Thorsten's Valve Output Stage with TDA1541DAC

[Bernhard]

What I can see from the simulation a few pages before, that filter has a big big peak around 20kHz.
Is that necessary only for the TDA1541 in non os ?

If I built a brickwall filter for a BB non os DAC with -60dB at 44,1kHz, should the cutoff point be at 20kHz or 22 kHz ?

Should it have a flat amplitude response or a peak around 20kHz ?

 

[Andypairo]

Hi,
the "bump" @20kHz is the anti-sinc filter that compensates the 3 dB loss of non-os DACs at that frequency.

Using a oversampling filter you can skip it (well, you'd better skip it).

A NON-OS brickwall filter? Sorry can't help about that


Cheers

Andrea

 

[JC951t]

Hi, Just need to confirm.
Is the phase at the output
inverted with one Ecc88 tube ?

Thank you

 

[mevang]

This weekend I'try to apply your suggestions. Thanks.

Is the phase at the output inverted with one Ecc88 tube ?

Yes, the KYW output stage is an inverting one.

Regards.
Paul

 

[mevang]

Hi Sándor,

I recommend the following:

1.) Disconnect the DAC from the tubes. If the hum is still there, it is in the tube stage. If not, there is a ground loop with the DAC.

2.) One end of the filament should be grounded.

3.) Inter-stage shielded wire is not always a good idea. Try twisted wire from UTP Ethernet cable.

4.) I would not put the AC socket so dangerously close to the analog stage.

5.) A good test is to disconnect the AC power from the three transformers one-by-one with a temporary switch (with caution, shock hazard!) for a few seconds, and see if the hum ceases.

1) Disconnecting the DAC does not help. The hum is generated by the output stage.
2) I have tied the filament to ground both directly at one end, and with two 120 ohm resistors and center tap. No change. During the short time immediatly after having disconnected the E88CC filament, the hum is still there. Then it strongly increases and short after the signal (of course) vanishes.
3) I use a shielded wire to connect the anodes, in the center of the board, to the output connectors at the left and right ends of the board. I have not tried with a twisted pair. Do you think it is a worthy mod?
4) During tests AC power was directly connected near that blue transformer (please see picture)
5) In the analog stage there is only one transformer. Those devices below the output stage board, that look like transformers, are the two anode chokes.

Hi,
do you have access to a scope?
If so give a look at the hum waveform and see if it is related to the PSU ripple.

Please look at the attached image taken from a Velleman PCS500 DSO (H=10mS/div V=5mV/div). The level of the signal is 6.25mV RMS and it is clearly related to the 50Hz power line.

The P2P assembly is way too big imho, and since we are dealing with mV levels at the tube's grids the AC inlet seems too close.
Try moving the I/V pcb outside the case.

I have pulled the output stage board off of the box. No change.

If you only connect a single channel output does the hum stop?

No

Try moving the choke and trannies.

No change

Do you think that I should completely rearrange the layout of the board?

Best regards.

Paul

 

[mevang]

I have also substituted the EZ90 with two solid state rectifier and changed the two 220uF capacitors with fresh ones.
No change.

Paul

 

[mevang]

Yes, the KYW output stage is an inverting one.

I forgot that the signal out of the TDA1541 is inverted.
So the KYW output stage restores the correct polarity.

Regards.

Paul

 

[JeroenR]

I have also substituted the EZ90 with two solid state rectifier and changed the two 220uF capacitors with fresh ones.
No change.

Paul

How is the grounding done? Starschema is the most safe (regarding hum) in my experience.

Is tda1541 ground connected to I/V ground? Did you disconnect ground this when disconnecting I/V from DAC and still got hum?

Do you have a metal tube/shield to shield the ecc82 (as it is close to the chokes)?

 

[JC951t]

Thank you Sandor

 

[Andypairo]

Sandor,
is the measurement taken on the output?

If so, how much ripple does the supply show?

Where are you exactly in Milan? I live not too far ;-)

I'd also investigate on the currents flowing and would try substituting a choke with a CCS for testing purposes.

Cheers

Andrea

 

[mevang]

HUM PROBLEM SOLVED!!!

Try moving the choke and trannies.

Carsten

I have rotated the two anode chokes (right channel 90 degrees and left channel about 45 degrees) and the hum is almost disappeared.

What is the cause of this strange behaviour?
Is there a way to shield the chokes (or supply transformer and choke) so that the hum level is independent from the orientation of the chokes?

Thanks to you all.

Paul

 

[carawu]

Re: HUM PROBLEM SOLVED!!!

I have rotated the two anode chokes (right channel 90 degrees and left channel about 45 degrees) and the hum is almost disappeared.

Fine!

What is the cause of this strange behaviour?

Magnetism.

Is there a way to shield the chokes (or supply transformer and choke) so that the hum level is independent from the orientation of the chokes?

Maybe shielding with Mu-metal.

Carsten

 

[jarthel]

is it possible to used the tube output stage on any DAC project?

here's the schematic again: http://img185.exs.cx/img185/2008/dacout6im.gif

thank you.

 

[Andypairo]

Hi,
this stage is specific for TDA1541A in NOS mode, but it can be easily adapted to current output DACs eventually changing some values for the input network.

Cheers

Andrea