The Melbourne Class A Headphone Amp and Pre-amp

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Founder of XSA-Labs
Joined 2012
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This amp is a spin-off of the Aksa Lender pre-amp, but now modified for DC-coupled output capability plus a revised mu-follower output stage for improved distortion performance. The Melbourne name was given as a designation for use with the Mark Johnson Pass M2X amplifier, where there are modular input stages named after cities of their origin (Mountain View, Tucson, Austin, Ishikawa, etc). As this input stage would be using Hugh Dean's Aksa-Lender topology, it seemed natural to name it The Melbourne. My goal was to make a modular self-contained input stage for folks who have the M2X amplifier to be able to try out this topology. The Aksa Lender preamp was a single rail design (+48vdc), but to use on the M2X required a dual rail (+/-24vdc) supply. So I redesigned it on LTSpice to be dual rail, and in the process, made a few modifications for improved performance, plus the ability to use the M2X's native power supplies (which, may not be the usual ultra-low ripple supplies that I normally specify for a SE Class A amp, and assuming it comes from a typical linear trafo with a CRC with circa tens of mV of ripple. As I was refining the design on LTSpice, Paul Bysouth gave a suggestion to use a mu-follower on the output. A mu-follower takes for example, the 39R source resistor from the CCS device (DN2540) and splits it in a 1:2 ratio with 12R:27R resistors and placing the output node in between the two resistors and the CCS gate feedback after the bottom resistor. This proved to be quite effective at reducing the THD by about 4x - a significant improvement. This allowed the pre-amp stage to easily drive 600ohm load requirement that is the Edcor auto-former load on the M2X. In doing this, I played around with putting lower impedance loads on the pre-amp. For fun, I connected my DT880-250's to the output and listened. First thing to notice was an absolutely quiet background, no hiss or hum even with volume up but source off. When I applied music source, I was shocked how good it sounded. Natural, dynamic, clean and no fatigue. I expected high distortion since the 250ohm load presented was much higher than the simulated 600ohm design load. The bias current is running at only 45mA and voltage rails for the flat wooden board mockup was only +/15.5v provided through a Mark Johnson designed BJT cap multiplier (layout by Prasi), and power from an Antek 25w 15v toroidal transformer (what I had on hand).

Here is the schematic on LTspice (ignore the series resistor on PSU rails - will be replaced with CRCRC):
717453d1542995688-diyaudio-watt-m2x-aksa-lender-m2xdb-melbourne-schematic-mu-follower-v1-3-png


Here is the schematic after JPS64 laid it out:
719786d1543880473-diyaudio-watt-m2x-melbourne-th-v1-schematic-jpg


Here is a photo of the finished Melbourne module:
720059d1544004769-diyaudio-watt-m2x-mb-th-verify-build-02-jpg


Note that there is also an SMT based layout with TH caps, although I have not built and verified that version yet:
717426d1542988871-aksa-lender-pass-hybrid-m2-alph-m2-amp-mel-smd-pba-bot-v2-png


Here is a photo of the module in the mockup amp with cap multiplier, input and output RCA's and volume pot (10kohm Alps RK09 series). Careful attention to star grounding (note green wires) with star hub located at the (10R/diode bridge/100nF) ground loop breaker connected to earth ground:
720432d1544167446-jumas-easy-peasy-capacitance-multiplier-mb-th-verify-build-04-standalone-pre-jpg


Here is a photo of the setup getting a first listen with headphones:
720693d1544279377-melbourne-class-headphone-amp-pre-amp-melbourne-standalone-preamp-hpa-02-jpg


Here is a photo of the testing setup with alligator clips attached to the RCA jack outs for the headphone outputs:
720692d1544279377-melbourne-class-headphone-amp-pre-amp-melbourne-standalone-preamp-hpa-closeup-04-jpg


Here are some initial measurements 2Vpp into 2k2 ohm load:
720694d1544279377-melbourne-class-headphone-amp-pre-amp-melbourne-th-2vpp-2k2-45ma-fft-01-png


4Vpp into 2k2 ohm load:
720695d1544279377-melbourne-class-headphone-amp-pre-amp-melbourne-th-4vpp-2k2-45ma-fft-02-png


