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Old 19th March 2012, 08:23 PM   #1031
tvrgeek is offline tvrgeek  United States
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I have been contemplating mods to my 220 from what I learned about my 120. What I have learned so far:
The HEXFRED bridge and modern caps were well worth it. Hexfred does not mean no snubbers, but reduced. Scope it out under heavy load. The 120 had a lot of extra wiring for mono and Danyaquad and input pots that were all unnecessary. Better jacks and binding posts while not audible, are reliable.

The gate stoppers are the same value for P and N. This is wrong. They should be selected based on the output capacitance. 680 and 470 or higher seem to be popular. It will do wonders for parastatics. Put the predrivers back in and at least put 2N5551's in instead of the 5550's. Half the noise. I would replace all the resistors in critical locations with 1%. Why go through the trouble to match the inputs if the degeneration resistors are not close? With bypassing on the heat sinks and careful ground routing, you can probably ditch half the compensation caps. My 120, I switched to standard miller compensation and a single feedback 3.3p. Going to much bigger input caps moves the distortion problems out of audio range. 47u? You can do the same on the feedback. The 220 ties the low power and high power grounds together through a cap. I would split them and return to the star ground independently. Then replace the output fuse with a protection circuit and relay. 3 different grounds. Yea, I got a problem with that. Do I see blue LED bias parts in there? Noise and hum are not about care, but about poor execution.
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Old 19th March 2012, 10:57 PM   #1032
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Quote:
Originally Posted by tvrgeek View Post
Put the predrivers back in and at least put 2N5551's in instead of the 5550's. Half the noise. 3 different grounds. Yea, I got a problem with that. Do I see blue LED bias parts in there? Noise and hum are not about care, but about poor execution.
Assuming this was directed at me... This amp had MPS8099/8599 for the diff pairs, and BC556/546 for the current sources. On the bad channel I plan on replacing all with 2N5551/5401, with new degeneration resistors.

The blue things are old school ceramic caps.
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Old 20th March 2012, 12:01 AM   #1033
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Quote:
Originally Posted by Stormrider View Post
Hello all,

... They removed and jumpered the VAS pre- devices, Q7/Q10, and re-did the output wiring.

This, IMO, is all going to far with one of these amps, and the extra noise and hum on the output says this amp was not treated with care. I intend to return the circuitry back closer to stock, as well as fix the DC offset problem with the left channel.
My 2 cents.
Removing the VAS pre-devices is a bad idea unless you radically change the input stage current or devices...

The ground return to the toroid securing screw may create a shorted turn and result in humm....

Probably a good idea to go back closer to stock...

Good luck

Fab
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Old 20th March 2012, 12:03 AM   #1034
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Quote:
Originally Posted by fab View Post
.....

The ground return to the toroid securing screw may create a shorted turn and result in humm....

.......
more than agree with that
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Old 20th March 2012, 01:03 AM   #1035
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I wasn't sure about that, glad you chimed in fab/zen mod

My previous Hafler experience is mostly with P125's and P230's with three wire power cords. In the original DH200 design the chassis is connected to the "signal" gnd through the RCA jacks. This unit has insulated jacks installed. Should the chassis be left floating, or connected to central gnd as it is now (only not through the transformer mounting bolt)?
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Old 20th March 2012, 02:40 AM   #1036
tvrgeek is offline tvrgeek  United States
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Originally Posted by Stormrider View Post
Assuming this was directed at me... This amp had MPS8099/8599 for the diff pairs, and BC556/546 for the current sources. On the bad channel I plan on replacing all with 2N5551/5401, with new degeneration resistors.

The blue things are old school ceramic caps.
Have you looked at 2SA970 and 2SC1085 where the SOA allows it?
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Old 20th March 2012, 03:07 AM   #1037
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Have you looked at 2SA970 and 2SC1085 where the SOA allows it?
Pin out of 2SA970 is different (bce instead of cbe).

Fab
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Old 20th March 2012, 06:46 PM   #1038
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Pin out of 2SA970 is different (bce instead of cbe).

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Old 22nd March 2012, 11:02 AM   #1039
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I conversed with John Hillig at Musical Concepts about this DH200. I was told the mods may have been done there, or inspired by his work.

He was very responsive and acknowledged that some of the mods may have been based on his work, but it was definitely not done by them. He agreed with Fab and zen mod about the central ground's location causing a shorted turn with the toroids, so I'll be moving that as I said earlier.
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Old 3rd April 2012, 02:51 PM   #1040
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The DH200 is back together and running perfectly. I replaced the input diff pairs in both channels and the current source devices with new ONsemi 2N5551/5401. Each group had the same date codes, and they were very closely matched out of the bag on my Heathkit IT-18. I ended up with 5-10mVdc offset on each channel. I also replaced the 22R emitter degeneration resistors for the diff. pairs with 1% metal film, which may have helped.

I also re-installed the VAS pre-drivers, Q7/Q10, moved the central ground location, and installed a three prong AC line cord. The amp is now very low noise, and sounds great.
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