Hafler Dh 220 muffled channel

mmerig · 26th January 2005, 10:48 AM

The left channel of my Hafler DH 220 is muffled. The sound out of the left channel is about half of the right, and it seems to have less bass than the right at comparable volume.

If I turn off the amp, but leave the preamp and a source going, the sound gets louder in the left channel just before it goes completely silent (about 2X for 1 second). The right channel rapidly goes silent when I do this test.

I set the bias at 275 mA (per Hafler manual), and the DC offset hovers between -3 to +3 mV. The B+ voltage at the fuses is the same for each channel with no input signal (about 63VDC).

I recently replaced the electrolytic capacitors on both boards (100uf 100v, 220uf 10 volts non-polar), and the 10Kuf power supply caps (they were ancient GE's). Someone (not on this site) recommended as a cheap mod to replace the C1 cap and the mylar caps near the output transistors with better film caps, so I did this too, but did not expect this to fix the problem.

I've checked and re-checked for cold solders, etc. and resistances at some nodes along the board (comparing left and right of course). I don't know that much about circuit design, but the short burst of sound when I turn off the amp seems to point at a capacitor or some other passive component, not a simple, poor connection. All of the MOS FET's feel similarly warm, but not hot (about 100 F).

Any ideas?

(BTW, I did the usual switching around of speaker wires, input wires, sources, used headphones, etc., and the problem is definitely in the DH 220).

Thanks for any advice from anyone.

slydwz · 26th January 2005, 10:58 AM

You will get something similar if the speaker fuse has blown. Have you checked it?

K-amps · 26th January 2005, 11:21 AM

In my experience, that symptom is sometimes related to bad capacitors.... sorry cant tell you which ones.

If you have already done that, check diodes especially any zeners.

fab · 27th January 2005, 02:05 AM

Hi mmerig,

See http://www.diyaudio.com/forums/showt...026#post359026 for some hints. Speakers fuse and zener diodes D12, D13 have to be verified so they are not shorted.

Good luck

Fab

mmerig · 27th January 2005, 04:24 AM

Thanks for the advice. The speaker fuses are okay. The sound is half again as loud, and quite a bit "thinner" when I pull the left speaker fuse.

I checked diodes 1 thru 8, and D12 and 13 (the zeners); all are okay. Other diodes are tough to get at. Voltage across R21 and R30 is 1.04 for both. Voltage across R33 is 1.46. (As per fab's suggestion, these are within specs).

Unless there are other diagnostic shortcuts, it looks like I'll have to start pulling some components off the board to test them, which would be very time consuming without a game plan.

One other mod that I did that did not seem worth mentioning is replacing the bias and offset trim pots with sealed multi-turn pots. I did this on both boards.

Any other hints?

mmerig · 27th January 2005, 05:45 AM

I pulled the board and checked the rest of the diodes. They are all okay.

K-amps · 27th January 2005, 11:20 AM

Well your next bet seems to be checking any FET/ MOSFET's. Being a BJT person myself, I do not know much about FET's but I knowthey "soft fail" sometimes....

fab · 28th January 2005, 01:09 AM

Quote:

Originally posted by mmerig
...The speaker fuses are okay. The sound is half again as loud, and quite a bit "thinner" when I pull the left speaker fuse....

Any other hints?

Fuses with aging or submitted to high current have sometimes their impedance increased. Have you checked that the resistance of the fuse is no higher than 0.1 ohms more than the resistance of your meter cabling shorted? or you can simply reverse the right and left fuses to see the effect on sound.

Fab

d3imlay · 28th January 2005, 06:47 AM

I'd leave the fets alone for the time being. if there was a problem with the fets it would sound distorted. The fets are very hard to find and expensive. Check the bias. If it's OK then the fets are probably OK.

It sounds to me like the electrolytic cap in the feedback loop is open. it helps set the gain of the amp. another possibility is the input coupling cap is open.

djk · 28th January 2005, 08:58 AM

Most likely the '220uf 10 volts non-polar' cap you put in was bad.

This cap is in the feedback loop to ground and if it is bad the amplifier will have very low gain and little bass.

