Hi Graeme,
I hope I'm not out of line but having followed this thread, it seems to me that your Phono has been constructed on a ASP (all spare parts) basis with components coming from here and there, some new but not the best type and others salvaged so there is no certainty that all parts are functioning correctly or appropriately selected. I think your earlier suggestion to build again with the caps and R's as recommmended by Nick is the way to go. The PCB makes it tidy but I'm not necessarily advocating doing this if you prefer to hardwire. A phono stage is very sensitive amplification and needs tight precission. Your (admirable) economic approach to building may not be so good in this particular application.
Keep at it. You'll get there in the end.
Best wishes,
Greg
WD phono/PSU III clone
#347
Not out of line in the slightest greg 
The way this phono was built was mainly down to finances. I figured i'd get it up and running and later on i could spend some money on it to bring it up to standard.
Doesnt look like thats working out too well though
Maybe i would have been better off waiting untill i could actually afford to do this properly
Anyhow, i think my caps are ok (except maybe the PSU caps).
I dont think im going to be able to make any real forwards progress untill i remove a few of the 'unknowns'
Ill order new resistors, and some new valve bases. Ill re-build it, hardwired into a nice metal case and use decent resistors and new caps in the PSU.
Ill also replace the diodes for ones i know for sure are up to the job.
May not be able to order stuff till after christmas but thats life.
In the mean time, ill try breaking the signal between V1 and V2 and see what happens.
The way this phono was built was mainly down to finances. I figured i'd get it up and running and later on i could spend some money on it to bring it up to standard.
Doesnt look like thats working out too well though
Maybe i would have been better off waiting untill i could actually afford to do this properly
Anyhow, i think my caps are ok (except maybe the PSU caps).
I dont think im going to be able to make any real forwards progress untill i remove a few of the 'unknowns'
Ill order new resistors, and some new valve bases. Ill re-build it, hardwired into a nice metal case and use decent resistors and new caps in the PSU.
Ill also replace the diodes for ones i know for sure are up to the job.
May not be able to order stuff till after christmas but thats life.
In the mean time, ill try breaking the signal between V1 and V2 and see what happens.
#348
I just cut the cap, disconnecting V1 from V2.
I just cut the cap lead, after c10/11 but before r23/24 and R21/22.
Noise has gone through the floor. Still there at full volume but inaudable at loud listening volumes from the listening position.
Out of interest, i plugged it into the 5687. It no longer varies volume.
I didnt chack the fluctuating voltage at the phono's outputs but i'd like to bet its gone. (ill check if needed)
So, disconnecting V1 from V2 solves all my problems.
Unfortunately it also stops the phono stage from working
Question is then, what has this proved? Has it narrowed down the 'problem options'?
I just cut the cap lead, after c10/11 but before r23/24 and R21/22.
Noise has gone through the floor. Still there at full volume but inaudable at loud listening volumes from the listening position.
Out of interest, i plugged it into the 5687. It no longer varies volume.
I didnt chack the fluctuating voltage at the phono's outputs but i'd like to bet its gone. (ill check if needed)
So, disconnecting V1 from V2 solves all my problems.
Unfortunately it also stops the phono stage from working
Question is then, what has this proved? Has it narrowed down the 'problem options'?
#349
Well, its proved that the noise is being generated by the first stage (as wasto be expected) and the problem with the pre is the result of power supply instability.
So the fix is reduce the noise in the first stage, and isolate the first stage from the pre.
Hopefully better R's will help the nosie, and decoupling should help the instability.
So the fix is reduce the noise in the first stage, and isolate the first stage from the pre.
Hopefully better R's will help the nosie, and decoupling should help the instability.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#350
Ok.
Im not in any way disagree'ing here, just clarifying.
How is the first stage causing the noise but the 2nd and 3rd stage hardly cause any? Or, is it just because of the extra amplification of the next 2 stages?
As for the instability, i dont get that either. Why would the first stage be unstable but the 2nd and 3rd not?
How is the first stage less decoupled from the pre than the 2nd and 3rd?
That really sounds like im questioning your knowledge/experience. Im really not, im just trying to understand
Are both cases because the first stage is the most sensitive, having the weakest signal?
Im not in any way disagree'ing here, just clarifying.
