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Old 18th December 2002, 05:50 PM   #31
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Quote:
Originally posted by phase_accurate
I'd like to throw in my 2 pence about the tradeoffs regarding the switching frequency of class-d amps (either digital like Equibit or the classic analog PWM).
Thanks, it's good to hear from someone who's worked with these things.

Any design involves making compromises, but it strikes me that with switching amplifiers it's even more of a balancing act.

Quote:
I was once thinking about air-cored output filters (the Tact actually uses such) but then you have additionall EMC issues.
One can't help but notice the enclosure that TacT uses. Unlike a lot of high end amps, I'm sure the TacT's is more than just eye-candy.

TI claims their reference design (with shielded bobbins and the circuit board mounted over an aluminum) passes CE emisions requirements outside of an enclosure. Pretty impressive for a fairly high power, high frequency switcher.

Quote:
Once it was thought that the complete (whatever that is) removal of the carrier is important but noone cares that much about that nowadays anymore, to which I can agree as well (our amp had a carrier suppression of > 80 dB).
This does seem reasonable, as long as any remaining power from the carrier isn't significantly cutting into the amp's efficiency or the driver's power handling.

So are you presently doing anything in this area?

Regards,
Brian.
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Old 18th December 2002, 06:35 PM   #32
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Thanks Brian!
That is what I wanted to hear!

Rarkov:
Sorry if you found yourself offended, but Brian is the only person who is actually doing something with Equibit chips, and his revious posts were very infomative. All credits that this thread is still alive go to him.
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Old 18th December 2002, 07:02 PM   #33
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Default About oscillators

You can request samples of crystal oscillators from Fox Electronics and MF Electronics at any specified frequency.

www.foxonline.com
www.oscillators.com
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Old 18th December 2002, 09:19 PM   #34
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Default Re: About oscillators

Quote:
Originally posted by Rookie
You can request samples of crystal oscillators from Fox Electronics and MF Electronics at any specified frequency.

www.foxonline.com
www.oscillators.com
Thanks for the links!

Fox doesn't list any jitter specs for their oscillators, so I'm assuming they don't have anything in the ultra-low jitter catagory.

MF Electronics has several products in the 100MHz range with jitter in the 5-6pS range, which is as good as I've seen from any company at this frequency. I just might give their custom sample service a try.

******
FYI:

With a regular DAC, jitter will affect the timing of the output. Increased jitter causes a type of IM distortion.

With a pwm-based digital amp, jitter affects output timing in the same way as a regular DAC, but it will also reduce dynamic range. (PWM output level is controled by variations in output timing.)

The main performance spec differences between the TAS5012 and TAS5015 are dynamic range and THD+N:

TAS5012:
dynamic range: 102-db
THD+N: 0.06%

TAS5015:
dynamic range: 112-db
THD+N: 0.01%

Using a variable-voltage power supply has the potential of increasing the dynamic range of either by an additional 30-db!

I believe the only principle difference that gives the TAS5015 better performance is lower jitter levels. It relies on the use of an external PLL (or oscillator) to generate its internal clock. The TAS5012 has a built-in PLL.

Wild speculation: It wouldn't surprise me at all if they used the same silicon. They might just be hooking up the bondout wires differently.

Just as with regular DACs, an Equibit stage typically would have to be sync'd to the incoming clock of the data source, hence the need for PLLs.

As we discussed earlier, the AD1896 ASRC allows us to have a local clock at the converter, which if done right will have less jitter than a clock recovered by a PLL. (A VXCO-based PLL might be an exception to this. I'm surprised I haven't seen anybody try this, even with a regular line-level DAC.)

(Another way to achieve jitter reduction is to have a high-quality master clock at the converter, and transmit it to the data source. The data source then sends the data to the converter along with a reflected clock. This data can then be sent through a FIFO to be reclocked by the original master clock. Since the reflected clock is sync'd with the master, there's no concern about a buffer underrun or overflow.)

Anyway, the specs in the TAS5012 and TAS5015 data are somewhat nebulus. Many data sheets list claims for best-case synarios, especially wrt front page claims. However, I'm speculating that in this case the claims are assuming a recovered clock. The story *might* change with stable master oscillators nearby.

So one of my questions comes down to:
Will it be better to use the TAS5012 with a 12.288MHz oscillator and <1pS jitter PLL'd times 8 (I'll use a local low-noise power supply for the PLL), or to use the TAS5015 with a 98.304MHz oscillator with 5pS jitter?

Enquiring minds want to know!

Regards,
Brian.
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Old 19th December 2002, 06:40 AM   #35
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Hi Brian

You asked whether I am doing anything in the direction of a digital amp at the moment.
I actually developed a PWM amp for my thesis about 10 years ago. Back then there was almost no info available on that subject. But we got one up and running whithin the desired specs (there would have been room for improvement though). But I was already interested in switching amplifiers before that and that was the reason why I chose this subject out of a few dozens, when they were presented.

