Musical Concepts PA-7 User manual

Musical Concepts
PA-7
ALL MODELS INCLUDING ULTRA
Kit Modification for Hafler amplifiers
DH-200, DH-220, DH-500, XL-280 and XL-600

Thank you for purchasing the Musical Concepts PA-7 modification. The PA-7 is the result of many years of
research. The sound quality you are about to enjoy has been achieved through careful engineering and careful
listening evaluation. The PA-7 is a triumph of simplicity using tried an true technology in a fine tuned state. The
PA-7 utilizes the highest quality parts, specifically chosen for their extremely musical qualities. You will enjoy
the state-of-the-art performance that until now was only a hidden promise of your amplifier.
APPLICATION
This modification is designed for the Hafler DH-200/220/500 and XL-280/600 power amplifiers. Dedicated constructors with consider-
able experience can use the PA-7 to drive a MOSFET output stage using from 2 to 12 lateral power MOSFETS in complementary source
follower mode. Such projects should be undertaken only by the most self sufficient types, since Musical Concepts cannot offer
assistance for custom installations.
DISCLAIMER
Musical Concepts accepts no responsibility for damages, direct or consequential, resulting from this modification. The user solely
determines his or her own ability to properly install this product. Musical Concepts accepts no responsibility for personal injury or death
resulting from electrical shock hazard.
CIRCUIT DESCRIPTION
The PA-7 first stage is a single-ended amplifier circuit with a low frequency 'corner' of about 0.10 Hz. It is a Single-Ended, Cascoded,
Current-Mirror Class-A front-end exhibiting the advantages of symmetry and high input impedance to the driving device. The following
Single-Ended Voltage Amplifier Stage(VAS) is independently current-sourced. It is operated at significant current for linearity. This
VAS stage uses wide-band video transistors for outstanding, clean sounding reproduction. It maintains that super-clean sound at the
highest volume levels, listen loud and comfortable. VAS stage is collector coupled to the MOSFET output stage. That is correct, there
are only three stages, making the PA-7 an incredibly simple, straightforward design. The output stage is the tried and proven lateral
Mosfets used in your original Hafler product. Power Mosfets have nearly 10 times the bandwidth of bipolar power transistors
minimizing the phase and bandwidth problems of the output stage. In addition, power Mosfets are known for their ruggedness under
adverse drive conditions.
TOROID POWER TRANSFORMERS
Musical Concepts offers the TP-202 toroid power transformer with internal and external shielding replace the standard, less efficient
E-core transformer of the DH-200/220(cannot be fitted to the XL-280 unless capacitor arrangement is modified with the PS-200 power
supply board). The TP-500 is a pair of transformers, 500 watts each, used to replace the single transformer of the DH-500/XL-600.
When used with twin capacitor power supplies, they are paralleled. They are used separately with the XL-600 or when installed with the
PS-500 dual-mono power supply kit. These transformers offer amazing improvements in bass solidity, dynamic range, ambience
retrieval and speed. There is a velvet-black backdrop to the sound revealing subtle imaging clues.
POWER SUPPLY CAPACITORS
The PS-200/PS-220 dual-mono style power supply circuit offers a great way to not only revive the flagging power supply of your
DH-200 but it excels way beyond the performance of the original power supply. It uses four power supply caps and superb twin
bridge rectifiers. The improvements are greatly improved bass solidity, wider dynamics at any volume level and greater ambience.
The sound is more liquid, delicate and revealing of subtleties always there in your music collection.
All mounting and wiring parts are provided, even solder, which reduces installation difficulty. Our PS-220 accommodates four stan-
dard 2-pin snap-in caps or Jensen 4-pole capacitors. PS-200M version is for Mundorf 4-poles for an upscale option.
The PS-100 is the more basic ‘stereo’ power supply with two capacitors and a single, but superb bridge rectifier. Mundorf caps can be
used with the PS-100. Upscale performance is very evident.

DUAL- MONO CONVERSIONS
Dual-mono versions of these amplifiers are available. In the DH-200/220/280, we use our TP-202 transformer. Transformer output is
separately rectified on our PS-220/500V2B dual-mono conversion circuit board with up to 88,000uF of capacitance. Consider the finest
power supply upgrade, PS-500M. In the DH-500/XL-600, we use the TP-500 transformer set, two 500 watt transformers stacked in the
space occupied by the original transformer. These transformers can be rectified into a single or even a pair of PS-200 boards or the new,
designed for DH-500, PS-500 power supply. The rail fuses are mounted directly on the PA-7, which makes any other proposed
dual-mono conversion much easier.
