Thanks Dick West
Today I got to change out a few parts in my DH200 Hafler. My LEFT channel was in need of a tweak. Changed out Q1,2,5,and 6 with a matched set of 2N5401's and 2N5551's. The DC offset plummeted from 300mV to 1.2mV...thats correct 1.2 mV!
Right channel was OK at 4.8mV
I love it when things turn out right.
Thanks for helping me out here Dick, and all others.
Today I got to change out a few parts in my DH200 Hafler. My LEFT channel was in need of a tweak. Changed out Q1,2,5,and 6 with a matched set of 2N5401's and 2N5551's. The DC offset plummeted from 300mV to 1.2mV...thats correct 1.2 mV!
Right channel was OK at 4.8mV
I love it when things turn out right.
Thanks for helping me out here Dick, and all others.
very good... just a question... was the offset channel noisy too?
when transistors begin to fail, they begin generating more noise. it's a method NASA uses for preventive maintenance on equipment. they do noise testing, and if for instance an amplifier begins to get noisy (compared to a baseline established after the equipment's burn-in period) they replace or repair it.
i'm thinking about testing their theory on more common things, like power amps.
it would be interesting to compare the transistors you removed against a new pair for their 1/f noise. or compare one to the other. to get 300mv offset one of them drifted more than the other.
when transistors begin to fail, they begin generating more noise. it's a method NASA uses for preventive maintenance on equipment. they do noise testing, and if for instance an amplifier begins to get noisy (compared to a baseline established after the equipment's burn-in period) they replace or repair it.
i'm thinking about testing their theory on more common things, like power amps.
it would be interesting to compare the transistors you removed against a new pair for their 1/f noise. or compare one to the other. to get 300mv offset one of them drifted more than the other.
Offset tweak for 9180 Hafler too
Thanks again for guiding me to minimizing the DC Offset in my DH200. the Q1,2,5,6 change out was the ticket! Now I need to perform a similar "fix" on my Hafler 9180 amp. It has 25mv on left and 50 mv on right channel. Normally this would be OK except I plan to direct drive a Morel EM1308 midrange driver from this amp. DC offset could push it out of its optimum GAP. What would you suggest I do to minimize DC offset in my 9189 series Hafler?
Thanks again for guiding me to minimizing the DC Offset in my DH200. the Q1,2,5,6 change out was the ticket! Now I need to perform a similar "fix" on my Hafler 9180 amp. It has 25mv on left and 50 mv on right channel. Normally this would be OK except I plan to direct drive a Morel EM1308 midrange driver from this amp. DC offset could push it out of its optimum GAP. What would you suggest I do to minimize DC offset in my 9189 series Hafler?
Haafler 9180 DC Offset
Did you start a new thread for this subject? You should. The 9180 used several FET parts in the VAS section and its troubleshooting/parts location tasks may be very different from the legacy Hafler amps that strictly use BJT parts.
As you study the 9180 schematic you can see that the input differential amplifier section (Q1-Q4) is comprised of FETs. Are these parts available? Does anyone provide matched sets of them? If not, what procedures does one use to match them?
Personally, I have no experience with this type of amp or parts so have no help to offer. I know from past experience that similar FET parts for the Hafler XL-280 amp in its diff amp section are no longer easily available. Several years ago one could get matched sets of them from Borbely Audio in Germany, but they were quite expensive.
Each FET in the diff amp has its own constant current source (Q5-Q8) which uses the very common BJT transistors found in the other Hafler amps. To what extent they must be in matched sets I don't know.
But, you have one channel that is OK which can be used as a reference. With extreme care and patience you could compare voltage readings at various points (probably emitters) of the constant current sources to see if any of them are different from the good channel. Ditto for other spots around the diff amp section.
There other troubleshooting tricks such as selective application of heat and cold to various devices while measuring DC offset, to see which ones make the meter reading jump the most.
And, of course, there is always the repair trick of carefully resoldering many points on the back side of the PCB.
This amp should have great sound but I have avoided it because of the complexity of any repairs or mods.
Good luck. I would like to see messages from others directed specifically at this amp, perhaps in the new thread you start.
The amount of DC offset is not HUGE so you could just ignore it. Or, you could insert a quality film cap in series with each mid range driver to block DC from getting to it. Its size and influence on the mid's high pass filter would have to be factored in.
Dick
Did you start a new thread for this subject? You should. The 9180 used several FET parts in the VAS section and its troubleshooting/parts location tasks may be very different from the legacy Hafler amps that strictly use BJT parts.
As you study the 9180 schematic you can see that the input differential amplifier section (Q1-Q4) is comprised of FETs. Are these parts available? Does anyone provide matched sets of them? If not, what procedures does one use to match them?
Personally, I have no experience with this type of amp or parts so have no help to offer. I know from past experience that similar FET parts for the Hafler XL-280 amp in its diff amp section are no longer easily available. Several years ago one could get matched sets of them from Borbely Audio in Germany, but they were quite expensive.
Each FET in the diff amp has its own constant current source (Q5-Q8) which uses the very common BJT transistors found in the other Hafler amps. To what extent they must be in matched sets I don't know.
But, you have one channel that is OK which can be used as a reference. With extreme care and patience you could compare voltage readings at various points (probably emitters) of the constant current sources to see if any of them are different from the good channel. Ditto for other spots around the diff amp section.
There other troubleshooting tricks such as selective application of heat and cold to various devices while measuring DC offset, to see which ones make the meter reading jump the most.
And, of course, there is always the repair trick of carefully resoldering many points on the back side of the PCB.
This amp should have great sound but I have avoided it because of the complexity of any repairs or mods.
Good luck. I would like to see messages from others directed specifically at this amp, perhaps in the new thread you start.
The amount of DC offset is not HUGE so you could just ignore it. Or, you could insert a quality film cap in series with each mid range driver to block DC from getting to it. Its size and influence on the mid's high pass filter would have to be factored in.
Dick
Matched Parts From...
I was able to order the matched pairs of Xistors from: http://tech-diy.com/
As you can see they worked great in my DH200. Now I need something for my 9180 series. That's going to be a little more difficult. I'll keep you all posted,,,,Please help me here if you can.
I was able to order the matched pairs of Xistors from: http://tech-diy.com/
As you can see they worked great in my DH200. Now I need something for my 9180 series. That's going to be a little more difficult. I'll keep you all posted,,,,Please help me here if you can.
New Thread started for 9180
Thanks Dick and all you amp mavens.
Per your suggestion Dick I have started a new thread specific to the Hafler 9180 series. see:
http://www.diyaudio.com/forums/showthread.php?s=&postid=1644723#post1644723
Thanks Dick and all you amp mavens.
Per your suggestion Dick I have started a new thread specific to the Hafler 9180 series. see:
http://www.diyaudio.com/forums/showthread.php?s=&postid=1644723#post1644723
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