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Old 23rd April 2014, 03:36 AM   #1
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Default Too simple MOSFET ESL amplifier

Is this too simple:

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sims very nice with the right bias
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Old 23rd April 2014, 10:29 PM   #2
jcx is offline jcx  United States
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too big mosfet - more Coss + Crss parasitic Q to drive to full Vout than most ES headphones

too little bias too 400V/200kOhm = 2 mA, which leads to

too slow with ~ 3 kHz pole, with

too short a feedback loop so the pole is open loop - directly rolls off the audio


there are a few sub 1 A high V mosfet being made, IXYS http://ixapps.ixys.com/DataSheet/DS9...-Y01N100D).pdf is a better fit with order of magnitude smaller parasitic C

Last edited by jcx; 23rd April 2014 at 10:56 PM.
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Old 24th April 2014, 05:52 PM   #3
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This started as a piezo driver project for a mechanical actuator, and morphed into an amplifier as I have a bunch of IRFPG40's left over from a HV power supply loosely based upon Horowitz and Hill. The OPA1632 is a nice chip for deriving the 2 out of phase signals.

The device you site is a depletion mosfet, so will require a bit more work --
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Old 24th April 2014, 06:38 PM   #4
jcx is offline jcx  United States
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depletion mode isn't really a big difference - just the Vgs DC component changes, but it does open the possibility of a self biased "srpp derived" modulating ccs

the opa1623 does look good - you can extend the SuSy feedback to the output with some compensation - my sims show a large SuSy "extra" distortion cancellation when the diff output is measured

ES headphones seem to use gnd referenced stator drive and bias V is applied to the membrane - I think this is motivated by safety concerns

so you usually see DC blocking output C with single supply ES headphone amp designs

and then you get the complaints that the C hurts the sound

if you like enhancement mode - another device I have on order: N-Channel Power MOSFET, 1500V, 0.1A, 150?, TO-220F-3FS
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Old 29th April 2014, 02:04 PM   #5
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Working on this with DN2540 depletion MOSFET as Q2, Q3
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Old 29th April 2014, 05:52 PM   #6
jcx is offline jcx  United States
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if you want to see what's being done in ES headphone amps its probably worth looking for Kevin Gilmore's pages, projects, posts - some found at Head-Fi.org, more at Do It Yourself - www.Head-Case.org

Class A ES headphone amps seem to typically run 5-10 mA bias current
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Old 8th May 2014, 05:57 PM   #7
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Hi,

just found this:
Click the image to open in full size.
1-kV Piezo Amplifier Keeps Cost, Noise Low | Components content from Electronic Design
Seems nice and easy to build, a simulation in LTSpice shows that almost any medium power, high voltage npn transistor can be used as well (commonly used in CRT screens...).
Just omit everything after R5 and duplicate the rest for BTL (invert one input, too). The CCS (Q2, R2 and the 9V battery) could be improved though...
Seems almost too simple Do I miss anything?

Best regards
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Old 12th May 2014, 04:21 PM   #8
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Quote:
Originally Posted by playOvation View Post
Do I miss anything?
I saw that design, or one similar in EDN -- you have to bias up the MOSFET driven by the OP27 to about 4 volts to get it to go.

As JCX points out, a depletion mosfet allows the possibility of self-biasing. In some experimenting, however, i can't get it to operate as "linearly" as I would like.
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Old 12th May 2014, 05:28 PM   #9
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Schematic in post #1 seems to run a common source amplifier + source follower buffer with no global negative feedback. So whatever lousy PSRR you get from R1+R2, is not reduced by the feedback factor. post#7 seems to eliminate this deficiency.

If it's challenging to make a low-Zout, low-noise, +15VDC supply, it's probably even harder to do the same at +400VDC.

Edit- whoops! If the headphone element is driven push-pull and if the two NMOS amps+buffers are very well matched, then injected supply noise is cancelled at the driver.

Vdriver = (LeftHandSide + SupplyNoise) - (RightHandSide + SupplyNoise) = LeftHandSide - RightHandSide.

Last edited by Mark Johnson; 12th May 2014 at 05:40 PM. Reason: differential drive
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Old 12th May 2014, 07:33 PM   #10
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Quote:
Originally Posted by jackinnj View Post
I saw that design, or one similar in EDN -- you have to bias up the MOSFET driven by the OP27 to about 4 volts to get it to go.

As JCX points out, a depletion mosfet allows the possibility of self-biasing. In some experimenting, however, i can't get it to operate as "linearly" as I would like.
If you mean Q1, there is absolutely no problem, just add an offset to the input signal of OP27.
Q2 is kind of a problem indeed. A depletion FET is one solution, another (more obtainable one) is a simple two-transistor-CCS.
My simulation looks fine, soon I will solder a prototype to test that circuit

Last edited by playOvation; 12th May 2014 at 07:41 PM.
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