SOLDERING
We recommend that you have some soldering experience before attempting this modification. We remind even the veteran that your
solder should be fresh rosin core type. Some fancy ‘audiophile’ solders that we have evaluated might devastate the sweet delicate
sound of the PA-7. Fresh 63/37 eutectic alloy rosin core solder is provided. Be wary of hype!
TOOLS
3-prong grounded 25/45 watt soldering iron--don't use a gun! A grounded solder station is the best choice, needle nose pliers, diagonal
cutting pliers, wire strippers, screwdrivers(Philips #2 and flat), miniature flat blade screwdriver, 1/4 & 3/16” nut drivers (optional),
DMM or multi-meter with 2 Amp or higher DC Ampere scale. Another ‘tool’ is the original owner manuals downloadable at
hafler.com on the Resources/Archive page for most of the amps. Pictorials in these manuals may add more clarity.
INVENTORY
2 - PA-7 Elite pre-tested circuit boards, Ultra versions come with the exotic parts already installed except on MOSFETs obviously.
These are pre-tested in an actual amplifier circuit and preliminary adjustments are made to DC OFFSET and BIAS.
Misc. parts: 1- length of solder, 4 - 22K ohm power bleeder resistors (Red-Red-Orange-Gold or 22K)[6800 or 8200 Ohm may be
packed for 200 series Haflers], 8 - 0.22 mfd film caps, 2 - 47pf caps, 4 - tie wraps, 2 - 4ft. lengths of 22 ga. wire(2 colors), 3 - 3ft.
lengths of heavy wire(3 colors), 4 - solder lugs for #10 screw, 2 - Gold-Plated female RCA jacks with isolating washers(2 flat, 2
shouldered), 1 - 10" length of heavy bus wire(solid and uninsulated), 1 - #6 ground lug for DH-200 mods. 8 - nylon #4 retaining washers,
mod tags. NOTE: Some included parts may not be used depending on your model; you may have leftover parts after the mod.
FOR ULTRA only : 8(200/220), 12(DH500/XL280) or 16(XL-600) - 1000 Ohm CC resistors.
INSTALLATION INSTRUCTIONS
We recommend that you read through the instructions before beginning the modification to determine if you will need the assistance of
an experienced friend or professional installation by Musical Concepts. Musical Concepts installations include a 1 year parts and
labor warranty. <<Kits do not have a parts or labor warranty. No refunds or exchange once the kit is soldered.>> Repair services
will be available for kit installations.
NOTE: Certain instructions may pertain only to specific amplifiers. These instructions will be proceeded by [200], [500/600] etc.
Unlabeled instructions are for all amps.
IMPORTANT: Read each step completely before starting work for that instruction.
1.( ) Turn unit off, remove all connecting cords and disconnect AC plug from wall.
2.( ) The screws holding the top cover on the DH-200/220 and XL-280 are located between the heatsink fins .
Remove all screws holding the top cover in place and remove the top cover. A 1/4" nut driver is the best tool for this with most amps.
#2 Philips screwdriver is used for DH-500/XL-600, very rarely it may use hex head screws.
3.( ) The input wires from the RCA jacks at eyelets 1 and 2 are at the ‘top’ of DH-200/220, XL-280 PCB, but the ‘rear’ of the
DH-500/XL-600 PCBs. You may re-use these wires to connect the PA-7 or replace them with supplied wiring. There may be a length
issue on some amps requiring replacement.

Note: As you install the mod the wire colors provided may vary but the point-to-point wiring is simple. See Fig B and C.
4.( )[200, 220,280] Remove the four screws, between the fins, holding the right channel heatsink to the main chassis. Let the heatsink
rest on an old towel to protect the finish if you like. You may find that doing both channels is more convenient.
5.( )[DH-200] Each stock L/R RCA jack is secured by 2 screws. Remove the connectors and attached wires. Reuse wires or discard.
6.( )[DH-200 only] Read the first sentence of step 7 before starting this step. Remove the two screws in the top of the power supply
capacitor, on the left, as you face the front panel. The filter capacitors are held to the bottom by screws in the feet of the capacitor clamps.
Remove the screw nearest to the center of the amp on the same capacitor's foot. Loosen the other screw slightly. 'Twist' the capacitor
out of the way and sand off the paint around the hole where you have removed the 'foot' screw. Select the #6 solder lug(ground lug).
