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 Post subject: Help troubleshooting a a Korg NuTube 6P1 audio input buffer
PostPosted: Dec Tue 31, 2019 6:58 pm 
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Joined: Sep Wed 18, 2013 7:18 pm
Posts: 147
Location: 50266
I recently built a Korg NuTube 6P1 audio input buffer, using a pcb and tube purchased from Pete Millett (http://www.pmillett.com/Nutube_buffer.html). Everything worked fine until I tried to power it from what was supposed to be a 24v regulated supply coming off a Class D amp board. The entire assembly started strobing on-and-off, approximately once a second; further research showed that this was expected behavior from that particular Class D board (and its _unregulated_ power supply. Sigh).

I removed the buffer board; while it still works (powered from the same brick I used to test it originally), it sounds as if there is a high-pass filter applied to both channels. No midrange or bass at all.

The bias voltages are within spec. Truth is, I have no idea as to where else to start troubleshooting.

Would any of you folks have any ideas? A schematic is at the web address above; I'll try to embed it below as well.


Last edited by htolino on Dec Tue 31, 2019 6:59 pm, edited 1 time in total.

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 Post subject: Re: Help troubleshooting a a Korg NuTube 6P1 audio input buf
PostPosted: Dec Tue 31, 2019 6:59 pm 
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Joined: Sep Wed 18, 2013 7:18 pm
Posts: 147
Location: 50266
Here's the schematic:


Attachments:
Nutube_buffer_sch.jpg
Nutube_buffer_sch.jpg [ 89.95 KiB | Viewed 574 times ]
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 Post subject: Re: Help troubleshooting a a Korg NuTube 6P1 audio input buf
PostPosted: Jan Mon 06, 2020 6:35 am 
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Joined: Jan Fri 03, 2020 7:54 pm
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Location: Malaysia Penang
Hi,
It sound like the oscillation positive feedback is coming from the power rail.. Maybe you can try to regulate 24V to 15 or 12V to your 6P1 board? maybe try something like 7812 or 7815? I not sure it can solve your problem , but you can give a try.


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 Post subject: Re: Help troubleshooting a a Korg NuTube 6P1 audio input buf
PostPosted: Jan Mon 06, 2020 1:49 pm 
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Joined: Sep Wed 18, 2013 7:18 pm
Posts: 147
Location: 50266
Thank you for taking the time to reply. I've removed the buffer from the Class D amp, and am currently powering it from a (clean and stable) 12v power brick. Same problem: there's no hum or anything, and I've adjusted the bias to the vendor's recommendations, but it still sounds like it's running through a high-pass filter.


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 Post subject: Re: Help troubleshooting a a Korg NuTube 6P1 audio input buf
PostPosted: Jan Mon 06, 2020 3:58 pm 
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Joined: Jan Fri 03, 2020 7:54 pm
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Location: Malaysia Penang
It sound like your Class D amplifier having low input impedance.. you may need a buffer to drive the class D amplifier.. or you can use op-amp to make it as voltage follower to drive your class D amplifier..

6P1 korg tube -> buffer (voltage follower can be op-amp..etc) -> class D amplifier.. the buffer act as impedance converter convert high impedance to lower impedance with gain of 1.


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 Post subject: Re: Help troubleshooting a a Korg NuTube 6P1 audio input buf
PostPosted: Jan Mon 06, 2020 6:14 pm 
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Joined: Nov Sat 26, 2011 4:09 am
Posts: 9661
Location: Texas. USA
I'd check the 2N7000s and, in particular, the ones on the output. Rapid power on/off may have popped them. That would make sense with the given symptoms since the output would then be high impedance and acting as a high pass filter with the output caps.


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 Post subject: Re: Help troubleshooting a a Korg NuTube 6P1 audio input buf
PostPosted: Jan Mon 06, 2020 6:37 pm 
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Joined: Sep Wed 18, 2013 7:18 pm
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Location: 50266
Thank you both. Given the low cost of the 2N7000, I'll just replace the lot and see if that helps.


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 Post subject: Re: Help troubleshooting a a Korg NuTube 6P1 audio input buf
PostPosted: Jan Thu 09, 2020 2:25 am 
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Joined: Sep Wed 18, 2013 7:18 pm
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Just a note to let you know that I've replaced the 2N7000s and all is well. Thank you again for the suggestion!


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 Post subject: Re: Help troubleshooting a a Korg NuTube 6P1 audio input buf
PostPosted: Jan Fri 10, 2020 2:35 am 
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htolino wrote:
Just a note to let you know that I've replaced the 2N7000s and all is well. Thank you again for the suggestion!
Super! Glad to hear it's working.


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 Post subject: Re: Help troubleshooting a a Korg NuTube 6P1 audio input buf
PostPosted: Jan Fri 10, 2020 6:30 am 
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Congratulation :wink:


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