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Old 25th February 2005, 08:41 PM   #101
djk is offline djk
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Pass amps are by and large class A designs, they draw maximum current all the time.

Most amps on the market are class AB, they draw current on demand.

Rectifier noise is when the diodes turn off.

Class A does not need the quietest diodes, class AB does.

Pass also uses Pi filters to reduce noise, most amps do not.

Nothing wrong with admitting you can't hear, but you have a strange hobby for same.

Diode noise may be seen with an analyzer, I use an HP8568B. Tucker has them used for about $15,000, the new model ones go for $80,000. What do you use?
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Old 26th February 2005, 03:29 AM   #102
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Quote:
Originally posted by cdsherman
...I'm currently awaiting delivery of some IXYS FRED bridge rectifiers for my 4 Hafler amps. I've been living with the stock bridges but I'm told these new soft recovery type rectifiers will be faster switching, quieter , and will increase reliability of my amps. Hopefully it will be a worthwhile upgrade and not just an exercise in soldering. Does anyone have any experience or opinions about upgrading the stock bridge? I picked this one because it came as a finished piece . I know I could have built one cheaper but I was concerned with aesthetics as well as performance. ....
Hi cdsherman

I do not know about these diodes but I can refer you to this address where it shows interesting stuff to improve your power supply:
http://www.zero-distortion.com/techn...ly/powersi.htm
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Old 26th February 2005, 04:53 AM   #103
KarlDL is offline KarlDL  United States
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No flame here, but ...

I installed 2 IXYS bridges, for dual-mono supplies. Theoretically, I tend to agree with the Nelson Pass assessment, especially when the PS has well-bypassed electrolytics and other appropriate transient-control precautions. But, since I needed two new bridge rectifiers anyway, why not try the FRED bridges? They don't make anything worse. One has the piece of mind knowing that he's taken this as far as can be done, so there's less "what-if" worry after the solder joints are cold and the unit's reassembled.

Had I to do it over again, I'm not sure that I would go the dual-mono PS route, especially since it's not really compatible with bridged (balanced or un-) operation of the amp.

I would like to see a valid double-blind test on the power cords, though (here come more flames). That's really the only way I've been able to reliably compare wines. Why should audio be any different? It's just as sensory. Regardless, the IEC is more convenient and perhaps I'll add that mod in the near future.
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Old 26th February 2005, 09:10 AM   #104
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Hi Dick,
I agree with you totally on the IEC socket. It is strictly for conveniece. I always replace the stock power cord in the DH200, however, whether the amp is for me or someone else . I find the stock cord just too short and too skimpy for my taste. It only takes me a couple minutes to put an 8ft. 16guage cord in place of the usual 5 ft. 18 guage cord, I normally find on these amps. I often have to replace the power switch anyway, so it's not much trouble to change out the power cord while I'm at it. I also replace the tin plated stamped steel input jacks with a little nicer quality gold plated ones and even replace the nickel speaker posts with some gold plated jobbies that have a much larger center hole for wire up to 10guage. These are very inexpensive mods I do routinely. The upgraded connectors I think do improve the amp aesthetically and maybe even sonically, the input jacks on most Hafler DH200 and 220 amp are too small and the right input jack on the DH200 is always somewhat recessed by design, to boot. That and the corrosion I often see are reason enough for me. Regarding the Ixys bridge, I tend to agree with Karl . I don't see the harm either and it is a higher rated part, if nothing else, in both voltage and amperage. It gives me a little more peace of mind. As for the bridging switch, that was Bogies question. All my amp are stereo and I have no desire to alter that. Sorry, no flames here . Just opinions, I have found this to be a very friendly place to share thoughts thus far. Very nice place, in fact.
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Old 26th February 2005, 11:01 AM   #105
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Karl,
BTW, the IEC sockets I purchased require a 3/4"x1 1/16" cutout. I bought a pair of sheet metal "niblers" that I'm going to try making the cutouts with, that and a file. I've heard of people using a Dremel tool, but they can get away from you pretty easily and I don't want to risk an accident . The cords I already have just 14 guage shielded Beldens I bought on closeout for $10 ea. nothing fancy. After the bridges and the IEC sockets I'm done with the Haflers . I'm already very satisfied with them, they are terrific little amps. I'll let you know if the nibbling tool does the trick or not.
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Old 26th February 2005, 03:25 PM   #106
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Default To Karl and Chris and djk

I like to do simple mods as much as the next guy but have to work hard at being objective about their sonic value. Karl, I agree with you on the DBT (double blind test) for the IEC power cords, and almost all other mods as well.

More importantly, I need to see some well controlled DBT's contrasting the same amp with and without the more exotic IXYS bridges. I have spent hours scouring the posts on many threads here and elsewhere to no avail. Many talk about installing soft recovery diodes because they "know" such will make a difference, and some claim they hear a difference. The degree to which that seemingly perceived difference really exists and is repeatable is the important question. And, whether more than one person can hear the difference in a well controlled DBT is another question.

djk, you got me there. I don't have a $15,000 analyzer. I read my first article about these diodes back in the Audio Amateur, some 12? years ago. However, this article was about a PS for a DAC, not for a DH-200. But then, and since, I've not found any well controlled DBT as to their value in an amp such as a DH-200. Not everything we read on an analyzer correlates with sonic improvements, unfortunately. Things are not that simple.

But, I agree with djk and Chris and Karl, one should keep an open mind. I have two identical DH-220 amps here and will try an IXYS bridge -- and will compare it closely to the stock DH-220. Maybe I'll even invite the neighbors in for a comparison listening test. Hopefully their auditory memory spans are long enough to span the time required to swap cables and amps. Whatta' hobby!
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Old 26th February 2005, 08:34 PM   #107
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Actually, with a good analyzer (and knowledge of how to use it), the effect of power supply bypass and snubbers may be seen.

Cords?

If the supply has noise, a shielded cord can help from re-radiating some of that noise. If the supply is quiet, it doesn't seem to do anything.

I first noticed noise on the supply from my old Maggotbox CD player. Then disc had pinholes in the aluminum, but sounded ok through the speakers. The error correction scheme was working.

But with the volume control all the way down I could hear clicks and whirrs through the speakers, mainly on the pinhole disc.

A small cardboard box between the CD player and the preamp took care of the noise, but looked bad and took up too much space.

After shoveling some caps in, an RF friend of mine suggested the diodes.

The test equipment is provided by my employer, and owned by the millitary.
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Old 2nd March 2005, 02:38 AM   #108
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Default How to do the cascode mod?

I recall reading a Dick Marsh article in Audio Amateur many years ago, plus what has already been covered in this thread.

Is there a schematic or something that details cascoding the Hafler DH-200?

Thanks and regards,

Rob
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Old 2nd March 2005, 04:18 AM   #109
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Default Re: How to do the cascode mod?

Quote:
Originally posted by weinstro
I recall reading a Dick Marsh article in Audio Amateur many years ago, plus what has already been covered in this thread.

Is there a schematic or something that details cascoding the Hafler DH-200?

Thanks and regards,

Rob
Hi Rob

Do you mean cascoding the diff input stage or VAS stage?

Diff Input stage cascoding plus CFP is indicated in post #22 of this thread. I recently sent some info on my own implementation of the input stage cascoding in a DH-200 to a diyer by e-mail.
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Old 2nd March 2005, 08:07 PM   #110
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Yes, cascoding the inputs.

If you don't mind, I'd appreciate it if you could send me that additional information you eluded to above.

I'm having trouble seeing how to integrate this into the standard input circuit.

Thanks and regards,

Rob
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