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PostPosted: 02 Dec 2010, 23:11 
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Joined: 15 Dec 2009, 20:49
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Hello all,

Mr. Gregg van der Sluys (a.k.a. Geek) has presented an interesting and compelling case for a Triode & MOSFET hybrid topology that creates a high-gain, high input impedance, high PSRR, low output impedance preamplifier stage, that he presents at following links:

1) http://geek.scorpiorising.ca/hybridmu.html

2) http://www.diyaudio.com/forums/blogs/ge ... lower.html

3) http://www.diyaudio.com/forums/blogs/ge ... lower.html

While I have a ton of questions about all of what Geek has presented, I'll start with just one question: What would be an appropriate power supply circuit for this topology?

Thanks & best regards,

Obe1


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PostPosted: 03 Dec 2010, 04:11 
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Hi,

I made a phono stage with it, so I went regulated for the heck of it. But it still had good PSRR without a reg. due to the fixed gate bias (like a regulated screen in a pentode)

I haven't really gone too much further with the experiments because of work commissions.

Cheers!

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
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PostPosted: 03 Dec 2010, 14:55 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, I have seen the references. Good write ups. On the PS question, any good one will do, however I would go for the regulated one. Try a LR8N as the regulator. Be sure to download the fact sheets on it. Buy a few extras as well as they can be fried fairly easily (any IC that can regulate at 450 volts is subject to damage). The advantage of using the LR8Ns is improved filtering in addition to regulation. I suggest you operate them at 1/2 or less of the dissipation ratings. If they get too warm, they shut down and you need to completely remove the power before they will restart. An alternative is to use a pair of 0B2s (gas regulators) in series for about 200-210 B+. They have a neat orange glow :pumpkin: and right now are pretty cheap.

Good listening
Bruce

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PostPosted: 03 Dec 2010, 19:16 
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Here's a soft-start for the follower.

If any one is interested, the power supply and final phono stage :D


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-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 03 Dec 2010, 22:24 
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Thank you Geek & Bruce for all the good information.


===
:cop: EDIT: 7 Dec 2010
Posts about LR8 regulators moved to: LR8 High Voltage Regulator thread.


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PostPosted: 14 Aug 2011, 09:38 
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Dear Mr Gregg,

I read with enthusiasm about your article on hybrid mu-follower and made a prototype for my amp.

I like the idea of having a valve amplification stage with a low output impedance which can be obtain, with an easy way, by using this method.

I built the prototype using ECC82 and IRF740 as active components, and set the bias around 5~6mA. I also run the simulation prior to the realization of the prototype.

The circuit works flawlessly during the simulation. However after I realized and started to measure the circuit, it turned out that some of the operating voltages were totally out of range:

(please refer to the attachment)

- level 2 is fixed at 150V with zener diodes;
- level 1 & level 3, 100V was measured (there is a 50V voltage drop across 4.7Mohm resistor);
- level 14, 149V was measured;
- level 8, 145V was measured.
Attachment:
mu_stage.jpg

How could this happened that Vgs is negative? What could be wrong?

Could you please kindly give some comment? thank you.


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PostPosted: 14 Aug 2011, 17:36 
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Hello wmincy,

If you took your measurements with a Digital Multi-meter (DMM), the DMM may not have a high enough input impedance and as such the DMM places a load on high impedance circuits like position "1" on your schematic. Other than very expensive DMMs, a typical DMM may only have an input impedance of 10M Ohms. If you were to add a 10M Ohm resistor from position "1" on your schematic to position "0" (ground), Ohms law tells you that the voltage at position "1" would be 102 Volts. :shock:

Best regards,

Obe1


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PostPosted: 15 Aug 2011, 01:26 
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Location: Chilliwack, BC
These do not Spice well vs. the real world.

The only Spice I've used that was reasonably accurate for DC voltages, is the el-cheapo Circuit Maker. Probably because the tube models (mostly made by users) were made to simulate the real world.

Cheers!

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 15 Aug 2011, 09:01 
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Thanks OBE1 and Gregg,

I rely too much on the reading of DMM instead of understanding its behavior (ohm's law)... too bad for me...

I checked again and this time I measure the right points and the reading of Vgs is around 2.5V. Not the way I obtained previously by subtracting V1 from V14... ah, what a mistake...

Thanks again :thumbsup: !!


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PostPosted: 16 Aug 2011, 22:06 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Several good lessons for all diyers to heed in the previous few posts. First while I certainly do use CAD in the early stages of a design / project, I use it only to get into the approximate ranges I want. Everything after that is try, test, measure, then refine the circuit. Spice works great for many things, but with the wide tolerance variations and general oddities of tubes, it is in my opinion only a good guess at what will happen. Inadequate tube models are likely a major cause of this. Second, lots of time we overlook the obvious (I certainly have on occasion). Measuring something changes it to some degree, In the circuits in this thread - a great deal in one instance. I personally am very cautious with measurements. What you get for a value is not necessarily what is really happening. I make extensive use of spectrum analyzers and distortion meters. Both can give bizarre answers. Noise or hum or poor impedance matching ( like with the DMM) can result in data that has no bearing on the circuit performance. My mantra in this case is ...if the circuit does what it is supposed to do, then most likely the measurements are invalid. (sorry Gio a bit OT, but this is important stuff for diyers). now off :soapbox:

Good listening
Bruce

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