I am still interested in switching amplifiers, though not so much in digital ones (like the equibit) but classic PWM ones, but one should never say never .......
The main reason why I am interested in class-d amplifiers is their efficiency and not the "digital" appeal. I have indeed an application where I could use some efficient poweramps. Currently I am doing some studies regarding delta-sigma amplifiers (class-T and the sharp ones belong to this group).

I knew that these TI chipsets exist but didn't know that they are available to everyone now, because TI first intended to sell them only to large companies and only if they pay huge licence fees.

I had the opportunity to listen to the TacT millennium (and also the newer TacT amplifiers including TacT's room correction system) and I must admit that it is definitely an outstanding amp. The amp's design is indeed an excellent showcase from both the technical AND aesthetical point of view. One thing I already mentioned somewhere else is its low end punch (which is theoretically inherent to any well made switching amp ). I assume the high sound quality doesn't come from the working principle alone. I have the distinct feeling that there is much more room for doing things wrong compared with ordinary linear amplifiers. A linear amp where the same care has been put into will also be excellent ........

If there are general questions regarding class-d (i.e. not the equibit chipsets themselves) or if somebody is interested in my "old one" I could of course assist.

Regards

Charles
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Old 19th December 2002, 10:20 AM   #36
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The TAS5012 costs 9$ at Digikey and the TAS5015 costs 32$. Assuming this I think that there must be some bigger difference between them, not only the internal or external PLL. And who knows how Equibit actually works?It is a big secret. Maybe the TAS5015 use better Equibit modulator than the TAS5012.

I would go for TAS5015 with a little higher jitter clock instead of
TAS5012 with ultra low jitter master clock. The internal PLL of TAS5012 still has to derive the high frequency clock from MCLK and its intrinsic jitter is higher for sure. Maybe it's on a level of DIR1703 which has 75 psec of jitter. I'm not sure for this.

This is from Hifi Choice review of TacT Millennium:

"The outboard DAC analogy extends to the Millennium's dependence on a digital source - typically a CD transport - and, like other two-box combinations, it has to deal with the jitter that's aggravated in between. The black trace on my jitter plot (Figure 2) not only shows a high level of power supply-related jitter (4) - to 900psec - but also a broad hump (5) that's never good news for sound quality."

900 psec of jitter! And it still sounds great!

Regards
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Old 19th December 2002, 06:16 PM   #37
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Quote:
Originally posted by Rookie
The TAS5012 costs 9$ at Digikey and the TAS5015 costs 32$. Assuming this I think that there must be some bigger difference between them, not only the internal or external PLL. And who knows how Equibit actually works?It is a big secret. Maybe the TAS5015 use better Equibit modulator than the TAS5012.
The cost may or may not be related to the cost of actually producing the part. It might just be a marketing decision. If there is a difference between them besides the PLL, it's probably that the TAS5015 has a larger FIR for interpolation.

Once I actually get something working, I think that it should be easy to tell if there's any actual processing difference by scoping the signals and feeding in some test code pulses.

Initially I don't want to sacrifice any parts. If I accidently damage a chip I might decap them to see if the silicon's different.

The general implementation of Equibit isn't a secret.
Like other oversampling DACs, the incoming data is interpolated to a higher sampling rate.
This high frequency PCM data is convolved into PWM by a single FIR filter. This FIR is look-up table based. The relative timing of the upper and lower transistors in the H-bridge is independantly controlled so that finer resolution can be achieved than could be obtained by simple integer multiples of the (384KHz) carrier. Non-linearity of the PWM switching output is also considered in the development of the filter coeficients.

The only thing that is really proprietary is the specific number and values of the filter coeficients, and the process that was used to create them.

There's lots of information out there on the Web if you do a search. The old Tocatta website had lots of good information, but unfortunately this disappeared after TI bought them. (I'm not going to complain too much since TI is willing to sell me their chips!)

Quote:
I would go for TAS5015 with a little higher jitter clock instead of
TAS5012 with ultra low jitter master clock. The internal PLL of TAS5012 still has to derive the high frequency clock from MCLK and its intrinsic jitter is higher for sure. Maybe it's on a level of DIR1703 which has 75 psec of jitter. I'm not sure for this.
The quality of a PLL is primarily affected by the quality of its power supply, and by the original stability of the incoming clock that the PLL is trying to recover. The DIR1703's SpACT obtains a typical 75pS jitter output, but that's starting with a highly jittered S/PDIF signal. By starting with a <1pS jitter clock, the TAS5012's x 8 PLL should output jitter somewhere between 1pS and 8pS. This is highly dependant on the overall implementation (of course the jitter could be much worse). The only way to know is to try it. Even then, I'll have trouble measuring anything less than 20pS jitter.

