There is no adjustment on the driver board to null out the DC offset. My old DH200 sounds good...but today I measured 8.9mV DC on RIGHT channel, and ~300mV DC on the LEFT channel (measured at speaker terminals. Is this extraordinarily high? A problem? Any solution / suggestions? ( I see the DH220 has a trim pot for this...not so in the DH200) THANKS !
If you do a search you will find much discussion on hafler dc2**
amps. The short answer is to match the differential transistors(q1,q2,q5,q6) for current draw. Transistor matching is an art unto itself, also a search is in order(ck out Nelson Pass DIY page for additional info).
There is a ton of info on the forum just search Hafler DH200/220 mods for the thread...Happy Discovery
Regards, Elwood
amps. The short answer is to match the differential transistors(q1,q2,q5,q6) for current draw. Transistor matching is an art unto itself, also a search is in order(ck out Nelson Pass DIY page for additional info).
There is a ton of info on the forum just search Hafler DH200/220 mods for the thread...Happy Discovery
Regards, Elwood
More Info Needed
I spent much time sifting through some of the materials mentioned....However...I'm not a very technical person...and I have little test equipment beyond a DMM and soldering iron. I was hoping there would be a resistor to tweak or a trim-pot I could solder in to use to null the offset to something under 25mv.
Perhaps a new set of driver boards (musical concepts?) could help but I'd rather not invest another $400.00 into my 1981 vintage Hafler DH200. ( I see where the "newer" Haflers i.e. DH220 has a trim pot to offset DC on output but no such function is on the DH200 driver board) . In the real world how big a problem is a 300mv offset on a channel?
Thanks
I spent much time sifting through some of the materials mentioned....However...I'm not a very technical person...and I have little test equipment beyond a DMM and soldering iron. I was hoping there would be a resistor to tweak or a trim-pot I could solder in to use to null the offset to something under 25mv.
Perhaps a new set of driver boards (musical concepts?) could help but I'd rather not invest another $400.00 into my 1981 vintage Hafler DH200. ( I see where the "newer" Haflers i.e. DH220 has a trim pot to offset DC on output but no such function is on the DH200 driver board) . In the real world how big a problem is a 300mv offset on a channel?
Thanks
Transistors are cheap(at least these are)buy 10 or 20 and use your dmm to match. all you need is a cheap power supply and a DMM.RadioShack has all you need to build a jig to match transistors, and MOUSER or DIGIKEY has the transistors. take a little time relax have a beverage of choice, and match a few transistors . put them in the board an check your offset, anything under 50 mv is good. goto www.Oldcrows.net/~patchell/matcher/matcher.html , for a simple circuit to build a jig to match transistors(bjt type) Go to Hafler.com for owners manuals with parts list for DC200 and DC220 to get transistor part numbers. both of these manuals have schematics and parts list.
Stay after it, You Know you want to...
Regards, Elwood(forgive the typing errors)
Stay after it, You Know you want to...
Regards, Elwood(forgive the typing errors)
Replacement Xistors
Perhaps trying the Xistor change out will be the way to go (for now). I recall somewhere in the depths of the recommended readings that there are better substitute transistors to use in place of Q1,2,5,6 in the DH200. Can anyone tell me what they are ? (If hope they won't require other component changes too).
Anyone know if there are "matched sets" pre screened available out there? THANKS
Perhaps trying the Xistor change out will be the way to go (for now). I recall somewhere in the depths of the recommended readings that there are better substitute transistors to use in place of Q1,2,5,6 in the DH200. Can anyone tell me what they are ? (If hope they won't require other component changes too).
Anyone know if there are "matched sets" pre screened available out there? THANKS
The 2N5401/2N5550 are ok for especially more modern versions.
Try our friends at http://tech-diy.com/Store/store.htm for matching and availability. You may also consider replacing all the electrolytic caps on the boards as well as the P/S as they are getting along in age, and doing some research on grounding schemes ...
Good luck.
Regards, Elwood
Try our friends at http://tech-diy.com/Store/store.htm for matching and availability. You may also consider replacing all the electrolytic caps on the boards as well as the P/S as they are getting along in age, and doing some research on grounding schemes ...
Good luck.
Regards, Elwood
offset...a possibility
To get 300 mV of offset, something has to be VERY unbalanced, or kind of leaky.
My best guess from far away is that C5, a 470 uF non polar electrolytic, has gone very leaky, or quite shorted.
That raises the DC gain from 1 to around 22. That's enough to make a LOT of offset at the output.
Don't operate the amp with the cap removed, as it probably won't be stable.
Replace the cap with a new one, and cross your fingers.
Best of luck...
Dan
To get 300 mV of offset, something has to be VERY unbalanced, or kind of leaky.
My best guess from far away is that C5, a 470 uF non polar electrolytic, has gone very leaky, or quite shorted.
That raises the DC gain from 1 to around 22. That's enough to make a LOT of offset at the output.
Don't operate the amp with the cap removed, as it probably won't be stable.
Replace the cap with a new one, and cross your fingers.
Best of luck...
