Edcor 150:600 output tx

[joaquins]

strange 100Hz bump in THD, that also make me suspicious about this. I will look at the schematics later, I don't have it now and I'm in a hurry, freq response has low end loss at +20dBu. It may have to do with the transformer saturating, could you measure the distortion at the primary of the transformer?

JS

 

[PRR]

> strange 100Hz bump in THD, that also make me suspicious

Just humm.

 

[ruffrecords]

strange 100Hz bump in THD, that also make me suspicious about this. I will look at the schematics later, I don't have it now and I'm in a hurry, freq response has low end loss at +20dBu. It may have to do with the transformer saturating, could you measure the distortion at the primary of the transformer?

JS

Yes, those measurements are all wrong. Got the levels messed up. I have made some more careful ones . Will post them later.

Cheers

Ian

 

[joaquins]

> strange 100Hz bump in THD, that also make me suspicious

Just humm.

Why only with  +20dBu output and not with 0dBu output, lower output should have more humm... That's what called my attention. Knowing they are wrong may explain some things...

JS

 

[ruffrecords]

Here are the more carefully taken results. First at 0dBu into 600 ohms:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/audiotests/sept13/6Uproto0dBudist13thsept.png

and then at +26dBu into 600 ohms:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/audiotests/sept13/6Uproto+26dBudist13thsept.png

All made using RMAA.

Cheers

Ian

 

[joaquins]

At higher level HF THD is reduced, (Hysteresis?)

At LF there is a little peak at 30Hz on the freq response @+26dBu, and of course the biig THD (about 50%!!??). @0dBu 0.2%THD doesn't looks so bad at 20Hz. If I'm looking at the correct schematics you have 4µ7 coupling cap, am I right? maybe it's reactance is to high at 20Hz, compared with the output impedance of the active circuit and the transformer when it's running into saturation. Maybe it is the core saturation but I would look to lower the LF output impedance which looks like rises somehow, rather than lower the overall output impedance which seems fine already.

JS

 

[ricardo]

If I'm looking at the correct schematics you have 4µ7 coupling cap, am I right? maybe it's reactance is to high at 20Hz, compared with the output impedance of the active circuit and the transformer when it's running into saturation. Maybe it is the core saturation but I would look to lower the LF output impedance which looks like rises somehow, rather than lower the overall output impedance which seems fine already.

Don't forget the inductance of an ungapped transformer varies with level.

I post measurements in another thread.  This behaviour is described by Williamson (of da amp) and probably much earlier.

Your 4u7 cap forms a 12dB/8ve filter (with the primary inductance) which can peak.  To damp this out, you Increase source resistance but Decrease load resistance.  And don't forget the peak & its frequency will vary with level.

If you make the cap bigger, you move its effects to a lower frequency but you increase the size of the peak.

 

[joaquins]

The major problem isn't the peak, is the big distortion here, you may not want the peak, would be nicer to have it at lower freq, then you can damp it. The resonance is there because the inductance has changed, the ~50% THD rising fastest around the resonance freq is because the source reactance (capacitive) at that point is the same as the transformer reactance (inductive) and as the transformer impedance is changing with level vs a comparable impedance feeding it the high distortion is there. I almost certain now that using a bigger cap there would end in less distortion, how big and what effects will it have in the freq response is another thing to look for. I wouldn't want that the freq response of my design is being limited in the last point of the chain, anyway we are needing a bigger cap here, we will have higher resonance, so it will need more damping, now it has almost no damping, but the output impedance of the tube stage which is rather low. How low, I think I would go with twice the capacitance is already there, I think it might end up close to -50dB THD @30Hz, +26dBu (based on the plot which has that figure for 60Hz now) and probably about 40dB for 20Hz, I think I could live with 1% THD in here at max output level.

JS

 

[ruffrecords]

The nominal turnover of the 4.7uF and the reflected 600 ohm load is about 13Hz which is why the response tends to be getting on for 2dB down at 20Hz.  On my to do list has been to try replacing this with a 47uF electrolytic to improve the LF response. I have done this and the response certainly flattens out. However, I have not checked what this mod does to the LF distortion. I'll do this and post the results.

Cheers

Ian

 

[ricardo]

Your 4u7 cap forms a 12dB/8ve filter (with the primary inductance) which can peak.  To damp this out, you Increase source resistance but Decrease load resistance.  And don't forget the peak & its frequency will vary with level.

If its THD you are concerned about, Increasing source resistance Increases THD while Decreasing load resistance Reduces THD

The ultimate is feeding a virtual earth.  Baxandall shows how to use a very small core for supa low THD in his "Loudspeakers as High Quality Microphones" http://www.aes.org/e-lib/browse.cfm?elib=3776

He expands on the transformer design in Microphone Design Handbook http://www.leonaudio.com.au/microphone.engineering.handbook..chapter.8.pdf

This is also Great Guru B's last word on Low Noise design and includes much stuff (eg transformer design) which will soon be Unobtainium.  Grab it now.