Again, I'm planning on trying both. I'm not nearly as concerned about clock jitter in my first digital amp board as other things. (Power supply impedance, snubber circuits, the output filter, and switching dead-time are my biggest worries.)

Quote:
This is from Hifi Choice review of TacT Millennium:

"The outboard DAC analogy extends to the Millennium's dependence on a digital source - typically a CD transport - and, like other two-box combinations, it has to deal with the jitter that's aggravated in between. The black trace on my jitter plot (Figure 2) not only shows a high level of power supply-related jitter (4) - to 900psec - but also a broad hump (5) that's never good news for sound quality."

900 psec of jitter! And it still sounds great!
I haven't seen this article, so I can't tell the whole context of this quote. However, I'm very sure the Millenium doesn't have 900pS of jitter controlling its Equibit modulator! TacT has clearly gone to great lengths in their clocking schemes.

TacT uses a reflected clocking scheme like I described previously:
If used with a TacT CD source, the Millenium provides the master clock and sends it back to the CD player. The CD sends the data to the Millenium along with a reflected clock. I'm guessing that the reviewer may have been looking at the reflected clock (which probably does have high jitter). This doesn't matter because the data is reclocked by the original master.

The S/PDIF and AES inputs use a recovered clock scheme.

Unless this reviewer was probing the internal clock signals right at the Equibit modulator, I don't think his comments would be accurate. Does the article contain any specifics about where he was examining the clock?

Regards,
Brian.
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Old 19th December 2002, 09:22 PM   #38
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Here is the link to the review:

http://www.hifichoice.co.uk/archive/...rintreview.htm
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Old 19th December 2002, 09:43 PM   #39
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Quote:
I had the opportunity to listen to the TacT millennium (and also the newer TacT amplifiers including TacT's room correction system) and I must admit that it is definitely an outstanding amp. The amp's design is indeed an excellent showcase from both the technical AND aesthetical point of view. One thing I already mentioned somewhere else is its low end punch (which is theoretically inherent to any well made switching amp ). I assume the high sound quality doesn't come from the working principle alone.
I haven't heard the TacT myself yet. I think I'd have to travel out of state. I want to sometime, though.

Quote:
I have the distinct feeling that there is much more room for doing things wrong compared with ordinary linear amplifiers. A linear amp where the same care has been put into will also be excellent ........
I'll be the first to admit that part of why I'm doing this project is to do something different. I'm learning tons of new stuff that will help me even if the sound quality of the amp isn't as good as I'm hoping.

Having said that, my expectations are very high for this project. Presuming that I can get all aspects of the amp to work well as a system, I think that it has the potential to exceed the sonic quality of a more traditional power supply, class A amp, and DAC for a given amount of money. Time will tell...


In the meantime, I just want to get something that works. I'm trying as much as I can to anticipate all of the issues affecting quality, and I'm trying to design the board for as much experimental flexibility as possible. I'll have a much better handle on things when I can start probing around active circuitry and listening to it. I thought really hard about spending the $499 on TI's eval board. I decided that even though it would take me much longer to start hearing it, that I would be better off starting with my own board. I'll have better features for what I want to experiment with, and it'll make for a much better head start on the second board.

Brian.
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Old 19th December 2002, 10:12 PM   #40
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Default Re: Here is the link to the review:

Quote:
Originally posted by Rookie
Here is the link to the review:

http://www.hifichoice.co.uk/archive/...rintreview.htm
Thanks for the link!

I couldn't see any of the plots being referenced, but that's OK.

The 900pS jitter mentioned was what was coming out of the NAD CD player. This clock is then recovered by a PLL inside the Millennium. They then go on to mention a reflected clock upgrade available for the NAD to work off of the TacT's clock.

I found this particularly interesting:

"But the final 11dB represents digital gain and can, depending on the peak level represented by the incoming digital data, cause the amp to crash into high distortion. Believe it or not, TACT was obliged to introduce this extra gain because various dealers were unsettled by the fact that the volume control could be advanced to full without creating distorted sound. "

One of the things that I've been wondering about is where to set the power supply voltage. Do I do it so that digital 0db is above the point of distortion, just at maximum output of the amp, or if the amp can overpower a driver (tweeters in particular) at the limit of the driver.

How do I decide the limit of a driver:
Rated power?
Audible breakup?
Some type of measured distortion threashold?

The analog part of my mind still wants to have some sort of 'headroom' to be able to push things past the limits. Sort of like setting the level so a VU meter bounces into the red.

The digital part of my mind tells me to set things for better dynamic range and no distortion.

Brian.
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