Pass DIY Addict
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Pass DIY Addict
Joined 2000
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Hi Zen Mod - that did the trick! Right away, I picked up 5dB in output compared to the unmodified amp. Using the scope, I was hitting max clean output with a 1.4vRMS input signal. Bandwidth (-3dB point) dropped from about 210kHz to just about 100kHz (a curious side effect...). Sine and Square waves look clean all the way through 100kHz, above that, the square waves begin to round over and look line sine waves, but this was normal before the mod.
Absolute DC Offset behaves as usual (starts at about 14v, drop to 10v with 2-3 mins, and by 30 mins is near zero) and there were no signs of oscillation that I could find. Relative DC offset remains below 2mV at all times.
Thanks again for the help!
Absolute DC Offset behaves as usual (starts at about 14v, drop to 10v with 2-3 mins, and by 30 mins is near zero) and there were no signs of oscillation that I could find. Relative DC offset remains below 2mV at all times.
Thanks again for the help!
So, these will be the last THD Plots.
I finally finished my BNC-XLR input cable
THD seems not to change much with balanced or unbalanced in
1st: DIY XA160 8 Ohms load, Balanced in
2nd: DIY XA160 4 Ohms load, Balanced in
3rd: DIY XA160 4 Ohms load, Un-Balanced in
Were your measurements of THD made with single-ended _output_ or differential output?
Jeremy
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Back to DC offset problems
Hey Guys - I've recently changed the configuration of my Aleph-X system and an old problem has resurfaced - DC Offset. It happens on all three of my amps when using unbalanced (RCA) inputs. I have installed a switch for selecting between RCA and XLR inputs. The switch is wired so that when closed, it shorts the XLR -IN to the Shield pin. All 3 amps were built without using any DC blocking input caps.
For the past year, I've been driving my amps directly from my Adcom CD player because it has a variable-level output (RCA) on the back. Things were going along smoothly and my amps didn't show any signs of offset related trouble. Relative DC offset (across the speaker terminals) was extremely steady at 0.0v whether the amp was hot or cold. Absolute DC Offset was close to 14v at poweron, but dropped very quickly and was gone within 30 mins or so. I've been using the amps this way so long, I figured this wasn't a problem for me any more.
This past weekend, I changed my configuration and replaced the Adcom CD player with an older preamp and CD player. All of a sudden, relative DC offset was at 2.2v DC! I checked the amp and preamp combination - they are not putting out any DC at all. My guess is that the 3-prong grounded AC cord from the Adcom CD player was providing some element of stability to the amp and I had inadvertently adjusted the amp to mate well with the Adcom CD player. The older preamp and CD player that I just put in place don't have a grounded AC cord.
So, I remember William's advice to install input caps as a way to solve offset problems. He was even kind enough to draw a diagram for me
I installed 4.0uF caps as indicated in the image above and remeasured things and now DC Offset is even worse! I'm measuring 6.7v DC across the speaker terminals when the amp is cold. It decreases to 2.5v within the first 5 minutes and is down to 0v (plus or minus a little bit) within an hour when using RCA inputs. If I open my switch for XLR use (though I don't have any XLR-output gear right now), the relative DC offset drops to 0.0v whether the amp is hot or cold.
I've spent several hours going back through this thread from almost 2 years ago where Ian and Nelson are discussing a solution to this, but most of what they are talking about is beyond my level of understanding.
Does anyone have any suggestions for how I can solve this problem? One item on the upgrade path as I complete the theater room is a new preamp that features balanced outputs. Will this solve the problem for me?
I can't imagine that people let their amps warm up for an hour before connecting there speakers, so what can I do?
Thanks for any help!
Eric
Hey Guys - I've recently changed the configuration of my Aleph-X system and an old problem has resurfaced - DC Offset. It happens on all three of my amps when using unbalanced (RCA) inputs. I have installed a switch for selecting between RCA and XLR inputs. The switch is wired so that when closed, it shorts the XLR -IN to the Shield pin. All 3 amps were built without using any DC blocking input caps.
For the past year, I've been driving my amps directly from my Adcom CD player because it has a variable-level output (RCA) on the back. Things were going along smoothly and my amps didn't show any signs of offset related trouble. Relative DC offset (across the speaker terminals) was extremely steady at 0.0v whether the amp was hot or cold. Absolute DC Offset was close to 14v at poweron, but dropped very quickly and was gone within 30 mins or so. I've been using the amps this way so long, I figured this wasn't a problem for me any more.
