Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Headphone Systems Everything to do with Headphones

Class-A Mosfet Headphone Amplifier
Class-A Mosfet Headphone Amplifier
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 14th January 2016, 10:47 PM   #1
jhorapalok is offline jhorapalok  Bangladesh
diyAudio Member
 
Join Date: Nov 2014
Location: Dhaka, Bangladesh
Default Class-A Mosfet Headphone Amplifier

I like to build the following circuit for my ear phone which has 16 ohms impedance. I like to play music from my mobile phone or portable mp3 player. Do you think the circuit will suit the purpose?

DIY IRF610 MOSFET Class-A Headphone Amplifier Project
  Reply With Quote
Old 15th January 2016, 11:46 AM   #2
xnor is offline xnor
diyAudio Member
 
xnor's Avatar
 
Join Date: Oct 2010
For portable use? Not a good choice.
For 16 ohm and sensitive headphones? Again, not the best choice.
  Reply With Quote
Old 15th January 2016, 03:10 PM   #3
HotIce is offline HotIce  United States
diyAudio Member
 
Join Date: Jan 2013
Location: USA/CA
Does not amplify (actually the gain is <1).
A more interesting, yet very simple approach, is a Zen-ish amp like:

Click the image to open in full size.


Of course, component values will need to be recalculated, for am HP amp.
  Reply With Quote
Old 15th January 2016, 03:31 PM   #4
xnor is offline xnor
diyAudio Member
 
xnor's Avatar
 
Join Date: Oct 2010
Well I guess he wouldn't need amplification for possibly very sensitive 16 ohm headphones, just a buffer.
  Reply With Quote
Old 15th January 2016, 04:47 PM   #5
HotIce is offline HotIce  United States
diyAudio Member
 
Join Date: Jan 2013
Location: USA/CA
I guess, but given the comparable simplicity, something like the Zen, allows you to change the gain by altering the feedback resistor (the 17k one in the schematic above).
So if tomorrow buys a new HP with different Z characteristics, he can still use the same amp.
And it is still a class A, if that is what the OP aimed for.

PS: There are also available PCBs and kits on eBay, for a Zen HP amp.
  Reply With Quote
Old 15th January 2016, 09:18 PM   #6
sgrossklass is offline sgrossklass  Germany
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
On the topic of the circuit originally suggested:
Quote:
Originally Posted by jhorapalok View Post
A Szekeres loaded with an LM317-based CCS at 250 mA, why not. As mentioned, it's clearly something for mains operation at half an amp of current draw (and requires adequate heatsinking if you do the math).

There are a few things I would definitely change about this one though. Here's the problems I spotted, which I think are fixable:
1. They specify a 10-20 V regulated supply partly because PSRR (power supply rejection) is not good. Assuming the bias pot is set to midpoint and a low-impedance source on the input, it's on the order of only 20 dB - that supply would have to be pretty darn clean indeed when you are planning to use typical 16 ohm earphones. That's the fault of the unfiltered input DC bias and highish source resistance.
2. Speaking of highish source impedance, I am not sure why that 4.7k series resistor is included at all, with a 150 ohm gate stopper already being present (which may or may not get the job done on its own, but would be easy to increase if needed). There is no filtering that would use it. Like this, its only function is to pointlessly increase high-frequency distortion (you want to drive MOSFETs with as low a source impedance as possible to minimize the effect of their parasitic capacitance), and reduce PSRR.
3. The pot for DC biasing is pretty quick'n'dirty breadboard DIY level and a more elaborate scheme would be advisable for the finished product. Adjustment range actually does not need to extend below Vgs,MOSFET + Vdrop,LM317 or about 4 V + 2 V ~= 6 V. As you can see, a 10 V supply would leave a maximum of about 4 V for Vdg, on 20 V it would be 14 V, which would reduce input capacitance nonlinearity quite a lot - or give room for much-improved level handling. (Best distortion for this kind of amp tends to be obtained when bias voltage is dialed down as far as voltage drop and signal peak amplitude will allow.) 20 V times 250 mA times 2 is 10 W, of course, plus power supply losses.

I therefore propose:
1. Remove 4.7k resistor and replace by direct connection
2. Remove the second 100k resistor (going to ground at 1 F - pot wiper - 150R junction).
3. Between the pot wiper and 1 F - 150R junction, add the following RCR filter components to clean up bias voltage:
* 100k series R
* about 10 F (>= 25 V), going off to ground, polarity observing
* 100k series R
(one may have to go 2nd order but this should do)
4. Add a series resistor in the ground leg of the 100k adjustment pot. Depending on supply voltage to be used, this would range from about 150k @10V to 39k (43k) @20V. Compute as 100k * (6V / (V+ - 6 V)), with 6 V being the approximate minimum drop discussed above.

I would also use a bit more supply buffering than the single lonely 100n - current drawn is not, actually constant here (though relative variation would normally expected to be fairly small). Maybe an additional 22 F per channel, and 470-1000 F for both depending on your power supply.

Our RJM makes another sensible suggestion to further improve PSRR, namely having the MOSFET drain on the clean "ground" circuit node - even if that means you need a "Negative Nelly", err, negative voltage power supply if you want to stick with the n-channel MOSFET, otherwise you need a p-channel counterpart. Also observe the classic R/RC bias voltage filtering. I went for a slightly more complex variety because with a pot, filtering action would be heavily reduced or eliminated with the voltage turned all the way up.

To demonstrate the effect of this idea on PSRR, I re-reversed the "Reverso" (p-channel) version of the circuit, i.e. converted it from positive to negative supply so it's equivalent to a positive supply n-channel job. The result of both variations is attached. You can see that high-frequency PSRR is normally worsened by the effect of feedback capacitance (note the 1k input series resistor) when the FET drain is on the "dirty" power supply node. Towards the low frequencies we are also limited by maximum gain, being Av = gm * Rd = Id/Vt * Rd for a MOSFET amplifier. (An amplifier's gain makes a follower's PSRR.)

I am guessing there'll be more to be found on CCS-loaded Szekeres variations elsewhere.
Attached Images
File Type: png szekeres-reverso-psrr.png (97.0 KB, 763 views)

Last edited by sgrossklass; 15th January 2016 at 09:47 PM.
  Reply With Quote
Old 15th January 2016, 10:19 PM   #7
sgrossklass is offline sgrossklass  Germany
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
Oh, and did I mention that the pull-down resistor at the output is a good idea? Otherwise you'd get pretty massive pop noise when plugging on headphones after turning the unit on. The 68k chosen by RJM seems a bit high for a 680-1000 F coupling cap though - I'd go for <10k, down to 2.2k if need be.
  Reply With Quote
Old 16th January 2016, 08:50 PM   #8
jhorapalok is offline jhorapalok  Bangladesh
diyAudio Member
 
Join Date: Nov 2014
Location: Dhaka, Bangladesh
Thank you so much for your detailed discussion. I appreciate your suggestion.
  Reply With Quote

Reply


Class-A Mosfet Headphone AmplifierHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
MOSFET Headphone Amplifier FOXYE Headphone Systems 1 16th January 2013 06:00 PM
Simplistic Class A MOSFET Headphone w Feedback: have I got my values right? monty78pig Headphone Systems 11 6th November 2012 04:56 AM
Class A mosfet headphone amplifier. How does it look? Plecto Headphone Systems 1 6th November 2012 04:11 AM
my Class-A MOSFET headphone amp davygrvy Headphone Systems 28 18th October 2011 07:42 AM


New To Site? Need Help?

All times are GMT. The time now is 05:22 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki