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 Post subject: MooseFET Preamplifier
PostPosted: 16 May 2010, 01:22 
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MooseFET is a low gain, power MOSFET based line stage that fills the gap between a passive preamp and a standard line stage.

The design goals were:

- Lower than usual linestage gain.
- Be simple.
- Be reliable.
- Be rugged.
- Be easily portable.
- Not sound "solid state".
- Not need a complex power supply, typically required of single ended MOSFET stages.
- Be inexpensive to build.



The Circuit

Let's take a quick look at MooseFET's circuit:

Image

The first thing you might say is, "Isn't that a BoZ?". The answer is not quite, but it was definately inspired by the magic of Nelson Pass (who contributed to this circuit with insights and suggestions).

The MOSFET chosen is the IRF510. It's a 100V device that can be easily obtained, inexpensive and has a low gate capacitance. You can use any of the IRF*10 MOSFET's without circuit modification.

The gain of the linestage is (Rd/Rs)*.8, a rule of thumb that's held true for every MOSFET we've dropped into the circuit. In this case, a measured gain of 5.3 is the result.

Should one change either of the source or drain resistors to change gain or Idd, the 14.8V drain bias voltage will no longer hold true. In that case you will need to use an oscilloscope and adjust the bias until the clipping of the amp at overload is completely symmetrical. It will then deliver similar performance as shown in the circuit above.

The 14.8V bias Vdd will hold true for any IRF510 (though I did not try other manufacturers than International Rectifier) and 14.95V for IRF710 (I used Siliconix here).

Another difference compared to other SE MOSFET preamps is the relatively low supply voltage. This does limit it's total dynamic range to the area of 7V RMS output. Remember, we're not going for typical linestage amounts of gain here, so you will be OK in most circumstances.

Some very critical components are the ferrite bead on the gate lead and the 470 ohm series output resistor. The ferrite bead is a must. Maybe add two. As with most high-bias MOSFET's, they like to act as VHF oscillators. In this design, position of the gate snubber and zener were irrelevant - it was a ferrite bead or instability! Most likely due to the tiny gate on these transitors. The 470 ohm series resistor on the output does the same if high capacitance and/or long patch cables are used, which could turn MooseFET into a "tuned line RF oscillator". If you look at the schematics of well designed pro gear, they all have the series resistor on the output.

I used metal oxide resistors in the drain and on the source not only to handle power dissipation, they are also low noise and non inductive.

The bias resistors were chosen for a centre-pot (or near to) when properly adjusted.

The 100K on the output side of the capacitor bleeds off any DC that might accumulate. Though, turn on MooseFET before your power amplifier, lest the output capacitor charging "thump" takes out your speakers. Low leakage, high quality caps like the Nichicon "MUSE" or Elna "Silmic-II" series are reccommended here and are not expensive at all (I personally like the MUSE better :smoking: ).


The power supply is part of this devices simplicity - you can't beat a wallwart and a LM317.

The LM317 is a wonderful regulator, low noise and stable. Once set to 24.00V, it will stay put within 100mV from cold to being in operation for days on end.

I had modified a 12V wallwart as a doubler, but since have found several 30-35V @ 500mA wallwarts at the local thrift store for $2. They work just fine and any noise they generate, the LM317 takes care of.


Being a simple circuit, it can be made quite compact. Here's the prototype and notes. The perfboard was slightly smaller than a buisness card.

Image


And here is the final PCB version, which will fit into a Dynaco PAS3 (sans tone controls):

Image


Performance

Sonic impressions are pretty impressive. I brought MooseFET to the diyAudio Festival Victoria and had no bad comments. Matter of fact, when it was inline with the downstairs system (where the 2A3's, 45's and 300B's were being used), I was not permitted by the guests to move it out of the circuit upstairs

It sounds like a cross between a triode and a SS device, yet both and neither.

Bah, rather than explain it, here's some FFT graphs of her output.

First is the spectrum of normal operation. The relation of fundemental and H2 remain constant, regardless on uncompressed volume level:

Image

The H2 is the reason for her "musical" attributes and the triode-like sonics.


As we go up in volume to compression, the higher order harmonics appear, but still in an orderly, decaying fashion:

Image


And finally, as MooseFET is starting to REALLY get annoyed:

Image


Conclusion

MooseFET is one of those little linestages you can use when you need a little gain, but not a lot. Easily toss into your suitcase and bring to audio festivals or friends houses and demonstrate that improving sound to most systems doesn't mean a 45Kg piece of equipment, or a second mortgage.

