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PA-6 Front-End Circuit Boards for Hafler DH-200/220/500 and XL-280/600. We do custom installations in some other MOSFET amps.


What is available: While our new PA-6 raises the bar in every aspect of performance the PA-5 V2 is still a fine sounding modification. For now the line-up will still have the PA-5 V2 as the introductory model and the PA-6 will be used for the Elite level models and above. During the transition we may have some PA-5 V2 Elite models at a discount!


The Design: John Hillig, our designer, worked a very long time to achieve enhanced clarity without brightness or ‘etching’ of the upper octaves. Finally he chipped away and a new level of excellence emerged. Design simplicity remains a trademark for Musical Concepts. The PA-6 is an enhanced, effective tweaked-by-ear amplifier circuit with a single-ended differential amp front-end called a LTP(long tail pair). The LTP pair are PNP transistors unlike all our previous boards which used an NPN architecture. A dual transistor circuit relying on transistor Vbe characteristics for reference voltage is used for the current source. This type bias source is also used for the VAS driver stage. It is a completely separate current source which does not rely on the first stage for its reference voltage. An enhanced front-end power supply design, called CRC, inherited from PA-5 series with four front-end power supply caps provides a much cleaner, quieter voltage for the critical front-end circuitry. Because MOSFETs are very easy to drive a traditional final driver stage can use high-performance, modest current video transistors. Also since MOSFETS don’t experience ‘thermal runaway’ there is no need for volt-amp limiting circuitry further simplifying the design and enhancing clarity.


PA-6! What’s important to your listening?





The PA-6 Sound: This is not a mere Hafler, not to demean this great basic amp design! No, this is a great amplifier in a Hafler chassis. You get a sense of a clean, clear response to the most intense transients/crescendos. The sense of dynamic range, bass solidity and yet majesty is striking. Overall delicacy is remarkable. The high frequency extension has a lucidity, liquidity and naturalness that makes you think you are listening to a vacuum tube amp. Then along comes bass detail and overall clarity that makes you reconsider that thought. The word fidelity, a really big word, is more than a marketing term. There is a connection to the instrument in a way that is striking. Why? Because you are hearing so much more of the true tonal structure and timbral clarity of each instrument. Voices are so clear, full-bodied and intelligible that emotion is conveyed in the performances at a new level. Many will hear new background vocals singing along with the lead vocal. There is a sense of gut solidity, simple majesty and air-moving power that must be heard. If your speakers play deep you’ll be thrilled. PA-6 modified Hafler amps will easily compare to the finest sounding amplifiers available today.


PA-6 Elite Modification


Superb high-end audio grade caps are used in the feedback loop and front-end power supply positions on the PA-6 Elite for higher resolution, crystal-clear frequency extension, clarity and just plain naturalness. PA-6 Elite has great bass, quick powerful transients, smoothness, natural warmth and great 3D imaging. That same word naturalness just keeps coming up. The PA-6 is available only in Elite model range.

PA-6 Elite: $499 kit*, $649.00 installed



PA-6 Elite Plus Modification

You not only want superior sound but extended reliability and confidence that you’re getting the best from your power amplifier. You want all this at a price that is unmatched considering the top quality of parts and sound. This more complete modification includes:



You will receive a PS-200/500 specialized for your amp, please specify your amp model when you order. See more PS-200/500 versions here, some of these items on Exotic Parts page.


Our latest PA-6 Elite Plus creates a truly a superb amp that demands comparison to any amp available!

PA-6 Elite Plus for 200 series amps: $829kit*, $1099.00 installed  >>>>>>>For PS-200J luxury version, with Jensen 4-pole caps, add $200 to either price.


PA-6 Elite Plus for DH500/XL600 amps: $879kit*, $1149.00 installed  >>>>>>>For PS-200J luxury version, with Jensen 4-pole caps, add $150 to either price. A second PS-200J can be fitted to the DH-500/XL-600. For a second PS-200J add $399/kit, $529/installed.




PA-6 Elite Ultra Modification

Our PA-6 Elite Ultra mod is truly State of the Art and beyond State of the Art as it is practiced by so-called high-end manufacturers. Although we at Musical Concepts have modified many expensive well known amplifiers we've never seen build quality even remotely close to the Ultra. The superb Takman resistor is used in nearly all slots for all Elite Ultra driver boards. For all amps the Elite Ultra also includes a new high in-rush current, silver-alloy power switch and a Rhodium-plated IEC power cord socket with hi-current cord. Just imagine what this incomparable product might sell for if it had some fancy over-hyped manufacturers name on the front panel. This is the finest mod we have to offer.


Depending on your original amp there are some differences in the power supplies.


The DH-200/220 and XL-280 has our TP-202 multi-shielded toroid power transformer. The main power capacitor supply is our PS-200J with IXYS HyperFred rectifiers and four high-resolution premium caps which are the finest you can buy for power supply applications.


The DH-500 and XL-600 have twin 500W toroid power transformers and twin PS-200J boards with Jensen premium/exotic caps - eight of them in total. This power supply operates with totally dual-mono performance. 


