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Old 24th November 2010, 12:38 AM   #2081
Eric is offline Eric  United States
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Thanks, Zen! I'll give that a try!
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Old 24th November 2010, 04:23 AM   #2082
Eric is offline Eric  United States
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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!
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Old 24th November 2010, 06:04 PM   #2083
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Quote:
Originally Posted by JBL4435 View Post
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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Old 26th January 2011, 01:52 AM   #2084
Eric is offline Eric  United States
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Default 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 Click the image to open in full size.

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
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Last edited by Eric; 26th January 2011 at 02:11 AM.
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Old 26th January 2011, 05:11 AM   #2085
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Hi Eric,

shame on me!

The connection between ground and -IN should be before the cap . Connecting -IN to ground via the cap. I think I have written it like that and then drew something else.......

Sorry!,

William
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Old 26th January 2011, 12:19 PM   #2086
Eric is offline Eric  United States
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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...
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Old 26th January 2011, 04:01 PM   #2087
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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
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Old 26th January 2011, 05:22 PM   #2088
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The X-ed amps are usually rather sensitive to different DC-impedances at their inputs, so AC-coupling is a good measure. Even more so for balanced vs. unbalanced operation.

Hannes
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Old 26th January 2011, 05:22 PM   #2089
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Hi Eric,

I just had a look. The connection is still on the right on your website.....

William
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Old 26th January 2011, 05:48 PM   #2090
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Quote:
Originally Posted by wuffwaff View Post
I just had a look. The connection is still on the right on your website.....
William, I'm flattered that you visit my website so often that you need to do a refresh rather than pull the cached image
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