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Old 7th April 2006, 10:40 AM   #1
MIKET is offline MIKET  United States
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Default solid state pentode

This all started with browsing a tube post that John Swenson stated that he and Gary Pimm were working on a circuit they call a "solid state pentode". http://www.diyaudio.com/forums/showt...450#post887450
I believe many will enjoy this work and look forward to hearing more about this.
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Old 8th April 2006, 08:07 AM   #2
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Here is the schematic for the first pass of the SSP preamp.

This is based on the self bias CCS from Gary Pimm. I had been using his CCSs for tube designs for awhile and was thinking about how they worked, its essentially a voltage reference and a follower that keeps that voltage constant across a fixed resistor, hence a CCS. With Gary's design you can vary the current by adjusting the voltage reference, I thought, well what would happen if you couple an audio signal onto that reference, you now have a CCS whose current is proportional to the audio signal, otherwise known as a transconductance amp!

Because Gary's CCS has such a low capacitance at the reference point its perfect for audio use, because of the way the cascoded CCS works the usual high and nonlinear capacitance of MOSFETs is negated.

In the circuit Q1 and Q2 are the main CCS, Q3 and Q4 form a low current CCS used to generate the voltage reference betwen Q1 and Q2. In this design I used 9V battery for the primary reference, but you can do it the way Gary does and use another resistor below R14 instead of the battery, but the input impedance is a little low that way, which is why I chose the battery for a first attempt.

Given the about 4V threshold on Q1 the voltage across R1 is 5V which means there is an idle current of 5mA. The transconductance is 1/R1 so in this case its 1000uS (micro Siemens) which with the anode resistor of 2.2K gives a voltage gain of 2.2. Notice that the voltage gain is R20/R1 no matter what the reference voltage is. One problem with this design is that the voltage gain also multiplies the voltage across R1, so in this case there is an 11V drop across R20, you have to take that into account when designing the power supply. If you go for more gain you also go for more voltage drop across R20.

The design sound better with more current, my present version runs at 30mA instead of 5mA, this sounds better but of course needs a lot more current. This also lets you go with a much lower output impedance. Say R1 = 150 and R20 = 330.

The input attenuator is a bit unusual, I wanted to have a balance control but didn't want a separate attenuator for that. What I came up with is a stepped shunt mode design, with the series resistor adjustable. When adjusting the balance the series resistor is decreased slightly in one channel but not the other, slightly changing the gain for one channel. This lets you have 2 db steps for the main volume control and .5 or 1 db steps for the balance control.

In subsequent tweaks I added a follower stage using almost exactly the same design, but without the battery or R20, the gate of Q1 gets connected directly to the drain of the first stage Q2 and the output is across the second stage R1. By using a low value for the second R1 it draws about 20mA which makes a VERY low distortion output stage that can drive very low impedance loads. It even makes a very good headphone amp!

This preamp sounds amazing, its very clear and open, incredibly smooth, yet highly detailed and lively

I also have a several power amp designs using this concept, but thats another post!

John S
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Old 11th April 2006, 10:29 PM   #3
MIKET is offline MIKET  United States
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Default PCB layout?

OK, I can breadboard the circuit, but would rather use a PCB if one has been designed. Checked Moser and the TO92 vesion of the LND150 are out of stock, I'll have to order some when they come back in stock.
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Old 12th April 2006, 06:52 AM   #4
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Gary Pimm has pc boards available, these are the "self bias CCS", these are small little boards (you can see pictures of them on his web site). I built my first 4 or so by hand on perf board, then gave up and bought a bunch of boards from him when I realized I was going to be using a lot of them. You'll have to talk to Gary about board layout files if you want to do your own using his layout. I've done several boards using the circuit but laying it out myself for some of the more complicated circuits where it was going to get a little expensive to have a whole bunch of his little boards.

The really only two things to look out for when laying out the board, the resistors connected to the gates are "gate stoppers" and need to be close to the gate, the other is that there can be significant voltages on these circuits, one of mine is running at over 100V (they are designed for 500V!) so make sure the traces have a little more than normal "minimum spacing" for the board process.

John S.
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Old 23rd April 2006, 10:22 AM   #5
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Hi John

Thanks you for sharing the schematic! I have a dumb question. I recently got a power unit with 4 heatsinks, each containing 6 of those Mosfets

75 amp, 55volt, 0.007 ohm, N-Channel ultra fets, TO-220AB

My idea is to breadboard your preamp using this mosfets, which are already nicely mounted and so on. I looked at the basic data, and would say it would work, if I don't go over the 36V from the schematic.

Someday I also want to try the other mosfets, but right now I am short on money and time, but would love to give this solid state pentode a try!

Thank you for the attention

Erik
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Old 23rd April 2006, 01:02 PM   #6
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Default MOSFET choices

ERik,
Q3 and Q4 can not be conventional MOSFET's, The part is a depletion mode MOSFET. The parts you hace that are "75 amp, 55volt, 0.007 ohm, N-Channel ultra fets, TO-220AB" would be better suited to a power amp or power supply. One important characteristic for Q1 and Q2 is low capacitance, the best parts for that application, the IRF 820 has an input capacitance of 360pF while a device just like you have is 3200pF, never mind the ultraFET's are meant to be driven by logic levels, not really intended for linear use. The good news is that the FET's stated in the design are under $1USD each so you don't need to be rich to buy them.
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Old 23rd April 2006, 01:30 PM   #7
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Hi Miket

Thank you very much for the reply! Actually I already thought that those mosfets would not be ideal, I even opened the datasheet of the LND and saw a diode within the symbol... But then, I am a complete noob on this.

I will spare some of those heatsinks to build regulated Power Supplies for cathode heaters, that seems to be a nice use!

Erik
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Old 23rd April 2006, 02:22 PM   #8
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Quote:
I will spare some of those heatsinks to build regulated Power Supplies for cathode heaters, that seems to be a nice use!
Or just go and build John Swenson's amp, and forget about heaters.
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Old 27th April 2006, 09:41 AM   #9
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Quote:
Originally posted by ErikdeBest


Or just go and build John Swenson's amp, and forget about heaters.

Any suggestion for the replacement of LND 150 ? I can't find it in local store .



Thanks,
Gede
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Old 27th April 2006, 10:35 AM   #10
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Default Where to buy LND 150

If you live outside the US follow yjis link for distributors in your part of the world.
http://www.supertex.com/contact_intl_distis.html
for North America this is the link
http://www.supertex.com/contact_n_amer_distis.html
I usually go to Mouser, www.mouser.com
Hope this helps
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