So far, it has lived up to all the design goals and it filled another that wasn't even on the original list of targets:

- To be fun.


Cheers!

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PostPosted: 17 May 2010, 22:15 
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Oh I just love the Canuck inspired name and logo! ;)

Suggestion, score / perforate the pcb so the power supply section could optionally be separated so that it can be located further away.

Link to Geek's build: http://geek.scorpiorising.ca/moosefet.html
Link to the PCBs: http://classicvalve.ca/tubepcb-1.html#MOOSE
Link to the PCB documentation: http://classicvalve.ca/docs/MooseFET_docs_v0.pdf

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PostPosted: 18 May 2010, 02:56 
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Thanks Gio!

Thanks also for adding "preamplifier" to the heading... by the time I thought of it, it had "timed out" for me to edit the post :blush:

To seperate the power supply would mess up the PAS case fitting. But I'll suggest leaving the power supply off-board to my PCB guy.

Cheers!

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* Ratings are for transistors - tubes have guidelines*
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PostPosted: 18 May 2010, 21:29 
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Or you could add a grounded PCB as shielding from the PS. May not be required at all but another inexpensive idea.

Image
from: http://diyaudioprojects.com/Tubes/6418- ... reamp-Kit/

In the end Mark had to remove the rectifier bridge from the chassis as it was adding noise.

Cheers

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PostPosted: 09 Jun 2010, 18:11 
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That is a very nice preamp circuit you've got there Geek, very simple indeed, so simple that I made my own using whatever parts I could find on hand. So far it sounds pretty darn good! :up:

I made a prototype class "A" amplifier a while back using a very similar singe mosfet circuit, and I loved the sound of it, but did not have time to construct the full amplifier. So far this circuit seems to give the same results as that amplifier, without the heat.

It doesn't look anything like the neat PCB version (it is done on experimental board like your first one). I can't believe what I am hearing! I am using my ipod and my 25EH5 amplifier with it's preamp because this one does not have enough gain to drive the amp stage alone..... but it sounds better using the mosfet preamp than just the tube preamp, it sounds smoother and detail seems enhanced! Or maybe it just seems that way because I didn't have to fix any mistakes, it just worked right from the start.

I am only running 20V and have the bias set to around 14.9 right now, the test power supply is using an LM317 out of coincidence, well it is my favorite voltage regulator. Most of the parts are reused, I have 270 & 220 ohm resistors to get 120 ohms, and the 18 ohm is a 39 & 33 ohm, (I could not find the right values on hand so I made them).

I found a pair of new NTE139A's for the 9.1V zeners, and the mosfet is IRF610A. I used two ferrite's before the Gate of the mosfet, but what likely made a big difference in terms of sound is the 1.5uF film capacitor and 4700pF silver mica along side the 47uF capacitor at the output. Try it and you may be suprised the improvement a film cap can make along side even the best electrolytic capacitor.

I will continue with the testing and Happy bulding!

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PostPosted: 09 Jun 2010, 20:37 
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Great to hear! :)

Bias point for other Vdd's is easy to adjust on a scope.... bring it to compression and adjust for symmetry. That also guarantees the FFT response.

Cheers!

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* Ratings are for transistors - tubes have guidelines*
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PostPosted: 16 Jun 2010, 09:45 
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Geek wrote:
the ferrite bead on the gate lead

Hi Geek.

Where on the PCB you placed the ferrite beads? I can't see them in the PCB picture yu posted.

c-J

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PostPosted: 16 Jun 2010, 20:51 
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cheap-Jack wrote:
Geek wrote:
the ferrite bead on the gate lead

Where on the PCB you placed the ferrite beads? I can't see them in the PCB picture yu posted.

I was asking myself the same question, I'm guessing it is there some where, I used two on mine with no problems what so ever so it doesn't hurt to use more than one.

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PostPosted: 17 Jun 2010, 00:10 
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Sorry, missed this question :(

They are on the gate lead.

Cheers!

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* Ratings are for transistors - tubes have guidelines*
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PostPosted: 17 Jun 2010, 10:00 
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Geek wrote:
They are on the gate lead

Hi Geek.

What type/model ferrite bead you used? They must be very small size as I can hardly see them in the photo of the PCB. Does it matter whatever ferrite bead we choose? Want to post us the picture of such ferrite beads you used?

How about those larger ferrite RFI suppressor ring we use on cable cords?

c-J

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