KT-88 Ella amplifier, how to lower noise? - diyAudio
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Old 5th February 2004, 11:17 PM   #1
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Default KT-88 Ella amplifier, how to lower noise?

Hi guys,

This is a repost from the Audio Asylum DIY HiFi forum. I apologize for the cross posting, but assume the two forums share many unique users. I'm posting because I'm running short on ideas, any help or ideas to get my wheels turning again would be greatly appreciated. My Ella preamp is currently wired ultralinear and has the powersupply choke installed. Thanks!

>>Hi guys, unfortunately my noise issue proved to be too much for the triode mod. I still can't get it down like I'd like. When wired ultralinear you can hear it when you get within about a foot of the 96 db sensitive speakers. When wired triode you can hear the noise from the listening chair. I've poked and proded and routed signal wiring where it results in the most quiet conditions. I've retwisted and rerouted all power wiring as well. The noise which sounds less like a constant buzz or hum and more like static, is present in both channels, and in both the woofer and tweeter, and constant unaffected by the volume control. Are there any particular parts which can slip out of tolerance and introduce noise into a system? Can the tubes themselves cause a static like background noise? I've tried different 6DJ8's/6922's/7308's but don't have any extra 6sn7 or KT-88's to swap. I'm currently biased at .57 mV and swapping tubes from socket to socket hasn't changed the situation. Also perhaps unrelated is a very high pitch whistle through the right channel only. It is also a constant volume, unaffected by the volume control, but is noticable from the listening chair and noticable over most music because of its high pitch. It is very intermittent occuring during music and while the amp is idle. If anyone has any ideas or can steer me in the right direction I'd appreciate the help. I've reheated the solder joints and everything appears to be layed out pretty cleanly. Maybe some additional tube substitution is in order, at least to rule out the tubes as suspect. Anyone have any tubes they know are good and wish to sell inexpensively? Thanks to all. Jerrod<<
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Old 6th February 2004, 12:27 AM   #2
SY is offline SY  United States
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Have you done some basic troubleshooting to see which stage is the culprit? I don't know this circuit (a schematic would help), but if you can isolate it to the output stage or the driver stage or whatever, then you can be a bit more systematic about curing it.
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Old 6th February 2004, 01:19 AM   #3
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Default schematic

Hi SY. I have a schematic but it's 700K! No, I haven't isolated it to a particular stage. I built the amp as a kit and I'm learning as I go. I think I need to learn more before proceeding. Thanks, Jerrod
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Old 6th February 2004, 01:25 AM   #4
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Hi,

Jerrod,

That 700K is not a problem for my mailbox, assuming it may be one for SY.

I can probably reduce its size considerable and send a copy to back to you and SY if you like.

I'll send you an e-mail through the from so you have my mail address.

Cheers,

P.S. The Ella is still commercialised, I think, so we'd better not publish it here??
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Old 6th February 2004, 01:35 AM   #5
cm is offline cm  Singapore
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Check the gnd when the power supply PCB is connected to the chasis. It is via a screw lug. Since the chasis is spray painted, contact may not be good.

Also check the gnd wire of the pot.

The high frequency noise could be due to the 1 K resistor at the input grid of the 6922. Check the soldering there.

I had the same hum problem after installing the choke. Poking around, it was due to the power supply PCB connections to chasis.
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Old 6th February 2004, 01:37 AM   #6
SY is offline SY  United States
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Frank, that would be great. My ISP connection is about morse-code speed. Figure a GIF couldn't be more than 10-20K or so.
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Old 6th February 2004, 01:57 AM   #7
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Frank Yes, we definately don't want to post a schematic.

cm The power supply pcb has a good ground. I now have the pcb isolated from the chassis with a 100 resistor connecting the star ground to the chassis. (don't know if this is good or not, read about it on a tube diy forum as a way to reduce noise) The 1K resistor at the input grid is of particular interest. The copper trace at this point on the pcb is slightly damaged, will investigate this situation further

Thanks, Jerrod
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Old 6th February 2004, 02:02 AM   #8
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Some points is this ella from diyhifi?

if so the pcb is not done properly. Their grid resistors are very far from the socket itself. You will have to mount the resistors on the lug of the socket for it to work properly. The ella's grid resistors are mounted on the pcb which is very far from the power tubes and far for the tubes mounted on the pcb. It sounds like oscillations for the sounds you are describing. Have you remove the feedback?
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Old 6th February 2004, 02:11 AM   #9
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Hi Nick,

Yes, this is the Consonance Ella from DIY HiFi. On this particular build the 1K resistors are located on the main pcb on the bottom side of the 6922 tube sockets. The feedback is currently hooked up. When disconnected the noise is audible from 10+ feet away. Thanks, Jerrod
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Old 6th February 2004, 02:40 AM   #10
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You have to remove the resistors and place them on the sockets itself for both 6922 and kt88. They can oscillate when the resistors are place far from the pins
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