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Class D Switching Power Amplifiers and Power D/A conversion

TDA7498e + Mohr SL10
TDA7498e + Mohr SL10
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Old 30th April 2019, 11:48 AM   #1
ventzitt is offline ventzitt
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Default TDA7498e + Mohr SL10

Hi
I am trying to build a garden low-cost active speakers so I ordered amplifier board TDA7498e(burned the first), power supply 36v 5A, and Mohr SL10 Speakers connected as the diagram shows below:

So far so good. The thing I don't understand is why the above-mentioned board plays much weaker than a vintage Pioneer SA 508 amp which is rated at 25w per channel at 4Ohms.
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Old 1st May 2019, 07:46 PM   #2
Mark Tillotson is offline Mark Tillotson
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You've tried the different gain adjustment settings?
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Old 1st May 2019, 08:33 PM   #3
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by ventzitt View Post
Hi
I am trying to build a garden low-cost active speakers so I ordered amplifier board TDA7498e(burned the first), power supply 36v 5A, and Mohr SL10 Speakers connected as the diagram shows below:

So far so good. The thing I don't understand is why the above-mentioned board plays much weaker than a vintage Pioneer SA 508 amp which is rated at 25w per channel at 4Ohms.
As Mark says, the sound level you get primarily depends on the gain. The gain in the power amplifier but also in the pre-amplifier and the output level of the source.

A small warning: I have a red TDA7498E board (the same as you show) I have been using with a 31V supply. Recently I got a 36V/7A supply and tried the two together. It played well for a couple of minutes and then the amplifier became silent. No smoke, no noise just became quiet. My conclusion is not to use that board at 36V though it should be able to handle 36V. If you can, adjust the voltage a bit down.

For your circuit diagram I have a few concerns unless you have used jumpers for the board to be in PBTL-mode (mono).

On one channel you have two 4-8 Ohm speakers in parallel which means a resulting speaker impedance down to 2 Ohm. 2 Ohm speakers is only for PBTL-coupling.
The other channel is unloaded which is OK for PBTL-mode but NOT for stereo mode.

Last edited by FauxFrench; 1st May 2019 at 08:49 PM.
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Old 1st May 2019, 09:24 PM   #4
ventzitt is offline ventzitt
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Quote:
Originally Posted by Mark Tillotson View Post
You've tried the different gain adjustment settings?
No, I haven't. The reason I haven't is that I am a bit worried that I am using 36V supply and I may burn it again.
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Old 1st May 2019, 10:48 PM   #5
ventzitt is offline ventzitt
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Quote:
Originally Posted by Mark Tillotson View Post
You've tried the different gain adjustment settings?

Thanks for replying Mark.

To your question. No, I haven't tried different gain setting because I am a bit worried that I may burn it again. The spec sheet got me ordering 36V power supply whereas it seems it runs better with 32V


Quote:
Originally Posted by FauxFrench View Post
As Mark says, the sound level you get primarily depends on the gain. The gain in the power amplifier but also in the pre-amplifier and the output level of the source.

A small warning: I have a red TDA7498E board (the same as you show) I have been using with a 31V supply. Recently I got a 36V/7A supply and tried the two together. It played well for a couple of minutes and then the amplifier became silent. No smoke, no noise just became quiet. My conclusion is not to use that board at 36V though it should be able to handle 36V. If you can, adjust the voltage a bit down.

For your circuit diagram, I have a few concerns unless you have used jumpers for the board to be in PBTL-mode (mono).

On one channel you have two 4-8 Ohm speakers in parallel which means a resulting speaker impedance down to 2 Ohm. 2 Ohm speakers is only for PBTL-coupling.
The other channel is unloaded which is OK for PBTL-mode but NOT for stereo mode.

Hi FauxFrench,

My drawing is a bit confusing. I will try to explain the connections I have done.

The amp is inside of the left speaker and the left channel of the amp is going straight to the "crossover"(only 1 capacitor and inductor highpass filter) of the Left speaker which is been desoldered from the terminal plate so I am only using that plate as an extension of the right channel to the right speaker. So eventually you get left channel ->left speaker, right channel -> right speaker

As far as the input signal of the Tv preamplifier, what I've done is play a 1Khz tone on youtube and adjusted the volume until multimeter shows 1V which corresponds to 41% volume on the TV scale.

What would you recommend:
1. Buy a 32V power supply and use the existing amp board.
2. Or use different amp board (witch) with power supply.


Please note that someone had tested the speakers and they go as low as 3.3-3.5 Ohm most of the time.


