PA-4 Front-End Circuit Boards for Hafler DH-200/220/500 and XL-280/600. We do custom installations in some other MOSFET amps.

The Design: Design simplicity has always been a trademark of Musical Concepts products. The new PA-4 is a straightforward, effective amplifier topology with a single-ended front-end. LED biasing is used for the current source supplying current to the differential 'front-end' to complement the LED biasing in the ultra-linear connected, cascode driver stage. The LEDs are not just for “pretty”, they produce reliable stable voltages to set the operating parameters of the front-end circuitry.

PA-4, what’s New? So as far as the general schematic is concerned there is evolutionary change. After all we were happy with the sound of the PA-3E. Then what gives? Well, the layout has been changed a bit for one. More physical space is provided for a superb new coupling cap. New wide-band, very linear driver stage transistors and heat sinks are used to increase quiescent Class A driver stage current providing greater linearity and lower distortion for a cleaner, full-bodied sound quality(in standard PA-4 and up models). The superb Dale,  widely-respected 1% metal film resistors are used in most PA-4 models. Yes, these resistors cost 5 times more depending on wattage rating than those used in many audio products but we believe the difference is worth every penny. They sound clearly superior in the sense of lucidity, soundstage and overall naturalness. There is a sense of a quieter, darker background that allows the instruments and voices to be more clearly located in transparent space. All PA-4 series driver boards,excepting the SE, have these superb sounding resistors. The PA-4 Elite Ultra version uses the wonderfully dynamic, dimensional, rich, powerful sounding Takman resistors. Yes, some things are worth the extra expense.

The PA-4 Sound: This is merely a Hafler you ask? No slight to Hafler, but think again. This is a really great amplifier in a Hafler chassis. It’s not just a great solid-state amplifier, but a great amp period! Beyond all the new technical improvements, the sound quality is the real treat. There is a sense ultra-low distortion and a clean, clear response to the most intense transients/crescendos. The degree of perceived dynamic range, bass power and overall delicacy is striking. You’re treated to a tremendous sense of effortless power. The high frequency extension has a delicacy, liquidity and naturalness make you think you are listening to a vacuum tube amp. Then along comes bass detail and overall clarity that makes you reconsider that thought. There is a sense of solidity, accurate timbre, simple majesty and air-moving power that must be heard. If your speaker plays really deep you’ll be thrilled. PA-4 modified Hafler amps will easily compare to the finest sounding amplifiers available today.

New! PA-4SE: Our PA-4 mod series has been further up-scaled in quality level of individual parts requiring a consequential price increase beyond the budget of some who wanted to own it. We had to bring the excitement of excellent audio reproduction to more affordable levels. 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 of the parts are different, we worked hard to maintain the highest possible sound quality. We think we succeeded. You will enjoy a sound quality well beyond what is possible with the original stock amps. The PA-4SE modified amps will compare to amps of far higher price.

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-4SE Modification <NEW!!>

$249 kit, $399.00 installed - kit includes all items required for installation. The hardware installation parts are of the same quality level as those provided with the original PA-4 (with two circuit boards) including specially alloyed eutectic solder. Only the actual driver boards have different components compared to the other models.

PA-4 Modification

$349 kit, $499.00 installed - 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. It's probably already installed on your computer. If not then get Adobe Acrobat Reader here

PA-4 Elite Modification

Superb Nichicon Muse high-end audio grade caps are used in the feedback loop and front-end power supply positions on the PA-4 Elite for higher resolution, crystal-clear frequency extension, clarity and just plain naturalness. PA-4 Elite has great bass, quick powerful transients, smoothness, natural warmth and great 3D imaging. That same word naturalness just keeps coming up.

PA-4 Elite: $449 kit*, $599.00 installed

PA-4 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 exotic modification includes all Elite mods, described above, plus:

Rhodium RCA jacks, the classic simple look

Rhodium binding posts, rather beautiful

PS-200 Power Supply - See it here. dual-mono power supply with wonderful sounding caps and superb IXYS HyperFred rectifiers. You will receive a PS-200 specialized for your amp, please specify your amp when you order. See more PS-200 versions here, some of these items on Exotic Parts page.

Our latest PA-4 Elite Plus creates a truly a superb amp that demands comparison to any amp available!

PA-4 Elite Plus: $899kit*, $1099.00 installed  >>>>>>>For PS-200J luxury version, with Jensen 4-pole caps, add $195 to either price.

PA-4 Elite Ultra Modification

Our PA-4 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 a 625W dual-secondary 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 premium/exotic caps - eight of them in total. This power supply operates with totally dual-mono performance. 

PA-4 Elite Ultra modification: $2495 installed only for DH-200/220 & XL-280, $3195 for DH-500, $3395 FOR XL-600 

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 greatly improved. 

More Power Amplifier options:

TP-200 and TP-500 toroidal power transformers

LC-200/LC-500 replacement power supply caps

Hafler amplifiers: SE Series, DH-120, DH-240, 9000 series* - We do THX stuff, Too! 

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 9505 amplifier.

Hafler LX Modification is $399.00/inst., Elite versions with Soft-Recovery rectifiers, SILMIC II and more caps is $599.00

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

Hafler Amplifier Modifications