Standby/Mute feature of 7498E

Crest

Member
2018-05-31 2:49 pm
Hi,

unfortunately the thread on the 7498 boards has been closed after I asked my question. Thus, I try to ask my question again, seperately. I hope the question is not too trivial but I could not find any answers.

I received the black board of the 7498E, a few days ago. looks good, but I couldn't try it out yet.

At the moment I try to understand the mute/standby function. Unfortunately, the datasheet of the 7498E doesn't say much about it. However, the datasheet of the 7498 includes more details, e.g., that the input current of STBY & MUTE should be less than 200µA. Is it ok to assume that both work similar in this context?

The standby and mute circuits of both datasheets (E and non-E) look very similar, but the circuit for the 7498E includes an additional resistor R19 with 33k. I don't understand why it is needed. Can somebody help? Is R4 >> than R2 to ensure that the chip is first enabled and then unmuted if both PINs are set to high at the same time?

I found another standby/mute circuit in the datasheet of the TDA7293 (Figure 6) that needs only one connector. It uses two resistors and a diode to ensure that the mute is first set to low if both PINs are connected to ground. Would it be possible to use a similar circuit for the 7498E to be able to control standby and mute with only one input? E.g., by splitting R4 into 20k and 100k and shortening the 100k by a diode?


Best regards,
Crest
 
Hi Crest,
You are not the only one not being impressed by the TDA7498E datasheet.

I believe we anyway can draw some important conclusions from the datasheet. On page 6 of the TDA7498E datasheet, you find a line (in the table) called "Function mode", "Standby & mute & play". This is the best information we get in that datsheet about stand-by and mute control. The information is that “low” means below 0.5V and “high” more than 2.5V (but not much more than 3.3V). Further if the stand-by pin in low, what is on the mute pin is unimportant because you will in any case have no output. If the stand-by pin is high, the output depends on the voltage on the mute pin. If the voltage on the mute pin is below 0.5V, the output is muted and you have no sound. If the voltage on the mute pin is above 2.5V, the amplifier is active and you have sound.
R2, R4 and R19 in Figure 3 (on page 7) of the datasheet show a way to control the stand-by and mute pins. The activation of the stand-by pin is faster than of the mute pin such that first the amplifier is activated and a bit later un-muted. For start-up, you can use the same switch to control the two inputs.
But, then comes a second issue. If you turn ON the general power to the amplifier and then activate a second switch that simultaneously controls the stand-by and mute pins, it works well. When you turn the amplifier OFF, you have to switch the second switch first, then wait a moment before switching off the general power. The switch OFF sequence is cumbersome as you have to operate two switches in the right order and wait in-between.
For start-up, the second switch can be avoided if the 3.3V to the stand-by and mute inputs is derived from the general supply voltage. The 3.3V will then arrive after the general supply voltage. The problem is at shut-down where the amplifier outputs should be disabled before the supply voltage has reduced much. A general but not trivial add-on is an early supply voltage detector that detects when the supply voltage starts falling for turn-OFF and disables the amplifier outputs, using the stand-by and mute pins, such that when the general supply voltage has fallen to a critical level, the amplifier is already inactive.
Have you thought of this problem?
 
Last edited:

Crest

Member
2018-05-31 2:49 pm
Hi FauxFrench,

R2, R4 and R19 in Figure 3 (on page 7) of the datasheet show a way to control the stand-by and mute pins. The activation of the stand-by pin is faster than of the mute pin such that first the amplifier is activated and a bit later un-muted. For start-up, you can use the same switch to control the two inputs.
But, then comes a second issue. If you turn ON the general power to the amplifier and then activate a second switch that simultaneously controls the stand-by and mute pins, it works well. When you turn the amplifier OFF, you have to switch the second switch first, then wait a moment before switching off the general power. The switch OFF sequence is cumbersome as you have to operate two switches in the right order and wait in-between.
Not sure which switch you are talking about :). The first is the power and the second the combination of STBY and MUTE as illustrated in Figure 3?

I want to power the amps with an SMPS. It can be assumed that it is always powered on. In fact, the MeanWell SMPS has a "5V STANDBY" signal to turn it on/off. In addition, it has a "DC OK" signal. The STBY/MUTE of the amp will be controlled by a raspberry. Using the above signals from the SMPS, it should be possible to control STBY/MUTE and the DC power correctly. However, I don't want to use two I/O ports of the raspberry per amp (I don't think that I need MUTE wothout STBY). Therefore I am interested in the circuit of the TDA7293 mentioned in my first post.



The issue I see with the circuit of Figure 3 is that if you control both STBY and MUTE with one switch, and if you set it to 0V to set the amp to STBY, first STBY will be set to 0V and then MUTE, because of R4 > R2. This is not how it should be, at least according to the spec of the TDA7498. The circuit given in the spec of the TDA7293 solves this problem by splitting R4 into two parts and bypassing one part with a diode such that unloading of C15 is faster than C7. E.g., I would use 30k and 90k for STBY, and 60K for MUTE. This should not exceed the maximum allowed current (the smallest resistor is still 30K), but when enabling the AMP STBY should be still enabled faster. Should that work?

In addition, I don't understand why R19 is needed and whether it has some influences to the above.

Best regards,
Crest