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Old 28th April 2012, 08:32 PM   #1
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Default TDA8924TH compatible replacements?

Anyone know which, if any, of the Class-D IC's from NXP are drop-in replacements for the obsolete TDA8924TH?

I'm hoping the TDA8950TH is compatible.
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Old 30th April 2012, 09:16 PM   #2
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I'm sure one of the series, including TDA8920BTH, TDA8950TH and the newest one TDA8954TH can be used as a replacement.

TDA8920BTH can be used as a true drop-in replacement. However, pay attention that the TDA8950TH has both pin 10 & 12 unconnected, whereas TDA8954TH chip has two different functions on pin 10 & 12, used for Diagnostics instead of VDD & VSS as in TDA8920/24TH.
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Old 1st May 2012, 03:37 PM   #3
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Thanks for the reply norture.

Not to doubt you, but how are we sure TDA8920BTH is a drop in replacement for TDA8924TH? I can't seem to find any reference to this.

If TDA8920BTH really is a pin for pin replacement, then TDA8950TH must be as well since the NXP site says they are both compatible.

I guess I'll just get a couple TDA8950TH and see. They're only $5.61 each, so I'll add them to my next avnet order.
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Old 1st May 2012, 03:53 PM   #4
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Yes, they are compatible as far as the pinouts are concerned. The difference lies only in the modulation frequency if you care to read their datasheets. So, why not give it a try and see?
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Old 1st May 2012, 04:03 PM   #5
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Yes, I will.

BTW, the circuit uses an external clock.
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Old 25th May 2012, 04:25 PM   #6
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Well I finally got a couple TDA8950TH and replaced the 8924's with them. They work!

Hopefully they are more reliable than the 8924.

Thanks
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Old 27th May 2012, 11:43 PM   #7
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Well, it seems this amp still has some demons.

I first purchased it knowing it had two channels not working and hoped it would be a simple fix. When I opened it up to check it out the first time I noticed a blown SMD cap by the rear channel IC. It was a 220nF ceramic that went from the negative rail to ground. It did some damage to the board and a trace that ran under the cap which I fixed. I replaced the cap with two parallel 100V 100nF caps I pulled off another board. After the repairs the rear channel still didn't work so I replaced both TDA8924 with the 8950's.

Today I was doing some cruising in my car and the amp was playing perfectly for a couple hours. I started to smell the smell of death and the amp began to cut in and out so I quickly reached behind my seat and yanked the power connector off. I tried to pick the amp up but the whole thing was insanely hot.

When I got home I connected it to a 12V supply and powered it up and immediately heard a "pop". After opening it up I noticed that the 220nF cap on the positive rail had now vaporized just like the negative rail had originally. I powered it up again without any further fireworks and tested the rear channels.... they still work!

Anyway, to get to the point; why would these 220nf ceramic decoupling caps be shorting like this and why ony on the rear channels? Also, another thing I noticed with this amp is there are two 63V 1000uF caps per IC across the rails, not one going from each rail to ground (+/-25.8V rails). This doesn't seem right since the outputs are single ended.
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Old 28th May 2012, 01:07 AM   #8
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The more I look at the layout of this board the more it looks like it was designed to be used in bridge mode exclusively, except it wasn't. Either that or the designers just didn't care much for the datasheet.

The datasheet shows 100nF going across VDDP and VSSP for each channel, and also 100nF from each supply rail to ground, for a total of 6 100nF caps. On this amp board there is only 1 cap of unknown value across VDDP and VSSP. The 220nF caps mentioned earlier are relatively far away from the IC leads.
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Old 28th May 2012, 01:12 PM   #9
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Quote:
Originally Posted by theAnonymous1 View Post
...why would these 220nf ceramic decoupling caps be shorting like this and why ony on the rear channels? Also, another thing I noticed with this amp is there are two 63V 1000uF caps per IC across the rails, not one going from each rail to ground (+/-25.8V rails). This doesn't seem right since the outputs are single ended.
I smell something fishy about your board. Do you have the schematic? I've never seen caps being put across the rails like that. And you seem to have severe ripple/oscillation problem.
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Old 28th May 2012, 02:16 PM   #10
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Quote:
Originally Posted by norture View Post
I smell something fishy about your board. Do you have the schematic? I've never seen caps being put across the rails like that. And you seem to have severe ripple/oscillation problem.
I've tried looking for a schematic for it (Blaupunkt THA-475), but no luck.

It does seems like what you drescribe and I can only assume it's related to the poor decoupling scheme chosen by the designers. I don't understand how a company like Blaupunkt could overlook such a thing though.

I also have a 5-channel version THA555 that is brand new. Taking the bottom off it looks like a similar layout to the other one, just with 4x IC's instead of two. Again the 63V 1000uF local decoupling caps by the IC's look to be all in parallel across the rails.

I can easily add some proper 100nF decoupling by the IC power leads on this wonky board, but I don't see an easy way to reconfigure those 1000uF caps.

EDIT: Oh, one thing I overlooked and forgot to mention. There are 4x 1000uF 35V caps across the rails (two each rail), but they are in a CLC configuration by the supply transformer a few inches away from the IC's.
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