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Old 15th October 2007, 02:39 AM   #1
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Default Hafler 120 modifications

I would like some help about some good modifications on a hafler 120 power amp .
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Old 17th October 2007, 08:25 PM   #2
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There is not much you can or should do to this amp's basic circuit topology. This same circuit is used in the SE-120, Pro1500 and Hafler 9180 amp. These later versions have both channels on one planar board but if you study their schematics you can see how identical they all are.

The Hafler 9180 amp is a good one to study as you can see where in the circuit improved (polypropylene) film capacitors are used and locations where 1% metal film resistors are used.

I am using two of these amps in addition to a DH-120 and can testify that, for example, the 9180's sound is more refined and detailed, especially in the upper mids and highs. Swap parts to make your amp more like the 9180 and the quality of your sound will improve.

Another mod consideration is to simplify input signal wiring. Get rid of the level controls, the stereo/mono switch and wiring, and for sure the ambiance speaker output circuit. You will be surprised how much "grunge" is removed by simplifying input and output signal wiring.

I built a custom version using DH-120 circuit cards and made an amp that sounds almost like the DH-220. I used a power trans with about 3 times the amount of current drive, 4 times the amount of PS capacitance, and a separate diode bridge for each channel -- plus the simplified wiring mentioned above. It made a great sounding little amp much improved over the stock DH-120 and with great sound stage and imaging. But, of course it required a larger chassis and heat sinks. Its picture is attached.

So, tell us why you want to mod your amp? What do you hope to gain from mods?

Dick
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File Type: jpg dh-120clone.jpg (68.6 KB, 902 views)
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Old 18th October 2007, 04:22 AM   #3
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Default Hafler Mods

Thank you for the reply and I will look over the circuits . I have 2 of these units . One I bought new in kit form and changed the wiring and input caps also bypassed the caps with poly p caps . It always sounded pretty good with good depth and image . I wanted a small nimble amp to knock around . I also own 2 hafler 200's and a 220 .They have musical concepts boards in them, I also have a modified 101 which I have had for years and just purchased a 110 preamp that was modified by musical concepts and also another stock 101 and 110 . I have quite a collection .Once these units get warmed up for a while they are fantastic . Thanks again. I am going deaf but the volume control still works and I still get chills when I listen to the Rose on my ariston turntable . If cd's would sound that good I would have sold my turntables and my record collection .I have about 300+ albums and three turntables . I love them .
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Old 15th November 2007, 07:01 PM   #4
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Default I meant Hafler 9130

In my previous post I mentioned the Hafler 9180 amplifier. I should have written 9130. The Hafler 9130 and SE120 are almost identical, same electronics, slightly different packaging.Their sound is slightly more "refined" than that of the DH-120, mostly a result of some superior passive parts in some of the circuit.
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Old 15th November 2007, 07:05 PM   #5
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Default DH-120 with 4 MOSFETs?

Is it possible to add another pair of MOSFETs to the DH-120 circuit? Does the present PCB circuit have enough drive to handle an extra pair of output MOSFETs?

Of course more MOSFETs would require a larger heatsink and MOSFETs used in parallel must be matched (10%) on Vgs.

I have an extra Hafler DH-200 chassis and heat sinks and wondered if a DH-120 could be transferred and beefed up?
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Old 15th November 2007, 07:18 PM   #6
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Default What Does P2 Do?

On the DH-120 PCB are two variable resistors, small pots. One sets BIAS current, the other is labeled "Balance."

What is "balance?" The manual says is should only be adjusted with the use of a 'scope. What would be the result if this balance control had been changed from factory setting?

If I don't have a scope how can I ascertain if a used DH-120 is still working OK as pertains to "balance?"
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Old 13th December 2007, 02:10 PM   #7
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Default BUMP

Is it possible to use 2 pairs of MOSFETs per channel with, of course, larger heatsinks?

Also, what does P2 adjust and how can I know, without a 'scope, if my used DH-120 is adjusted properly?

Thanks for responses. This is a nice little amp with a tried and true circuit.

Dick
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Old 13th December 2007, 02:35 PM   #8
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I don't know for sure, but would guess that P2 might adjust the offset voltage, if the other trimmer controls bias.

If that is correct, then you can use a DVM - measure the DC voltage across the speaker terminals with the input shorted, and adjust for as close to zero as you can get. But don't touch unless somebody can confirm my guess.
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Old 13th December 2007, 10:38 PM   #9
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Good guess but its place in the circuit probably has nothing to do with DC offset. I would guess that it has to do with the shape of the output waveform, both positive and negative going, and is adjusted for minimum distortion.

To bad we can't get a real expert to chime in here.
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Old 15th December 2007, 03:28 PM   #10
ppa is offline ppa  Italy
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the mosfet used probabilly are the famous hitahi 2sk135 2sj50 with 100W max dissipation, It's possible using the magnatec mosfet buz901D and buz906D , they have 250W of max power dissipation at 25C , in pratics these component have two 2sk135 or 2sj50 inside the same pakage, so you can doubling the mosfet without heatsink problems.
But the buz current polarization must be 200-240 mA for each.
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