HF roll-off on left channel

https://www.youtube.com/playlist?list=P ... F23jLjEpOC
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xraysandwich
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Location: Portland, OR

HF roll-off on left channel

Post by xraysandwich »

I finished my build of the single ended KT88 last weekend and I'm seeing some high frequency roll-off in the left channel. The right channel is just about flat from 20hz-20kHz, while the the left starts to roll-off around 10kHz. The signal is down -1dB @ 14kHz and -2.5dB @ 20kHz relative to the output at 1kHz.

I think I've narrowed the issue down to the output transformer (Edcor CXSE25-5K) and was hoping to have some smarter eyes double check my work and let me know if you think it's reasonable to ask Edcor for a replacement.

My build is identical to the Blueglow schematics with the exception of omitting the volume knob and changing the value of the grid leak resistor on the driver stage to 120K.

I'm making all of these measurements with a function generator and a scope.

Here's what I've done so far to troubleshoot:

-Swapped tubes. Problem stayed on the left channel.
-Confirmed signals were balanced and frequency response was flat at the grids of the KT88's.
-Confirmed all components in the output stage were good.
-Swapped the transformers between channels. Problem moved with the transformer.

Video of some of the troubleshooting:
https://youtu.be/2LdERSiM3ic

Here are my measurements (driving an 8 ohm dummy load):

Output @ 1kHz = 8 Vpp = 2.8284 Vrms
Output @ 14kHz = 7 Vpp = 2.4749 Vrms
Output @ 20kHz = 6 Vpp = 2.1213 Vrms


Gain @ 14kHz relative to 1kHz:

20log(2.4749/2.8284) = -1.2dB

Gain @ 20kHz relative to 1kHz:

20log(2.1213/2.8284) = -2.5dB

The Edcor OPT is rated 20hz-20kHz, <1dBu. I'm not quite sure how to apply the dBu in their specs to my dB calculations, but it seems significant especially compared to Mark's results and the near flat response I'm seeing on my right channel.

Any insight is appreciated. Thanks!
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4thtry
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Re: HF roll-off on left channel

Post by 4thtry »

I watched the video a couple times and it looks like you have a bad transformer. When you swapped transformers, did you physically de-solder, unbolt and remove the two transformers, and then swap them back in after completely removing and re-dressing all the wiring? Doing so would eliminate the possibility that the HF roll off is being caused by a lead wire dressing issue in one of the two output stages. Or did you swap them out as shown in the video by simply installing temporary jumper cables? Bill
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xraysandwich
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Location: Portland, OR

Re: HF roll-off on left channel

Post by xraysandwich »

Thanks, Bill. Enjoyed following your Stromberg-Carlson project, BTW. Nice work!

You're right, I did only jumper with clip leads. I did not physically move the transformers, so the wire dressing remained the same relative to the OPTs.

Looking at the layout also got me thinking - my variac and isolation transformer are probably less than 12" from the OPT that's exhibiting roll-off. I wonder if their proximity could possibly be affecting frequency response?

I'll take a look at all this later tonight.
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Tom_S
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Re: HF roll-off on left channel

Post by Tom_S »

My money is on a bad OPT too.

Just thinking (typing) out loud....I do see what looks like a ground return from the driver stage (the cathode resistor?) on one channel running along side the primary lead of the OPT. Induced phase cancellation - acting kind of like a form of counter feedback? Yeah, probably not.

Here's an online calculator for Volts - dBu

http://www.sengpielaudio.com/calculator-db-volt.htm
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xraysandwich
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Joined: Wed Feb 24, 2021 6:07 pm
Location: Portland, OR

Re: HF roll-off on left channel

Post by xraysandwich »

Sorry to take so long posting back.

I did try removing the transformers and moving the one from channel 1 to channel 2 and the HF roll-off moved with the transformer. Here's a video documenting it:

https://youtu.be/cHiN7vlitj4

Moving my test equipment away from the unit also had no effect on the output.

I'll shoot Edcor an email in the morning and see what they say.

Tom, thanks for that link. Plugging in my output voltages, I get 11.25dBu @ 1kHz and 8.75dBu @ 20kHz which aligns with the -2.5dB I calculated previously.
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xraysandwich
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Location: Portland, OR

Re: HF roll-off on left channel

Post by xraysandwich »

Just to update this, Edcor asked me to send back the transformer so they could test it. Hope to hear something in the next week or two.
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