First Post - Hum on 5687 Pre-amp - Page 3

Shoog · 17th July 2006, 10:22 PM

Some people have said it is possible to build the "simple 5687 preamp" without any hum, using any 5687. I personally believe this tube is more prone to heater hum than say a 6922, and some brands more than others. I therefore suggest that DC heaters are almost a must when building a 5687 circuit. With this circuit I would go DC as the first stage to any hum fault finding. With other tubes I would say DC heaters would be the last thing I would try.

Shoog

colinB · 18th July 2006, 07:15 AM

Thanks for all the comments & suggestions guys,

So my plan is now:-

1. Implement Sheldon's rectification/filter schematic (rectifier bridge/10mf/1r/10mf/1r/10mf) to give DC for the 5687 heaters. (This is imminent probably the next couple of days)

2. Review the level of 'hum' at this point - hopefully very little.

3. If 'hum' is still present then I'll go ahead with placing insulating material between transformer and steel chassis - probably initially try rubber washers or something like that.

4. Assuming hum is then ok I'll reduce the gain by using the potential divider suggestion of Shoog's on the power amp using best combination of metal/carbon film resistors for the sound.

BTW the 5687 is Philips JAN WB version (good or bad?)
Volume pot is cheap model made by Alpha (not Alps)- grounded through chassis.

Thanks
Colin

colinB · 21st July 2006, 11:00 PM

Hi,

Just a quick update regarding DC supply for 5687 filaments. Tried Sheldon's suggestion of SS rectifier (bridge)/10mf/1r/10mf/1r/10mf for DC from 6.3VAC. Without valve installed Voltage at valve socket for filament was 8.2VDC. With 5687 installed, voltage at heaters dropped to 5.5VDC. So filament voltage is too low.

Will revisit earlier attempt of

6.3VAC/Bridge/4700uF/0.56R(10W)/4700uF

as this seemed to give ~6VDC at filament with 5687 valve installed.

Colin.

Shoog · 22nd July 2006, 07:45 AM

A part of Sheldons plan was to use Shottkys diode, did you use them?

Shoog

colinB · 22nd July 2006, 07:53 AM

Hi Shoog,

When I came to build the rectifier/filter I realised I didn't have any schottky's but went with a SS bridge that I had to hand.

If this bridge is the problem then I need to get some Schottky diodes

Will try to locate some to-day.

Colin

Shoog · 22nd July 2006, 08:13 AM

The reason they are important is because they have a lower forward voltage drop. Using them should give you the extra voltage. Remember that 6VDC is good and in some ways better than 6.3VDC.

Shoog

Sheldon · 22nd July 2006, 04:16 PM

A Schottky like a 1N5821 or 5822 has a forward drop of about 0.3V at 1Amp. A regular bridge will have about double that. Remember, you have two diode drops per bridge, so a net difference of about 0.6V. Should get you right there. I agree with Shoog on the voltage too. Most filament ratings are +/- at least 5% - remember, they were originally spec.'d for battery use. Most of what I've read suggests that you can achieve significantly longer tube life at filament voltages on the low end of the ratings, especially if you are not running them near max. plate dissipation.

Sheldon

colinB · 23rd July 2006, 04:48 PM

Hi Sheldon, Shoog

Here's a quick update regarding Hum on 5687 pre-amp:

1. Couldn't source any 1N5821 or 5822 schottkys locally but was offered 31DQ06 as alternative so bought these.

2. Wired up circuit as per Sheldon's suggestion (10mf/1r/10mf/1r/10mf) on stripboard. Connected to 6.3VAC but disaster - immediate smoke

Looks like I made an error on the stripboard (How useless am I?) Need to investigate this later may have damaged a diode.

3. So on another stripboard wired up KBPC601 bridge rectifier/4700uF/0.5R(6W)/4700uF as an alternative. This time this worked with 6.35VDC at valve (5687 in socket). Hum was reduced but audible with ear pressed to speaker - so this looks ok. Will use this temporarily until I sort out the other damaged board.

4. Also wired in potential divider on power amp using 2x 100k resisitors. Initially used cheapo carbon film resistors. Gain is reduced - vol pot is now at 10-11 o'clock rather than 8 o'clock. Perhaps some loss of detail is evident but otherwise no major deterioration in sound quality. Will try 1 carbon film + 1 metal film resistor combination when I track down the 100k metal films I bought earlier!

5. Thanks for all the help guys but I think I'm virtually there now. Just need to sort out the damaged strip-board and use this for DC supply

6. BTW I haven't yet investigated Sheldon's other suggestion of inductive hum from your main transformer. Will see about this eventually.

Colin.

Sheldon · 23rd July 2006, 05:57 PM

Nice to hear that it's progressing. I've had a couple of times with hum/noise issues that turned out to have multiple causes. This is not too surprising, as the loudest problem masks the lesser ones. Once the loudest is nulled, the next ones down become apparent. But I've learned a lot about causes by systematically chasing them down. It's good exercise and, if you keep at it, you can make your gear as quiet as the best.

Sheldon

colinB · 25th July 2006, 07:16 AM

Hi Sheldon,

I rectified my mistake with my implementation of your suggested circuit for the DC filament supply on another stripboard last night. So using 31DQ06 schottky diodes and filtering as 10mf/1r/10mf/1r/10mf connected to 6.3VAC supply gave 8.9VDC without the valve in place.

With 5687 installed this reduced to 7VDC. This is too high but with 0.5R in both 'feeds' to the valve heater the voltage dropped to about 6.2VDC

The hum is now only audible with my ear pressed against the speaker so I'm happy!

BTW sorry it took such a long time to get this right but I'm learning only very slowly..

Many thanks
Colin.

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17th July 2006, 10:22 PM	#21
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	Some people have said it is possible to build the "simple 5687 preamp" without any hum, using any 5687. I personally believe this tube is more prone to heater hum than say a 6922, and some brands more than others. I therefore suggest that DC heaters are almost a must when building a 5687 circuit. With this circuit I would go DC as the first stage to any hum fault finding. With other tubes I would say DC heaters would be the last thing I would try. Shoog

18th July 2006, 07:15 AM	#22
colinB diyAudio Member Join Date: May 2006	Thanks for all the comments & suggestions guys, So my plan is now:- 1. Implement Sheldon's rectification/filter schematic (rectifier bridge/10mf/1r/10mf/1r/10mf) to give DC for the 5687 heaters. (This is imminent probably the next couple of days) 2. Review the level of 'hum' at this point - hopefully very little. 3. If 'hum' is still present then I'll go ahead with placing insulating material between transformer and steel chassis - probably initially try rubber washers or something like that. 4. Assuming hum is then ok I'll reduce the gain by using the potential divider suggestion of Shoog's on the power amp using best combination of metal/carbon film resistors for the sound. BTW the 5687 is Philips JAN WB version (good or bad?) Volume pot is cheap model made by Alpha (not Alps)- grounded through chassis. Thanks Colin

21st July 2006, 11:00 PM	#23
colinB diyAudio Member Join Date: May 2006	Hi, Just a quick update regarding DC supply for 5687 filaments. Tried Sheldon's suggestion of SS rectifier (bridge)/10mf/1r/10mf/1r/10mf for DC from 6.3VAC. Without valve installed Voltage at valve socket for filament was 8.2VDC. With 5687 installed, voltage at heaters dropped to 5.5VDC. So filament voltage is too low. Will revisit earlier attempt of 6.3VAC/Bridge/4700uF/0.56R(10W)/4700uF as this seemed to give ~6VDC at filament with 5687 valve installed. Colin.

22nd July 2006, 07:45 AM	#24
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	A part of Sheldons plan was to use Shottkys diode, did you use them? Shoog

22nd July 2006, 07:53 AM	#25
colinB diyAudio Member Join Date: May 2006	Hi Shoog, When I came to build the rectifier/filter I realised I didn't have any schottky's but went with a SS bridge that I had to hand. If this bridge is the problem then I need to get some Schottky diodes Will try to locate some to-day. Colin

22nd July 2006, 08:13 AM	#26
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	The reason they are important is because they have a lower forward voltage drop. Using them should give you the extra voltage. Remember that 6VDC is good and in some ways better than 6.3VDC. Shoog

22nd July 2006, 04:16 PM	#27
Sheldon diyAudio Member Join Date: Dec 2003 Location: San Diego	A Schottky like a 1N5821 or 5822 has a forward drop of about 0.3V at 1Amp. A regular bridge will have about double that. Remember, you have two diode drops per bridge, so a net difference of about 0.6V. Should get you right there. I agree with Shoog on the voltage too. Most filament ratings are +/- at least 5% - remember, they were originally spec.'d for battery use. Most of what I've read suggests that you can achieve significantly longer tube life at filament voltages on the low end of the ratings, especially if you are not running them near max. plate dissipation. Sheldon

23rd July 2006, 04:48 PM	#28
colinB diyAudio Member Join Date: May 2006	Hi Sheldon, Shoog Here's a quick update regarding Hum on 5687 pre-amp: 1. Couldn't source any 1N5821 or 5822 schottkys locally but was offered 31DQ06 as alternative so bought these. 2. Wired up circuit as per Sheldon's suggestion (10mf/1r/10mf/1r/10mf) on stripboard. Connected to 6.3VAC but disaster - immediate smoke Looks like I made an error on the stripboard (How useless am I?) Need to investigate this later may have damaged a diode. 3. So on another stripboard wired up KBPC601 bridge rectifier/4700uF/0.5R(6W)/4700uF as an alternative. This time this worked with 6.35VDC at valve (5687 in socket). Hum was reduced but audible with ear pressed to speaker - so this looks ok. Will use this temporarily until I sort out the other damaged board. 4. Also wired in potential divider on power amp using 2x 100k resisitors. Initially used cheapo carbon film resistors. Gain is reduced - vol pot is now at 10-11 o'clock rather than 8 o'clock. Perhaps some loss of detail is evident but otherwise no major deterioration in sound quality. Will try 1 carbon film + 1 metal film resistor combination when I track down the 100k metal films I bought earlier! 5. Thanks for all the help guys but I think I'm virtually there now. Just need to sort out the damaged strip-board and use this for DC supply 6. BTW I haven't yet investigated Sheldon's other suggestion of inductive hum from your main transformer. Will see about this eventually. Colin.

23rd July 2006, 05:57 PM	#29
Sheldon diyAudio Member Join Date: Dec 2003 Location: San Diego	Nice to hear that it's progressing. I've had a couple of times with hum/noise issues that turned out to have multiple causes. This is not too surprising, as the loudest problem masks the lesser ones. Once the loudest is nulled, the next ones down become apparent. But I've learned a lot about causes by systematically chasing them down. It's good exercise and, if you keep at it, you can make your gear as quiet as the best. Sheldon

25th July 2006, 07:16 AM	#30
colinB diyAudio Member Join Date: May 2006	Hi Sheldon, I rectified my mistake with my implementation of your suggested circuit for the DC filament supply on another stripboard last night. So using 31DQ06 schottky diodes and filtering as 10mf/1r/10mf/1r/10mf connected to 6.3VAC supply gave 8.9VDC without the valve in place. With 5687 installed this reduced to 7VDC. This is too high but with 0.5R in both 'feeds' to the valve heater the voltage dropped to about 6.2VDC The hum is now only audible with my ear pressed against the speaker so I'm happy! BTW sorry it took such a long time to get this right but I'm learning only very slowly.. Many thanks Colin.