Thanks for trying to help
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Old 2nd May 2019, 12:32 PM   #6
FauxFrench is offline FauxFrench  France
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My drawing is a bit confusing. I will try to explain the connections I have done.
The amp is inside of the left speaker and the left channel of the amp is going straight to the "crossover"(only 1 capacitor and inductor highpass filter) of the Left speaker which is been desoldered from the terminal plate so I am only using that plate as an extension of the right channel to the right speaker. So eventually you get left channel ->left speaker, right channel -> right speaker

OK, left amplifier channel is to the left speaker and right amplifier channel is to the right speaker? For a class D amplifier, the speaker serves to dampen the output filter resonance and should be permanently connected. Speakers on all outputs!

As far as the input signal of the Tv preamplifier, what I've done is play a 1Khz tone on youtube and adjusted the volume until multimeter shows 1V which corresponds to 41% volume on the TV scale.
What would you recommend:
1. Buy a 32V power supply and use the existing amp board.

I have 24V and 36V power supplies similar to the one you show.
I have looked at them with the hope to tell you how to reduce the output voltage to around 32V. Unfortunately, the type of the controller IC has been grind off. However, for one of the ICs (in a 24V supply) they forgot to grind it off. It is a ON-Semiconductor "1200P60" low-power fly-back controller. As the IC is low-power and the board output considerably higher, power transistors have been added to handle higher currents. Without a circuit diagram I cannot tell you how to adjust the output voltage. In principle the board should handle 36V. I just have a bad experience.

2. Or use different amp board (witch) with power supply. Please note that someone had tested the speakers and they go as low as 3.3-3.5 Ohm most of the time.

The amplifier as such is fine. 3-3.5 Ohm is rather low and I will suggest you a 24V power supply instead. Cheap and standard. For the same TDA7498E board.
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Old 2nd May 2019, 03:02 PM   #7
chermann is offline chermann  Austria
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Hi


additionally to FF statement i want to add that the decoupling caps 12x 470/50V are fake...in this thread i was sure that the size is not correct ot have this voltage/or capacity.....Ebay cheap TDA7498 boards


so it would be better to keep the voltage lower than 35V


chris
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Old 2nd May 2019, 06:40 PM   #8
ventzitt is offline ventzitt
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то FauxFrench
First of all, I want to thank you for your willingness to help not only me but almost every member of diyaudio (read lots of your posts) and when you said you recommend 24v PSU that's it, I am no longer searching for an alternative solution that would be it. Ordered it already.
So once again thank you very very much...



To chermann
Thanks for your input chermann, it is much appreciated. Since I will be switching PSU to 24V I would think the fake capacitor would be in the safe zone you recommended "lower than 35V"
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Old 2nd May 2019, 07:53 PM   #9
chermann is offline chermann  Austria
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Quote:
Originally Posted by ventzitt;5778740[B
]то FauxFrench
First of all, I want to thank you for your willingness to help not only me but almost every member of diyaudio (read lots of your posts) [/B]...

....yes he is really so helpful and a very good engineer....he feels the electrons
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Old 2nd May 2019, 08:50 PM   #10
FauxFrench is offline FauxFrench  France
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Thank you very much to both of you for the kind words.

A comment on "fake" electrolytic capacitors. I am a bad example to other as I buy cheap capacitors from where I am warned. I have only once seen 25V rated capacitors blow-off at 24V with moderate ripple. Only allowing up to 75% of the rated voltage, I have never seen any other capacitors self-destroy. I actually build a capacitance tester for testing all my bigger decoupling capacitors (quite a lot - old, new "fake", reused items, quality brand items etc.). All capacitors were within +/-25% of the stated capacitance, most far closer. All passed a half-voltage leakage test, almost all a full voltage test. A few (possibly fake) I considered should be rated 10%-20% below the actual voltage rating though the original specs are very wide for leakage current. The eldest capacitor was a 2x13000uF picked-out from a 50 year old Marantz 1050D. It passed all tests.

Your 470uF/50V Sanyo capacitors (2x6) will do fine even at 40V. Is there no downsides from "fake" capacitors? Yes - probably on long term life (reliability) and ESR. Thus, less (indirect) performance but no "destroying the gear".

My conclusions:
NEVER use components where you do not know their origin and performance if you take money for your services. Your reputation cannot stand a major mishap.
If you want top-performance from your own gear, buy original components so you have the best performance.
If you are mainly interested in electronic circuit experiments (like I am) you can also use "fake" capacitors though the lifetime may be shorter and the performance somewhat less. If it works with "fake" components, it almost certainly works with genuine components as well.
Recover (salvage) components of reasonable value from any old amplifier/receiver. One day a weirdo, like me, on the forum suggests you to try a component you don't have for overcoming a problem. Then, you dive into your box with recovered components and find something like what is needed. If it works, you can order better for replacement. Else, we spend too much time waiting for components to arrive.

NB: In particular old transformers were often of excellent quality.
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