ZOTL and ferrites

[Lampie519]

The core material is from a standard PSU common mode choke i found in an electronics dump store. Therefore i am not sure about the exact value. I tested this and found it satisfactory. The schematic of the amp is attached but the Zotl part can be found on David's website (user manual of Microzotl). Instead of 12v i used 7v as it was convenient for me.

 

[savu]

And how does it sound? It kinda has a carrier of 250Khz as I remember... I've done classD amps and I can say that with the right parts and the right filtering you can't hear the difference from solid state AB to solid state class D.

Can you share some more of your ZOTL experience? I find it fascinating being a switching guy that started playing with tubes

 

[Lampie519]

This is not my first Zotl (i am a fan). I can say from my experience that it sounds very clean like any good OTL amp and you can hear the differences of the used tube but the class is not class D or C at all. It is class B or even A if you would like. However it switches the tube on/off therefore the duty cycle is 50% and the tube will get less hot even in class A. In my case i use 500Khz as a carrier frequency. It has a bass response of a transistor amp and goes all the way down to 20Hz (easy) even a small amp like this.

 

[savu]

I also built an Circlotron OTL and it sounds incredibly clean even if I use smps for output tubes and I always wondered how would a ZOTL sound because It has to be clean.. the group delay at 500khz carrier should be nonexistent + I love the way that DHT transmission tubes sound... some like 300B...
I have two 833A and I was planing to use them in this ZOTL technology...
I have some dual mosfets for classD (halfbridge TO220 looking but with 5 pins) and some HIP4081 I was thinking to buy some ferrite pots or toroids for the HV transformers.
What other ZOTL stuff have you built?

BR, Silviu

 

[Lampie519]

I have build a single ended Zotl (300b, SV527-10 and 6L6) plus some small push pull amps. Nothing real big until now. I have build HV amps as well (direct drive ESL amps).Please see images.
Building big Zotls need higher voltages and big Mosfets plus double/parallel impedance converters. Big tubes are not required with Zotl because with small tubes you can get enough power, opposite from standard OTL's.

 

[savu]

That's the most beautiful thing I have seen, tubes and switching electronics... It can't get better than that...

You use pulse transformers because it's easier to wind or because you don't need the precision that specialized PWM mosfet drivers have?

Have you considered using SiC diodes for the output bridges?

How do you manage the single ended? You sit it on a CCS?

Sorry for the high number of questions... But you are the first person that I know built one except the inventor himself.

Best Regards,
Silviu

 

[Lampie519]

I use the transformer cores as these were at hand , no special reasons.
No SiC diodes here as these are too slow (maybe). IN4148 are faster anyway and sufficient in my case.
No CCS as load but simply a heater of an unused tube (EL86 in this case depending on output tube used as i am lazy).

 

[savu]

Lazy is my middle name [emoji1787][emoji1787][emoji1787]...
Lazy people think outside the box because they always cut corners

Thanks so much for the photos

I've designed and built an OTL (hope the pictures attached) it sounds great in my Quad ESL 63's... the problem is taht if I ever want to move to a warmer I have to get read of it because of the heat it produces soZOTL is the next step for me... so glad I met someone who did it DIY.

 

[Lampie519]

Some other Zotl projects and images:

 

[savu]

Frank,

Did you ever probed the anode of a tube driven by this contraption? I am expecting it to be very noisy thanks to the high frequency and no shielding.
Do you have any more scope traces?

Silviu

 

[Lampie519]

Nice big amp ! I also have build many OTL amps in the past but that is now really in the past. No more for me haha!
I do like to build HV amps to drive electrostatic loudspeakers directly.

 

[savu]

Very nice stuff Frank...

I don't have the courage to start working on Direct Drive amps for Electrostats...
That OTL amp is my first tube project.. I still have lots to learn.

Silviu

 

[Lampie519]

Yes, it is very noisy but at high order harmonics of the 500 KHz and can be filtered out very easy. I will get more pictures of traces.

 

[savu]

Why don't you use some filtering on the high voltage side of things...
Just enough that you don't have a horrible phase delay. Teflon or silver mica capacitors behave very good at high frequency, COG also.

 

[Lampie519]

Not sure what phase delay you are referring to. Please explain.

 

[savu]

If you filter the high voltage you create a reservoir/buffer of energy because that cap will be charged.
When the tube starts to conduct it starts to draw energy from the primary but it won’t draw it from the energy in the core it will take it from the capacitor thus creating current lag for the primary side of the transformer which is connected in series with the load itself. This is why this ZOTL thing is so great, because the inventor relies on very fast and linear energy transfer from the 12V external supply to the reflected load.

 

[Lampie519]

The high voltage side does not have any capacitor therefore no delay i guess. It will draw it from the core directly, no other source available for the tube.

 

[Lampie519]

I am building a new Zotl amp at this moment to see if i can get more power with six 1P24B tubes per channel and still using a low voltage PSU (12V). I have a lot of these tubes plus they sound great in my headphone amp (two tubes per channel).

 

[savu]

nice stuff... I'm still very interested in this topic... unfortunately did not find the time to build one

 

[Lampie519]

Slow progress..