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Old 19th October 2012, 09:58 AM   #1511
DF96 is offline DF96  England
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Quote:
Originally Posted by fas42
There has been a misunderstanding of what I meant, I most certainly am not talking about average levels of the signal being near clipping, rather whether the peak level ever clips.
OK, sorry for my misunderstanding what you said.
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Old 19th October 2012, 10:53 AM   #1512
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Quote:
Originally Posted by fas42 View Post
At least one take on an indicator, by that ever prolific chap, Rod Elliot, is here: Power Amp Clipping Indicator.
If you have an inverting (shunt feedback) amp, there's a slightly simpler way to detect clipping - put a window comparator on the inverting input. Normally the voltage here will be very low - the output voltage divided by the OL gain at any particular frequency. When the amp clips, this input is no longer a virtual earth. This method does away with the need for the clip detect circuit to look at the PSU rail voltage - that's now taken care of by the f/b network and amp itself. This method would also respond to slew limiting which can be thought of as a kind of temporal clipping.

I once built a compressor using this method to detect clipping. The basic premise was that I didn't want to adjust the gain until the amp was very near to clip. So I used a CMOS 'model' opamp (which swings between the rails) and ran this from a scaled down (using a resistor network) version of the main supply rails. The gain was adjusted so the CMOS opamp clipped around 80-90% of the output of the main amp and the inverting input was then amplified and fed into a diode pump to reduce the gain via a JFET element.
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Old 20th October 2012, 05:09 AM   #1513
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Quote:
Originally Posted by fas42 View Post
I have got my chip amps to overload, but this was thermal overload, the sound starts to chatter as the protection cuts in and out; interestingly, this is never on pop but rather operatic -- the soprano hits a big note and sustains it, a very pure, high level tone over a decent period was enough to start overcooking the chips.
Bi-Amp/HT setup with the mids-n-treble section, "TDA chip" powered up to 20w on clean regulated power? Looks doable for hi-fi. Which chip is it?
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Old 20th October 2012, 06:36 PM   #1514
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Quote:
Originally Posted by DF96 View Post
The key words are 'consistently just below the point of clipping'. Music dynamics are such that this is not possible. You can either run significantly below the point of clipping and accept an occasional clip, run just below and have frequent clipping or a mechanism to avoid this (soft clip or fast compression), or listen to 'music' with no dynamic range like many modern pop CDs. In the latter case you are not in a position to judge the sound of the equipment as the signal has already been well mangled by the recording engineer.
Sorry to be so late in chiming in.

I would have to assume they meant it was not clipping after taking into account the dynamic range of the music.

On a related note, the point of the Cmin analysis is to find combinations of C, Vrail, VA, and Pmax that make it impossible for a square wave to clip at Pmax, even by a little bit, which would mean that no other type of signal could ever clip. And it can easily be done. Then it is guaranteed that the lowest trough of the ripple waveform cannot intrude into the headroom reserved for the signal (load) and amplifier voltages, as long as the output signal voltage peak stays at or below its specified maximum level.

I have a new spreadsheet that calculates it much more accurately, taking into account the transformer droops, and even the cap's ESR. I will try to upload it today or tomorrow.

I have been comparing its predictions with my simulation results, to validate it, and it seems to be able to account for everything, within about one percent. One of the last things that I still need to add is the variation of the rectifier voltage drop versus the average rectifier current.

In order to do the calculations needed, I had to resort to an iterative method, since some of the equations have no closed-form solution, and some are approximate until you work backwards to the transformer primary winding and then forward again through the rectifier, enough times. But it's quite compact and only needs three iterations, although I provide four because it's nice to see the smallness of the final changes in the values.

Cmin is good to know, for any combination of transformer size, output voltage, and Pmax. But I realize it's not what is needed when designing a power supply, except when it points out that a particular combination is marginal, or impossible.

So I will be using the information I have found, while researching for this, to create a more-design-oriented spreadsheet.

Cheers,

Tom

Last edited by gootee; 20th October 2012 at 07:03 PM.
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Old 22nd October 2012, 01:26 AM   #1515
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Quote:
Originally Posted by danielwritesbac View Post
Bi-Amp/HT setup with the mids-n-treble section, "TDA chip" powered up to 20w on clean regulated power? Looks doable for hi-fi. Which chip is it?
Sorry for the late reply, Daniel, bits of life happening ...

