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Old 19th July 2013, 03:27 AM   #1
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Default Hafler XL-600/XL-280 PC-40 upgrade to 9300/9500 predriver

I have had a Hafler XL-600 amplifier in my system for about a year now. I bought it off of Craigslist from a local seller who was the original owner of this amp. It is super clean and is about as close to new as you can expect from a 20+ year old amplifier. The only thing it really needed once I got it home aside from cleaning the fan blades off was to adjust the output stage bias. However, being an avid DIYíer I can never leave well enough alone.

I know that the later generation 9300/9500 amplifiers were very well regarded and highly reviewed back in the 90ís. Not that the XL series was any slouch, but they didnít seem to have the same reputation as the later gen 9300/9500 amps. I started wondering what the differences were between the older XL series versus the newer non-balanced input 9300/9500 Hafler amplifiers. After examining the schematics [which are attached] for the XL-600 and the newer 9300/9500 it looks as though the main change was a simplification of the predriver circuit. The later amps gained generic 3 pin 24V regulators to the DC rails and the elimination of several parts including the cascode transistors (did I identify that correctly?). This offered ďregulatedĒ DC rails in the later 9300/9500 amps.

After thinking about it for a while it seems it would be reasonably simple to construct a PC 40 replacement board based on the later 9300/9500 predriver and marry it to the XL-280 or XL-600ís output stage. In fact Iím sure it could be used for the DH200/220 output stage as well although hI have not investigated it. From the schematics it appears that the output stage between the earlier XL series and later 9300/9500 generations of Haflers are identical except for the fact the DH and XL amplifiers used the more rugged metal can TO-3 devices, which I much prefer. I know that the XL series input differential SK163/SJ74 JFETís have been obsolete for a long time, but the original JFETís could be carefully removed from the working original PC 40 boards and installed into a new board. Not to mention that there are still a few sources for matched 2SK170BL/2SJ74BL devices albeit at a premium.

Aside from the JFETís the other components in the 9300/9500 predriver all look to be easily sourced and cheap. I was thinking that a simple zener + emitter follower regulator could be integrated onto each board so each board would have its own dedicated +/- regulator. The zener + emitter follower regulator would also have a much lower noise floor than the standard 3 pin variety used in the 9300/9500.

I know based on other threads here that others have modified and changed the front end of the old Hafler amps over the years, but I havenít seen anyone that has done this type of upgrade. Musical Concepts offers their PA-4 kit, but at a very hefty price. I think that these updated PC-40's could be built at a cost of < $100 even with having to go and pay a premium for the input JFETís. Without the cost of the JFETís I would think these could be built very cheaply at < $50.

Now some are probably wondering ďWhy not just sell the XL-600 and buy a 9500?Ē So Iíll go ahead and answer that question.
I like to mod my electronics.
I like the looks of my XL series better.
I like the metal can TO-3ís better than the plastic TO-3Pís.
I like the fan cooled heatsink better. Many would say the later 9500 is the better amp because of this, but the fan affords much better cooling. I am running 575 mA of output bias and can hold my hand on the outputs no problem and at this bias level the fan barely speeds up. Sitting in my chair the fan is inaudible when music is playing and it is barely audible with no music playing. It is my understanding that the 9500 cannot get much past 400 mA without getting too hot. If Iím wrong on this last point let me know!

One thing that I do not like is the reports I have seen of overheating on the 9300/9500 boards around the output mosfets and trace failures as a result. This seems like some marginal engineering was at work here. Due to this fact I would be quite leery of buying one without being able to inspect the main board.

Just wondering what others thoughts are on this possible upgrade for the PC 40? Would it be worthwhile? If it is a worthwhile upgrade would anyone be interested in some bare boards if a small batch were to be built and sold at cost?
Attached Images
File Type: jpg HAFLER_9500.jpg (128.0 KB, 139 views)
File Type: jpg Hafler_XL600.jpg (219.5 KB, 136 views)
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Old 19th July 2013, 07:47 AM   #2
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Bit of thread jacking here ....

