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Old 8th November 2010, 01:47 AM   #931
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Quote:
Originally Posted by djk View Post
450mA bias times 60V rail voltage....
Ok. That mention of "50 watts" threw me. I follow what you meant. I thought you drastically lowered your rail voltage to compensate for a higher-than-spec bias setting. I measured about 63V on 2 of my 280s. One of them idles hotter, not loaded, than the other at moderate volume levels.

Next stop is adjusting the bias on those amps.

Being a fellow 280 owner, can you tell me how to measure & adjust DC offset?

Thnx tony
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Samsung 60" LED, Rotel RSP 1068, LG BD-C6500, Charter DVR, Denon LDP, TDM EXO->Hafler XL280/2 Belles 1s->Polk RTi A7 (Bi-amped!), Rotel 985 ->Polk CSi A6(Tri-amped!), Mackie 1400->DBX 15 Sub, Haf XL280->Polk RTi A1
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Old 16th November 2010, 05:09 PM   #932
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Hi all,
I have a DH220 which sounds great and have no particular problem.
Though, i find the input stage a bit too noisy to my taste.
I think that's because the input transistors are not low noise ones. Though i might be wrong.
Is there a known mod to fix that?
I am too lazy to read the whole thread, so, help would be appreciated.

I have seen the XL280 uses cascoded jfets.
Should it be possible to cascode the input stage and use sk170/sj74 Jfets instead of those used in the XL280.
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Old 28th November 2010, 05:22 PM   #933
Rush is offline Rush  United States
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Default DH200 in Class A

I have very efficient speakers (96db/1w/1m) and would like to play with a low power Class A using the Hafler DH200 chassis, heat sinks and outputs.
I have read this thread from one end to the other and the promise of new driver boards has never materialized.
Then I read the Pass forum F5 page and I think that driver board would be great to try here. I have a toroidal 24-0-24 vac transformer and plan on 4 39,000 uF of capacitance per Mr. Pass' F5 power supply design.
My question is do I need to change anything to drive 4 Mosfet outputs or due to the heat sinks should I only use a pair of outputs.
Also somewhere on the Pass forum I found this F5 mod and would like opinions. It shows only 20 VDC +- rails is that a problem as I will have 32 + - VDC? (see jpg)
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File Type: gif f5 mod 1.gif (8.8 KB, 333 views)
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Old 29th November 2010, 01:26 AM   #934
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Rush,

Can you be more explicit about the location of the description of the schematic you showed? Thanks........
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Old 29th November 2010, 02:01 AM   #935
Rush is offline Rush  United States
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I really can't remember. I was plowing through the Pass F5 forum and saved a couple of schematics. Too many posts, too many pages, I should have made a note in the title, but didn't.
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Old 29th November 2010, 07:20 AM   #936
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Anyway, cascoding should be better than current sources at this voltage.
See Cviller F5 v2 wiki.
On the other hand, dh220 heatsinks are on the small side even for stock F5.
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Old 29th November 2010, 03:07 PM   #937
Rush is offline Rush  United States
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I may need to find a lower voltage transformer and just use one pair of output devices.
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Old 29th November 2010, 05:25 PM   #938
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I know Jackinj has tried F5 with lateral mosfets and was not satisfied.
At the early pages of F5 thread.
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Old 29th November 2010, 05:54 PM   #939
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Rush,

All of Pass's designs use vertical MOSFETs, not the lateral MOSFETs as in the Hafler amps.

I don't have engineering data but I can tell you from reading about using Hafler heatsinks in a class A amp, the heat sinks are just not large enough for any amp with a reasonable amount of watts.
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Old 29th November 2010, 06:10 PM   #940
Rush is offline Rush  United States
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I know the heatsinks are smallish. I figure if you lower the voltage enough, you can get 5 or 10 watts and that is all I need. I have found that good sounding amps (the ones that sound powerful) all have great big large current power supplies whether they are tube or SS regardless of output wattage.
I currently use a 2 watt 45 tube amp with great success. The only thing I don't like about the 45 is the 60 hz hum due to the ac of the 2.5 volt heaters (please don't tell me how to DC the 2.5 volts, you really don't understand dht.) It really isn't noticeable when the music plays, but it is bothersome because I know it is there.
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