This is installed where you removed the 'foot' screw, by sandwiching it between the bottom cover and the foot of the clamp. Orient it
toward the rear of the amp so it will be convenient to solder wire to it in the following steps. Re-tighten both of the 'foot' screws securely
so that you will have a rock solid ground point. Reinstall the two screws/wiring in the top of the capacitor, tighten them securely.
7.( )[DH-200 only]You may find this step easier if you temporarily remove the power supply caps.
You may find that you have to slightly enlarge the RCA jack holes for some of our jacks - a multi-step
drill bit is handy for this. Select the new Gold-Plated, Teflon® insulated RCA jacks and isolating
washers (one shouldered, one flat per connector - both might be shouldered). Pre-solder the wires to
the RCA jacks before they are physically installed into the chassis. Make sure the shouldered plastic
washer will fit ‘inside’ the chassis hole. Hole may need to be enlarged. The required wire lengths are
as follows.: [11" blue and 11" black for L. Ch., 11" blue and 11" black for R. Ch] - colors could vary.
These lengths will vary if you are modifying the power supply with say our PS-220 during the mod.
Solder the small light gauge wires(just ‘small’ going forward) to the center lug of the jack for R&L
Ch and the 2 small gauge black wires to the separate ground tabs. Next install the shoulder washers onto the RCA jack, as shown. From
the outside, install the jack assembly thru the original jack hole with the shouldered insulator against the back panel. On the inside, install
the flat insulator, gnd lug and nut in that order. Tighten securely(Hardware is 12 MM)! Both channels are done the same way. The
shoulder washer must fit inside of the hole to be electrically isolated. These will be wired to eyelets E1 and E2 of the PA-7 in a later
step. Once the RCAs are installed and tightened, twist the wires 1 to 2 turns per inch awaiting installation into the PA-7 boards. You
can put a drop of enamel paint on the threaded and nut part so that the jack will not loosen in the future.
If you are replacing power supply caps or installing our PS-xxx power supply board now is a good time to do that - but if in-
stalling new binding posts do that now after removing the cap clamps and rectifier in the case of PS-XXX.
8.( )[All amps except XL-280] Find the center tap of the power transformer secondary. It is one of the wires connected to the existing
'star ground' bus wire between the large filter caps and it goes inside the power transformer housing. Note its color(typically red/yel),
remove it from the star ground and leave the other wires there connected. This may take some serious heat, so clip it off if you prefer.
Select 5 in. of heavy bus wire. Now install this wire to the power supply caps just like the original 'star ground'. Make a slight 'V' shape
pointing toward the power transformer(DH-200/220). Point the 'V'(s) toward the front panel in the DH-500/XL-600. The idea is
illustrated in Fig. B. The XL-600 requires that this be done with both pair of filter caps - use the #10 lugs. After the wire is formed and
secured around the lugs on each filter cap, solder securely. Faster heating of the solder joint can be accomplished by temporarily
loosening the screws connecting the 'star ground' to the large filter caps. This wire is the primary 'star ground'. Now solder the power
transformer secondary center tap, which you have just removed, to the center of this new 'primary star ground'. This is the only wire
connected on this new bus wire. This removes the charging current gradients from the sensitive secondary star ground, reducing power
supply noise.
9.( )[DH-200 only] Cut a piece of heavy wire about 4.5 inches long. Strip both ends. Solder one end into the ground lug installed in
step 6. Secure the other end around the rear wire linking the two large power supply caps, i.e. the secondary star ground. Solder
securely near the center of the secondary star ground wire. All wires on the secondary star ground should be as close as possible to
the center. See Fig. B.
It is time for DH-500 and XL-600 owners, to make a choice. Now is the time to decide if you want to remove the power module
during the modification. If you have an original DH-500 with PC-10 circuit boards it will probably be necessary to remove the module.
Why? Because you will find it difficult and clumsy to replace the old RCA jacks with the module in place. Also there must be a wire
from the 'secondary star ground' to the ground lug between the RCA input jacks(marked "A" in Fig. B). Some later amps and PC-19
amps will already have this wire, earlier amps will not. If you have a DH-500 and you know this ground wire is already in place and
you are satisfied with your present RCA jacks, then you can elect to leave the module in the amp. The same can be said for the XL-600.
10.( )[500, 600] If you prefer to do the mod without removing the module, skip this step. There are several wires routed beneath the
module, to the fan switch and thermal breakers. Carefully desolder(Keep your own records or snap a photo for reconnection!) at the
terminal strip end(PC-43 end in XL-600). This will allow you to remove the complete power module from the amp.