Given the opportunity and while many Hafler experts may look at this i wonder if musical concepts PA4 boards is an option for the XL 600

Farther more i have just installed PA2 boards in XL 600 and i am runing into troubles so i need schematics and help

Kind regards
Sakis
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Old 19th July 2013, 11:41 AM   #3
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Quote:
Originally Posted by sakis View Post
Bit of thread jacking here ....

Given the opportunity and while many Hafler experts may look at this i wonder if musical concepts PA4 boards is an option for the XL 600

Farther more i have just installed PA2 boards in XL 600 and i am runing into troubles so i need schematics and help

Kind regards
Sakis

Sakis,

The Musical Concepts PA-4 boards are an option for all of the old DH and XL series Haflers, but an expensive one IMO. I mentioned this about half-way through my original post. I'm a bit of a cheap skate when it comes to this kind of thing. I would rather come up with my own upgrade solution and avoid the $$$ where possible.

The schematics are readily available from Rockford Fosgate. I called their tech support and within 5 minutes the schematics were in my email. While I had them on the line I went ahead and had them send me the Hafler Iris preamp schematics as well.
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Old 19th July 2013, 12:00 PM   #4
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sorry to hump into your thread asking for info ...

yes Rockford people been the same helpful to me in more than one occasion In my case the original boards work as is with no probs

problem is that costumer has already the PA2 ( not ! PA 4 ) boards and wants them in ... so gate capacitors and implementation is one issue , bias behavior is the other plus it seems that one of the boards has some trouble as described here :

HAFLER xl 600 PA 4 / PA 2 boards

Thanks for the info so far ...

Kind regards
Sakis
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Old 20th July 2013, 11:24 AM   #5
djk is offline djk
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Transnova requires two transformer secondaries.
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Old 20th July 2013, 02:32 PM   #6
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Transnova requires two transformer secondaries.
Good point!

Options,

1) A new 25 VA toroidal is $20 from Mouser.

2) A higher voltage rated transistor (i.e. 2SC5171) could be used in the zener emitter follower to drop the 100V rail down to 24V. The CB voltage drop would be around 75V so a resistor of around 5W would be needed in that location which would eat up some board space. Plus decent heatsinks would be needed on the transistors.

In the end, option 1 would be the better solution even though it is more expensive. Isn't that the way it always goes, the more expensive solution is the better one!
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Old 20th July 2013, 05:45 PM   #7
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"1) A new 25 VA toroidal is $20 from Mouser. "

You're building a 10W amplifier?

You only need one transformer for how ever many channels you need for the front end, you need a separate secondary for each output stage channel.
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Old 20th July 2013, 05:55 PM   #8
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"1) A new 25 VA toroidal is $20 from Mouser. "

You're building a 10W amplifier?

You only need one transformer for how ever many channels you need for the front end, you need a separate secondary for each output stage channel.
I am not looking to copy all aspects of the Transnova, only the front end. Trying to recreate the whole circuit would be too costly. I was just going to stick with the XL-600's original single secondary x-former with dual mono rectifier & filter capacitor setup on the output stage.
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Old 20th July 2013, 08:56 PM   #9
mlloyd1 is offline mlloyd1  United States
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i understand the need to tinker with it.
i suffer from that disease myself.
:-)
isn't the XL600 the high power version of the XL280?
if so, i'm thinking that the XL600 design front end may a better performer overall than that of the transnova design.

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Old 20th July 2013, 10:01 PM   #10
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MLLOYD1,

Yes that is the one. Same PC40 front end as the XL280 and 4 pairs of Hitachi outputs per channel with 25 volt higher rails. Basically it was what evolved out of the DH500.

Well I certainly would not want to take a step back or even laterally in performance.

I recently did some upgrade work on a Hafler SE100 and there was a lot of improvement to be had by modifying that design. Economic constraints greatly affected the parts used in the original manufacture of that unit. With the XL600 I may just need to look at ways to improve what is already there. I'll have to investigate some more...
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Last edited by Chamberman; 20th July 2013 at 10:16 PM.
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