How is the first stage causing the noise but the 2nd and 3rd stage hardly cause any? Or, is it just because of the extra amplification of the next 2 stages?
As for the instability, i dont get that either. Why would the first stage be unstable but the 2nd and 3rd not?
How is the first stage less decoupled from the pre than the 2nd and 3rd?
That really sounds like im questioning your knowledge/experience. Im really not, im just trying to understand
Are both cases because the first stage is the most sensitive, having the weakest signal?
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richardcooper2k
- Old Hand
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- Joined: Thu Jun 07, 2007 2:32 pm
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#351
the noise from the power supply will be going into all the stages. but the noise going into the first stage gets amplified in that stage and all the others as it goes on down through the amp. the noise entering through the power supply in the latter stages is the same but gets amplified less.
in its current state the amp is unstable as a whole
you need to 'decouple' the power supply HT to the first stage from the earlier ones. the easiest way to do this is with RC filters between the stages. ie between one or more stages in the HT supply you have a series resistor and a cap to earth. this 'decouples' one stage from the next
at present if i remember correctly, no stage is decoupled from any other including the stages in the phono from the pre (via the ht supply)
in its current state the amp is unstable as a whole
you need to 'decouple' the power supply HT to the first stage from the earlier ones. the easiest way to do this is with RC filters between the stages. ie between one or more stages in the HT supply you have a series resistor and a cap to earth. this 'decouples' one stage from the next
at present if i remember correctly, no stage is decoupled from any other including the stages in the phono from the pre (via the ht supply)
#352
Yes, there are two factors that makes the first stage the most sensitive to noise and instability. First, noise. Each triode will generate its own noise, as will the load resistor, grid resistor and cathod eresistor. But you then need to consider the signal to noise ratio. In the input to the first stage, the signal will be (say) 2mv, at he output of the first stage, its amplified maybe 50 times, so the signal is now 100mv, and thats the input to the next stage, so you can see that the ratio of the signal and noise will be 50 times more in the first stage than the second stage, so noise generated inm the first stage is 50 times mor important than the second stage.
Then instability, for a amplifie rto be unstable takes two things, gain, and positive feedback. For the feedback to be positive the signal fed back must be in phase with the input, otherwise the signal will cancel (negative feedback). Each of the first stages invert the signal, or in other words they provide 180degrees of phase shift, the cathode follower doesn't invert its output, and also it doesn't provide any gain, so can be ignored to a large extent. Its normally hard to get a two stage amp to provide positive feedback, because the input to the first stage doesn't see the signal on the B+ supply created by the later stages, so in terms of the feedback path through the B+ supply a two stage amp will only provide negative feedback via its power supply. However once you add the preamp stage, you have added a third stage sharing the same supply, so now you have the conditions needed to allow positive feedback.
The frequency of the feedback (if it happens) depends on how the gain of the system varies with frequency. If it was a simple three stage amp, the frequency response of the feedback path would be controlled by the frequency response of the power supply. At high frequency the supply provides a low impedance to the signal so feedback is reduced, and the amp is stable. As we consider lower frequencies, the caps in the supply start to look like larger resistors, so the impedance rises, and more signal is allowd to be fed back. So thi ssort of instability due to power supply feedback normally happens in the low frequency area, and is called "motorboating". The way to stop it, is to either reduce the gain of the amplifier at low frequencies, which for audio we don't want to do, or reduce the feedback at lower frequencies, and we can do this by adding additional filter stages in the B+, so a RC filter between the first and second valve will help to prevent the feedback from getting to the point wher its unstable.
Matters are worst with a phono stage, as the RIAA eq ,eans that the maximum gain is at low frequencies, so that adds to the problem.
The other way of preventing feedback through the supply is to use a activly regulated supply that provides a low impedance to signals down to DC, but thats a level of complexity that we nont need here. As additionsl choke in the supply to the preamp, taking it off before the existing choke and with another cap, would provide a higher level of decoupling and probably stop the instability.
