Thorsten's tube Stage for TDA1541A

[new2hifi]

Hi:

I am planning to build TDA1541 based DAC using boards from a diyaudio member and I/V & Tube stage as posted by Thorsten Loesch in 2005 (see attached image).

I am just a beginner thus need some help. Here are two questions,

1) What changes I have to do in this Thorsten circuit if I want to try 6H30 instead of ECC88? Apart from heater current requirement, ECC88 has amplification factor of 33 and 6H30 has 15/16. Please guide in detail about the changes in circuit.

2) Is this the last version by Thorsten or is there any upgraded version? I mean this is from 2005, any 2011 version?

Regards

[SY]

Just curious- since excellent ECC88 are easily available and the design for that tube has been worked out, why not start with the original circuit, get it working, then start experimenting once you understand its strong and weak points?

[analog_sa]

Originally Posted by new2hifi

Please guide in detail about the changes in circuit.

Not much you can do about the drop in gain. Fixed bias will only have a minor effect. An active preamp may be essential.

[studiostevus]

Originally Posted by analog_sa

Not much you can do about the drop in gain. Fixed bias will only have a minor effect. An active preamp may be essential.

Wouldn't a cap across Rk help improve gian?

[new2hifi]

Originally Posted by SY

Just curious- since excellent ECC88 are easily available and the design for that tube has been worked out, why not start with the original circuit, get it working, then start experimenting once you understand its strong and weak points?

Thanks, Sy. What you are purposing is my probable plan. However, I want to learn some without experimenting. When you fine people have established some facts/opinions by hard work, I don't want to establish them again. I am beginner (but have been reading tubes), this circuit is simple, thus from your replies I'll learn.

Originally Posted by analog_sa

Not much you can do about the drop in gain. Fixed bias will only have a minor effect. An active preamp may be essential.

analog_sa, thank you. can you please explain what changes you expect due to fixed bias but change to different tube, 6H30, in this circuit?

Originally Posted by studiostevus

Wouldn't a cap across Rk help improve gian?

I believe you are referring to increased gain if we bypass the cathode resistor with a capacitor. But what are the effects on sound quality if we increase gain this way?


Kindly help me learn. What is the disadvantage of lower gain in this circuit, what is the advantage of maintaining the 33 times gain here? I am rather very confused. With a CD player's output of 2V, I cannot use more than 1/3 of the volume control on my preamplifier. That is, gain structure is already high and most of the gain is going into drain. Some friends claim that, keeping the overall sound level constant, it sounds nicer if the volume pot is at higher end, i.e. less resistors.

Besides gain, please help me understand other changes in circuit if needed with why and how explanation.

[new2hifi]

Hi:

The question was originally asked in digital section. I am puzzled by lack of responses. Either there were few experts on tubes there or everybody was avoiding touching Master Thorsten's work.

Nonetheless, here is information about this curcuit in Thorsten's own words as copied from audioasylum, 03/15/05.

"It is a simple ECC88/7308 with a choke or CCS load, biased negative minimally (around -0.1V) from a 47R I/V resistor (and the -2mA offset current for digital silence) and with a 10R Cathode resistor. Anode Voltage around 75V IIRC. The one added trick is an "anti sinc" filter using the most primitive methode of a parallel RLC Circuit. Good output coupling capacitor (2.2uF Mundorf Silver or AN Copper)"

Kindly help!

[SY]

Originally Posted by studiostevus

Wouldn't a cap across Rk help improve gian?

Barely. Close to "not at all." 10R raises the plate resistance by about 300R, and the choke impedance is thousands of time higher.

My advice still stands- build it as-is first, get a feel for what it does well and what it doesn't, then direct your experimentation toward the latter, rather than deciding a priori that the tube needs to be changed.

[pieter t]

A bit of calculation:
TDA1541 has full scale current output of 4 mA p-p, that is 1,41 mA RMS.
The 47 ohm resistor will convert this current into 66 mVRMS at the grid of the ECC88.
The current source loaded ECC88 will have a gain about equal its mu, so full scale output of the complete stage is a little over 2 VRMS which is what a CD output stage is "supposed to do".
In my opinion the most elegant way to reduce the gain is to lower the 47 ohm I/V resistor.
When I remember well in the original Philips datasheet of the TDA1541 there is an indication of the maximum value of the I/V resistor being 27 ohm for distortion reasons.
A 27 ohm I/V resistor would give some 1,25 VRMS output which might better suit your "gain structure".
Besides I would not be surprised the lower value resistor resulting in better overall sound.

[Rick Miller]

Has anyone built this I/V, and how did it sound with the ECC88?

[new2hifi]

Originally Posted by pieter t

A bit of calculation:
TDA1541 has full scale current output of 4 mA p-p, that is 1,41 mA RMS.
The 47 ohm resistor will convert this current into 66 mVRMS at the grid of the ECC88.
The current source loaded ECC88 will have a gain about equal its mu, so full scale output of the complete stage is a little over 2 VRMS which is what a CD output stage is "supposed to do".
In my opinion the most elegant way to reduce the gain is to lower the 47 ohm I/V resistor.
When I remember well in the original Philips datasheet of the TDA1541 there is an indication of the maximum value of the I/V resistor being 27 ohm for distortion reasons.
A 27 ohm I/V resistor would give some 1,25 VRMS output which might better suit your "gain structure".
Besides I would not be surprised the lower value resistor resulting in better overall sound.

Thanks all for the replies.

pieter t, I am very thankful to you for your analysis, however, can you please also guide me about the operational parameters of the 6H30 in this circuit, i.e. changes to B+ and cathode resistor?