Hafler DH-200/220 Mods

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djk

djk

R.I.P
Joined 2001
"Large inductors in series with the transformer primaries and secondaries
can be used to stretch the duration of the charge pulse to the power
supply capacitors, improving regulation and reducing noise. Large
inductors in combination with multiple power supply capacitors can form
"pi" filters to reduce the noise on the supply lines."
 
O

ouimetnick

Member
Joined 2011
Lots of nifty mods and improvements. Very impressed. I have a DH-200 and the DH-101.

You can read all about it

Got My Hafler DH-200 & DH-101 - AudioKarma.org Home Audio Stereo Discussion Forums

to bad I broke it. The right channel has 1.3 Volts of DC offset, and the left blue a rail fuse when adjusting the BIAS. saw a spark from the pot, and blew the fuse that the DMM wasn't connected to. Problem: bad soldered legs on the pot. I basically have it all apart in a mess. Just want to throw in some PC-19C boards to replace the PC-6 ones, upgrade the internal wiring, and be done with it. Already have new binding posts and RCA jacks.
 
eyoung

eyoung

Member
Joined 2007
I did a bad thing... I bought a dh220 on fleabay. I have not got it yet to see what kind of basket case it is , I have not bought anything on ebay electronics that was working when I got It ....That said I have been lucky that I have been able to fix everything reasonably cheap. Stay tuned It looks like a pawn shop special (was working the last time I plugged it in)...

I am an amplifier addict. the fact that i happen to be drinking when I buy these wonderful amps means nothing....

Does anyone know how deep in class A you can bias or is there a sweet spot(to paraphrase another:cool:)

Burning in South Carolina... EL
 
O

ouimetnick

Member
Joined 2011
eyoung said:
I did a bad thing... I bought a dh220 on fleabay. I have not got it yet to see what kind of basket case it is , I have not bought anything on ebay electronics that was working when I got It ....That said I have been lucky that I have been able to fix everything reasonably cheap. Stay tuned It looks like a pawn shop special (was working the last time I plugged it in)...

I am an amplifier addict. the fact that i happen to be drinking when I buy these wonderful amps means nothing....

Does anyone know how deep in class A you can bias or is there a sweet spot(to paraphrase another:cool:)

Burning in South Carolina... EL
Click to expand...

Normal BIAS for the DH-200 is 250mA. For the DH-220, it is 275mA.

You can go higher, but at the expense of more heat, and a slightly shortened life span. I'm not sure what the maximum you can go before something smokes, but I wouldn't go higher that 300mA.
 
fab

fab

Member
Joined 2004
fabaudio.online
Paid Member
ouimetnick said:
Normal BIAS for the DH-200 is 250mA. For the DH-220, it is 275mA.

You can go higher, but at the expense of more heat, and a slightly shortened life span. I'm not sure what the maximum you can go before something smokes, but I wouldn't go higher that 300mA.
Click to expand...

Hi eyoung

For more clarification, the output Lateral Fets are self protected against over temperature. The temperature coefficient (about 110-130 ma range per mosfet) change after some bias current value, so the more the temprature rises the more the current drops. Therefore, even if you try to increase the bias current, after some times, it will automatically reduces due to high temperature. The amount of reduction is also function of the size of the heatsink. I remember noticing that effect around 300ma ( bias for 2 pairs) or more - I think. I have already described it earlier in the thread.

The "sweet spot" for these Lateral mosfets is high bias current thus would require a bigger heatsink...or to reduce the voltage (I remember Nelson Pass indicating that this design can make a nice sounding class A amp)

Also, these amps have thermal cutoff switches when heatink temperature exceeds 70C thus you can try different bias current values without too much risk.

Good luck :)

Fab
 
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woody

Member
Joined 2002
Here is another idea. Convert the power supply from a full wave BRIDGE to a simple full wave supply. This way instead of having + and - 60v you would only have about 60v. Then use the type circuite used in one of the Quad amps to make a + and - supply from one voltage. Use the first circuite to get the 60V dc then the last 1/2 of the next circuite to get the split supply voltages. Then we should be able to run the bias a whole lot higher.
 

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fab

fab

Member
Joined 2004
fabaudio.online
Paid Member
Class A higher current

woody said:
Here is another idea. Convert the power supply from a full wave BRIDGE to a simple full wave supply. This way instead of having + and - 60v you would only have about 60v. Then use the type circuite used in one of the Quad amps to make a + and - supply from one voltage. Use the first circuite to get the 60V dc then the last 1/2 of the next circuite to get the split supply voltages. Then we should be able to run the bias a whole lot higher.
Click to expand...

You mean a virtual ground for a power amp and using half de power capacity of the actual tranformer.... :scratch2: why not change the transformer to a smaller voltage with same power rating then...;)
 
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