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Old 6th December 2008, 09:17 PM   #1
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Default Volume swinging problem

Hi, ive built a phono amp and a pre amp.

Im running my CD player direct into the pre-amp as an output stage with the volume control before the power amp.

All is well.

The phono amp is fine but a little quiet so i tried putting it through the same pre amp stage as the CD player.

When i do this the volume just swings all over the place, loud/quiet/bit loud/silent/loud etc etc.

This is the pre amp stage, the volume controll is after this, there are 1k grid stoppers on the input.
Click the image to open in full size.

This is the phono stage

Click the image to open in full size.

they both work fine independantly but feeding the phono into the 5687 results in the mad volume fluctuations.


I tested the phono outputs with nothing connected, they have fluctuating DC varying from 0v to 0.05v. I dont know if thats normal or not?

Im a bit new to all this so its got me proper stumped.
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Old 7th December 2008, 02:27 AM   #2
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Sounds like either one or the other is oscillating at a very low frequency. Both schematics look alright... I would guess maybe a ground loop?
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Old 7th December 2008, 10:37 AM   #3
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Motorboating. You need to decouple the first stage from the second two. Use a separate RC circuit to the first stage's plate resistor istead of counting on R41 for all three stages. It's OK (and maybe preferable) to feed the second stage and its cathode follower from the same rail, but the first stage needs a rail of its own.
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Old 7th December 2008, 10:46 AM   #4
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I dont quite understand, im new to the world of electronics.

Are you talking about decoupling the stages of the phono or decoulping the phono from the 5687?

The 5687 stage has its own rc filter before the 2 20k resistors (i 10k in my case), it also has a 1k/100uf rc stage.
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Old 7th December 2008, 10:47 AM   #5
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Ill try and draw the 2 stages together to show exactly how there linked.
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Old 7th December 2008, 11:04 AM   #6
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This is how they are linked.

Forgive the shoddy 'paint' skills

Click the image to open in full size.
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Old 7th December 2008, 11:22 AM   #7
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Hi,

What they meant to say is; how do you supply these stages? Are they running from the same PS?

It seems they give eachother feedback through their Power supply.

"decoupling" means an R/Cstage to prevent them from feedbacking to eachother to their power supply.
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Old 7th December 2008, 11:33 AM   #8
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Yes, they share the same power supply.

The PSU is designed to power a phono and a pre (just not this pre).

Both the phono and pre have their own 1k/100uf RC filter inside there cases to isolate them from each other and the PSU. (the power supply is cased seperately)

I normaly run my CD players dac chip pirect into the 5687, still with the 1k grid stoppers.

I can have both units working at the same time, no problem.

Issues only arrise when i link the 2 together.

Ive been given a couple of suggestions to try on another forum, but all suggestions welcome, 2 brains are better than 1 and all that

Thanks.
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Old 7th December 2008, 11:38 AM   #9
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Break the wire between R29 and C12, then supply C12 (and the first tube) through its own resistor connected to the main power supply. First guess would be 2k to get the DC correct.
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Old 7th December 2008, 11:43 AM   #10
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Ill try it

Just to clarify though, you know R29 and R19 go to the same valve right? The first 2 valves in the schematic are 2 halves of the same valve.

You probably know that, but just checking.
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