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Old 10th December 2001, 03:29 PM   #21
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HaaiFaai in a pwm amp an analog signal goes in (or it could be a digital signal that gets converted to analog) but the output i assure you is not analog. The amplified digital appoximation of an analog waveform (such as that which leaves the output of a pwm amp) fails in a few ways namely the slew rate of the output stage and it's inability to accurately (if at all) produce the subtle nuances that make music what it is.

[Edited by AudioFreak on 12-10-2001 at 10:33 AM]
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Old 10th December 2001, 05:45 PM   #22
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Default TI

TI have evaluation boards for sale. I have looked at the data sheets and it appears that substituting the TAS5015 high-end "processor" into their evaluation board is relatively simple. If you use the IRF output stage as used by Crystal in their solution, you have what appears to be a pretty interesting package.

Of course, this involves some hacking on the eval board, but hey ...

Search http://www.ti.com for TAS50 and then TAS51 then you will find lots of cool info.

Petter
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Old 22nd February 2002, 05:27 PM   #23
alvaius is offline alvaius  Canada
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Default Digital Amplifiers......

Let's see if we can get this forum on track......

Class-D is just that, a class of amplifiers. Remember that transistor amplifiers sounded terrible when they first came out. (Okay, hold the comments on they still sound terrible.)

Digital audio sounded pretty terrible when it first came out, but even some die hard turntable people I know have to admit SACD and DVDAudio with a good quality player may even be better than their turntable on two channels.

Digital amplification is probably at that stage. Give it a few years, and it will likely kill off a good portion of analog amplifiers (and probably a lot of companies with it.)

Digital amplification can allow one of those holy grails many of us like out here, zero overall feedback, and it will be able to do it without high distortion. Heck, tone controls without excessive phase distortion, etc. are all possible in the digital domain.

My concern, as a hobbyist, is that amplification may start to go the way of the car in the 70s. The things are just getting too complex to work on without specialized tools, etc. and realitically, what you can do is limited. That, or it will end up like the Personal Computer world, where you plug some modules together and the end result is only 10% different from anyone else.

The real "science" of digital amplifiers is not just in circuit design, but in Mathematics and digital signal processing, probably beyond the realm of most DIYers. If not because of knowledge, but because of the time needed.

So, lets get and keep this forum on track. Maybe some primers on Class-D, Class-T, Sigma-Delta modulation (for pure digital amplifiers), cool circuit ideas, etc.

The early digital amplifiers sounded bad because they did a simple PWM. To get better sound, they just switched faster, however. However, you need orders of magnitude to get a significant improvement. In comes signal processing, which is why we are seeing significant improvements, just like how sigma-delta conversion can give us $5.00 DACS at 24bits and 192KHz, that sound as good as $100, 20 bit DACs of not too long ago.

Happy computing.....
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Old 22nd February 2002, 05:45 PM   #24
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Well now.
Switch mode psus use PWM and they seem to work rather well for controlling dc voltages. High accuracy and high damping.
Is one approach to try to extend the frequency response of a switch mode psu out to 20kHz? How hard would that be?

I haven't given this topic much thought. I'd be interested in what peoples' opinions are about how to make an audio PWM amp - such as switching speed and output filtering and so on. Is the switch mode psu topology applicable?
BAM
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Old 23rd February 2002, 05:43 AM   #25
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Default trying a Tripath EVM (long)

Like the others above I have been interested to follow recent developments in switching amps. Tripath and amplifiers using Tripath technology have had great press - and the evm modules are reasonably priced so I decided to give one a go.

Its summer here in NZ and despite a cold summer, 1kW of class A from a Plinius SA250 in my lounge is a bit much, so I though maybe I could try a Class D amp and maybe oust the Plinius and save a few bucks on the way past. My set up is not bad, SACD, TT, good valve pre, SA250 and N801's - and is going rather nicely (at last!). So I'm in a very good position to compare results from the Tripath amp and the Plinius (which I find increasingly hard to fault).

Just to add interest my profession involves me with design of very high power class D amplifiers (to about 750kW) - better known as inverters. The techniques of these are completely similar to those used in making class D work well - at least from an electrical engineering perspective.

I had little difficulty in getting the EVM module going (forget the number but itís the largest one - about 300+W into 8 ohms). For the sake of peace of mind I fitted the speaker protection components which Tripath leave off the big module (I'm not running it near full power).

The result is a very "nice" sound. Very smooth, nothing offensive makes cds and records that are unlistenable through the Plinius quite pleasant by taking the harshness away. Unfortunately this is definitely a filtering process and the life and energy (air, feeling of energisation in the room) in the music go out the back door too. The bad cds I refer to are bad - it isn't the Plinius mucking them up - it just plays them as they are.

Most of the smoothing effect is in the highs. The mids seem OK. The upper bass is not especially tight (but this may be my power supply not being big enough - I'll borrow 47kuF and add that tomorrow and see what difference it makes). My 801's need very well controlled bass or the 15inch woofers take over the sound - they sound relatively boomy with the Tripath amp.

I've incorporated all tweaks I could find on the net - except input transformers (but I have wound the input coax cable around large common mode cores appropriate for rf suppression at the frequencies used in the amp module).

I'm open to any suggestions as to how to get better sound out of the amp. Please let me know your thoughts!

My gut feeling is that the sound is quite dramatically different to that which I would expect from a normal amplifier, and I don't see that it is likely to be result of the fact that it is class D (Class T is really a marketing name - its a switching amp that shuffles the carrier frequency around and compensates for switching times - nothing very unusual in that - it has been done for years in inverters to stop the ear latching on to the carrier frequency (we use much lower frequencies 2 - 16kHz) and to produce reasonable fidelity output waveforms).

All the indications to me are that Tripath have an algorithm built in to their control chip that processes the sound to achieve the soft sound people like (in fact Tripath actually refer to this at their website). One can never be sure as someone stated above - once manufacturers start processing with DSPs they can do as they wish (just look at the cheap home theatre units with their various sound effects - "hall", "church", "jazz club" etc). This is a hobby horse for me as I recently heard a top end cd player which clearly (to me anyway) does the same thing - it is well know for sounding "live" - but in doing its thing it kills attack and makes electronica sound live too - which it shouldn't!!

Any tweak I have tried with the Tripath EVM has had little influence on the basic sound. Research on the net finds generally that others have found similar results with the Be Canto EVO 200 - which tends to point to a character of the Tripath processing rather than a character of the particular implementation (Bel Canto make their own pcbs which are quite different to the EVM modules).

Don't get me wrong - the sound is very good and outside of direct comparisons with hi-end gear or plenty of listening experience I think most people would find it quite beguiling.

Anyway, this is just my experience in my set up. I have no barrow to push, except having invested the money in the kit I would rather have liked the new amp to be replacing my Plinius!

Look forward to comments/feedback/experience/suggestions from others!

MurrayP
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Old 23rd February 2002, 07:06 PM   #26
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Default This would be fun!

If you have an SACD player, pry it open, and hook the signal to the gate of a switching MOSFET capable of operating in the 3 MHz range. Put a 100-500KHz first order filter on the output and -- voila.

Now that would be fun.

Petter
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Old 23rd February 2002, 07:51 PM   #27
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Don't tempt me!

Well, not far removed I suspect is.... (ok its not a power circuit) but a similar appraoch maybe....

http://www.vacuumstate.com/son.htm

Murray
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