Please comment on my design -and a few resistor values..

[Miniwatt]

Hello everyone,

I tried to design an output stage for my TDA1541 dac using a D3a. Well, not really designing this time (my second), I got most of it from the data sheet..
But I couldn't completely copy it, as some values aren't mentioned..



The troublemaker here is R6, I haven't got a clue how to determine this one, couldn't find it in my books..
Also I chose 1K for the gridstopper R2, but it's kind of a guess..

Designing this stuff is quite new to me, I'm terrible at maths but I'm eager to learn. I'm sure posting this here will get me some usefull extra info. All comments are very welcome!

Thanks in advance!

[ervinl]

Originally Posted by Miniwatt

Hello everyone,

I tried to design an output stage for my TDA1541 dac using a D3a. Well, not really designing this time (my second), I got most of it from the data sheet..
But I couldn't completely copy it, as some values aren't mentioned..


The troublemaker here is R6, I haven't got a clue how to determine this one, couldn't find it in my books..
Also I chose 1K for the gridstopper R2, but it's kind of a guess..

Designing this stuff is quite new to me, I'm terrible at maths but I'm eager to learn. I'm sure posting this here will get me some usefull extra info. All comments are very welcome!

Thanks in advance!

R6 is usually defined by the tube (next stage resistor), but it is not critical. The main function instead of defining output impedance, also to discharge the coupling capacitor if it is disconnected from next stage.

Usually, a value around 100k-470k is enough.

Grid stopper is commonly using 1k, carbon composite (like AB, Riken, Takman TRex), or Tantalum (AN, Sinkhoh). Can be ranged from about 200 ohm or up. Lower value may increase a possibility to self-oscilate(unstable). But the higher, may make high freq roll off.

Another comment, Input Coupling 100nF may be to small (to high of low freq roll off), as the grid leak is about 47k. I like about 1uF (PIO or good MKP), or bypass it as long as there is no DC offset from previous stage.

Thanks,

Ervin L

[Miniwatt]

Originally Posted by ervinl

Thanks,

Ervin L

No, thank you

[Miniwatt]

BTW, I'll be using a very low value I/V-resistor, I'm not quite sure if the operating point is best suited for this. I was looking at the curves, and liked the -0.5V line at around 95V and 30mA.. In the end I decided the TFK guys probably knew their stuff a lot better than I do.

http://www.hifitubes.nl/weblog/wp-co...funken-d3a.pdf

[rock4016]

Originally Posted by Miniwatt

BTW, I'll be using a very low value I/V-resistor, I'm not quite sure if the operating point is best suited for this. I was looking at the curves, and liked the -0.5V line at around 95V and 30mA.. In the end I decided the TFK guys probably knew their stuff a lot better than I do.

http://www.hifitubes.nl/weblog/wp-co...funken-d3a.pdf

For the TDA1541 I/V resistor, I'd start at 50 ohms, and go no lower than 25 ohm.

[Miniwatt]

I've done some experiments before, started with 100 ohm as suggested on Lampi's website, went lower step by step and settled with 28 ohm if I remember correctly. 100 ohm is waàày too high btw, you get a nice distortion 'sauce', and a decent output voltage, but it's very much 'my-fi'..
I believe some people even used 18 ohm..

[nigelwright7557]

C2 is the wrong polarity.
Should be positive end to cathode.

[Miniwatt]

Originally Posted by nigelwright7557

C2 is the wrong polarity.
Should be positive end to cathode.

Thanks, but that was a 'typo'

[ErikdeBest]

Isn't the plate resistor way too low in value? If you don't have more B+ for the stage I would recommend a CCS or gyrator instead of that 625R thing (which is about 1/4 of the D3a internals resistance, when triode connected).

[Miniwatt]

Originally Posted by ErikdeBest

Isn't the plate resistor way too low in value? If you don't have more B+ for the stage I would recommend a CCS or gyrator instead of that 625R thing (which is about 1/4 of the D3a internals resistance, when triode connected).

Thanks, this is the kind of comment I've been aiming for
And I'm still in the dark about what a plate resistor actually 'does', apparantly it can have a 'too low' value. What consequences would this have?
I've got my eyes on a small 125 V tranny, it would fit in my cd player, that's why.