I think Tom said he was working on a SMPS in an earlier post. Here is a quote from the data sheet I linked for the IR board on bus pumping:
"When the IRAUDAMP7S is running in the stereo mode, bus pumping effect takes place with low frequency high output. Since the energy flowing in the Class D switching stage is bi-directional, there is a period where the Class D amplifier feeds energy back to the power supply. The majority of the energy flowing back to the supply is from the energy stored in the inductor in the output LPF. Usually, the power supply has no way to absorb the energy coming back from the load. Consequently the bus voltage is pumped up, creating bus voltage fluctuations.
Following conditions make bus pumping worse:
1. Lower output frequencies (bus-pumping duration is longer per half cycle)
2. Higher power output voltage and/or lower load impedance (more energy transfers between supplies)
3. Smaller bus capacitance (the same energy will cause a larger voltage increase)
The OVP protects IRAUDAMP7S from failure in case of excessive bus pumping. One of the easiest counter measures of bus pumping is to drive both of the channels in a stereo configuration out-of-phase so that one channel consumes the energy flow from the other and does not return it to the power supply. Bus voltage detection monitors only +B supply, assuming the bus pumping on the supplies is symmetric in +B and -B supplies.
There is no bus pumping effect in full bridge mode."
-Doug
I was wondering if any of yall could help me out here. I hooked up my ti based class d audio amp board last night. Everything worked very well except there was some high frequency noise.
It wasn't real load but is kind of sounded like a hiss or white noise. Has anyone had this problem and how did they remedy it?
Thanks,
Nick
It wasn't real load but is kind of sounded like a hiss or white noise. Has anyone had this problem and how did they remedy it?
Thanks,
Nick
The PSRR is -65 dB.Hi Tom (and All), just wondering if you can tell me what the PSRR of the input stage is and if you are producing (or have produced) an SMPS for this amp.
Also, what does bus pumping do and why is it a benefit that it is never an issue with this amp?
thanks
As for bus pumping, bus pumping only occurs in half bridge mode. The TI amps are actually two full bridge (BTL) amps, and in bridged mode (PBLT) are two full bridged amps tied together, so bus pumping is not an issue.
Ye, we will have the SMPS (switch mode power supply) finished soon for these amps. We are just waiting for the new PCB's to arrive here so we can complete testing and make sure there are no problems before they are for sale. The power supplies put out 600 watts, with short term 1200W peaks. They are designed specifically for audio.
We do have the DC/DC SMPS available now for car, mobile, and portable applications. They take a DC input of 8.5 volts to 16.5 volts DC. We have 3 different models, 250 watts, 500 watts, and 1000 watts, and you can see them on our website.
Tom
Hi Tom, a PSRR of -65 dB is relatively low, although not a problem if compensated for by your SMPS. If you don't mind my asking, what is the size of the SMPS's output ripple (p-p) and the output impedance?
thanks
What type of pre-amp, you used with the classD?
I have bought one also and considering the a most suitable type pre for it
Another DIY effort on my part.
The source I use is a pair of PCM1794s in dual mono mode, so I've got a balanced output to deal with. The DAC itself has a TPA6120 headphone amplifier chip from TI to drive the cables to the preamp. Just to ensure that there's no problem with driving the cables as the preamps input impedance is rather low.
Inside the preamp, the balanced signal is then fed into an AD797 which sums the signal and then drives a DACT 10k stepped attentuator, which in turn feeds another pair of AD797s that then drives another TPA6120.
The output from the TPA is specifically designed to drive headphones and that's how I use it. The preamp is also a 4 way active crossover for my loudspeakers, where the signal after the DACT pot is switchable either into the headphones or the crossover.
The distortion on the headphone output, with a 48k sampling frequency measures better then 0.0005%. When I was using the TAS5630 amp I was driving it with the headphone output on the Preamp/Active crossover, so it had a rather good source and preamp ahead of it.
If you were after a nice simple preamp I'd recommend having an opamp as an input buffer, with a high input impedance, say 200k or so. Have that opamp feed a pot with reasonably low impedance say 5-10k. Then feed the output of the pot into another opamp, then feed that into a TPA6120 chip. The 6120 has great driving capabilities and rather excellent overall specifications that make it really quite good to use as a buffer for driving cables.
Input > Opamp buffer > Pot > Opamp buffer > TPA6120 > Output
Something like that. It will give you excellent performance whilst being very simple and compact.
Use whatever opamps take your fancy, I do like the sound of the AD797 but you need to take care when using it as it requires some special attention if its to not oscillate. It's not hard to design around you just need to read the data sheet properly before hand!