The HT setup uses a real basement unit, TDA7269A, specs look pretty dreadful! These fellows actually shut down completely, I has to do a power off/on reset, from memory, to get them going again. Otherwise, they do remarkably well because they directly drive a widerange speaker, no crossover to place extra current stress on them; with something like Foo Fighters I have no trouble getting my ears to start ringing within a few minutes ...

An earlier effort was my own LM3875 gainclone design, these chips start chattering when they thermally overload. These were driving decent sized 2 ways, so had to contend with crossover demands. The heatsinks were oversized for the job, but even so the transient power dissipation was too much for the heatsink contact area to get rid of fast enough, so the protection cut in ...

Frank
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Old 23rd October 2012, 01:41 AM   #1516
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Originally Posted by fas42 View Post
The HT setup uses a real basement unit, TDA7269A, specs look pretty dreadful!
Considering the performance you got, it is mostly likely to be a singleton input amp or similar audio delight with dreadful specs. Those work best with clean power, low gain and a light load. . . and you did that.

So, when you added the regulators, what power caps did you put on the amp board, the 1000u in the datasheet or something more like 330u or maybe the 10u~47u range suggested by regulator datasheets?

Normal regulators don't help much at HF unless they're set a lot lower than the transformer voltage. What's the voltage difference between the regulated versus unregulated? You could add RC's across the transformer's primary and secondary. Is the bass amp powered by the same transformer as the mid-n-treble amp?

Do you have additional HF filtering at the amp, like a picofareds cap from in+ to in- (the biggest size that didn't change the audio) and/or at the RCA jack? I was thinking that maybe some additional filtering could prevent the amp from getting stuck in protection mode.
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Last edited by danielwritesbac; 23rd October 2012 at 01:44 AM.
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Old 23rd October 2012, 04:39 AM   #1517
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Quote:
Originally Posted by danielwritesbac View Post
So, when you added the regulators, what power caps did you put on the amp board, the 1000u in the datasheet or something more like 330u or maybe the 10u~47u range suggested by regulator datasheets?
Don't confuse 2 different projects, Daniel, I'm confused enough by life as it is, !! The HT used tweaking essentially with the supply as is, everything could be better in myriads of ways but I've done it a certain way to try some other ideas. The regulator technique was done on a LM3875 gainclone, which was all my own design ...

Quote:
Normal regulators don't help much at HF unless they're set a lot lower than the transformer voltage. What's the voltage difference between the regulated versus unregulated? You could add RC's across the transformer's primary and secondary. Is the bass amp powered by the same transformer as the mid-n-treble amp?

Do you have additional HF filtering at the amp, like a picofareds cap from in+ to in- (the biggest size that didn't change the audio) and/or at the RCA jack? I was thinking that maybe some additional filtering could prevent the amp from getting stuck in protection mode.
The HF issue with the regulator approach was addressed by having very low ESR local decoupling, power and earthing planes, minimal lead lengths -- I was aiming to get absolutely the lowest supply impedance at HF without going really silly. One of the key qualities the power supply had was effectively high reserves of energy, as I've mentioned before: the amp could keep working for a few minutes at moderate volumes with no audible change if you pulled the mains plug ...

For the HT on the other hand, no such fanciness: why it can do the job is that, as you guessed, the subwoofer is completely separate, electrically and physically, it has its own mains cable. So part of the tweaking was to isolate the glitching of the mains caused by the current draw of each, from the other to some degree. It all could be done better, but it performs well enough to prove some points, for me ...

Frank

Last edited by fas42; 23rd October 2012 at 04:43 AM.
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Old 23rd October 2012, 04:40 PM   #1518
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In a previous thread , I hinted that it maybe better to get your preamp supply from a higher rail voltage that is gradually stepped down using voltage references. So rather than use a +/-15 transformer, use a +/- 30 . Its might even be better if the power supply is in a different casing
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Old 23rd October 2012, 04:50 PM   #1519
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Aaah.. have just located it Power Supply Resevoir Size

Later there was a science fiction piece that went un answered Power Supply Resevoir Size
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Last edited by OnAudio; 23rd October 2012 at 04:58 PM.
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Old 23rd October 2012, 05:04 PM   #1520
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