Amplifier Distortion, DC-Offset, and You!
- Thread starter EchoWars
- Start date
Excellent post, quick question
After lurking for quite a while, I decided to join because of all the excellent info and friendly/knowlegable people here.
Echowars, will this test work with an amplifier that is set for bridging? For example, I have 2 NAD 2400 amps, both set bridged, each amp running one speaker in a 2 channel setup. I am using both positive outputs from the A and B channels with speaker B mode selected.
Will I have to set the amp to normal and test it or will it give usable/meaningful results in bridged mode?
Thanks again for the great info.
Ron
After lurking for quite a while, I decided to join because of all the excellent info and friendly/knowlegable people here.
Echowars, will this test work with an amplifier that is set for bridging? For example, I have 2 NAD 2400 amps, both set bridged, each amp running one speaker in a 2 channel setup. I am using both positive outputs from the A and B channels with speaker B mode selected.
Will I have to set the amp to normal and test it or will it give usable/meaningful results in bridged mode?
Thanks again for the great info.
Ron
Hi Ron!!
The concept is the same whether bridged or not - an amplifier should pass only AC along to the speakers. You do not have to set to stereo mode to check offset, just connect a meter between the two positive terminals that supply the signal in bridged mode.
Setting the offset, if the amp has internal provisions to do so, would likely require that the amp be set to stereo mode. But just to see where you are now would require you do nothing more than begin measuring.
The concept is the same whether bridged or not - an amplifier should pass only AC along to the speakers. You do not have to set to stereo mode to check offset, just connect a meter between the two positive terminals that supply the signal in bridged mode.
Setting the offset, if the amp has internal provisions to do so, would likely require that the amp be set to stereo mode. But just to see where you are now would require you do nothing more than begin measuring.
Philips 388 dc readings
Echo,
I checked the readings on my philips 388. the right side read 7.1 mv, the left side read 34.7. the reading came up as negative with the leads hooked up to their respective poles (is this a phase issue? that could be causing problems?).
Could this difference between left and right cause the low end on this amp to sound somewhat anemic?
Pleas let me know your thoughts
Echo,
I checked the readings on my philips 388. the right side read 7.1 mv, the left side read 34.7. the reading came up as negative with the leads hooked up to their respective poles (is this a phase issue? that could be causing problems?).
Could this difference between left and right cause the low end on this amp to sound somewhat anemic?
Pleas let me know your thoughts
3 amps tested 2 ok 1 YIKES
OK, here is what I found.
I have 3 power amps all vintage. 2 of them are NAD 2400's running in bridged mode, and 1 is a Denon POA 2400A.
NAD 1 -24mV
NAD 2 - 320mV YIKES!!!!
Denon L-14mV R-16mV
I am going to switch the NAD's back to stereo and retest them again and find out which channel in #2 that is causing the problem. I will post back with the results.
Methinks with the info provided by Echowars on this simple yet revealing test, it is time to pull the NAD amps out of the system and put in the Denon. Now to find someone that will work on NAD.
Thanks again Echowars.
Ron
OK, here is what I found.
I have 3 power amps all vintage. 2 of them are NAD 2400's running in bridged mode, and 1 is a Denon POA 2400A.
NAD 1 -24mV
NAD 2 - 320mV YIKES!!!!
Denon L-14mV R-16mV
I am going to switch the NAD's back to stereo and retest them again and find out which channel in #2 that is causing the problem. I will post back with the results.
Methinks with the info provided by Echowars on this simple yet revealing test, it is time to pull the NAD amps out of the system and put in the Denon. Now to find someone that will work on NAD.
Thanks again Echowars.
Ron
stewey
Active Member
Hi reznik, EW will no doubt correct me if I'm wrong, so here goes.
The dc-offset centers around 0v, but it does not matter if the reading you get is positive or negative. It is the absolute value we are concerned with.
From everything I've read on this, 35mv is not enough to make the low-end 'anemic'. If you do adjust it down closer to zero, you may notice a slight improvement in sound, but probably nothing too significant.
Good luck, Stew.
The dc-offset centers around 0v, but it does not matter if the reading you get is positive or negative. It is the absolute value we are concerned with.
From everything I've read on this, 35mv is not enough to make the low-end 'anemic'. If you do adjust it down closer to zero, you may notice a slight improvement in sound, but probably nothing too significant.
Good luck, Stew.
Stew,
Your thoughts are the same as mine. The 34.7mv in and of itself still falls into the acceptable category, but it does raise a flag towards the root problem. Which brought me to the thought, there may be a grounding problem at fault here.
All I really do know for sure right now.... My wife is getting pretty sick of me trying to find out what the problem is by cranking old Rhino Bucket music from the early 90's. It's sort of like repairing a Juke Box that only has one 45 rpm record in it.
Your thoughts are the same as mine. The 34.7mv in and of itself still falls into the acceptable category, but it does raise a flag towards the root problem. Which brought me to the thought, there may be a grounding problem at fault here.
All I really do know for sure right now.... My wife is getting pretty sick of me trying to find out what the problem is by cranking old Rhino Bucket music from the early 90's. It's sort of like repairing a Juke Box that only has one 45 rpm record in it.
OMG!
Echo,
Picked up a Marantz 2230 yesterday at a garage sale, the owner said it had scratchiness in the pots, but worked fine. Just got done checking DC offset and coudn't believe my meter. It read OL on the millvolt scale. Set it to volts --- 29.68 in one and 29.38 in the other. Thought maybe that I wasn't using the right scale on my meter, I checked 3 other receivers and all had low readings in the millivolt scale. Rechecked the marantz, OL in millivolts and 29.68 and 29.38 on the volts scale. I don't think it's my meter- fluke 87. Haven't hooked up any speakers yet- Thank God. Popped the cover off and it is pretty dusty on the back half, haven't cleaned any pots or anything. Nothing looks suspicious from what I can see of the boards ( No popped zenners like my Pioneer 1250 you fixed for me last year) Any thoughts or warnings?