Dan
I've seen several DH200s that developed those kinds of offsets, and in both cases it was the input transistors that were doing it. Rather than sending off for new ones, I fixed the amps by changing the input stage into a cascode arrangment using different transistors (unmatched 2n5210 and 2n5087) as the 4 input parts and the existing high voltage (but apparently a little leaky) 4 transistors as the upper output parts. It's a pretty easy change, using the same PC layout and only needing the transistors and a few wires.
It would be easier to replace the existing transistors with the same type (no wires to jumper, leads to twist, or circuit changes to make), though, but matching transistors is a fair bit of effort to arrange to do. The matching probably isn't needed anyway (I doubt Hafler did any when they made the amps), and you'd get a much lower offset by just putting in new (nonleaky) replacements of the same type transistors. A 300mV offset is a lot(!) and is NOT going to happen just from differences in Vbe between the transistors, more likely to be collector-base leakage.
It would be easier to replace the existing transistors with the same type (no wires to jumper, leads to twist, or circuit changes to make), though, but matching transistors is a fair bit of effort to arrange to do. The matching probably isn't needed anyway (I doubt Hafler did any when they made the amps), and you'd get a much lower offset by just putting in new (nonleaky) replacements of the same type transistors. A 300mV offset is a lot(!) and is NOT going to happen just from differences in Vbe between the transistors, more likely to be collector-base leakage.
OK Lads...
back to basics(things we know about old amps). Replace the electolytics(as we all know they are old and leaky) on the driver boards, look for poor solder joints(fix as needed), then attend to grounding schemes(a good example is.....found at http://www.musicaldesign.com/mc_amp_mods.htm,
download manual .
then check your offset if it is still out of range, replace your front end trasistors with new fresh matched ones
Regards,Elwood
back to basics(things we know about old amps). Replace the electolytics(as we all know they are old and leaky) on the driver boards, look for poor solder joints(fix as needed), then attend to grounding schemes(a good example is.....found at http://www.musicaldesign.com/mc_amp_mods.htm,
download manual .
then check your offset if it is still out of range, replace your front end trasistors with new fresh matched ones
Regards,Elwood
Hola Y'all...
I think Ronzeman has the older PC6 boards that do not have offset pots just bias. So it is more likely to be a leaky cap,poor connection on the board, or a wayward differential transistor or two. all of those parts a cheap enough to warrant freshening up.
The end result is a great amp for not a lot of money, also there maybe someone on the forum that has a good set of PC6 or PC19c they would be willing to part with.
Regards, Elwood
I think Ronzeman has the older PC6 boards that do not have offset pots just bias. So it is more likely to be a leaky cap,poor connection on the board, or a wayward differential transistor or two. all of those parts a cheap enough to warrant freshening up.
The end result is a great amp for not a lot of money, also there maybe someone on the forum that has a good set of PC6 or PC19c they would be willing to part with.
Regards, Elwood
Update Parts
CORRECT: Mine Hafler is an old DH220 no trim pot on DC offset. My right channel has less than 9mV offset; left channel is 300mV.
Thanks folks. I ordered a set of matched Xistors (1,2,4,5) from DIYStore. What caps would you recommend to replace ? ( in both Left and Right boards)
BTW; I just purchased a Series 9180 Hafler. I measured the DC Offsets as: R= 25.1mv L= 50.9 mv. I can live with that.
I hope to use both Haflers to Bi Amp a pair of Audionics TL-90 Speakers that I'm refurbing and updating.
CORRECT: Mine Hafler is an old DH220 no trim pot on DC offset. My right channel has less than 9mV offset; left channel is 300mV.
Thanks folks. I ordered a set of matched Xistors (1,2,4,5) from DIYStore. What caps would you recommend to replace ? ( in both Left and Right boards)
BTW; I just purchased a Series 9180 Hafler. I measured the DC Offsets as: R= 25.1mv L= 50.9 mv. I can live with that.
I hope to use both Haflers to Bi Amp a pair of Audionics TL-90 Speakers that I'm refurbing and updating.
back when that amp was made, +/-100mV was the commonly acceptble range for offset, and +/-300mV was "tolerable" (but definitely not desireable). that you have an amp where one channel is within the 100mV range and one outside of it, tells me that you have what right now is a minor problem. if you look at the schematic you have C5, a 470uf, 6.3V electrolytic. this is probably leaky. it's probably better to use one that's rated at 10 or 16V, and with a 105 degree C temperature rating. it's probably a good idea to keep an extra for the other channel, since it's likely that not only the two boards are the same age, but the caps in both channels came from the same batch. same idea for the transistors, keep a set of spares on hand for the other channel.
btw, looking at the DH220 manual, the original spec for offset was +/-10mV, not 100mv.
btw, looking at the DH220 manual, the original spec for offset was +/-10mV, not 100mv.
hola again...
on my DH200 / pc6 boards I replaced C1 with a film cap(wima etc.) 2mfd/50v, C5 with a low esr 470 mfd / 10v non polar cap,
C7 and C12 with 220mfd/100v. I also replaced the p/s filter caps
15000mfd/80v bypassed with a 470mfd 250v . The other hafler thread has quite a bit of good info for futhe r mods and bias settings. Do your homework, thats why the call it diy. Some basic bench items like a good dvm or 3 ,an audio signal generator, oscilliscope, and a variac can be invaluable. Most of my stuff i have bought on Ebay or in pawn shops.
