Hafler 9270 Toroidal Xfo Mod - A Different Hafler Mod - diyAudio
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Old 11th April 2011, 12:23 AM   #1
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Default Hafler 9270 Toroidal Xfo Mod - A Different Hafler Mod

Hello amp-tweakers,

Here's a different sort of Hafler modding thread. I've found a few threads that deal with the older pre-Fosgate Haflers (DH series), but none that deal with the Series 9000 amps. I have a mid-90's Hafler Series 9270 power amp, which is 135W/ch into 4 ohms., and sounds quite good. I have no idea about the amp's topology, except that the front panel of the amp states "High Gm JFET Topology Lateral MOSFET Power Amplifier". I scored the amp used, and it sounds good and seems to be in good working order except that one side of the amp idles slightly (barely noticeably) warmer than the other (and both sides run hot). This amp uses two special-design transformers, but they aren't toroids. See below for a photo of the internals of this amp (I can disassemble mine and provide better photos if needed):

Click the image to open in full size.

I've been considering upgrading to a toroidal power transformer. This amp is listed at up to 1200W of AC power draw from the wall socket, which confirms the rumors of bridgeability at 2 ohms, but also means that I'll need a big toroid to avoid degrading performance. I bought the 9270 used, and I don't think I'll be able to make a significant amp upgrade in the near future, so I'm looking for ways to improve the performance of this classic amp, and I thought an upgrade to the power transformer seemed as simple a mod as any (since 60 hz is 60 hz, so there'd be the least likelihood of screwing anything up). This amp does produce a very faintly audible hum at about 12 inches from the housing when powered on and not passing a signal, but I haven't heard the hum through the speakers.

Would you agree that upgrading from the dual power transformers to a single toroid with dual secondaries would probably improve the performance of this amp? Would it be a fool's errand?

Manual (with circuit diagram) http://www.hafler.com/techsupport/pd...70_amp_man.pdf

Thanks!
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Last edited by Taterworks; 11th April 2011 at 12:25 AM.
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Old 26th May 2011, 04:33 AM   #2
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Default how did it turn out?

Hello! I need to replace the T301 transformer and was wondering if you completed your mod. My amp is non functional right now, and I am trying to troubleshoot it with a multimeter with little skill under my belt. If the mod did work, which transformer did you use and where did you get it? Did you mod anything else? Thanks for the help.
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Old 14th December 2012, 05:42 PM   #3
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Bump for an OLD thread. Did you ever end up upgrading your Hafler to a Toroid transformer? I have a Hafler 9303 TransNova amp that also has an audible hum from the transformer. I tested several other 9303s and they all had this transformer hum. The hum is not audible through the audio stream but it's pretty noticeable to me from the case, unless I damp it by stacking things on top of it. I was surprised to see an amp in this price class had an EI transformer in it (the 9303 was around $1300 new I think?)

Thanks

Steve
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