PA-6 Elite Ultra: $2495 installed only for DH-200/220 & XL-280, $2995 for DH-500, $3195 FOR XL-600 





PA-5 V2 Elite Modification


Superb high-end audio grade caps are used in the feedback loop and front-end power supply positions on the PA-5 Elite for higher resolution, crystal-clear frequency extension, clarity and just plain naturalness. PA-5 V2 Elite has great bass, quick powerful transients, smoothness, natural warmth and great 3D imaging. That same word naturalness just keeps coming up. The PA-5 is available only in Elite model range.

PA-5 Elite: $349 kit*, $499.00 installed –you may sub the PA-5 V2 boards into any of the above PA-6 versions at a -$150 reduction.




PA-4 Series Driver Boards


PA-4 Series: for Hafler DH-200/220/500, XL-280/600

The Design: See PA-5 V2 description but PA-4 lacks the current mirror, special Asian transistors, CRC power supply.


The PA-4 Sound: See PA-5 Sound comments. While is is not quite as clear it has a warm, rich quality which has thrilled customers who know their stuff.


New! PA-4SE: Our PA-4 mod series has been further up-scaled in quality level of individual parts - prices rose. We wanted to deliver the excitement of excellent sound to a more affordable price. The PA-4SE, still a very hi-quality product, is our answer. The PA-4SE is shipped to you with the very latest version of the PA-4 PC board. The circuit topology is the same but some operating points are carried over from our excellent PA-3 series. Though many parts are different, we chose carefully to maintain the highest possible sound quality. We think we succeeded. At this point the PA-4SE has a track record of enthusiastic reception from those who have purchased it.


The PA-4SE Sound: After you’ve read above about the sound quality of the PA-4 imagine a close runner-up position for the PA-4SE. Yes, in a side by side comparison the PA-4 will finish first. Taken by itself the PA-4SE is very enjoyable. It would serve at the heart of your audio system for many years putting a smile on your face all the while.


Attention kit builders --- you must set the output stage bias current with a multi-meter (digital type with 2 Amps DC or more capability preferred). Then you set the DC offset voltage at the speaker outputs. These are simple, well-documented procedures explained within your excellent PA-4 instructions.
Your kit will have eutectic alloyed solder. This means that the “solidus” point is the same as the melting point. When soldered correctly this produces a solder joint that has the appearance of a drop of mercury, a mirror like shine, while non-eutectic alloys may produce a matte finish appearance. Eutectic properties are said to decrease the number of crystal boundaries in the hardened solder and produce a better sounding solder joint. Notice: our solder does contain lead.



PA-4 Modification <<<<<Limited number, hurry>>>>>>

SPECIAL CLOSEOUT PRICE  $199 kit, $349 installed, $349 kit, $499.00 installed - kit includes all items shown (with two circuit boards) including specially alloyed eutectic solder. 



PA-4 Elite Ultra Modification boards  <<<<<just a few left>>>>>

This is a pair of Elite level boards with installation kit!! SPECIAL CLOSEOUT PRICE  $299 kit, $449 installed, $449 kit, $599.00 installed - these have audio grade caps and Takman CF resistors for an open sweet sound. kit includes all items shown (with two circuit boards) including specially alloyed eutectic solder. 



---------View the installation manual! --------- This is a large file and requires Adobe Acrobat Reader in order to view and/or print it on your computer. Adobe PDF reader is probably already installed on your computer. If not then get Adobe Acrobat Reader here or your choice of PDF reader.



Dual-Mono Power Supply Conversion


See our PS-200 page for dual-mono power supply conversions for some amps. It is recommended for new dual-mono conversions. Improve the transparency, focus, imaging, bass solidity and resolution abilities of your amplifier with the tried and true optimization of your power supply for a mono amplifier level of sonic results. The ability of the amplifier to sound more "together" and unstrained is significantly improved. 


More Power Amplifier options:

TP-202 and TP-500 toroidal power transformers


LC-200/LC-500 replacement power supply caps





Hafler amplifiers: SE Series, DH-120, DH-240, 9000Series*

LX modification techniques include a very thorough mod with changes in the Feedback Loop, existing RFI input filter, Bias Circuitry, Power Supply, bypassing, compensation caps, etc. The result is a remarkable improvement in perceived wider dynamics, greater naturalness, warmth and transparency! Save yourself from dry, bright sound today!


ELITE modification uses all the LX techniques and more. ELITE replaces your main power supply bridge rectifiers, front-end power supply rectifiers(some amps) and important small value electrolytic caps, such as the DC restoring cap in the FB loop. We use the remarkably natural sounding Silmic II capacitors. Additional power supply bypass caps linearize the highest frequency performance of the power supply.


ELITE SOUND - Picture a smooth, extended top-end, wide but perfectly scaled dynamics, tremendous coherency and a fatigue-free listening experience. Picture cymbals with light airy frequency extension and not a bit of harshness. Now picture yourself enjoying instead of analyzing!


*We don’t service the 9500/9505 amplifier.


Hafler LX Modification is $399.00/inst.

Elite versions with Soft-Recovery rectifiers, Nichicon Muse caps and more is $599.00/installed


See our "Connector" page for hi-quality RCA connectors and binding posts that we can install for you!


Hafler Amplifier Modifications