2Vpp into 240R load (representative of DT880-250's):
720696d1544279377-melbourne-class-headphone-amp-pre-amp-melbourne-th-2vpp-240r-45ma-fft-05-png


4Vpp into 240R load:
720697d1544279377-melbourne-class-headphone-amp-pre-amp-melbourne-th-4vpp-240r-45ma-fft-06-png


And even with only 45mA bias current, if one plays at reasonable volumes into 50R load, this is what you get for 1Vpp:
720698d1544279377-melbourne-class-headphone-amp-pre-amp-melbourne-th-1vpp-50r-45ma-fft-08-png


And here is a more typical normal listening level for 1Vpp into 50R load:
720699d1544279377-melbourne-class-headphone-amp-pre-amp-melbourne-th-2vpp-50r-45ma-fft-07-png


As you can see, the harmonic profile is always classic SE Class A with 2nd order dominant harmonic and decreasing 3rd order and higher harmonics. With a ratio of -17dB for H3/H2, the sound is very euphonic and the right balance of sweetness and articulation, and the absence of higher orders makes it non-fatiguing to listen to.

I would like to acknowledge the help and inspiration of fellow DIY'ers AKSA, JPS64, Mark Johnson, Prasi, and Paul Bysouth for this design. Without your help, passion, and energy, this would not be possible. Thank you!

Update Aug 6, 2019: yourownbuzz made a nice headphone amp with Thai:
772524d1564896110-melbourne-class-headphone-amp-pre-amp-top-jpg
 

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Founder of XSA-Labs
Joined 2012
Paid Member
So you say you use +/- 15.5V
That looks very reasonable for a Headphone Amplifier.

Yes, the cap Mx board has pots for adjusting the output voltage drop. The Aksa Lender topology can actually benefit from an asymmetric rail (larger -ve vs +ve) as it clips asymmetrically from the bottom first. So judicious selection of a power trafo for headroom and adjusting voltages at say +18v and -21v will give significantly higher output power due to added negative clip headroom.

Gerbers and schematic for Cap Mx here:
Juma's Easy-Peasy Capacitance Multiplier

Photo of cap Mx:
720207d1544046993-jumas-easy-peasy-capacitance-multiplier-smt-cap-mx-build-01-jpg


720208d1544046993-jumas-easy-peasy-capacitance-multiplier-smt-cap-mx-build-02-jpg


This preamp will run happily with +24v and -27v rails for instance and that will give some folks who have low sensitivity (82dB/mW) planars the chance to drive some more SPL. Heatsinking will need to be beefed up and use of 2SA1837 vs the KSA1381 will be mandatory for high bias and high voltage.
 
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Founder of XSA-Labs
Joined 2012
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The modules are available now, however, these units have an error that prevents you from slapping them onto the M2X (incorrect pinouts for M2X main board as pins 1 & 3 are reversed - my biff). They work perfectly fine as standalone preamps or headphone amps. Send me a PM if you are interested in ordering. I may have to start a GB thread as this is the general DIY section and should not do GB's from here. If you have no intention of using on M2X, these are fine.

attachment.php


TH stuffing guide:
719787d1543880473-diyaudio-watt-m2x-melbourne-th-v1-stuffing-guide-jpg


Parts list:
https://www.diyaudio.com/forums/att...73-diyaudio-watt-m2x-melbourne-th-list-v1-txt
 

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SS Buffer based on the Aksa preamp Lender or the spin off, Melbourne

Very interesting, again. One question: what breakfast? I ask for your creative hyperactivity, surprising. I want to take it too :spin:

I have had an idea for weeks, before to know the new Nelson Pass gadget (H2 generator). A really good SS buffer based on the Aksa preamp Lender or the new Melbourne, with similar harmonic profile. With the H2 level easily adjustable within a range to be able to listen to all kind of music. If you listen to jazz or small acoustic group, H2 at 70-75 dB. Orchestral, electronic... maybe with H2 at 85-95 dB. A jumper?

H2 and H3, nothing else.

Without the noise and distorsion of a tube. And with very clean power, with low pick at 50/60Hz.

Clean DAC with good/very good digital preout -> optional SS buffer H2 adjustable -> H2 preamp or clean poweramp.

****** ******* ********

Like DAC, my choice is easy: RME ADI-2 DAC. I have been appreciating the ADC of that company like a tool to make very good vinyls rips for years.

