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kasey197 11th July 2012 05:13 AM

Ticle Zen Amp
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After a few years of on-off again interest in diy audio, I finally got round to actually building something. I was inspired by the Zen V5 and later by the F5. I wondered if I could perhaps marry some of the ideas there with something I always wanted to try - transformer coupled inputs doing some of the voltage gain duties feeding a fet in common source configuration.

Some searches basically yielded some interesting topologies with pretty good results (see the Zeus Amp, Tubecad, ciuffolis Circlotron). So that gave some comfort that this wasn’t a completely barmy idea. I got some great ideas and insights here too:

So I wondered if we could get measured performance somewhere between a ZV5 and F5 with respect to THD, bandwidth and low output impedance. Much ruminating, reflection and reading yielded two or three ways of slapping on a tx front end to a ZV5 and I settled on this one:

p.s i'm not too good with schema software so used circuitlab an online thing - sorry for the poor layout but its manual. also watch out for the wrong mosfet symbol - I have noted it in the schema..[EDIT: Phase for top secondary winding is shown incorrectly, it needs to be flipped] if anyone can do a better one to aid discussion that would be great!

kasey197 11th July 2012 05:19 AM

1 Attachment(s)
After a while, I decided to just suck it and see and build a prototype.... i tried hard to get it messier but failed :)

kasey197 11th July 2012 05:21 AM

Some details..
Mosfets: I tried the 2sk1058/2sj162 laterals and the IRF240/9240 as well. Six mosfets were killed during the making of this movie (well so far..)

In the end I settled on the laterals because I just found I didnít have to work so hard stabilizing bias but will go back to the IRF parts at some point once I have decent heatsinking in place.

Input transformers:

I bought and tested quite a few power transformers. I took a look at the audio types but the ones with wide quoted bandwidth were pretty expensive and so I left that as a possible future upgrade. In general, toroids were far better than EI types. A local cheap n cheerful one seemed to work fine so I went ahead and ordered a few types from Digikey to check results so that others could try it as well if they liked. The one I used for testing was a 50VA VPT24-2080 from Triad Magnetics ($16.45 from digikey). There are plenty more on my bench and I'll report back later on results in the amp. In general though, it looked like you needed a decent size (I tried 2va and 5va and didnít get very good results), dual split primaries (which I'd use as secondaries) and the rights turns ratio (2x 12 volt secondaries, 2x115V primaries) - and we're in business. Since the transformer is outside the FB loop so any response anomalies or bandwidth restrictions aren't addressed by the amp. Wideband/broadcast quality audio coupling transformers run north of $100 bucks and perhaps worth the dough once the basic topology is proven. 600 ohm/10k impedance seem the type I'd want if I went down this route....

kasey197 11th July 2012 05:24 AM

3 Attachment(s)
Results (so far .. need help to improve..)

This is a bench-test only but initial results seem ok. After frying so many fets, I didn’t want to drive things too hard so just settled on 850mA bias and 20v rails.I didn't take the o/p beyond 14.5V pk-pk this round. Test load was 6.8 ohms, so I guess that equates to 15W or so (only the first third of which is in class A at this stage due to the bias). That is if I got my math right!

Initial measurements results at 14.5V Pk-pk into 6.8R resistive load:

Bandwidth: +/- 0.5db 5Hz to 40kHz.
THD (20) 1kHz: 0.0144%
THD (20) 10kHz: 0.051%

Gain was ca 15x closed loop and approx 50+ open loop. The open loop gain was roughly in line with calcs assuming Gm of 1S into 8 ohms irc...
Distortion spectrum is largely 2nd harmonic. The measurements were made with my cranky 35665a DSA... if any suggestions on how to improve my measurement technique with this machine please let me know - what I have is just from learning through trial and error!

I haven’t made any special efforts at optimizing apart from choosing the feedback resistor value. The tx secondary needs to be loaded with a high value R to reduce the slight gain at HF, and perhaps some multiple parallel mosfets might help load it a bit too :)

Output impedance was a disappointing 2.3R ... this is something I really hope to be able to reduce down to 0.5R or so if I can.

kasey197 11th July 2012 05:32 AM

sooo... i was *hoping* to get some input/suggestions/feedback on the project and any tips would be welcome. In particular though, I have a few questions at this stage:

a) ZV4 Biasing arrangement: Why do we take the bottom leg of the bias resistor network to the drains i/o ground ? It works both ways, but there must be some benefit of doing so but I'm not sure I understand what it is. Taking it off the amp outputs means that there is some feedback here as well - so is that the only benefit ? But you cannot vary the feedback in this config without also varying the bias so this seems a little superfluous since I have a separate feedback loop ?

b) This amp feedback arrangement: I tried and tried and tried to think of ways I could get rid of that DC blocking cap in the f/b loop but cannot (and no i don't want to move it to the input drives:) .
With the current feedback resistor chosen, you cannot see it, but at some higher levels of feedback, I get weird freq response (within +/- 0.5 to 1db but just looks strange). I guess I can live with this since the practical results seem decent enough to warrant tweaking and listening but this also means I cannot just vary fb with a free mind as I try to get a lower output R.

c) If i cant mess with the feedback, any other suggestions to lower output impedance significantly - i can only think of more o/p devices and maybe lower the 0.47ohm resistors. not sure if the latter makes that much diff if i had a properly set up fb network ...

If anyone wants to try this or has questions, feel free to shoot.... I'm travelling the next few days so no soldering iron time, but I thought the time might help with, you know, reflections and discussion... thanks in advance for the help and most of all to NP for his wonderful, inspiring articles and projects....

kasey197 24th July 2012 08:12 AM

Just a note that the THD measurements and screenshots in post #4 are just plain wrong! Looking at the amplitude makes it obvious I guess (well only in hindsight!) ... Freq response is right though a tonne of ideas and insipirations from the F6 thread (and leads from there as well) so I'll fire up the iron again tonight and see what happens !

padamiecki 24th July 2012 01:49 PM


Originally Posted by kasey197 (Post 3088213)
Output impedance was a disappointing 2.3R ... this is something I really hope to be able to reduce down to 0.5R or so if I can.

do not be disapointing, this is V to I converter amp, it has to have high output impedance.
I can bet that the sound is incredible, all you have to do is flatten with RC filters impedance peaks inside your speakers like this Project CS-8 - Current-Drive - The Natural Way of Loudspeaker Operation

kasey197 25th July 2012 02:09 AM

thanks for the encouragement and the pointers Pawel - I'll certainly take a look... Youre right though, that the high-ish output impedance is not really that a big a deal. The elephant in the room is fixing the input transformer driving :)

Nelson Pass 25th July 2012 03:02 AM

I think it's very cool. You can try using feedback to the primary like in
the F6 if you want to lower the output impedance.


kasey197 25th July 2012 07:38 AM

thanks np for the encouragement and will try that as part of my pre-F6 adventures :)

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