26th January 2005, 10:48 AM	#1
mmerig diyAudio Member Join Date: Jan 2005 Location: south east Idaho, Teton Basin	Hafler Dh 220 muffled channel The left channel of my Hafler DH 220 is muffled. The sound out of the left channel is about half of the right, and it seems to have less bass than the right at comparable volume. If I turn off the amp, but leave the preamp and a source going, the sound gets louder in the left channel just before it goes completely silent (about 2X for 1 second). The right channel rapidly goes silent when I do this test. I set the bias at 275 mA (per Hafler manual), and the DC offset hovers between -3 to +3 mV. The B+ voltage at the fuses is the same for each channel with no input signal (about 63VDC). I recently replaced the electrolytic capacitors on both boards (100uf 100v, 220uf 10 volts non-polar), and the 10Kuf power supply caps (they were ancient GE's). Someone (not on this site) recommended as a cheap mod to replace the C1 cap and the mylar caps near the output transistors with better film caps, so I did this too, but did not expect this to fix the problem. I've checked and re-checked for cold solders, etc. and resistances at some nodes along the board (comparing left and right of course). I don't know that much about circuit design, but the short burst of sound when I turn off the amp seems to point at a capacitor or some other passive component, not a simple, poor connection. All of the MOS FET's feel similarly warm, but not hot (about 100 F). Any ideas? (BTW, I did the usual switching around of speaker wires, input wires, sources, used headphones, etc., and the problem is definitely in the DH 220). Thanks for any advice from anyone.

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26th January 2005, 10:58 AM	#2
slydwz diyAudio Member Join Date: Oct 2002 Location: Woodstock, GA	You will get something similar if the speaker fuse has blown. Have you checked it?

26th January 2005, 11:21 AM	#3
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	In my experience, that symptom is sometimes related to bad capacitors.... sorry cant tell you which ones. If you have already done that, check diodes especially any zeners.

27th January 2005, 02:05 AM	#4
fab diyAudio Member Join Date: Mar 2004 Location: Canada	Hi mmerig, See http://www.diyaudio.com/forums/showt...026#post359026 for some hints. Speakers fuse and zener diodes D12, D13 have to be verified so they are not shorted. Good luck Fab

27th January 2005, 04:24 AM	#5
mmerig diyAudio Member Join Date: Jan 2005 Location: south east Idaho, Teton Basin	Thanks for the advice. The speaker fuses are okay. The sound is half again as loud, and quite a bit "thinner" when I pull the left speaker fuse. I checked diodes 1 thru 8, and D12 and 13 (the zeners); all are okay. Other diodes are tough to get at. Voltage across R21 and R30 is 1.04 for both. Voltage across R33 is 1.46. (As per fab's suggestion, these are within specs). Unless there are other diagnostic shortcuts, it looks like I'll have to start pulling some components off the board to test them, which would be very time consuming without a game plan. One other mod that I did that did not seem worth mentioning is replacing the bias and offset trim pots with sealed multi-turn pots. I did this on both boards. Any other hints?

27th January 2005, 05:45 AM	#6
mmerig diyAudio Member Join Date: Jan 2005 Location: south east Idaho, Teton Basin	I pulled the board and checked the rest of the diodes. They are all okay.

27th January 2005, 11:20 AM	#7
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	Well your next bet seems to be checking any FET/ MOSFET's. Being a BJT person myself, I do not know much about FET's but I knowthey "soft fail" sometimes....

28th January 2005, 06:47 AM	#9
d3imlay diyAudio Member Join Date: Sep 2004 Location: Ohio	I'd leave the fets alone for the time being. if there was a problem with the fets it would sound distorted. The fets are very hard to find and expensive. Check the bias. If it's OK then the fets are probably OK. It sounds to me like the electrolytic cap in the feedback loop is open. it helps set the gain of the amp. another possibility is the input coupling cap is open.

28th January 2005, 08:58 AM	#10
djk diyAudio Member Join Date: Feb 2001 Location: USA	Most likely the '220uf 10 volts non-polar' cap you put in was bad. This cap is in the feedback loop to ground and if it is bad the amplifier will have very low gain and little bass.

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