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Old 22nd June 2001, 12:44 PM   #11
ergo is offline ergo  Estonia
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One good place to look for information and ideas is

http://www.borbelyaudio.com

Mr. Borbely is a big fan of FETs and has many kits available online. I myself have not built any of these...
But if anyone has comments, it's alvays interesting to read them.
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Old 22nd June 2001, 02:09 PM   #12
ppl is offline ppl  United States
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Borberly is Right up their with John Curl, Walt Jung , and Nelson Pass just to name a few, as one of the Classic Audio designers. An interesting point about these guy's is the fact that thay all Eather worked for IC manufactures or have learned alot of their Knolage from IC Data Books. I Know i have learned alot from IC Data Books and APPlication Notes. I have been playing around with an All transistor Design using a Input stage topology like Borberly uses and is Full Complimentry as per Mark Levenson and John Curl. Uses Casscode Stages Throught inCluding the Driver and output stages. The Proto-Type has worked and sounded Great the First time. Most of my Audiophile Frends Thought the sound was way Better than my IC Designs and these same people thought my ic designs were some of the Best sounding Line stages thay have Heard. But thought the Prototype i just discribed had Better sound stage more Delineation of detail and A term i have not heared nor could thay explane to me what thay ment "Space". I thought that indeed while the Prototype Circuit sounded better The Differencewas not as Great as I expected it to be and the IC design fit on a board 1/4 the area of the Discreet design, only took a few days to Tweek. andworked rather as expected right after The Circuit was Powered up. The Discreet component circuit has been in the Tweeking stage for about 1 year.It also worked somewhat as expected at first except i had to lower the Open-Loop gain by using Larger Source Resistors on the Input stages, This cured an oscilation problem with some loads. So in conclusion I think that if you want the Last word in Sonic quality then A discreet component circuit is the way to go but expect it to be complex create Headaces and require alot of effort to get compleatly right. on the other hand if you Know what kind of sound you like and want to get the Circuit up and Running in a short time and have space limitations then A good IC circuit is hard to beat.
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Old 22nd June 2001, 02:17 PM   #13
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Jam,

Thanks for your tips on alleviating the leanness part. I think I'll try that out sometime. What I will do tonight is to get this little gain block to work off a proper power supply; actually, I ran it off a lower than intended 32 volt dual supply, the boards had no supply decoupling caps whatsoever. The reason is that I made these as modules, so that I could solder a few pins and stick the board into an IC socket for a direct comparison - IC in and next Discreet module in; this test was to be done in a crossover unit but I never got to actually doing this. The crossover board with the IC sockets would have contained the decoupling caps anyway, and if correctly rated, it was only a matter of switching over to the correct voltage in each case. Given the right supply and layout, I expect this to sound great.

As already mentioned, the only good comparisons I had at hand were the Bride of Zen, OPA604 and MAT02,03s. I cannot find better opamps in my country (India) and my only source would be RS Components.

The Electronic Workbench simulations show the following data: Differential pair run with a current source of 7mA. Second stage (MPSA92) run at 3.6mA and output stage run at 2.95mA. This is for configurations with a gain greater than unity. I found that for unity gain (Buffer) the current sources have almost got to be reversed in their order to obtain the best figures.

At a gain of 4 times, the frequency response is flat upto 200KHz. Second harmonic distortion is 0.00002% upto 5 KHz rising upto 0.000045% at 20KHz and still 0.0001% at 40KHz. Third harmonic distortion is 5 decimal places down all the way to 40KHz.

I know these are ideal figures, but the actual model seems very good too.

At a gain of ten, the frequency response remains the same. Second harmonic distortion is 0.0005% upto 5KHz and 0.001% at 20KHz. These figures look quite realistic although still ideal.

Just give me a hint as to how to upload circuit diagrams onto the web and I'll probably try and put this up. This is no claim to being the best or anything of that sort. But it is a good point to begin with, especially if the purse doesn't allow for more.

If one can try out the Low Memory Distortion type of discreet model, the results sonically could be astounding, compared to even the best 'Audio' ICs available.
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Old 22nd June 2001, 02:44 PM   #14
ppl is offline ppl  United States
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Samuel While maby unable to get Good Opamps in your area you arev truly blessed to be able to still get the MAT-02 & Mat-03's I have a supply of these left over from years ago but i have been unable to locate any hear in the US nowdays. The closest i have came to these is Linear Intergrated Systems LS-312 & LS-352 complimentry Low noise Monolithic Dual PNP & NPN. BTW These work great also. Please let us Know How your Comparrison works out.
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Old 22nd June 2001, 05:03 PM   #15
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ppl

I'm surprised that You're unable to find the MAT-02 and MAT-03 from Analog Device in the US, since they're still being manufactured!
In my own project I'm using the MAT-01 and MAT-03, since I have easy acces to them, and that's the reason for my surprise. Have You tried to find a dealer through http://www.analog.com ?
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Old 22nd June 2001, 06:39 PM   #16
ppl is offline ppl  United States
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Thanks, My AD rep sed thay were Discontinued. will check with AD.
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Old 22nd June 2001, 06:49 PM   #17
dorkus is offline dorkus  United States
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Default FET or bipolar diff inputs?

hmm so are most of you using bipolar devices for the differential input of your discrete inputs, or FETs? in general i thought FET inputs are more linear, at least in IC devices. has anyone tried hybrid design, i.e. FET inputs with bipolar gain/output stages?

P.S. anyone who needs a place on the web to upload their schematics, design info, etc., please let me know - i have a web server with plenty of space and am interested in eventually setting up a nice audio site w/DIY stuff. having different people's ideas, etc. in one place would be a great start - just email me at marcyun@dorkus.org.
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Old 22nd June 2001, 07:54 PM   #18
jam is offline jam  United States
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Default Fet vs. Bipolar

Marc,

The advantage the fet input device has is that the offset voltage can be very low, there is almost no gate current involved. Bi-polar input devices can have some offset due to changing base currents with different source impedences (example. volume control). So if the input on a bi-polar input device is not cap coupled the offset would change with changes of the volume control. (Since we all know there is no cap like no cap, why use one?)

A Fet input devices has the potential to have lower noise at impedences above 1k (source impedence).
I myself like fet input devices but still prefer discreet designs with mosfet inputs, even though there is a higher noise penelty. This should not be a problem unless you are designing a phono stage.

Jam

PS. Great idea about the web site, we could all pool our resources and probably come up with some nifty designs.

[Edited by jam on 06-22-2001 at 01:57 PM]
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Old 22nd June 2001, 08:12 PM   #19
dorkus is offline dorkus  United States
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cool. i definitely agree about the offset advantages of FET devices... so you prefer MOSFET to JFET? hmm wait isn't JFET a type of MOSFET? i forget the terminology... anyway, my mention of nonlinearity with bipolar input stages is in reference to walt jung's article on thermal distortions in opamps, but that probably applies more to monolithic IC devices where there is more thermal coupling between stages. also, the right amount of emitter degeneration on the diff devices can probably alleviate any possible nonlinearities with bipolars.

anyway, let's definitely pool information for an audio site. i'm a lead web developer at a dot-com firm (yes, we're still in business) so can help come up with a nice slick site.

marc
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Old 22nd June 2001, 08:30 PM   #20
jam is offline jam  United States
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Marc,

Well, jfets and mosfets are slightly different. I like mosfets because you can get some really low rds. devices and these seem to work better in low Z circuits.(Like bi-polars)
Bi-polars usually have a lot higher transconductance and usually some emitter degeneration helps (improves linearity).

Please let us know when you are ready, and I am sure the web site will be a big success.

Thanks,
Jam
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