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Old 17th Mar 2016, 2:24 pm   #101
paulsherwin
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Default Re: Solid state preamp for EL84

You are right, that's not clipping, it's unstable.

Have you double checked the construction, particularly the way the capacitors are wired? What have you used for the 1uF cap? If it's polarised, have you wired it the right way round?
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Old 17th Mar 2016, 2:51 pm   #102
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Default Re: Solid state preamp for EL84

I only had polarised 1uf caps and I put the negative side to the top of the emitter.
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Old 17th Mar 2016, 3:20 pm   #103
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Default Re: Solid state preamp for EL84

Looks like mains ripple.
Set the scope to 'line trigger', and the time base to 10 or 20mS/DIV.

If the wobble is then stationary it's main frequency related (ie 'line' frequency).

dc
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Old 17th Mar 2016, 4:11 pm   #104
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Default Re: Solid state preamp for EL84

I thought it might be my sig gen playing up so I tried a test record with tones on it and I did the settings that you suggested and got this.

https://m.youtube.com/watch?feature=...&v=c0eMPlWoLGU
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Old 17th Mar 2016, 4:39 pm   #105
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Default Re: Solid state preamp for EL84

I watched the last video, it didn't tell me a lot to be honest, if you think your sig gen's output is at fault then maybe check that with the 'scope, if all is ok I would feed a signal from the sig gen (say 1Khz) to the input of your preamp at a level that's realistic to the pickup cartridges output, then check the waveforms from there, input at preamp, output from preamp, output across the loudspeaker or dummy load, make a note of the peak to peak amplitude remembering to take into account 'scope probe settings (X1, X10 etc)

As has been already suggested, set the 'scopes time base to 20ms, that should show up any anomalies that are derived from mains hum.

Try at different volume settings for the speaker/dummy load waveform, noting the peak to peak amplitude.

Without proper coherent data etc to asses what's what usually means stabbing in the dark, you might be lucky or might be not.

Lawrence.
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Old 17th Mar 2016, 4:46 pm   #106
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Default Re: Solid state preamp for EL84

I'm going round in circles with this thing. Can't believe something so simple is confusing me so much.

Am going to put it away for today.
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Old 17th Mar 2016, 4:53 pm   #107
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Default Re: Solid state preamp for EL84

I admire your doggedness and patience Tim. I am afraid I might have given up and tried another solution by now.
It does seem to indicate however that even developing a simple circuit can turn out not to be a trivial task.

A.
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Old 17th Mar 2016, 6:11 pm   #108
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Default Re: Solid state preamp for EL84

Dave Cox suggests it may be mains related, and from watching the youtube video, I agree. There is an unwanted 'signal' rolling throught the scope picture.

As he says, put the scope onto mains triggering (AC line trigger in Tek parlance_ and you should see the ripple stop moving, and the main signal from your sig gen run through the picture.

One hint: Always try to have the scope channel gain pots in their "Cal" positions and tell people how many volts/div and seconds/div are selected. Otherwise people looking at your photos are left guessing what the scales of things are.

The wave shape from the Nombrex looks to be distorted, on the input photo, just after it starts to fall from the top of each cycle, there seems to be a chunk 'missing'

One difficulty is that you have several people advising you and we all have different viewpoints, and so the guidance you're getting is causing you to jump around. Our fault, not yours.

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Old 17th Mar 2016, 6:35 pm   #109
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Default Re: Solid state preamp for EL84

To add another variable, increased gain brings with it increased susceptibility to hum pickup. What's just background to a one valve amp may become a problem in the typical simple player being discussed. One more committee member, sorry!
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Old 17th Mar 2016, 8:34 pm   #110
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Default Re: Solid state preamp for EL84

Hi guys,

Thanks for all your input.

I really do appreciate all your help!

I will get the pp data tomorrow. I must say that I did lose patience with it today more at my lack of experience using a scope than anything else. It would be so much easier if I knew how to use it properly. I'm pretty adept with a multimeter but this is uncharted territory for me so please continue to bear with me.

I'm sure we'll get there eventually. This preamp has real potential to sound great.

What are the thoughts on this unwanted signal, what could this be?
Is even more decoupling possible with this circuit?