One thing to bare in mind is the output filter on the Class D amp. The roll off varies depending on what load your loudspeakers present to the amp. It's entirely possible that the amplifier itself is rolling off the high frequencies a little bit. To combat this you can either alter the output filter of the Class D amp or change the crossover in the loudspeakers such that the input impedance at high frequencies matches the output filter of the amplflier, so you end up with the frequency balance you're after.
For example if you've got a 4 ohm load between 10-20khz and as a result you had rolled of highs - you could alter the L pad such that the tweeter would now run a couple of dB too loud. The impedance would remain the same so that the crossover on the tweeter wouldn't have to be changed, but now you're tweeter is playing too loud. All you need to do is put a resistor in series with the input of the tweeter. This will bring the output level back down and also increase the presented load by a couple of ohms. The higher impedance should help to flatten out the rolled off highs.
Why is a low value pot better than a high value one?
An opamp buffer isn't cabable of driving a cable itself?
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Thanks 5th, I believe the amp is adjusting to some AC filter tweaks I've got set up (Alan Maher Hammond 193L diy from AA). I'd forgotten the effect it has at first on amps: first a roll-off on HF, then the opposite (HF came back in force day a few days ago), and then eventually things settle to a balance over a few days. So I am just going to give it a few days and then re-assess.
Hi Scott, my main amp has been an Audiosource Amp 3. But I recently had the Sure board going as well, til I fried it w/ too much modding. heh.
Audiosource Amp2
I actually had an Audiosource Amp2 (not Amp200) home for a while and it was one of the better commercial amps that I have tried. Still not quite in the league with a 3875 PBA100, UCD180, the previous favorite 7250/TIP or the current king of the hill The uber modified Sure 2X100. You have saved me the trouble of trying the IR as I place transparency and clean treble extension above smoothness.
There are certain attributes of the Sure that I miss for sure, but I don't want to give the impression that the IR amp is not transparent and/or compromises the treble especially as it's still settling in and I think 5th E has a point about the load. I love it's sound; it's so enveloping and clean that when I try to step back and assess it like I could my previous amps I just get sucked in after a few seconds and give up
. It makes me surrender every time. Reminds me of the opium-filled days of my youth (just kidding).
Noise. And more specifically a lower value pot reduces whats known as Johnson noise. Now whether or not any difference would be perceived going from a 5k to a 50k or a 100k pot is another matter, but it doesn't hurt to use the lower value pot. This is why low value resistors have to be used if you're after very low noise for some critical application. Of course you want to keep the resistors above a certain value so as not to increase distortion, but most decent opamps can drive local 1k ohm loads without any problem.
Well yes it should be. However the TPA6120 has outstanding specifications, is relatively inexpensive and will drive any cable and source you could ever really think of attaching to it. It's not so much a case of the opamp won't be able to drive it. I rather think of it as, why not use the chip that's designed specifically to drive cables and difficult loads - sure the opamp should do fine, but why not go a little bit further to make sure it's guaranteed do fine. The TPA6120 is a relative of the THS6012 which is DSL line driver.
You could always add a headphone socket in aswell and have an excellent headphone amp to boot
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Seems to me like this low resistance attenuator needs to be pushed to the exstreme to have the benefit you suggest
And it introduces all sorts of other crazy problems
I have been there
And honestly, I really experienced nothing but the problems
I think its a dead end, only useful fore advertising
Im sure other things are a lot more important
And it introduces all sorts of other crazy problems
I have been there
And honestly, I really experienced nothing but the problems
I think its a dead end, only useful fore advertising
Im sure other things are a lot more important
Crazy problems? Such as what? I can't see how you could create problems unless your implementation was iffy.
The only issue I can think of is the issue of the opamp before the pot. The lower the impedance the harder it is to drive. However for all intents and purposes a 10k pot should be fine in almost all situations. I guess what I'm really saying is avoid pots with very high impedances. 10k-100k should be fine.
Sorry, I realised too late that you do a specific implementation with buffers
Quite different from waht I had in mind about the low impedance noise realtions
Yeah, I messed it up
I was thinking more in general terms
Like having very low input impedance on a power amp
Please ignore what I said
Quite different from waht I had in mind about the low impedance noise realtions
Yeah, I messed it up
I was thinking more in general terms
Like having very low input impedance on a power amp
Please ignore what I said
Well even so it's good that you mention it because it's worth saying that you should keep the input impedance high enough that your preamp isn't going to have trouble driving it. However if you've got a TPA6120 driving the cables you'd be okay!
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