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ETI 477 article
ETI 477 article
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Old 17th February 2013, 01:36 PM   #101
David Tilbrook is offline David Tilbrook  Australia
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Join Date: Jan 2011
Location: NSW
Default Line amps

Originally Posted by owdeo View Post
Hi David,

Deliighted to see you on here. I'm another fan (probably of Suzy's generation) that grew up lusting after your amp designs. To this day I still think the 6000 is my favourite power amp and I too have some Renesas (Hitachi) 162/1058s stashed away and have been planning to do my own 6000 PCB layout as, while Suzy's version looks wonderful, I wanted to stick with the higher power and through-hole components. I must confess to really liking the sound of the 5000 too, though not as much...

The reason I'm writing is I still have the line stage of your UF 6000 preamp. I never really thought it was as good as the power amp, and not long ago I decided to see if I could work out why. I believe it is perhaps not as stable as it should be as it shows quite pronounced ringing on square waves. I found this both in simulation and measuring the actual preamp with a 'scope. I also discovered a track missing in one channel of the original PCB, which probably didn't help...

Anyway, I played around in spice with different forms of compensation and found the the best looking result by ditching the input compensation (1n8 + R) and instead slugging the next differential stage with a 68pF cap across C-B of each transistor of the diff pair. This seemed to tidy things up nicely in the real circuit, with no visible signs of overshoot or ringing on square waves.

Unfortunately this was very much trial and error on my part, I don't have the necessary skills and knowledge to really know if this method of compensation is optimum, but it does seem to sound quite a lot better now. I'd be stoked if you could have a quick look at my mods and see if you think this is really a good idea. I don't have the means to measure distortion, but it certianly looks very low in simulation. I've attached the simulation screen grabs and can of course send the files if you're interested. The waveforms look identical on the 'scope. Please don't feel in any way obliged though, you've probably got much better things to be doing.

Do you think it's still a topolgy worth playing with? Douglas Self and others seemed to have dismissed the differential VAS as flawed in their publications. I'm curious to know whether the FET preamp you mentioned is based around that sort of discrete opamp toplogy?

Cheers, and also many thanks for the inspiration,
Sorry to take so long to reply. I did not realize that this post had progressed.

I think you are on the right track with those mods, if you have observed any signs of instability, I.e. overshoot on square wave signals, etc. Even very small instability has enormously detrimental effects on amplifier sound.

I am surprised that there was instability, but one of the things that makes publishing designs for the DIY area difficult, is that the designer has no control over the type and quality of components used. Your mods would make the line stage a little more robust with respect to component variations.

I like the differential VAS. Roughly a year after developing the 6000 I designed a fully symmetric MOSFET 200w power amp which uses a fully symmetric differential VAS. I have used this amp as my personal reference ever since, and over the years have compared it to a large number of high - end commercial power amps. I think it beats most of them for overall sound quality, although there have been a few power amps which may have been superior in some facets. I think part of the reason is the symmetric differential VAS, which provides large voltage gains with low open-loop distortion.

I have not seen Doug Self's comments that you refer to... do you have any references I could look up?

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Old 18th February 2013, 07:23 AM   #102
owdeo is offline owdeo  Australia
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Join Date: Jun 2003
Location: Sydney
Hi Dave,

No worries, great to hear from you and thanks for having a look. I've played around with it some more since and have increased the input stage currents by a factor of 4 while keeping the degeneration constant (ie increasing emitter Rs in the LTP). I figured this would improve the noise figure as I'm using a lower impedance volume pot and no balance pot in front of it. I also replaced the small film coupling cap at the input with a 22uF bipolar electro. These two changes have further transformed the sound quality and it's now the best preamp I've built and sounds far better than anything else I can compare it to. The midrange has snapped into focus and the sound is transparent and musically involving. I'll post the updated schematic if anyone's interested.

Regarding your updated design following the 6000, would you care to share any more details? The 6000 had a fully symmetrical differential VAS too didn't it? Reason I ask is I still intend to redesign the 6000 PCB so I can build it up - I already have all the parts ready. But if your later design is an improvement I'd rather spend the time on that, though of course only if you feel inclined to put it out there (or perhaps publish it in Silicon Chip? You never know, they might!).

Self's dismissal of balanced VAS stages was originally published in Electronics World in the early 90's in his series on amplifier distortion and is included in his Audio Power Amplifier Design Handbook - I have the 4th edition and it's in Chapter 4 under "The Balanced VAS". There's not much to read really, his summary is "...all seem to be open to the objection that the vital balance of the input pair is not guaranteed, and that the current through the bias generator is not well defined." These comments may make more sense if you consider that his work focuses almost exclusively on the Lin topology as though it is the only way to design a good amplifier.

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