TDA1305 / TDA1549 bypass opamps

[jockel77]

I just bought a mini USB DAC with TDA1305T Chip inside from China (which is on the way to me now) (review: USB DAC using ancient technology)

As I've seen on lampizators webpage, the TDA1305T has got identical structure as TDA1549. There's shown a trick on lampizators page how to bypass the internal opamps of the DAC Chip (see attachment file).

I wonder If anyone here has already tried this modification. As I don't have much electronic background knowledge, I've got some questions about it:

- Why does the PIN VDDC 10 has to be cutted ?
- Are the PINS 5 + 6 connected to + or to - signal input of preamp ?
- which output values (mV and mA) does the DAC Chip itself have after modification ?

Thank you in advance for help.

Best Regards
Horst

[erin]

Why do you want to bypass the op-amp?
Are you connecting it to a valve output stage?
If you are not connecting to a valve you dont need to do anything.

[jockel77]

Yes, the reason for bypassing the opamps is that I'd like to get the pure (unamplified) analog output signal from the DAC itself to connect it to an discret analog pre-amplifier (DIY-Version from a friend of mine).

My friend had some very good results with the same mod for a TDA1547 based CD-Player. The Diagramm of TDA1305T says that it should offer the same possiblity.

As I've read some tweakers said that they got a distorted signal after bypassing the internal opamps. So I hope, there's someone who can give me a good advice how to get best result (swictching of the circuits CEXT1 and CEXT2 and then taking I-out signals from PIN 23 and 24 ?)

Have a look what lampizator's result was:
DACMAGIC Cambridge tubed DAC player TDA1305

[QSerraTico_Tico]

Contrary to the TDA1547 the R-IV is still in the circuit.
Perhaps you can disconnect the analog supply to the TDA1305?

[jockel77]

Originally Posted by QSerraTico_Tico

Contrary to the TDA1547 the R-IV is still in the circuit.
Perhaps you can disconnect the analog supply to the TDA1305?

Hmm ... are you sure ?
look:
http://digilander.libero.it/paeng/a__tda1547_tweak.htm

here's a detailed tweaking description of TDA1547.
The opamps are also inside the DAC Chip.

What would happen if the analog supply of the opamps would be capped ?
Does it still give out a signal when it's capped ?

[fzn9bl88]

jockel77, I'm wondering if you made any progress in your quest to bypass the internal opamps of your 1305? I too have a TDA1549 and thinking of bypassing the internal opamps and connect the current output directly to a valve stage. However, I too have very little electronics knowledge so currently searching for the ways to do it.

cheers!

[EUVL]

Simple answer is that it does not work, of course :

Philips_tda1549

"After intense testing i concluded that the signal is distorted. So I just reversed to the power to leg 10, and signal from legs 4 and 7....."

Removing the power supply to an opamp does not equal to removing the opamp altogether.

Pity. Would have been a nice idea to play around with.


Patrick

[tiefbassuebertr]

Originally Posted by jockel77

I just bought a mini USB DAC with TDA1305T Chip inside from China (which is on the way to me now) (review: USB DAC using ancient technology)

As I've seen on lampizators webpage, the TDA1305T has got identical structure as TDA1549. There's shown a trick on lampizators page how to bypass the internal opamps of the DAC Chip (see attachment file).

I wonder If anyone here has already tried this modification. As I don't have much electronic background knowledge, I've got some questions about it:

- Why does the PIN VDDC 10 has to be cutted ?
- Are the PINS 5 + 6 connected to + or to - signal input of preamp ?
- which output values (mV and mA) does the DAC Chip itself have after modification ?

Thank you in advance for help.

Best Regards
Horst

Interesting question. Unfortunately I cannot disconnect the line between the output switches and the inverted input of the internal OP-AMP so as the 2K2 resistor. Thus the influence of the input load of this internal I/V stage is still the same.