Hi all,
I asked this over at Audiokarma but haven't received any answers yet.
Is there a way to test Hafler DH-series amps with the Mosfet outputs removed?
I've searched and haven't found a totally clear answer.
I'm trying to locate a significant current draw and the MosFets test fine individually when removed (just testing on a cheap eBay transistor tester, so I understand it still could be an output).
Tested with Right rail fuses installed, left removed. Only got to 20vac on Variac before I noticed problems. Measured 10vdc on positive rail at filter cap, 6vdc on negative rail at filter cap. At speaker relay I measured about 4vdc on output.
So, obviously something is dragging negative rail down and dumping a portion of the rail voltage onto the speaker output.
I would like to rule out the board before proceeding.
(EDIT: also at 20vac on Variac, meter is showing about 5A current draw on AC line. Obviously not a good sign.)
I've read in one place that you can remove all the outputs and put a 1kR across terminal #6 to #7, and #9 to #7 and this will be enough load (sic?) for the board to function at a base level? But, I have not found anywhere to confirm this before I try it.
This is for a DH-500 that landed in my lap as a salvage/parts unit. Thought I would ask here instead of starting a whole new thread since this is by far the largest Hafler DH thread I could find.
I asked this over at Audiokarma but haven't received any answers yet.
Is there a way to test Hafler DH-series amps with the Mosfet outputs removed?
I've searched and haven't found a totally clear answer.
I'm trying to locate a significant current draw and the MosFets test fine individually when removed (just testing on a cheap eBay transistor tester, so I understand it still could be an output).
Tested with Right rail fuses installed, left removed. Only got to 20vac on Variac before I noticed problems. Measured 10vdc on positive rail at filter cap, 6vdc on negative rail at filter cap. At speaker relay I measured about 4vdc on output.
So, obviously something is dragging negative rail down and dumping a portion of the rail voltage onto the speaker output.
I would like to rule out the board before proceeding.
(EDIT: also at 20vac on Variac, meter is showing about 5A current draw on AC line. Obviously not a good sign.)
I've read in one place that you can remove all the outputs and put a 1kR across terminal #6 to #7, and #9 to #7 and this will be enough load (sic?) for the board to function at a base level? But, I have not found anywhere to confirm this before I try it.
This is for a DH-500 that landed in my lap as a salvage/parts unit. Thought I would ask here instead of starting a whole new thread since this is by far the largest Hafler DH thread I could find.
Last edited:
Those pin numbers don't look right. You remove the FET's and link the feedback point to either Q12 or Q13 emitters.
So I would say either 4 or 9 to 8.
Easier than all that to begin with is to monitor the voltage across R33. If that rises above more than about 1 volt or so then the FET's will start to conduct. See what you have across there first.
The amp will run OK with one pair of FET's fitted for test purposes.
So I would say either 4 or 9 to 8.
Easier than all that to begin with is to monitor the voltage across R33. If that rises above more than about 1 volt or so then the FET's will start to conduct. See what you have across there first.
The amp will run OK with one pair of FET's fitted for test purposes.
Update: issue solved. I feel really stupid for this, but I think it's probably an easy mistake to make.
It turns out that there is a difference in the connections for PC-10 and PC-19 boards.
I knew I had the earlier PC-10 boards, but the only assembly manuals available online show the PC-19 boards in the schematic and wiring pictorial.
Well, I was trying to solder up the dummy 1k resistors and something wasn't clicking so I dug around and managed to find a fuzzy PC-10 schematic only (no wiring diagram).
Turns out that between the board versions terminals 4 and 5 (Gate and Line out to Relay) are flipped. And Terminals 6 and 7 (Source + feedback loop and Ground) are flipped!
No wonder it wasn't playing nice.
This amp's prior life was with a Pro-sound company and it came to me labelled for "parts" after the fan failed and someone tried and only partially succeeded in repairing the fan circuit. Whoever was working on it left it partially disassembled, which is why I had to reconnect all the wiring to the amp/heatsink module.
Happy to report I got the wiring corrected and it has been playing sweet music. And yes, I did repair the fan circuit. The fact that attempts to fire it up with the wrong connections didn't blow it up really impresses me with how rugged these old amps are!
Now to go through it and get it updated and cleaned up.
Thanks for the help.
It turns out that there is a difference in the connections for PC-10 and PC-19 boards.
I knew I had the earlier PC-10 boards, but the only assembly manuals available online show the PC-19 boards in the schematic and wiring pictorial.
Well, I was trying to solder up the dummy 1k resistors and something wasn't clicking so I dug around and managed to find a fuzzy PC-10 schematic only (no wiring diagram).
Turns out that between the board versions terminals 4 and 5 (Gate and Line out to Relay) are flipped. And Terminals 6 and 7 (Source + feedback loop and Ground) are flipped!
No wonder it wasn't playing nice.
This amp's prior life was with a Pro-sound company and it came to me labelled for "parts" after the fan failed and someone tried and only partially succeeded in repairing the fan circuit. Whoever was working on it left it partially disassembled, which is why I had to reconnect all the wiring to the amp/heatsink module.
Happy to report I got the wiring corrected and it has been playing sweet music. And yes, I did repair the fan circuit. The fact that attempts to fire it up with the wrong connections didn't blow it up really impresses me with how rugged these old amps are!
Now to go through it and get it updated and cleaned up.
Thanks for the help.
Recently finished my Hafler DH-500C. I acquired the Hafler amplifier many years ago with one dead channel. The dead channel was carnage with an apparent explosion that blew a hole in the PC19B circuit board. I bought the DH-220C circuit boards May 2021 and I am finally enjoying the finished product. I rebuilt it using original MOSFets in one channel and Exicon replacements in the other.
