DIY tube output in DAC - needs advice...

[ThorstenL]

Re: May I jump in?

Konnichiwa,

In the Vasfda article is discussed the value of the I/V resistor for the TDA1541, which cannot withstand the common mode voltage of a 270 Ohm resistor.

However a 100 Ohm figure is given as a maximum value for the PCM63, saying that similar limitations apply to the 1541 also.
Looking in the other threads I noticed that for this DAC are suggested values between 20 and 50 Ohm.

Thge article states clearly what changes need to be made to the circuit to make it work with DAC's having a output current of 4mA PP for full scale instead of the more common 2mA or less.

Ultimatly selecting the I/V resistor is a choice between too high distortion from the DAC and too low output. You need to select the Resistor apropriatly and design the analogue stage apropriatly.

With 4mA peak-peak current and 100 Ohm I/V resistance you get 400mV peak-peak or 141mV. To get 2V from 141mV we need a gain of around 14. The ECC88 SRPP will have a gain of approximatly Mu/2 where Mu for the ECC88 is 33, so the gain becomes 16.5, so the output level will be a little higher than most normal CD-Players.

If you reduce the I/V resistor further your output voltage will drop unless you increase the gain of the SRPP Stage. If you use an ECC81 whose Mu is closer to 60 you get a gain of 30 and thus you could use a resistor of around 50 ohm.

Anybody?

cheers

Andrea

Please appreciate that those people who post here and provide free help and advise live lives of their own. They are not at your immediate beck and call but will answer when they have time. This may at times take several days, not a few hours.

Sayonara

 

[Andypairo]

Hello Thorsten,
thank you for your help.

I had seen which changes need to be made for a DAC like the TDA1541A, but the I/V resistor value confused me a bit.

Lowering that resistor seems the only way to me (I just bought some Mullard ECC88... ), even if it means a lower output.
This output stage is somewhat an experiment for me to see how can tubes affect the sound and maybe this can be a new start...

Cheers

Andrea

PS Point taken, I didn't want to hurry a reply but rather give more visibility to my question ... if I should do it again me

 

[fdegrove]

Hi,

Lowering that resistor seems the only way to me (I just bought some Mullard ECC88... ), even if it means a lower output.

Depending on the DAC chip used an I/V resistor of 100-120R should be fine with the ECC88 in SRPP.

Make sure to use a metalfilm resistor of good quality for lowest noise.

Cheers,

 

[Andypairo]

Hi,

Depending on the DAC chip used an I/V resistor of 100-120R should be fine with the ECC88 in SRPP.

Make sure to use a metalfilm resistor of good quality for lowest noise.

Cheers,

The DAC is, as said before, a TDA1541A.
Or do you mean "depending on how good is your TDA1541A?"

BTW is it possible to test if a tube works properly without a tube tester?

Cheers

Andrea

 

[fdegrove]

Hi,

Or do you mean "depending on how good is your TDA1541A?"

No, just the type of DAC. For a TDA1541A a 100R should work fine.

BTW is it possible to test if a tube works properly without a tube tester?

Yes, but most are basically just emission testers.

Cheers,

 

[Andypairo]

Hoping not to abuse,
If I should feel the need to squeeze some more gain from the poor ECC88, can you point me to a suitable circuit?

Cheers

Andrea

 

[ThorstenL]

Konnichiwa,

If I should feel the need to squeeze some more gain from the poor ECC88, can you point me to a suitable circuit?

Use the same, but connect a large value capacitor (at least 47uF)across the lower cathode resistor. Voila, your gain goes up a little. Otherwise try 12AT7/12AV7/12AZ7, especially the 5965 is underrated and will work well....

Sayonara

 

[fdegrove]

Hi,

Another solution to what Kuei suggested ( I don't like electrolytic bypass caps) is to use a mu-follower iso a SRPP.

This will probably yield too much gain but, hey, you asked for it, right?

