TDA7293 with OPA2604 suggestions - diyAudio
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Old 17th March 2005, 08:10 AM   #1
Tihi is offline Tihi  Slovenia
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Default TDA7293 with OPA2604 suggestions

I am planing to build these combination, but i dont know which schematics to use. It is my first attempt with opamps. Wich do you think is best. I prefer the second one.
They all have the same common gain of aprox. 30dB.
My first question: Is it better to have more gain with OPA2604 like in a) or less like in b)?
Second : do i use imput impedance 100k, 47k or none impedance adaptation?
Third : How high should be the max tension of input (for my calculations of output)?
For any other suggestions I would be grateful

(sorry for my bed english, and i tryed to get my hands on better opamp - opa627, 637, opa2134 ... - but here they just dont seem to have them )
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Old 17th March 2005, 07:54 PM   #2
Tihi is offline Tihi  Slovenia
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Still noone???
Well anyway, these is almost my final decision. Later on i will first try without R2 and later with it. Suggestions are still welcome.
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Old 17th March 2005, 07:54 PM   #3
Tihi is offline Tihi  Slovenia
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I almost forgot, here is my power supply for opamp. Uin=+/- 50V ,Uout=+/- 18,5V.
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Old 21st April 2005, 02:12 PM   #4
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The gain is too low for the TDA: you need 26db (ie *20) closed loop gain for the TDA to ensure stability.

You may try also an inverted configuration, as well as a tube buffer (you can get rid of one cap this way)
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Old 21st April 2005, 04:22 PM   #5
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For input voltage considerations: 0dBu = 775mV rms = ~1V peak.

Gain of OP2604 is 26dB (aka x20) => very high for a buffer, in my humble opinion (too much gain => floor noise of source more audible, risk of clipping at high listening level, and maybe risk of distorsion from OP2604 due to limited slew rate?)
Feedback impedance is also too high for my taste (thermal noise from resistor becomes worse than TDA noise floor, risk of noise picking in OP2604 feedback loop).

If one plays inverted configuration with TDA7293, one cannot use mute/standby anymore.

I see no transfo, no rectifier, no smoothing capacitor in the supply schematic, is that normal?

Tihi: almost all discussions in this forum about lm3875 and lm3886 can apply to tda7293, with minor adaptations. My 2 cents...
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Old 22nd April 2005, 07:33 AM   #6
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Yoghourt,

why can't you use standby/mute in inverted configuration?

Concerning the gain, because you already need 26dB gain for the TDA only, I agree with you that additional gain around the buffer is unnecessary!

Eric
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Old 22nd April 2005, 09:37 AM   #7
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Hello eborgard,

- datasheet recommends that TDA is muted when stand-by is operated.
- From my experience, stand-by seems to control supply of TDA output transistors
- spec says that mute pin commutes TDA positive input between pin Vin+ and pin Vin+mute (I have not checked that)

In inverter configuration, Vin+ is tied to ground. So:
- if you also tie Vin+mute to ground and play with mute, nothing will happen.
- if you tie Vin+mute to Vin-, TDA output has small chances to return to ground when TDA is muted. It won't output music, but will probably output DC. And I'm not sure that TDA remains stable during commutation of mute pin.

Thus my conclusion that inverter configuration prevents to use mute. On second thought, leaving mute and playing only with stand-by may be ok (or maybe a "pop"?).
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Old 22nd April 2005, 10:20 AM   #8
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I have never experienced a plop with my TDA7293.

I guess that activating the mute/standby avoids that the output transistors draw any current from the power supply.

So you have a very high output impedance, hence very low DC. (0,07mV on my meter)

You can also mute the Vin- pin with a transistor, but the differential pair will anyway try to ensure Vin-=Vin+(=0).
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