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The Melbourne Class A Headphone Amp and Pre-amp
The Melbourne Class A Headphone Amp and Pre-amp
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Old 8th December 2018, 01:29 PM   #1
xrk971 is online now xrk971  United States
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The Melbourne Class A Headphone Amp and Pre-amp
Default The Melbourne Class A Headphone Amp and Pre-amp

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):
Click the image to open in full size.

Here is the schematic after JPS64 laid it out:
Click the image to open in full size.

Here is a photo of the finished Melbourne module:
Click the image to open in full size.

Note that there is also an SMT based layout with TH caps, although I have not built and verified that version yet:
Click the image to open in full size.

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:
Click the image to open in full size.

Here is a photo of the setup getting a first listen with headphones:
Click the image to open in full size.

Here is a photo of the testing setup with alligator clips attached to the RCA jack outs for the headphone outputs:
Click the image to open in full size.

Here are some initial measurements 2Vpp into 2k2 ohm load:
Click the image to open in full size.

4Vpp into 2k2 ohm load:
Click the image to open in full size.

2Vpp into 240R load (representative of DT880-250's):
Click the image to open in full size.

4Vpp into 240R load:
Click the image to open in full size.

And even with only 45mA bias current, if one plays at reasonable volumes into 50R load, this is what you get for 1Vpp:
Click the image to open in full size.

And here is a more typical normal listening level for 1Vpp into 50R load:
Click the image to open in full size.

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:
Click the image to open in full size.
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Last edited by xrk971; 6th August 2019 at 09:18 PM.
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Old 8th December 2018, 01:42 PM   #2
avtech23 is offline avtech23  Australia
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The Melbourne Class A Headphone Amp and Pre-amp
Looks like a nice compact design. Good work!
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Old 8th December 2018, 01:50 PM   #3
xrk971 is online now xrk971  United States
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Thanks, Avtech! Are you by chance, anywhere close to Melbourne?
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Old 8th December 2018, 01:54 PM   #4
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So you say you use +/- 15.5V
That looks very reasonable for a Headphone Amplifier.
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Old 8th December 2018, 02:07 PM   #5
xrk971 is online now xrk971  United States
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Quote:
Originally Posted by lineup View Post
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:
Click the image to open in full size.

Click the image to open in full size.

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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Last edited by xrk971; 8th December 2018 at 02:31 PM.
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Old 8th December 2018, 02:18 PM   #6
avtech23 is offline avtech23  Australia
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The Melbourne Class A Headphone Amp and Pre-amp
Quote:
Originally Posted by xrk971 View Post
Thanks, Avtech! Are you by chance, anywhere close to Melbourne?
Not this week. I live in Adelaide but spend plenty of time bouncing around the country, so I do get to visit Melbs quite often.

When do you think a final board layout will be ready for ordering
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Old 8th December 2018, 02:27 PM   #7
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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.

The Melbourne Class A Headphone Amp and Pre-amp-melbourne-smt-th-pcbs-jpg

TH stuffing guide:
Click the image to open in full size.

Parts list:
https://www.diyaudio.com/forums/atta...th-list-v1-txt
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File Type: jpg Melbourne-SMT-TH-PCBs.jpg (569.8 KB, 1919 views)
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Last edited by xrk971; 8th December 2018 at 02:29 PM.
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Old 8th December 2018, 02:31 PM   #8
maty tinman is offline maty tinman  Spain
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Default 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

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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Last edited by maty tinman; 8th December 2018 at 02:41 PM.
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Old 8th December 2018, 02:37 PM   #9
xrk971 is online now xrk971  United States
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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):
Click the image to open in full size.



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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Last edited by xrk971; 8th December 2018 at 02:40 PM.
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Old 8th December 2018, 03:04 PM   #10
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This is truly insane. Class A on steroid.
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