Have fun and be careful in the amp big mojo in those caps can be leathal.
Regards, Elwood
on my DH200 / pc6 boards I replaced C1 with a film cap(wima etc.) 2mfd/50v, C5 with a low esr 470 mfd / 10v non polar cap,
C7 and C12 with 220mfd/100v. I also replaced the p/s filter caps
15000mfd/80v bypassed with a 470mfd 250v . The other hafler thread has quite a bit of good info for futhe r mods and bias settings. Do your homework, thats why the call it diy. Some basic bench items like a good dvm or 3 ,an audio signal generator, oscilliscope, and a variac can be invaluable. Most of my stuff i have bought on Ebay or in pawn shops.
Have fun and be careful in the amp big mojo in those caps can be leathal.
Regards, Elwood
yeah, what he said.... the charge in those caps is enough to vaporize the tip of a screwdriver, imagine what it can do to you...... use 10 OHM 5 WATT resistor mounted in a dual banana plug as a handy discharger. it will still spark a little, but not like a screwdriver. keep that discharger handy on the workbench it'll save your screwdriver blades and cut down on 911 calls.... 
also.... NEVER do any work inside an amp (except powered adjustments like bias) with it plugged in EVER!!
getting "bit" is an occupational hazard, but proper safety procedures will hopefully 1)eliminate the hazard altogether 2)minimize the probability or effects of a shock hazard.
we shoot for #1, but if we DO get bit, proper safety procedures will minimize the effects of getting bit.
another good one is even with the unit unplugged and caps discharged, get in the habit of using only one hand to work on the equipment, preferably your right hand (even if you're left handed)... if you must use both hands inside of the equipment, get in the habit of resting the heels of both hands on the chassis while you work (better to have sprained fingers than a sprained heart muscle).
also.... NEVER do any work inside an amp (except powered adjustments like bias) with it plugged in EVER!!
getting "bit" is an occupational hazard, but proper safety procedures will hopefully 1)eliminate the hazard altogether 2)minimize the probability or effects of a shock hazard.
we shoot for #1, but if we DO get bit, proper safety procedures will minimize the effects of getting bit.
another good one is even with the unit unplugged and caps discharged, get in the habit of using only one hand to work on the equipment, preferably your right hand (even if you're left handed)... if you must use both hands inside of the equipment, get in the habit of resting the heels of both hands on the chassis while you work (better to have sprained fingers than a sprained heart muscle).
300mv sure seems excessive, even for a DH200. Replace the caps and see if it changes. Then check the ground wiring to the schematic. If this was originally a kit amp whoever built could have screwed up the wiring and not realized it because the amp will still work with that much offset.
I'm just a dumb kid that's been at this stuff for 4 years and never been shocked, so you guys have no excuses.
I'm just a dumb kid that's been at this stuff for 4 years and never been shocked, so you guys have no excuses.
ronzeman,
Go to the link below for an explanation of a small and simple circuit to null out DC offset on a DH-200. It works fine for small amounts of DC offset. The parts are cheap and only a small amount of time is required to install the circuit:
The reference is message # 623 in the Hafler DH200/220 Mods thread.
http://www.diyaudio.com/forums/showthread.php?postid=1477093#post1477093
In the message I stated to use a 1 megohm resistor. I should have stated to use a 2 megohm resistor. I used a small wire drill to make 3 tiny holes in the PCB through which the legs of the variable resistor were pushed, then the legs were soldered to the wiring to attach the PS rails and the 2 megohm resistor.
The bottom leg was bent down and soldered to the PS rail on the left side of the PCB. The topmost leg has a wire that is attached to the PS rail on the right side of the PCB. The middle leg (wiper) has the 2 megohm resistor attached to it that feeds the small amount of either + or - voltage required to balance out the DC offset.
For me this was quicker and cheaper than getting matched sets of small signal transistors for the input differential amp.
Good luck.........
Go to the link below for an explanation of a small and simple circuit to null out DC offset on a DH-200. It works fine for small amounts of DC offset. The parts are cheap and only a small amount of time is required to install the circuit:
The reference is message # 623 in the Hafler DH200/220 Mods thread.
http://www.diyaudio.com/forums/showthread.php?postid=1477093#post1477093
In the message I stated to use a 1 megohm resistor. I should have stated to use a 2 megohm resistor. I used a small wire drill to make 3 tiny holes in the PCB through which the legs of the variable resistor were pushed, then the legs were soldered to the wiring to attach the PS rails and the 2 megohm resistor.
The bottom leg was bent down and soldered to the PS rail on the left side of the PCB. The topmost leg has a wire that is attached to the PS rail on the right side of the PCB. The middle leg (wiper) has the 2 megohm resistor attached to it that feeds the small amount of either + or - voltage required to balance out the DC offset.
For me this was quicker and cheaper than getting matched sets of small signal transistors for the input differential amp.
Good luck.........
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