Thanks.................. Kim
Echo,
Picked up a Marantz 2230 yesterday at a garage sale, the owner said it had scratchiness in the pots, but worked fine. Just got done checking DC offset and coudn't believe my meter. It read OL on the millvolt scale. Set it to volts --- 29.68 in one and 29.38 in the other. Thought maybe that I wasn't using the right scale on my meter, I checked 3 other receivers and all had low readings in the millivolt scale. Rechecked the marantz, OL in millivolts and 29.68 and 29.38 on the volts scale. I don't think it's my meter- fluke 87. Haven't hooked up any speakers yet- Thank God. Popped the cover off and it is pretty dusty on the back half, haven't cleaned any pots or anything. Nothing looks suspicious from what I can see of the boards ( No popped zenners like my Pioneer 1250 you fixed for me last year) Any thoughts or warnings?
Thanks.................. Kim
If it doesn't pop a main fuse, then a pre-output stage is shorted...or something like that 
Odd that both channels exhibit the same behavior. Check all the fuses. I suspect that this might have a fuse on one of the rails that is toast. If all that looks OK, then again I'd suspect a shorted device on the driver boards.
Odd that both channels exhibit the same behavior. Check all the fuses. I suspect that this might have a fuse on one of the rails that is toast. If all that looks OK, then again I'd suspect a shorted device on the driver boards.
Checked my Sherwood S-7300. Left side 2.2, Right side 28.8, odd thing is the Left side has NTE TO-3's and Right side has Sony's. There are two pots, I suspect they are bias adjustments, made no change. I also suspect the Sony's are probably original and the NTE's were replaced somewhere down the road, thus the low 2.2mv reading I got from that side. The Right side isn't "bad" but I think it's probably weak, I also found a ground wire loose on the Right (Sony) side and soldered it back. I'm considering rebuilding both input stages and replacing all the TO-3 devices, just for my learning experience. I found some on National Semiconductor's site, 80wpc rated.
Echo, what do you think of these as replacements?
http://www.national.com/ds/LM/LM12CL.pdf
Echo, what do you think of these as replacements?
http://www.national.com/ds/LM/LM12CL.pdf
The Sony devices are 2SD339 and the NTE's are NTE280, doing a search I notice these devices have different wpc output, the Sony 80wpc and the NTE 100wpc, I know this is dependant on the input stage but it appears the Sony is the older of the two.
Loosened input driver boards, found PNP device on NTE side changed from A706 to NTE189, also found about 4 newer looking resistors.
Great thing about this receiver, every section is separate for quick repairs or upgrades. Both boards disconnect with just 2 screws and are slot mounted in the chassis and come right out. Of course the wiring needs to be desoldered to be removed completely, but there are only about 8 wires total per board, 4 on each side of each board. And all those are soldered on posts.
According to NTE's site, the NTE181 is suppose to be the direct replacement for the 2SD339.
Loosened input driver boards, found PNP device on NTE side changed from A706 to NTE189, also found about 4 newer looking resistors.
Great thing about this receiver, every section is separate for quick repairs or upgrades. Both boards disconnect with just 2 screws and are slot mounted in the chassis and come right out. Of course the wiring needs to be desoldered to be removed completely, but there are only about 8 wires total per board, 4 on each side of each board. And all those are soldered on posts.
According to NTE's site, the NTE181 is suppose to be the direct replacement for the 2SD339.
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NTE uses whatever source they can...their matching is non-existent, and the likelyhood of it actually being an 'exact' replacement is slim to none. Their devices work, but that's as much as I'll grant them.
A good replacement for the outputs, if you'd like matching devices, would be 2SD287's (NPN) and 2SB539's (PNP's). Not terribly expensive, and usually pretty easy to find.
The devices for the output are not really chosen for any real reason relating to the input...they are chosen according to the supply voltage and expected output power, and well as the load the amp is intended to be able to drive.
I would suggest that if we want to explore these issues further that you start a new thread in DIY or here in Vintage, and I can lend a hand if need be.
A good replacement for the outputs, if you'd like matching devices, would be 2SD287's (NPN) and 2SB539's (PNP's). Not terribly expensive, and usually pretty easy to find.
The devices for the output are not really chosen for any real reason relating to the input...they are chosen according to the supply voltage and expected output power, and well as the load the amp is intended to be able to drive.
I would suggest that if we want to explore these issues further that you start a new thread in DIY or here in Vintage, and I can lend a hand if need be.
Thanks Echo, I'll do that, I rather not just "get by" I would like to get some really good output devices. And I want to put in BG caps since there are only 3 per side. Plus replace the other NPN devices with good quality ones, and maybe even non-inductive resistors. I'll post in there when I'm ready to purchase the parts for your recommendations. Right now I have three computers I need to fix for some extra scratch so I afford those parts.
MJ Clemmer
Well-Known Member
Hello Echo WArs, I hear that you are the BEST in the field of fixing some Sansui rigs... I am sure that you have read my post about my G 8000 and NEED to getit working like its suppoesed to. How much do you charge to look at one? When are you locaated in the midwest? I live in Milwaukee Wisconsin and would LOVE your input about this problem. (its on the Exclusivly SANSUI) and the POST says HELP with distortion on G 8000.. thank you so much hope to hear from you soon....In DIRE NEED,,,Michael...