In Europe, RME ADC / DAC with AKM chips is almost a standard in many recording studios, I think.


Note: have you received my last mail? Hugh had the mailbox full.
 
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Founder of XSA-Labs
Joined 2012
Paid Member
Hi Matty,
Nice to hear from you... I have a strong cup of coffee with milk for breakfast at 6am and eat a pre-lunch snack of granola cereal with "trailmix" (peanuts, raisins, M&M candy, chocolate) and a yogurt with a second coffee. Sometimes, I indulge in a ramen noodles for the snack. But what I did this morning for this amp was just after only coffee... :)

You can adjust the emitter resistors R5/R6 for balancing H2/H3 (larger is more H2 and lower is less):
717453d1542995688-diyaudio-watt-m2x-aksa-lender-m2xdb-melbourne-schematic-mu-follower-v1-3-png




Changing feedback resistors R10/R11 to metal oxide will increase relative H3, I am using carbon film on R10 for best ratio of H2 to H3. Metal thin film here is less H3 than metal oxide thick film, but perhaps may sound a bit harsh compared to carbon film.
 
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Founder of XSA-Labs
Joined 2012
Paid Member
I just removed the output coupling caps and had a listen. Wow, it really opened up the sound and the overall visceral impact has gone up several notches. Bass extension appears to go down to infrasonic now (tested by listening to “Lullaby” by a Perfect Circle), digs really deep. The drums on “Texas” by Natalie Merchant sound deeper than I have heard them before. Very nice. Amazing for 45mA bias current driving 50ohm OB-1’s. Articulation of high end is super. Very enjoyable listening experience.
 
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Well, if you want to do a VERY HARD TEST to your system:

Gregorio Paniagua, Atrium Musicae de Madrid - La Folia de la Spagna (2003), SACD, Harmonia mundi, Germany

Now Playing + What are you listening to?

I have other interpretation, in SACD too: Jordi Savall - La Folia 1490-1701 (2003)

And the 2015 vinyl reprise from AudioNautes: La Spagna - Atrium Musicae De Madrid - Gregorio Paniagua (1980)

Hi, I am maty and I am an Spanish audiophile man :)

But of those who think and inform themselves before making decisions, never impulsive!
 
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Founder of XSA-Labs
Joined 2012
Paid Member
Yes, use 2SA1837 instead of KSA1381. Increase bias current to 150mA and drop voltage to +12v and -15v rails. That’s about 2w dissipation per output device. They will need good bigger heatsinks. Will need to adjust source resistors. Right now 39R total gives 45mA. 3x increase in bias current suggests 12R total or 4R and 8R on mu-follower. I would have to run sim to see how much power you could run before onset of clipping. It’s easy to use flying leads to the output devices mounted on main Al floor panel of amp case. 8w total dissipation for both channels is mageable on non-finned 2-3mm thick Alum floor plate if about 12in deep by 6in wide.
 
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Nice compact boards, this could be my next project. :)
regards,
marjohn


The modules are available now, however, these units have an error that prevents you from slapping them onto the M2X (incorrect pinouts for M2X main board as pins 1 & 3 are reversed - my biff). They work perfectly fine as standalone preamps or headphone amps. Send me a PM if you are interested in ordering. I may have to start a GB thread as this is the general DIY section and should not do GB's from here. If you have no intention of using on M2X, these are fine.

720727d1544282856-melbourne-class-headphone-amp-pre-amp-melbourne-smt-th-pcbs-jpg


TH stuffing guide:
719787d1543880473-diyaudio-watt-m2x-melbourne-th-v1-stuffing-guide-jpg


Parts list:
https://www.diyaudio.com/forums/att...73-diyaudio-watt-m2x-melbourne-th-list-v1-txt
 
The bias current is running at only 45mA and voltage rails for the flat wooden board mockup was only +/15.5v provided through a Mark Johnson designed BJT cap multiplier (layout by Prasi), and power from an Antek 25w 15v toroidal transformer (what I had on hand).

I'm confused. You state the cap multiplier is a Mark Johnson design yet when I click on your link I see it being called a "Juma" design.
Which is it?

Also, I'm not really into SMDs. So what readily available through-hole transistors can be used for A1 and T1 in the cap. multiplier?

Thanks.
 
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