TDA1543 DAC with resistor I/V and tube amp

[wa2ise]

I modified my old Magnavox CDB600 (really a Philips machine) CD player. It uses a TDA1543 "twin DAC" DAC chip, and it used to feed op-amp I/V converter circuits. But I found that this DAC chip seems quite happy to feed a resistive load for the I/V circuit. This DAC chip has, for each channel, a constant source current generator, and a varying (to the music) a sink current generator. The difference current between these two goes into/from the resistor load. It wants a bias of about 2.2V for this resistor load (going directly to ground with the resistor doesn't work, sounds awful like severe clipping). This 2.2V bias needs to source or sink current. A conventional voltage regulator chip would only source current but not sink current, so that won't work. But an easy way to create this is to use a voltage divider network, 3k resistor from +5V, 2.4K to ground, to create a thevanin equivalent of a 1.3k resistor going to 2.2V. The DAC develops something like 1Vp-p of audio, which feeds a cathode follower triode grid. I used existing power supply voltages inside this machine to get about 40V for this tube (not a lot, but it seems happy).

I used the light bulb symbol to indicate vacuum tube usage...

[analog_sa]

sounds awful like severe clipping

Probably something wrong with Vref. This topic has been discussed to death - just do a search. But what is the purpose of the valve? Just to add lots of ugly distortion and capacitor colourations?

[wa2ise]

"Probably something wrong with Vref." It's working well now, the chip wasn't designed to drive a resistor going to ground.

"purpose of the valve? Just to add lots of ugly distortion and capacitor colourations?" In some circles, that's a FEATURE!

[dddac]

Just a few comments:

1. The tda1543 is so popular, because it works fine at a supply voltage 8-9 Volts, allowing an almost 2 Volt RMS output. Therefore no need for an extra TUBE stage .....

2. From your design jpg it is unclear what supply voltage you use for the tda1543. If this is also 5 volt, this will most likely explain the distortion, as you clip the music signal.....

3. An extra tube stage "amplifying" the signal to 0.7 Volt seems useless in regard to point 1.

4. If you want to know about biasing properly a tda1543 and loading it with a resistor correctly you can do a few things:

a) search the DIY as suggested before
b) read my detailed articles on this subject on my website


best regards
doede

[wa2ise]

Originally posted by dddac
Just a few comments:


2. From your design jpg it is unclear what supply voltage you use for the tda1543. If this is also 5 volt, this will most likely explain the distortion, as you clip the music signal.....

Okay, should have mentioned it 5V.

3. An extra tube stage "amplifying" the signal to 0.7 Volt seems useless in regard to point 1.

Well, I'm a tube head....

4. If you want to know about biasing properly a tda1543 and loading it with a resistor correctly you can do a few things:

Well now I got it to work well, it must not be incorrect Seriously, I wanted to maintain the average voltage bias the TDA1543 saw with the op-amp circuir, so operating it into a resistor equivalent into a voltage similar to Vref seemed a sensable thing to do. Looked at your web site and your TDA1543 circuit and I do see a resistor going only to ground. You must have set your Ibias to be larger than the max the Idac ever sinks. And the current Ibias - Idac goes into your resistor. As long as Ibias is high enough to keep Idac operating corrrectly (voltage higher than something like 0.6V?) that should work. A concern I had was that the Idac circuit might not work quite right if the output voltage got too close to ground. The clipping I experienced before I did my above circuit must have been do to insufficient Ibias on Idac peaks. My use of the 2.2V equivalent source avoids that. It allows Idac to exceed Ibias.

b) read my detailed articles on this subject on my website

Been there. Unless I missed it, there's no mention what Ibias does or what happens if it's misadjusted. Using several DAC chips in parallel is interesting though. But I'm not convinced that oversampling is bad. There's more than one right answer to many questions...


best regards
doede