I did observe wandering bias in both channels. There is more than 100 mA higher bias current with cold amplifier versus after 15 minutes of warmup. Behavior is the same in both channels. Is this normal?
I am curious how many of the Cordell Hafler amplifiers exist in the world. Any feedback from builders?
Regards,
Glenn
I did observe wandering bias in both channels. There is more than 100 mA higher bias current with cold amplifier versus after 15 minutes of warmup. Behavior is the same in both channels. Is this normal?
I am curious how many of the Cordell Hafler amplifiers exist in the world. Any feedback from builders?
Regards,
Glenn
I use my DH-220C nearly every day since I built it almost two years ago. It's in my basement rig with my EconoWave speakers and Playstation/Roku TV/Logitech Squeezebox as sources. It sounds superb and is still one of my favorite amps.
See my posts in this thread: #1933, 1937, 1944, and 2027.
See my posts in this thread: #1933, 1937, 1944, and 2027.
Due to the LatFETs' PTC characteristics and the lack of any compensation scheme, I'd say yes.
Best regards!
All of your encouraging contributions make me consider to replace the two pair Crescendos in my 40 years old existing build by Bob's designs, reusing the chassis/enclosure, the PSU and the 'housekeeping' 
.
Best regards!
Addendum: ...as soon as I manage to get my hands on a set of PCB's and the schematics...
.Best regards!
Addendum: ...as soon as I manage to get my hands on a set of PCB's and the schematics...
Last edited:
I saw your Crescendo amp pictures in another thread. They look great as donors for a newer design. Overbuilt power supply, plenty of K135/J50 pairs. What is the DC voltage out of that power supply? Probably just right for the two-pair Hitachi outputs per channel.
@rsavas would be able to tell you if boards/schematics are available
What is the DC voltage out of that power supply? Probably just right for the two-pair Hitachi outputs per channel.@rsavas would be able to tell you if boards/schematics are available
The build features two 500 VA toroids, each with 55-50-0-50-55 Vac secondaries. Hence, there are two rail pairs of ±70 Vdc, filtered by 10 mF, for the power devices, and ±77 Vdc, filtered by 2.5 mF, for the small signal section. I think that would suffice . But I also could add one more pair per amplifier if necessary, as I still have plenty of SJ50's/SK135's.
Best regards!
. But I also could add one more pair per amplifier if necessary, as I still have plenty of SJ50's/SK135's.Best regards!
Has anyone heard both the DH-220c and a 220 with Ed Fantasia's boards? I'm interested in a comparison. I've installed Ed's boards and the result is quite satisfactory, and I'm considering doing a second amp with the 220c boards, or building one from scratch with Exicons.
I've searched the forum but have not located a reference for a toroidal transformer for the DH200. I have a 1981 vintage one that I'd like to upgrade. Looked at the Concepts listing but it seems there should be an Antek of similar option someone has explored also... While I'm asking - any user feedback on their power supply the PS100 w/ Munford caps option? Thanks
I'm in the same boat. I have a nice DH200 that I did the full monty with Qua-co (Ed Fantasia) kit from eBay. I really like it although there does seem to be a slight up-tilt in the upper frequencies that is just ever so slightly annoying (I'm very sensitive to "bright" components).
I've seen so many posts lavishing praise on the Cordell boards, but none that dive into exactly what they like so much about them. I'd be really interested if someone would do say a single paragraph or two on their listening impressions compared to stock, or any of the other well known upgrade paths.
If I've missed out on an extensive review, pardon my ignorance. But would appreciate a link.
@dogwan, I am not one for extensive prose on what an amp sounds like, but I will say that after spending several years with bargain higher quality class D designs and well-preserved vintage amps and receivers, listening to the DH-220C was beyond a breath of fresh air. It was like the cliche veil was lifted and there was more resolution into the recordings than before. From top to bottom of the audio spectrum. It is certainly not “bright” and has an overall balance to amplification that is rare in my experience of futzing with audio components over the years.
The DH-220C whet my appetite (after MANY years of lurking) for building amplifiers designed by very, very intelligent and wise designers (Bob Cordell and Nelson Pass, especially) and I have not stopped for two years. I expect that makes me a young’un still in diyAudio, but I think I’ve learned a heck of a lot about what makes me happy with this hobby in that time. I will unashamedly promote the DH-220C as a fabulous amplifier well worth its modest cost whenever it seems appropriate for me to do so. I use mine all the time and will likely never part with it. Bob‘s and Rick’s documentation of the DH-220C are first rate and great education on top of being practical for a specific build. All praise from me. 👍
The DH-220C whet my appetite (after MANY years of lurking) for building amplifiers designed by very, very intelligent and wise designers (Bob Cordell and Nelson Pass, especially) and I have not stopped for two years.
I expect that makes me a young’un still in diyAudio, but I think I’ve learned a heck of a lot about what makes me happy with this hobby in that time. I will unashamedly promote the DH-220C as a fabulous amplifier well worth its modest cost whenever it seems appropriate for me to do so. I use mine all the time and will likely never part with it. Bob‘s and Rick’s documentation of the DH-220C are first rate and great education on top of being practical for a specific build. All praise from me. 👍A few things I can add is that Dh-220c retains the original 200/220 designers topology of complementary input stage allowing for a push-pull Voltage Amplifier Stage. Dh-220c improves the original design by using complementary jfets in the IPS and a DC servo to allow removal of the Evil ecap in the FB loop. We also designed a special OPS pcb specific for each Hafler design we support. How much influence each design technique add to the listening experience is debatable
I struggle to tell any difference between the Dh-200c and the new BC-1 I am currently using
I struggle to tell any difference between the Dh-200c and the new BC-1 I am currently using
Last edited: