DC Darling Amp – Buzzing on both channels. - diyAudio
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Old 1st October 2007, 08:42 AM   #1
colinB is offline colinB  United Kingdom
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Question DC Darling Amp – Buzzing on both channels.

Hi,

Having pretty much failed with three other valve amp ‘projects’ I thought I’d go ahead with one final attempt to build a SE power amplifier before I give up completely and accept that I’m too dumb to build a valve amp.
So I decided to build Bob Danielak’s DC Darling amplifier as it seemed simple enough for a newbie like me. Anyway, I finished it a few days ago but as usual there are problems

I built the amp as per attached schematic (except I didn’t share the cathode resistor/bypass capacitor.)

On testing the DC voltages were a bit low compared to targets on schematic but probably near enough?

B+ voltage = 400V,
Voltage at Pin 8 (1626) = 130V
Voltage at Pin 5 (1626) = 110V
Voltage at Pin 2 (8532) = 1.05V.
Heater Voltages were slightly low at 5.9V for 8532 and 11.9V for 1626.

However I had a big problem with very loud buzz on both channels without anything connected to the inputs. However, when I inserted shorted RCA plugs on the input sockets the buzz disappeared. Similarly with MP3 player connected (no music playing) amp is quiet but as soon as I disconnect the MP3 player the loud buzz returns.

My first guess was that the amp was oscillating so I put 220ohm grid-stoppers on pin 1 of 8532 but this had no effect on the buzz. Could it be the 1626 valve oscillating?

Also, noted that 47K/2W anode resistor (for 8532) had also discoloured (due to overheating?) Could this also be related to the problem?

Anybody out there experienced this problem and able to suggest a simple fix?

Cheers
Colin.
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Old 2nd October 2007, 12:23 AM   #2
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Don't give up now, you must be close!

The voltages seem close enough. Though if the amp is oscillating at RF, it may be drawing more current, causing the voltages to be low.

You could add grid stoppers on the 1626 but it's not a very high Gm tube, so I doubt that it will fix things. Worth a try though, it can't hurt.

As for the discolored resistor... it's being run very close to the limits for a 2W resistor (1.8W in your case), so I'm not surprised. It will work but will have a limited life. At some point it would be best to replace it with a 5 watt resistor.

A few suggestions:

1. Verify for sure that the Rg resistors are connected correctly, well soldered, etc.

2. Make sure the input wiring (from RCA to grid stopper) doesn't go near the wiring on the 1626 plate.

3. Try a really big grid stopper - maybe 10k. If it still buzzes it's probably not the input tube oscillating. Make sure the stopper is really close to the tube.

If you have a camera, shoot a picture of the underside wiring and post it here. Maybe somebody will see something...

It would be worth finding an oscilloscope (or a friend with one) to have a look around.

Pete
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Old 2nd October 2007, 05:10 AM   #3
arnoldc is offline arnoldc  Philippines
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I don't see a cause for alarm. Normally you'll have a source conected to the input.
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Old 2nd October 2007, 09:03 AM   #4
nkg is offline nkg  Australia
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Hi Colin
As Pete said don't give up I'm not that bright either, most of the threads I read on this site go over my head and will probably never design any thing from scratch either and will be content to build other peoples designs and it doesn't matter either. I have just finished mine it took me two years I couldn't make a decision till I had read and reread until it sunk in and that was on simple things like positioning components and grounding techniques all the more complicated stuff is still magic to me . The thing that helped me alot was finding circuit diagrams and photos and comparing the two to see how works visually , mine worked so can yours.
Nigel
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Old 2nd October 2007, 09:52 PM   #5
colinB is offline colinB  United Kingdom
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Hi,

Firstly, thanks for the positive advice from Pete and Nigel to stick with the project.

Just a quick update following suggestions by Pete.

Quote:
As for the discolored resistor... it's being run very close to the limits for a 2W resistor (1.8W in your case), so I'm not surprised. It will work but will have a limited life. At some point it would be best to replace it with a 5 watt resistor.
I thought this was likely to be the case - thanks for the tip! I'll look to change this to a 5 watter when I next place an order with Farnell or RS.

Pete, regarding your suggestions:

Quote:
1. Verify for sure that the Rg resistors are connected correctly, well soldered, etc.
Checked this all connections seem good.

Quote:
2. Make sure the input wiring (from RCA to grid stopper) doesn't go near the wiring on the 1626 plate.
I think I'm ok with this also. See attached picture.

Quote:
3. Try a really big grid stopper - maybe 10k. If it still buzzes it's probably not the input tube oscillating. Make sure the stopper is really close to the tube.
Put 10k carbon resistor as gridstopper on 8532 on one channel only to hear the effect of this change. Sadly, the buzz was still present on both channels - suggests input valve isn't oscillating.

Quote:
You could add grid stoppers on the 1626 but it's not a very high Gm tube, so I doubt that it will fix things. Worth a try though, it can't hurt.
Tried this a few days ago with a 1K5 carbon resistor but this had no effect on the buzz - maybe value was too low?

Posted a picture of the underside as suggested - it ain't too pretty so feel free to suggest changes to the layout


Quote:
It would be worth finding an oscilloscope
I might be able to borrow an old oscilloscope from work but I haven't ever used one before so I need some serious advice in how to use it to track down the source of the buzz in this amp. Will look into this.


Quote:
arnoldc: I don't see a cause for alarm. Normally you'll have a source conected to the input.
I agree but I was curious enough to want to remove the source of the buzz just in case this resulted in further issues a few months time.

Cheers
Colin.
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Old 2nd October 2007, 11:39 PM   #6
arnoldc is offline arnoldc  Philippines
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Isn't your choke too close from the input?
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Old 4th October 2007, 07:05 AM   #7
colinB is offline colinB  United Kingdom
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Hi ArnoldC,

Quote:
Isn't your choke too close from the input?
Thanks for the tip! It looks like I need to make some changes to the layout to overcome the buzzing problems.

Cheers
Colin
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Old 4th October 2007, 09:54 AM   #8
chrish is offline chrish  Australia
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Hard to see in your photo, but it looks like you are using tag strips with components soldered to the tag that connects to the chassis. With those tag strips, make sure you don't solder any components to the tag that continues through to the chassis connection, otherwise it will be connected to chassis ground. If you wanted to have this component connected to ground, you might have a ground loop through the chassis. If you did not mean to connect the component to ground - then you have bigger problems!

Cheers,

Chris
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Old 4th October 2007, 03:05 PM   #9
arnoldc is offline arnoldc  Philippines
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Hi Colin, I turned on my Darling (hmmm, he he he) without anything on the input. Didn't hum nor buzz. BUT, I'm using 2C51 as a driver for this.
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Old 5th October 2007, 10:19 PM   #10
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Default DC Darling amp +buzzing

Do you have a capacitor between the chassis and the electrical ground?

I just built the capacitor coupled Darling (see Darling club) and it worked perfectly on power-up.

I am just getting back to tubes after a 50 year absence. My first amp buzzed. A friend suggested a capacitor between chassis (connected to the third AC plug wire) and the circuit ground. It cleared the hum right up.
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