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Reply with quote  #76 
Achim, all SSRs need to be delayed in my opinon (see schematics).

Kelvin, I tried to decrease the resistor value in several steps difference regarding noise. With 2 SSRs it needs to be now 1k instead of 2k.

I calculated the discharging time (delay)for 5uF and 15uF and get 4ms and 13ms. Is this correct (5V at trigger-2V at the SSR when active=3V as basis for the discharging)?

Anyway, the crackling noise has changed form and is now much more subdued and at a lower frequency (softer), rather some dark little rumbling noise. Attachment 1 shows the switching with 5uF/100us/div and #2 shows the switching with 15uF/200us/div.

The difficulty if I increase the delay time more: I get the 3 way switch problem (noticeable loudness difference) through the back door so to say because for some short amount of time both ground connections are connected. Now I am caught between a rock and a hard place. Any advice? Or are SSRs simply not the right switch for this purpose and I have to let it go? I don't see any alternatives for the SSRs at the moment.

Attached Images
Click image for larger version - Name: SSR delay test.png, Views: 6, Size: 24.08 KB  Click image for larger version - Name: 5uF 100us_div.png, Views: 6, Size: 23.01 KB  Click image for larger version - Name: 15uF 200us_div.png, Views: 4, Size: 23.79 KB 

Kelvin Tyler
Reply with quote  #77 

I guess you are looking for a fast up /slow down type of action similar to your 'release time' arrangement. I would do this before the final Schmitt drivers rather than slugging the input to the SSRs. It is best to present as snappy an action as possible to the SSRs.
You can of course switch MOSFETs in nano-seconds, but that would require a bit of a redesign.

Reply with quote  #78 


the switching speed of the Schmitt triggers and the SSRs is ok in my opinion. My goal is  to keep the AT SSRs still open (delayed turn off) after the trigger has already gone from high to low so that there is no interrupt due to different turn ON times of the "OTL direct" SSRs and vice versa. You can see it works if you compare the scope readings from post #72 and #76. I am just mildly shocked that even 4ms more are not enough – if my calculation is right.

Interesting also in this context  that the regular  relays of my original test which didn't make any switching noise  had an average of 12-15ms turn on/off time.

The only way to increase delay time even more in order to eliminate even the tiniest residue of noise without getting the loudness change due to double ground connection of AT/OTL direct would be a switch for the AT ground (switch  # 6!) which would operate without turn off delay while all the other SSRs  would operate with delay. If this wouldn’t work and wouldn't make the circuit dead silent it means it cannot be done with SSRs.



Kelvin Tyler
Reply with quote  #79 
Browsing through the ZEROs manual earlier today, I noticed that the symmetrical 2:1 impedance  connections do not in fact use the whole winding. In post No.73 I had wrongly assumed that they did, and so the comments I made there in this context were incorrect. The total number of turns used in  the 5-switch and the 3-switch schemes are the same, or very nearly so.
I repeated manually switching from OTL to having the A-T connected with my Pinnacle amp,an 8 ohm speaker and with constant input level. In these tests there is a small, but distinct, drop in volume in the 3-switch setup, yet virtually no drop with the 5-switch arrangement. I do not really understand how this difference arises. Anyone have any ideas on this?
There is more to this project than meets the eye (mine at least!).
Reply with quote  #80 

In the 3 switch version we have parallel to the voice coil the AT making a ground connection. If we switch to OTL direct it’s only through the voice coil. In the 5 switch version it’s in both cases only one connection to ground through the voice coil/AT or voice coil OTL direct. This is the only obvious difference between the two versions. In the impedance readings we can find a little but distinct difference between 3 and  5 switch version as I have mentioned before. That looks very little (0,3 ohm, maybe more or less at different frequencies) but obviously it’s more noticeable than it appears considering only the tiny amount of  impedance/phase change.

The whole current vs voltage drive of speakers is a discussion of its own. But that’s what we are doing, or at least it points in the same direction as this discussion IMO, if we switch impedance. The effect of this might be more complex than meets the eye because it includes loudspeaker parameters and  the speaker design as well. Some go so far and even consider the room-speaker-air-volume interaction as part of the speaker parameters which will appear at the amp's output as reactive force if I have understood these studies right.

On the topic of switching I can finally conclude that at least with SSRs it’s always a compromise between more loudness difference and less switching noise and vice versa. The switching noise can be brought down to hardly noticeable levels  (increase of delay time for the SSRs for switching OFF) but then loudness difference through double ground connection becomes more prominent which is even worse than the switching noise itself. Depending on the music style I can hear the little dull clicks more or less,  sometimes they simply disappears in the music but this is not a solution for High End or if one aims for perfection.

Kelvin Tyler
Reply with quote  #81 
If I dare make a final point on impedance ratios with the Zero A-Ts. If we take the 2:1 and 4: 1 figures given in Fig.1 of the manual to be exact for the connections indicated, and we assume overall symmetry in the winding, then the impedance ratios for the 5-switch and 3-switch cases are not exactly the same. The respective ratios are 2:1 and 2.25: 1. This may well account for the small difference in Wolfgang's measurements which his elegant graphs show, but would hardly account for the respective volume drops. I wonder just how exact are the tappings?
I do feel that Wolfgang should have the last word (if there is one!) on this interesting project. Quite a bit of ground has been covered and I greatly admire his persistence in seeking a good outcome. I have learned a lot. Many questions answered, ideas justified, and so very, very close. Well done Wolfgang!
I always thought that acoustic design and the like involved black arts.
Reply with quote  #82 

Thank you for your kind words, Kelvin. Yes, it was a roller coaster ride but that’s anyway my experience with building electronic circuits. You never can be 100% sure that it works until it really works. But it’s always fun no matter how the outcome is.

I have a quote hanging over my work space where I do my little experiments which always cheers me up if things don’t go my way. It’s from T. Edison and everybody knows it: “I haven’t failed. I have just found 10,000 ways that won’t work.”


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