The waveform is a bit kinked on the nombrex I really need to recalibrate it when I have learnt how to.

Thanks again guys.
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Old 17th Mar 2016, 8:43 pm   #111
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Default Re: Solid state preamp for EL84

Stick with it kiddo and hopefully you'll get there, stick to the simple stuff first with regarding using the 'scope, and as I said in a previous post set the Y gain control to the cal position, that's the inner red control knob on the volts per division control.

Lawrence.
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Old 17th Mar 2016, 9:02 pm   #112
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Default Re: Solid state preamp for EL84

We would benefit from some comments from Chris (Herald1360) who suggested this design in the first place. I'm suspicious about the suggestion that this instability is mains derived, as that doesn't explain why it only kicks in when the input reaches a certain level. The instability seems plausibly close to the resonance of the 1uF capacitor, but I can't suggest what the mechanism might be.
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Old 17th Mar 2016, 9:40 pm   #113
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Default Re: Solid state preamp for EL84

The bootstrap is positive feedback, of course, but the gain ought to be fractionally less than unity.

At the moment, I don't know whether there is instability or hum pick-up, so I'm hoping some work with the scope will show what's happening.

David
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Old 18th Mar 2016, 7:22 am   #114
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Default Re: Solid state preamp for EL84

Simon, Refugee showed me a way to see if the interference on a signal is mains related by feeding a 50hz/100hz signal into the DUT. I can't remember all the details or describe the process without being at the scope.

I was too had difficulty using a scope at first Tim, see http://www.allaboutcircuits.com/work...gger-controls/ and https://www.youtube.com/watch?v=g0tBJlOEz00 also https://www.youtube.com/watch?v=OFGm-Pel4Hg
Try inputting a 1khz signal from your sig gen into your scope, then adjust the trigger controls to get a good display, then try with a 50hz signal using the mains triggering as suggested. It's easier to suss out the scope controls when not testing something.

Andy. .
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Last edited by Diabolical Artificer; 18th Mar 2016 at 7:29 am.
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Old 18th Mar 2016, 8:31 am   #115
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Default Re: Solid state preamp for EL84

The designers of decent scopes got there already. Looking for mains hum is a common problem, so they put an extra position in the trigger source selector switch so that you can watch the input signal, but trigger (synchronise) the timebase with mains. What happens when you do this is that mains related junk is made stationary while your normal signal stuff isn't synchronised and moves through the picture.

At the moment you're triggering off of the signal, so the signal is seen fixed and you have something you don't want moving through the picture. Select 'Line" trigger and if the unwanted stuff is mains related, it gets frozen and the signal moves. If it isn't mains related, they both move.

Once you get used to driving a scope, and start picking up ways of interpreting its traces, you'll use it as your primary test instrument because you can see so much more of what's going on. Multimeters are more accurate for voltage measurements, so they have their place when you need accuracy.

David
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Old 18th Mar 2016, 12:51 pm   #116
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Default Re: Solid state preamp for EL84

Hi guys,

I set the scope to 1 ms and 2 volts/div.
I'm using the 10x setting on the probe.

I switched from int to line source and the jittering stopped.

I have uploaded a video but it's very hard to see the line setting with my camera.

There seems to be a bit of distortion now.

https://m.youtube.com/watch?feature=...&v=uCpzBPya_SM

Can somebody talk me through getting pp data?

Many thanks,
Tim
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Old 18th Mar 2016, 1:16 pm   #117
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Default Re: Solid state preamp for EL84

Quote:
Originally Posted by vampyretim View Post
I switched from int to line source and the jittering stopped.
That's solved one issue. You don't have instability, it's just mains hum. Line trigger is a handy way to find out, it's a common problem, so Tek made it easy to check.

As said earlier, the lower output cartridge makes less volts, so any hum picked up is going to be relatively larger.

Is the sig gen a battery powered one, or is it mains? Hum could come from the latter - a possibility not a certainty.

Hum could also come from a ground loop involving record player, scope, sig gen. They can be difficult to track down.

Anyway, you've learned one scope trick. Time for another. Probe not the signal line, but the ground line and see if the hum signal is also there. Use line trig if necessary.