Cheers,

 

[Andypairo]

Well if the gain is too much I can lower the I/V resistor...
If the SRPP gain was about 16, this stage gains... ? (yes I don't understand a **** about tubes... but if this project turns out well I promise I'll get a book and will start reading...)

Cheers

Andrea

 

[fdegrove]

Hi,

Simply put:

SRPP mu/2 so 6N1P 33/2 = 16

Mu follower, as the name suggest tracks the mu of the valve more closely so you roughly arrive at mu = 33.

about tubes... but if this project turns out well I promise I'll get a book and will start reading...)

Not IF but WHEN....

Cheers,

 

[Andypairo]

again on 22uF cap

Konnichiwa,

I find them supremely easy to get:

www.cricklewoodelectronics.com


Well, I know off hand of at least five or six locally (UK) available Manufatcurers who have Polypropylene capacitors in such values readiluy and fairly inexpensively (compared to premium grade "audiophile" electrolytic capacitors), Ansar (UK), ICW (UK), SCR/Solen (FR), Arcotronics (IT), Ducati (IT). All these companies make polypropylene capacitors up to around 50..60uF.

Sure, plenty. Just look it up.
Sayonara

I came across a couple of 10uF/630V Solen caps, can they suffice or is the capacitance value mandatory?

Cheers

Andrea

 

[ThorstenL]

Re: again on 22uF cap

Konnichiwa,

I came across a couple of 10uF/630V Solen caps, can they suffice or is the capacitance value mandatory?

You can try them, but I'd recommend to stick to my original value. Due to my STRONG preference for non-polar capacitors I usually use the lowest value I feel I an "get away" with. Lower values MAY work, but I do not guarantee it.

Sayonara

 

[Andypairo]

To be true I have some russian PIO caps that I could parallel to the Solens to reach the 20uF, the only problem is their size....

Btw for the power supply at the moment I have a 250V trafo (but it measures more like 270V ac), so, using a double RC cell (2k7 + 470 uF) I get it down to about 230V.. is that safe or should I use higher value resistors?

Cheers

Andrea

 

[ThorstenL]

Konnichiwa,

To be true I have some russian PIO caps that I could parallel to the Solens to reach the 20uF, the only problem is their size....

If you must recycle junkbox parts, try this:

100uF electrlytic capacitor, then 100R series resistor and the the the best 5-10uF capacitor you have. I would advise against Solens, in the past I found them to give sonic problems.

Sayonara

 

[fdegrove]

Hi,

I would advise against Solens, in the past I found them to give sonic problems.

NOT the FC series IMHO.

Cheers,

 

[Pedja]

What problems?

 

[ThorstenL]

Konnichiwa,

What problems?

A hard, unpleasant sound. This may be very well a result of the microphonic behaviour of these capacitors, just sweep one with a nice high level sinewave while applying a few volt bias....

Sayonara

 

[Pedja]

Interesting, I found them to be among more pleasant ones.

 

[argo]

Hi,

Another solution to what Kuei suggested ( I don't like electrolytic bypass caps) is to use a mu-follower iso a SRPP.


Cheers,

Frank

Here

You are showing that mu follower with ECC88

Hi,

Simply put:

SRPP mu/2 so 6N1P 33/2 = 16

Mu follower, as the name suggest tracks the mu of the valve more closely so you roughly arrive at mu = 33.

Not IF but WHEN....

Cheers,

And here you are talking about 6N1P. Can I just replace the tube of above schematic with 6N1P (I have loads of them but nil ECC88 or 6N23P)?

Regards,

Argo

 

[ThorstenL]

Konnichiwa,

Interesting, I found them to be among more pleasant ones.

In what application and compared to what other options?

I have compared them to the ICW's, Ampohm and Ansars, with the SCR/Solen types coming out poor seconds compared to these. Between Ansar & ICW the race is less clear cut, it often depends upon the specific position, the Ampohm are solidly encased and are best of all these MKP types I came across.

Sayonara