some special attention on;
THD of the Class-T above 1K and the UcD.
The true flat frequency response of the UcD, load independed (!) instead of the piek of the LC filter in the Class-T with different load (2-4-8-16 ohm). We even show a flat Frequency response in 1 ohm!!

You don't have to be worry about the lower DF as the 800. A wire of 2x4qm length 1 meter has already a DC resistance of 0.009. Thereby you have the internal wiring in the loudspeaker, four time the resistance of the Speakon connector. The Speakon will have 0.002-0.005. So at least you have a DC resistance > 0.02. What kind of output impedance the amplifier has in practice you will never come closer to 0.02!

One or our potential PA customers will test in a few weeks a dual UcD700 in bridge mode (2 amps bridged, in total 4 amps!) build in a dual 18" subwoofer....

When the DF of these amps were not ok, the PA manufacture would NEVER use the UcD700 on one of their 'most powerfull' subwoofer.....

Jan-Peter
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Old 9th January 2006, 07:36 PM   #7
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Quote:
Sorry I don't wants to be rude, but an UcD can never beat Class-T in full range??? Please read first the Datasheet of the both amps, and put some special attention on;
THD of the Class-T above 1K and the UcD.
The true flat frequency response of the UcD, load independed (!) instead of the piek of the LC filter in the Class-T with different load (2-4-8-16 ohm). We even show a flat Frequency response in 1 ohm!!

You don't have to be worry about the lower DF as the 800. A wire of 2x4qm length 1 meter has already a DC resistance of 0.009. Thereby you have the internal wiring in the loudspeaker, four time the resistance of the Speakon connector. The Speakon will have 0.002-0.005. So at least you have a DC resistance > 0.02. What kind of output impedance the amplifier has in practice you will never come closer to 0.02!

One or our potential PA customers will test in a few weeks a dual UcD700 in bridge mode (2 amps bridged, in total 4 amps!) build in a dual 18" subwoofer....

When the DF of these amps were not ok, the PA manufacture would NEVER use the UcD700 on one of their 'most powerfull' subwoofer.....
First, I have to ask you, are there any listening tests performed on your amplifiers in full range amplifiers?
Another question. Do you supply any pro-audio manufacturer with your modules for full-range amplifiers?

Also, please, keep in mind that it was not me who told that we need damping factor of 800. I can't even hear the difference between two amplifiers from same manufacturer with DF 400 and 800. I know that Speakon and wiring have some resistance which CAN degrade damping factor. So, please, you do not have to tell me.

The only thing I would like you to, is to help me with RB-TDA2500 DF maximization.
I hope that it is not too hard to understand.
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Old 9th January 2006, 07:57 PM   #8
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I did some experiment on two conventional Class AB amps(monoblocks) , which was driving my subs. Really short speaker cables were used. Findings: varying the feedback-factor showed that there, in my opinion , was an optimal amount of negative feedback. This isnīt the same as maximal NFB. You can make an amp with very high open loop gain , then apply lots of NFB and get a very low appearent output impedance. But then I suspect that back- EMF from the speaker will make things worse .

A low output impedance is perhapes not the only way to good bass reproduction?
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Old 9th January 2006, 09:15 PM   #9
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Thanks for posting, this is a kind of post I have been awaiting.

How did you found out, where is that "right" point of feedback amount? How did the system behave at maximum NFB? What was worse at maximum NFB than at yours "optimal" NFB?

I will try to explain this from an easy point of view, because I like that
It is right, that all speakers in this world are not ideal resistances, but they have also a kind of "capability" to accumulate energy, so they behave as a complex load to their supply (amplifier). And, because, the speaker is basically an inductor, it tends to resist the supply either in pumping the current into, and getting it out of it.

The final results of this is, that we need a strong supply to keep the speaker cone moving how we want, and not to let him move how it wants. And my opinion is, that we have to overcome the results of accumulating energy by driving the speaker strong. SO we need a strong power supply, and strong connection between supply and speaker

Am I right or not?

So, what did you mean with "EMF from the speaker will make things worse"?
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Old 9th January 2006, 11:19 PM   #10
mskeete is offline mskeete  United Kingdom
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I was under the impression that the relatively large resistance of the voice coil meant that having an extreamely high DF was pointless

http://www.audioholics.com/news/edit...AudioMyths.php

http://www.audioholics.com/techtips/...pingfactor.php
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