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Old 19th April 2008, 09:04 AM   #1
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Default ecl86 preamp critique please

ok - help required at a pretty basic level.

I fell in love with poinz's mobius preamp - its elegant simple design (visually) and its easy to follow (sorta) electrical design. Way cool.

I had a similar presenting problem - a largeish MOSFET output power amplifier (NZ build McLaren 701) is lurking around my place and is underutilized cos it has no preamp. I'd built a set of three 12ax7/el84 SE amps for my kids recently and caught "the bug". So a tube pre is the only reasonable option...

Heres the rub - I tend to play out of my depth, so immediately I was drawn to all sorts of complex or esoteric solutions - no simple 12xx7 grounded cathode fix me me - no sir! Immediately it was clear - I had no idea what to do or how to do it!

Then recently I was led to valvewizard who, after the complexity of John Broskie's fantastic but waaaaay deep stuff or the once over lightly at boozhound (no disrepec' here - that site certainly helped understand the kids el84 amps), well the wiz is able to put over the right combo of maths, intuition and real ife application. For me.

Anyway, the wiz has a real simple explanation of how to apply feedback on a triode stage. Up until then it seemed a black science driven by incantations and chicken entrails. Not true - its simple maths - and the idea is just application of parallel circuits and ratios. Whoa.

And then the wiz describes cathode followers!!!! What a guy! Same approach. I have a new hero.

*Sits back, fans self with hat and recovers*

OK, so now I'm real excited. I have the parts (I think) and the knowledge (kinda) to produce the solution - a preamp using 2 ecl86 tubes, the triode sections providing amplification of the line signal with local feedback to tame any nasty stuff and to temper the triode's over-exuberant mu. THe output goes to the pentode sections configured as a cathode follower to give plenty of current drive for the butty mosfet power amp.

Known issues: Gotta lift the heaters to around +80v to get within the required heater-cathode rating.

Unknowns: How do I separate the HT DC from the feedback circuit? I guess a cap of some sort, but how to work it out?

I appreciate there are no values shown - I figure I may be scrapping the lot on your collective advice, so why bother with all that calculus?

Feel free to criticise liberally but not along the lines of "Oh but a ecl86 is sooooo passe, everyone is using the NOS Unobtainium Deluxe triodes hand-built by Ukranian virgins and available only by invitation..." Just the electrical design stuff please.
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File Type: pdf cathode follower preamp.pdf (10.9 KB, 557 views)
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Old 19th April 2008, 09:37 AM   #2
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Hello aardvarkash10
please note:
The LED voltage drop is non linear with its current.

Kind regards,
Darius
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Old 19th April 2008, 11:08 AM   #3
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aard, problem 1 is that your feedback level (and hence distortion spectrum) varies with volume control setting. This topology choice also forces you to run things through two more capacitors than necessary.

Although I've never been able to measure the effect of LED nonlinearity in an actual circuit (the dynamic impedance is so low that any changes with current are down in the noise), the 1mA or so that your first stage will run does put it on the high slope portion of the Vf/If curves. You can ameliorate that by running a resistor from B+ to the cathode, sized to run an extra 8 or 9 mA through the LED.

If you're ambitious, you can increase the feedback and remove any question of LED nonlinearity by using a CCS as the first stage plate load. That will also give you better power supply rejection.
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Old 19th April 2008, 01:35 PM   #4
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I think overall its a terrible approach to the preamp problem.
The ECL86 is an excellent valve in the right place, but the front end valve has much to much gain which forces you to use a lot of feedback to bring it down. Final result will be a dull sounding circuit little better than a simple opamp.
There are an infinity of better designs out there,

Shoog
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Old 19th April 2008, 04:11 PM   #5
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'Vark,

I'm in agreement with Shoog. The triode in the 6GW8/ECL86 is "identical" to those found in the 12AX7/ECC83. The mu of 100 is not well suited to line stage service.

Let's back up and start with the SS power amp that is to be driven. What's the drive voltage needed to achieve full power O/P? What is the I/P impedance of the SS power amp? Those numbers tell us what sort of circuitry is appropriate, in this particular situation.
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Old 19th April 2008, 09:35 PM   #6
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Default well, there you go...

THanks for that guys - back to plan 1 and developing triode curves for my 4 6CH6 tubes. Back to poinz'schematic, but nothing lost. I love this time consuming hobby!
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Old 20th April 2008, 08:27 AM   #7
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Quote:
Originally #3 posted by SY


Although I've never been able to measure the effect of LED nonlinearity in an actual circuit (the dynamic impedance is so low that any changes with current are down in the noise), the 1mA or so that your first stage will run does put it on the high slope portion of the Vf/If curves. You can ameliorate that by running a resistor from B+ to the cathode, sized to run an extra 8 or 9 mA through the LED.

Hello SY,
have a look at the datasheets of LED's.
You find forward current vs. voltage curves there.
From the curve you can calculate the dynamic impedance.
I always put a bypass cap in parallel to LEDs in
stabilizing circuits to get no "LED noise".

Kind regards,
Darius

edit: You are right, I tested it out, noise is not a problem.
thanks.
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Old 20th April 2008, 11:20 AM   #8
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FWIW,

why don't you just build the moebius? Its a very good sounding preamp with few parts, an easy to achieve PS and the tubes are cheap. Signal goes through 1 resistor and 1 cap IIRC. Very simple design with few parts, much simpler than an aikido too (of which I have 2 and think are really great).

Another option would be a 24V aikido for many of the above reasons.

What more do you want??

Fran
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Old 20th April 2008, 04:42 PM   #9
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Default Re: ecl86 preamp critique please

Quote:
Originally posted by aardvarkash10

Unknowns: How do I separate the HT DC from the feedback circuit? I guess a cap of some sort, but how to work it out?
R9 x C4 > R8 x C1

Darius
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Old 21st April 2008, 12:02 AM   #10
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aardvarkash10,

Bung this thru' babelfish to translate from german. It just shows that there are many ways of "skinning the cat".

A few variations based upon a 2000 Elektor article.
http://www.jogis-roehrenbude.de/Vers...r/ECL86-VV.htm

Cheers,
Ian
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