David
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Old 18th Mar 2016, 5:26 pm   #118
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Default Re: Solid state preamp for EL84

I think the mains (or line) ripple you are seeing on the scope, with a test signal, is an artifact of the measurement. That is, it only exists in the 'scopes view of things when you have the signal generator attached As David pointed out there can be many causes and it is always quite a problem to eliminate them !

Anyway, on a previous videos the hum is clearly audible but on the latest video it's much less evident (after you added the extra decoupling) when you actually play a record and, from experience, you probably would see very little on a scope at that remaining level of hum.

The distortion I CAN hear on the latest video (which strongly depends on the volume setting) is very likely due to the EL84 amp itself. At a guess it is clipping but someone who knows one end of an EL84 from the other can confirm the correct operating point for it

dc
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Old 18th Mar 2016, 7:47 pm   #119
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Default Re: Solid state preamp for EL84

It was written..."Can somebody talk me through getting pp data?"

By pp it is assumed that it's P to P aka Peak to Peak, there are a couple of measurements that are handy to know, peak voltage and peak to peak voltage, peak to peak voltage is the one that is most commonly required, in simple terms with AC waveforms the peak voltage is generally referred to as the voltage amplitude from the zero crossing point of the waveform to the highest amplitude point, it can have a positive or negative term, the waveform above the zero crossing point is positive going (positive voltage) the waveform below the zero crossing point is negative going (negative voltage) the amplitude of those two voltages added together is the peak to peak voltage, eg: one can say that if a waveform swings from 1 volt negative at its lower peak to 1 volt positive at its highest peak then it's peak to peak voltage is 2 volts and it's peak voltage is 1 volt.

To measure P to P with a 'scope within a reasonable degree of accuracy is quite straight forward, the most common way is to AC couple the signal to the 'scopes Y amplifier, you are most probably aware of the selector switch on the front panel, it can select for either AC coupling or DC coupling, usually there's a ground position as well, in that position the 'scopes input will be grounded, effectively shorted out.

Before measuring the amplitude of a waveform (P or P to P etc) you need to check if the 'scopes calibration is within reason, to check for calibration across it's rated bandwidth is a bit involved so we'll skip that as you are only interested with audio frequencies in this thread.

There's a calibration waveform output on the front panel, it will be a square wave, it's amplitude and frequency should be on the front panel somewhere, if you connect the 'scopes probe to it you should be able to check the calibration of the 'scopes Y amplifier to a reasonable degree by measuring the peak to peak value of the calibration waveform and comparing it to that stated on the front panel.

The calibration waveform can also be used to adjust the frequency compensation on an X10 'scope probe or a 'scope probe that has an X1 X10 switchable setting on it, the adjustment should be made using a non metalic tool, the adjustment should be made so that the calibration square wave has minimum overshoot and undershoot, eg: the top and bottom of the square wave should be flat and level with nice square corners on the rising and falling edges.

Before commencing measurement the 'scopes variable gain pot should turned to the cal position, switch the input selector to ground, that should give a nice thin flat trace, set that to the center horizontal line on the graticule, that will represent the zero crossing point or zero line if you will, switch the selector to AC and the Y volts per division switch to a suitable range and away you go, the highest point to the lowest point point of the waveform is the peak to peak and it's value can be read off accordingly.

This isn't meant to be an all there is to know about waveform measurements, it's just a basic guide to help you on your way, if I've missed anything out then guilty as charged.

Good luck.

Lawrence.
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Old 18th Mar 2016, 8:47 pm   #120
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Default Re: Solid state preamp for EL84

Thanks a lot guys.

That's a very helpful guide Lawrence, I will have a go at measuring the pp voltages. I do understand what you are saying and am grateful to you for explaining it all so clearly.

I really think I'm going around in circles with this and am going to come back at it fresh on Monday. I think I'm over complicating it all and am going to reduce the gain of the transistor amp first. I'll see if that fixes it.

The Nombrex is battery powered so the unwanted mains frequency isn't coming from that.

I will try probe the ground line too.

I do think that the scope is playing up a bit and I'm chasing phantoms.

Anyway, to be continued. I will get back on Monday.

Thanks again,
Tim.

Last edited by vampyretim; 18th Mar 2016 at 8:53 pm.
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