This past weekend, I changed my configuration and replaced the Adcom CD player with an older preamp and CD player. All of a sudden, relative DC offset was at 2.2v DC! I checked the amp and preamp combination - they are not putting out any DC at all. My guess is that the 3-prong grounded AC cord from the Adcom CD player was providing some element of stability to the amp and I had inadvertently adjusted the amp to mate well with the Adcom CD player. The older preamp and CD player that I just put in place don't have a grounded AC cord.
So, I remember William's advice to install input caps as a way to solve offset problems. He was even kind enough to draw a diagram for me
An externally hosted image should be here but it was not working when we last tested it.
I installed 4.0uF caps as indicated in the image above and remeasured things and now DC Offset is even worse! I'm measuring 6.7v DC across the speaker terminals when the amp is cold. It decreases to 2.5v within the first 5 minutes and is down to 0v (plus or minus a little bit) within an hour when using RCA inputs. If I open my switch for XLR use (though I don't have any XLR-output gear right now), the relative DC offset drops to 0.0v whether the amp is hot or cold.
I've spent several hours going back through this thread from almost 2 years ago where Ian and Nelson are discussing a solution to this, but most of what they are talking about is beyond my level of understanding.
Does anyone have any suggestions for how I can solve this problem? One item on the upgrade path as I complete the theater room is a new preamp that features balanced outputs. Will this solve the problem for me?
I can't imagine that people let their amps warm up for an hour before connecting there speakers, so what can I do?
Thanks for any help!
Eric
Last edited:
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Thanks, William! I'll give this a try! I think originally you drew it up with the switch before the cap and then sent me a follow up message indicating that the switch should be after the cap instead, so I modified your drawing accordingly.
I figured it would turn out to be something simple since my amps have been working so well...
I figured it would turn out to be something simple since my amps have been working so well...
Pass DIY Addict
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I've updated the image on my Aleph-X page and as a result, the image I linked in my message last night has been updated as well. In the original post, the switch was indicated to the right of the input caps. Hopefully, this will prevent confusion when others read this thread later...
Eric
Eric
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William, I'm flattered that you visit my website so often that you need to do a refresh rather than pull the cached image
Pass DIY Addict
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Pass DIY Addict
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William - that solved the problem! I moved the switch to the other side of the cap and it worked like a charm! Relative DC offset now holds constant at 0.0v as the amp warms up. Absolute DC offset behaves as before: it starts off somewhere near 9v from each speaker terminal to ground and reaches 2v within 10-15 mins, and hits 0v within 45-60mins and stays there.
Thanks again!
Eric
Thanks again!
Eric
Rich,
I have a couple of the BrianGT rev 1 boards available. PM me if interested.
Peter Daniel just ran another batch of AJ-X boards. You could substitute ZVP3310s in his boards if you don't want to go with the jfet version. Just watch the pin outs - you will put the ZVPs in the opposite orientation as depicted on the silkscreen
I have a couple of the BrianGT rev 1 boards available. PM me if interested.
Peter Daniel just ran another batch of AJ-X boards. You could substitute ZVP3310s in his boards if you don't want to go with the jfet version. Just watch the pin outs - you will put the ZVPs in the opposite orientation as depicted on the silkscreen
The Version 1 boards are set up for 1 output device per quadrant (~30W out) with connections for additional off board devices. You could use the Burning amp single ended output boards or create your own. I'm sure there's a BOM or three somewhere in this thread.
Peter's X boards are set up for up to 3 output devices per quadrant. There are some suggested BOMs in the AJ-X thread.
Peter's are probably easier to use for 100W.
Peter's X boards are set up for up to 3 output devices per quadrant. There are some suggested BOMs in the AJ-X thread.
Peter's are probably easier to use for 100W.
Absolute DC Offset jumps up as I lower CCS current
I posted on the Alephx-x thead about the problems I am having with Absolute DC offset but didn't get a response.
I was wondering if I should have posted it in the builder thread instead.
Here is the link to the post on the Aleph-x thread.
http://www.diyaudio.com/forums/pass-labs/3748-aleph-x-320.html
I hope someone can help me sort this out.
I posted on the Alephx-x thead about the problems I am having with Absolute DC offset but didn't get a response.
I was wondering if I should have posted it in the builder thread instead.
Here is the link to the post on the Aleph-x thread.
http://www.diyaudio.com/forums/pass-labs/3748-aleph-x-320.html
I hope someone can help me sort this out.