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Lundahl Transformers for TDA1541 Tube Output stage

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Posted on February 12, 2008 at 03:56:24
FibCouple
 Dear Kevin and fellow inmates

This is my first post here & I would be grateful if I could have some advice please about what I am trying to achieve.

I am a newby but have made a fully working tube output stage for my TDA1541A S1 dac. This was all done after following on the circuits by Lukasz Fikus as www.lampizator.eu, who has done a great job putting up step by step procedure for this wonderful sounding output stage. Improvement was remarkable over my discrete op-amp output stage, even though I already had all the previous mods. They included superclock 4 with separate regulated power supply, super-regulators for all DAC voltages, black gates in PSU & decoupling etc etc. The sound is more natural, bass is better. The sound now begining to come close to my vinyl front end, which consists of EMT930, EMT TSD15 and EAR 834P phono stage.

Presently I am using 80R resistor for i/v conversion. This produces
around 96 mV which is amplified by 6N1P tube stage (anode follower, active
dynamic anode load) to around 2V at RCA output, with an amplification factor of I suspect around 20. Please see the circuit on the link.
http://www.lampizator.eu/LAMPIZATOR/TDA1541%20corner/schematics%20003.jpg

So why am I looking to tinker with the player? I guess trying to make the best possible cd player it can be. I understand best cap is no cap & audionote have gone down that route for their more costly dacs. Also I know TDA1541A likes to see zero volts at its output & certainly doesnt like more than 25mV.

So here goes my rough plan:

1. Instead of 80R i/v resistor, maybe use 20R so the voltage at current
output of the dac is less than 25mv. So I am looking for a step up transformer to do the rest of the voltage amplification before the tube stage. Mind you this dac has a few millivolts od DC at its output. What Lundahl transformer would you recommend?

2. Instead of output coupling capacitor, use an output transformer. I have seen 1660AM being used by Andrea Ciuffoli for AD1865.

http://www.audiodesignguide.com/DacEnd/index.html

I will be grateful for your advice as to:

a. Is there any advantage of this instead of say Mundorf Silver/Oil capacitor
for coupling ?
b. If you think this is useful, what transformer would you recommend & in
what configuration as my DAC has a single output?

Many thanks for your time.

Best regards

Fib
RE: Lundahl Transformers for TDA1541 Tube Output stage, posted on October 2, 2009 at 14:54:34
WF
Audiophile

Posts: 1
Location: Taipei
Joined: October 2, 2009
 Yes, 80 ohm for the passive I/V in TDA1541A is too high to keep the dynamic range. I had picked up a lower resistance such as 36 ohm or so, it worked much better in bass and dynamics but trade-off in SN ratio.

Besides the resistor I/V, active I/V converters are worth to try. An OPA861 "transconductance amplifier" I/V stage is used in my current TDA1541. It is very good at all. Compared to the resistor I/V, the sound from OPA861 is solid, powerful, liveness, and well-balanced over the wide-band.

RE: Lundahl Transformers for TDA1541 Tube Output stage, posted on August 18, 2008 at 09:56:24
lukasz
 USE Only 6N2P tube and it will blow your mind !!! good for 20 Ohm resistor. 100X amplifiction.
Lampizator ;-)

RE: Lundahl Transformers for TDA1541 Tube Output stage, posted on April 27, 2008 at 23:40:08
Newbie
 Is it possible to email me a clearer diagram as I am new to reading hand written electronic diagram.
Thank you very much for any help.

RE: Lundahl Transformers for TDA1541 Tube Output stage, posted on May 3, 2008 at 14:52:31
fibcouple
Audiophile

Posts: 8
Joined: February 12, 2008
 Hi

If you look at this site, there is plenty of info as to how I built my tube output stage.

http://www.lampizator.eu/LAMPIZATOR/Lampizator.html

Lukasz Fikus has put pictures of many dozens of cd player he has modified & has a detailed word document as well. Any queries do not hesitate to ask me.

http://www.lampizator.eu/LAMPIZATOR/photogallery/STEPS/Lampizator.doc

Regards

Fib

RE: Lundahl Transformers for TDA1541 Tube Output stage, posted on February 12, 2008 at 18:19:00
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 The presence of a DC "offset" at the output pins of the DAC complicates the issue of using a transformer. If you want to try a transformer without the capacitor you need the largest appropriate transformer you can get your hands on. I would use an LL7903 connected 1:4 for this purpose. It's a great sounding transformer and it is unusually large, so the voltage offset will have a smaller effect on transformer performance. You can evaluate the effect of the DC offset by listening for bass level and quality with and without a coupling capacitor.

The AD1865 application you quote uses the Lundahl transformer in the anode circuit of the tube output stage, not for interfacing the DAC to the tube stage.

Kevin Carter
K&K Audio
www.kandkaudio.com

RE: Lundahl Transformers for TDA1541 Tube Output stage, posted on February 13, 2008 at 05:54:32
fibcouple
Audiophile

Posts: 8
Joined: February 12, 2008
 Thanks a lot Kevin

Would 7905 work equally as well here? (with 7905's step-up I can possibly use it for MC step-up if it fails to help here)

The link I posted does use the 1660 in anode circuit but it appears it is being used as an output transformer after the tube output, instead of a coupling capacitor. I wonder if it can be used for my project, but in a different way?

Best regards

RE: Lundahl Transformers for TDA1541 Tube Output stage, posted on February 13, 2008 at 06:43:17
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 Yes, you could use the LL7905 equally well. With either the LL7903 or the LL7905 you won't really need an I/V resistor, because the primary plus reflected secondary resistance will be adequate for your goals.

Yes, the LL1660AM transformer is being used as an output coupling device in place of the usual capacitor. With the right tube (medium or lower Rp), the LL1660AM is, in my view, a better sounding alternative to any output capacitor. What way of using it do you have in mind?

Kevin Carter
K&K Audio
www.kandkaudio.com

RE: Lundahl Transformers for TDA1541 Tube Output stage, posted on February 13, 2008 at 07:01:36
fibcouple
Audiophile

Posts: 8
Joined: February 12, 2008
 Thanks Kevin. Looks like 7905 is the ticket then for i/v interstage part of the mod.

My other planned mod was to try to use a transformer instead of the output coupling cap, which has sonic benefits as you said. However, my circuit (anode follower, modified srpp) is different to the link I posted. As I am only starting in tube diy, I am not sure how to inegerate the 1660 into my circuit. Any help will be much appreciated. Here is my circuit again





Best regards

Fib

RE: Lundahl Transformers for TDA1541 Tube Output stage, posted on February 21, 2008 at 16:05:54
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 It is not straightforward to integrate an LL1660 into this circuit with the aim of removing the coupling cap without changing it substantially. You would lose much of your output level due to the step down that would be required to ensure low output impedance. Do you really want to do this?

Kevin Carter
K&K Audio
www.kandkaudio.com

RE: Lundahl Transformers for TDA1541 Tube Output stage, posted on October 5, 2009 at 06:09:56
reVintage
Audiophile

Posts: 120
Location: Eskilstuna
Joined: June 19, 2008
 There is an easy to get, nice russian triode/pentode called 6F12P. The triode section is mu=100, S=19mA/V, and Ri just above 5k. Together with a LL1660/10mA, 4,5:1 it could help you get rid of the SRPP and still have gain enough.
Brgds
Lars

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