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DCPP - 'notch' in frequency response
DCPP - 'notch' in frequency response
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Old 20th October 2019, 11:25 AM   #1
tristanc is offline tristanc  United Kingdom
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Default DCPP - 'notch' in frequency response

Any ideas for what would cause the wierd 'notch' in my frequency response at 40kHz (attached) for the right channel? My OCD side would prefer that both channels be closer.

Schematic: http://pmillett.com/file_downloads/dcpp_sch.pdf

I have the 6dB NFB connected. The notch appears with differing valves in both pre and power positions. But I haven't gone to the extent of swapping OTs and seeing if the problem follows.

Should I even care? I won't hear it... (attached plot of audio frequency sweep) But am hoping it's not a sign something else is wrong.

This amp is being used for a few hours a day, for the last 2.5 years without issue apart from the odd valve failing.
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File Type: png DCPP-Sweeps.png (70.2 KB, 318 views)
File Type: png DCPP-Sweeps-Audio.png (60.5 KB, 322 views)
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Old 20th October 2019, 01:43 PM   #2
jackinnj is offline jackinnj  United States
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DCPP - 'notch' in frequency response
Quote:
Originally Posted by tristanc View Post
Any ideas for what would cause the wierd 'notch' in my frequency response at 40kHz (attached) for the right channel? My OCD side would prefer that both channels be closer.

Schematic: http://pmillett.com/file_downloads/dcpp_sch.pdf

I have the 6dB NFB connected. The notch appears with differing valves in both pre and power positions. But I haven't gone to the extent of swapping OTs and seeing if the problem follows.

Should I even care? I won't hear it... (attached plot of audio frequency sweep) But am hoping it's not a sign something else is wrong.

This amp is being used for a few hours a day, for the last 2.5 years without issue apart from the odd valve failing.
Are you sure you've got the secondaries correctly terrminated?

This amplifier is in almost continuous here without that issue.
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Old 20th October 2019, 02:19 PM   #3
DF96 is offline DF96  England
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The design appears to have push-pull feedback. With two feedback paths it is vital to ensure that the two paths are identical. Check component values, especially C9 and C15.
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Old 20th October 2019, 03:01 PM   #4
jackinnj is offline jackinnj  United States
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DCPP - 'notch' in frequency response
My DCPP has somewhat lower THD% than Pete's example (at 10W mine is ~1%, his a hair over 2%), but as illustrated below the two channels differ:
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Old 20th October 2019, 03:34 PM   #5
tristanc is offline tristanc  United Kingdom
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Quote:
Originally Posted by jackinnj View Post
Are you sure you've got the secondaries correctly terrminated?
Here's a pic I took a while back during assembly. You can see the secondary wires - unconnected wires snipped at this point in the build. Not sure if I covered them with tape though...

Quote:
Originally Posted by DF96 View Post
The design appears to have push-pull feedback. With two feedback paths it is vital to ensure that the two paths are identical. Check component values, especially C9 and C15.
Good shout - just noticed on a picture I took during populating the board that one of those two capacitors is mounted higher up from the board than the other one (I think the leads were a tad tight for the holes). Perhaps any difference here (additional capacitance) could cause this?

Can potentially check the solder connections too.

Quote:
Originally Posted by jackinnj View Post
My DCPP has somewhat lower THD% than Pete's example (at 10W mine is ~1%, his a hair over 2%), but as illustrated below the two channels differ:
That's interesting and good to know. All I have is a picoscope to measure THD and I can never be sure if I've the FFT set up correctly.
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File Type: jpg IMG_2167.jpg (187.5 KB, 290 views)
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Old 20th October 2019, 05:18 PM   #6
Alllensoncanon is offline Alllensoncanon  United States
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DCPP - 'notch' in frequency response
What is your 1KHz & 4 KHz square wave output looks like. Looks to me one channel rings more than the other.
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Old 20th October 2019, 06:32 PM   #7
tristanc is offline tristanc  United Kingdom
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Quote:
Originally Posted by Alllensoncanon View Post
What is your 1KHz & 4 KHz square wave output looks like. Looks to me one channel rings more than the other.
Eash... Check these out.

1k, 4k & 40k square waves - taken using 0.6Vpp (~1W output for a sine wave) input, output in to 8ohm.

Where do I start? I chop-sticked about the two capacitors C9 & C15 inside the plate-to-plate loop. Also moved the NFB wire from the speaker terminal back to the board. No change in the wave...
Attached Images
File Type: png 1k L square_01.png (33.2 KB, 280 views)
File Type: png 1k R Square_01.png (35.9 KB, 51 views)
File Type: png 4k L square_01.png (51.4 KB, 47 views)
File Type: png 4k R square_01.png (56.2 KB, 49 views)
File Type: png 40k L square_01.png (53.6 KB, 49 views)
File Type: png 40k R square_01.png (52.5 KB, 53 views)
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Old 21st October 2019, 01:51 AM   #8
jackinnj is offline jackinnj  United States
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DCPP - 'notch' in frequency response
Oscillating at 128kHz?
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Old 21st October 2019, 04:14 AM   #9
Alllensoncanon is offline Alllensoncanon  United States
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DCPP - 'notch' in frequency response
Change C9 and C15 value might reduce the right channel ring a bit (ie: Change it to 1000pf. See if that reduce the 40KHz notch.
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Old 21st October 2019, 01:39 PM   #10
DF96 is offline DF96  England
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I notice that the frequency response plots in post 1 show a slight rise in the left channel at the same frequency as the right channel has a dip. It may be the same effect, but of opposite polarity.
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