TDA1540 passive I/U Stage ???

Hello,

I hope there is someone who could help me.
I want to use a older TDA1540 only with a Restistor as the
I/U Stage, without the normaly taken OP-Amp. Has somebody done this bevore ? What Resistor should i take, I tried 100ohm and 330 ohm, but there is a sprankling dirstortion on the less loud passages...

I hope there is someone who could help me.

Best regards from bavaria,
Manfred.
 
Hi Manfred,

I have played around with different resistors, from 7.5 ohms to 47 ohms as I/U with a valve output. I also used a 1:2 step up transformer. I found that the sound with step up by itself was lacking in dynamics and does not produce enough drive. Higher I/U resistors do cause distortion, I think anything above 50 ohms is too much for the 1540.

I’m currently using a 1:2 step up transformer with a valve output which is rather nice, I found the bass is better with the transformer in the chain, rather than just the plain I/U resistor before the valve. I have a 20 ohm which, in parallel with the transformers primary, gives an I/U of about 8 ohms. The transformer I’m using is a vintage one from some medical laboratory equipment (10Hz – 60kHz), but I recall someone here using a Lundahl 7905 with good results.

You should adhere to advice from Rfbrw & Oshifi and keep the I/U resistor value low so that the voltage compliance is near the dac’s +/-10V limit.

You may also find the following thread useful
http://www.diyaudio.com/forums/showthread.php?threadid=111259&perpage=25&pagenumber=3

Cheers,

Richard
 
Hello,

thanks a lot,
and o.k. i will try the prozedure,
i allready have done this with TDA1543.
I know the dddac article. Very helpfull indeed.

Probably I should try the transformer, but they are not cheap I think, and not so easy to get in germany.

Thanks again and I wish you all a good
and interesting year 2009 !
 
Initially, I wanted to make it exactly as I did with TDA1541A (22 Ohm resistor) but TDA1540 apparently needs a lower impedance. With TDA1541A it sounds great, very analog, relaxed, and detailed sound, however with TDA1540 I am getting ladder steps on the scope and the sound is awful.

I would like to try with IV converter (with bipolar transistors - works great in LTSpice with a current source) with 5 Ohm input impedance but I'm not sure if even 5 Ohm is not too much for this DAC. Definitely, I don't want to use opamp, especially when I disabled oversamplig.

Guys, do you have any experience with TDA1540 and non-opamp IV converters?