Then instability, for a amplifie rto be unstable takes two things, gain, and positive feedback. For the feedback to be positive the signal fed back must be in phase with the input, otherwise the signal will cancel (negative feedback). Each of the first stages invert the signal, or in other words they provide 180degrees of phase shift, the cathode follower doesn't invert its output, and also it doesn't provide any gain, so can be ignored to a large extent. Its normally hard to get a two stage amp to provide positive feedback, because the input to the first stage doesn't see the signal on the B+ supply created by the later stages, so in terms of the feedback path through the B+ supply a two stage amp will only provide negative feedback via its power supply. However once you add the preamp stage, you have added a third stage sharing the same supply, so now you have the conditions needed to allow positive feedback.
The frequency of the feedback (if it happens) depends on how the gain of the system varies with frequency. If it was a simple three stage amp, the frequency response of the feedback path would be controlled by the frequency response of the power supply. At high frequency the supply provides a low impedance to the signal so feedback is reduced, and the amp is stable. As we consider lower frequencies, the caps in the supply start to look like larger resistors, so the impedance rises, and more signal is allowd to be fed back. So thi ssort of instability due to power supply feedback normally happens in the low frequency area, and is called "motorboating". The way to stop it, is to either reduce the gain of the amplifier at low frequencies, which for audio we don't want to do, or reduce the feedback at lower frequencies, and we can do this by adding additional filter stages in the B+, so a RC filter between the first and second valve will help to prevent the feedback from getting to the point wher its unstable.
Matters are worst with a phono stage, as the RIAA eq ,eans that the maximum gain is at low frequencies, so that adds to the problem.
The other way of preventing feedback through the supply is to use a activly regulated supply that provides a low impedance to signals down to DC, but thats a level of complexity that we nont need here. As additionsl choke in the supply to the preamp, taking it off before the existing choke and with another cap, would provide a higher level of decoupling and probably stop the instability.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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richardcooper2k
- Old Hand
- Posts: 780
- Joined: Thu Jun 07, 2007 2:32 pm
- Location: sheffield
#353
i think i made an error in my last post and reading nicks has confirmed this in my mind
since the Ht comes in at the anode at each stage it is after the amplification at that stage. so noise coming in will be amplified at subsequent stages as it will enter the grid at these
since the Ht comes in at the anode at each stage it is after the amplification at that stage. so noise coming in will be amplified at subsequent stages as it will enter the grid at these
#354
Great, makes sense now, thanks
Ill look into how i can achieve the extra decoupling later when i get home.
You say an RC filter between the first and second valve. Is that not what i created when i added the 1k resistor between R29/30 and C12/13? Did that not create a RC filter in the Hv between V1 and V2?
Ill look into how i can achieve the extra decoupling later when i get home.
You say an RC filter between the first and second valve. Is that not what i created when i added the 1k resistor between R29/30 and C12/13? Did that not create a RC filter in the Hv between V1 and V2?
#355
Yes, in fact before being amplified by the next stage, it will be reduced by the PSRR (power supply rejection ratio) of that stage. Thats a measure of how much the stage "ignores" any signal on the power rail. For a simple triode stage, its the ratio of the load resistance and the valve anode resistance. It can be thought of as a potential divider with the upper resistor as the anode load and th elower resistor as the valve anode resistance.richardcooper2k wrote:i think i made an error in my last post and reading nicks has confirmed this in my mind
since the Ht comes in at the anode at each stage it is after the amplification at that stage. so noise coming in will be amplified at subsequent stages as it will enter the grid at these
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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Mike H
- Amstrad Tower of Power
- Posts: 20189
- Joined: Sat Oct 04, 2008 5:38 pm
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- Contact:
#356
Either it's still wired wrongly or 1k isn't big enough. I'd used 10k minimum. Will still only drop about 10V.Graeme wrote:Great, makes sense now, thanks
Ill look into how i can achieve the extra decoupling later when i get home.
You say an RC filter between the first and second valve. Is that not what i created when i added the 1k resistor between R29/30 and C12/13? Did that not create a RC filter in the Hv between V1 and V2?
#2: I'm a bit confused, wasn't the PSU an actual WD kit? But you were 'given' the PSU caps. You might be able to test them to see if they're leaking current, which they may be sporadically ~ how about a resistor inserted into the negative lead(s) then monitor the voltage across that, if any?
#3: you can get positive feedback around a two stage system if you add a phase shift.
The signal cap twixt V1 anode and V2 grid is large @ 100nF, given the 1 Meg grid resistor, nevertheless at low enough frequency the voltage and current will be out of phase producing a signal delay, if it becomes 180 degrees out of phase then you've got oscillation.
Note that the max gain of V1 + V2 = 50 x 80, = 4,000, and the EQ networks don't attenuate much at the low frequency end. So that's the sort of overall gain of the amp at the point where this LF instability is occurring! Eeek...
So just needs a tiny bit of phase shift to set it off"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
#357
No, the PSU is cobbled together too
I have some 10k resistors, ill swap the 1k to 10k and see if anything changes.
It was also mentioned on DIY Audio about having a seperate Hv feed to V1.
I think the suggestion was to break the Hv between R29/30 and C12/13 and run a seperate feed, after C20 to supply V1. Still using the extra resistor before C12/13.
I dont get how that would be any different (electricaly) than how its wired at the moment though, maybe im missing the point?
I have some 10k resistors, ill swap the 1k to 10k and see if anything changes.
It was also mentioned on DIY Audio about having a seperate Hv feed to V1.
I think the suggestion was to break the Hv between R29/30 and C12/13 and run a seperate feed, after C20 to supply V1. Still using the extra resistor before C12/13.
I dont get how that would be any different (electricaly) than how its wired at the moment though, maybe im missing the point?
#358
Yep, arn't phono stages fun.Note that the max gain of V1 + V2 = 50 x 80, = 4,000, and the EQ networks don't attenuate much at the low frequency end. So that's the sort of overall gain of the amp at the point where this LF instability is occurring! Eeek... Shocked So just needs a tiny bit of phase shift to set it off
Though you only have a gain of say 100 between the second stage anode and the first stage anode, and the PSRR of the first stage will reduce that to 40 or so, so it would take a fair bunch of phase shift to get those two stages to take off (and a very high impedance power supply). Especially if the phase shift was done (as it is here) by a low pass RC network, as the maximum phase shift would correspond to the minimum gain.
But once you add that third stage with the common power supply all bets are off.
Though you will never get more than 90 deg phase shift in a simple RC stage.The signal cap twixt V1 anode and V2 grid is large @ 100nF, given the 1 Meg grid resistor, nevertheless at low enough frequency the voltage and current will be out of phase producing a signal delay, if it becomes 180 degrees out of phase then you've got oscillation.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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Mike H
- Amstrad Tower of Power
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#359
Yes that's true ~ but with the supply caps charging and discharging as well... 
Oops ...Graeme wrote:No, the PSU is cobbled together too
Yes does sound a bit like 'overkill'...It was also mentioned on DIY Audio about having a seperate Hv feed to V1.
I dont get how that would be any different (electricaly) than how its wired at the moment though, maybe im missing the point?
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
#360
We have a breakthrough
Just swapped the 1K resistors between R29/30 and C12/13 for 10k's.
(i remembered to reconnect C10/11
)
Noise is back, obviously.
It doesnt swing volume when run through the line stage
With the cart plugged in the high frequency tone has gone
There is still a little hum, but not untill high volumes and there is a little something, interferance maybe, but considering its not cased i think thats ok.
So, just noise now, and thats probably down the the cheapo resistors.
I think a nice, neat hardwired version with metal films and an earthed case will finish the project off nicely
I still have fluctuating voltage at the outputs though. Its quite alot (nearly 1v) soon after switch on, then drops to around 0.05v with plenty of fluctuation.
Could it just be the multimeter picking up the noise?
Just swapped the 1K resistors between R29/30 and C12/13 for 10k's.
(i remembered to reconnect C10/11
)Noise is back, obviously.
It doesnt swing volume when run through the line stage
With the cart plugged in the high frequency tone has gone
There is still a little hum, but not untill high volumes and there is a little something, interferance maybe, but considering its not cased i think thats ok.
So, just noise now, and thats probably down the the cheapo resistors.
I think a nice, neat hardwired version with metal films and an earthed case will finish the project off nicely
I still have fluctuating voltage at the outputs though. Its quite alot (nearly 1v) soon after switch on, then drops to around 0.05v with plenty of fluctuation.
Could it just be the multimeter picking up the noise?