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Old 15th Mar 2016, 4:18 pm   #81
peter_sol
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Default Re: Solid state preamp for EL84

OK sorry to mislead.
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Old 15th Mar 2016, 5:42 pm   #82
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Default Re: Solid state preamp for EL84

Hi David,

Thanks for your suggestion, it worked a treat.

I have put 2.7M in the signal line and it's reduced the distortion considerably. However somethings still not right. The distortion does seem to diminish with the volume.

It sounds like there's a ripple on the audio much like a leaky coupling cap.

I've uploaded another video.

https://m.youtube.com/watch?feature=...&v=m0DBbag21Pc

I do have a dvm and a scope so can check out biasing tomorrow. I did have a cursory check with my avo and the emitter voltage off the top of my head was 21.5 v and the base was 7.5v the collector was 81v.

Shouldn't the base voltage be higher than the emitter?
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Old 15th Mar 2016, 5:58 pm   #83
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Default Re: Solid state preamp for EL84

The base is fed through a very high resistance, so the DVM will load it and give an erroneously low reading.

The loading effect on the emitter will be much less, so we'll trust the emitter voltage 21.5v in 15k Ohms means 1.43mA not a bad value.

Now 1.43mA in 150k should drop 215 volts. If your supply really was 250v, that would leave you (250-215) = 35v on the collector.

You've got 81v (possible a bit more due to loading by your meter) So is your HT more like 300v?

81v on collector 21.5v on emitter leaves about 60v across the transistor. It's a 300v rated device, but it pays the price for that rating with a fairly low Hfe spec.

David
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Old 16th Mar 2016, 12:59 am   #84
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Default Re: Solid state preamp for EL84

I've been out of the loop for a bit- this is getting quite exciting!

That design (post 19) was cobbled up playing with a simulator and seemed to work OK bias wise, but that would have been at the particular "typical" gain of the transistor model. I was trying for minimum parts count but ended up adding the bootstrap to get the input R up to something high enough for the cartridge without losing most of the gain with a series resistor.

Gain was intended to be about x10 (150K/15K) or a bit less if the transistor gain was lowish. Missed a trick with the decoupling requirement

Gain could be reduced simply by reducing the collector load resistor. For about x4 use 68K or x2, 33K. Simulated results looked reasonably linear, with plenty of headroom for a signal big enough to drive the EL84 fully.

Op amps are really easy by comparison, and come with all sorts of built in performance that would need at least a dozen transistors and half a box of passives to get anywhere near in a discrete design. My preferred breadboarding technique for opamp designs uses single sided copper clad matrix board hand wired on the back. Through holes are cleared on the top with a Vero spot face cutter or 3mm drill bit and anything connected to 0V just gets top soldered to the copper plane.

I'll follow proceedings with interest
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Old 16th Mar 2016, 12:04 pm   #85
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Default Re: Solid state preamp for EL84

Morning all,

The H.T was running a bit high so I've reduced the cathode bias by putting 370 ohm in place of 470 ohm. I now have 245v at the valve anode.

Voltage checks with my dvm is. E-16v C-84v B-16v

I'm guessing that the collector voltage is way too high.

Would you guys like me to inject a signal and trace it?

I know we're close to getting this thing working. I can hear it working so much better than the bc109 ever did.

Thanks again guys.
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Old 16th Mar 2016, 7:43 pm   #86
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Default Re: Solid state preamp for EL84

This is good news Tim!

That's a good solution if you are competent and happy working with high HT voltages.

For the less experienced i will be watching how the op amp experiments go. I can't do any myself as I do not possess a vintage record player to work with.

However I was wondering, if it's a transformer player with 6.3V heaters what are the issues with using a voltage doubler followed by a voltage regulator PSU? In particular are there any earthing issues especially if the heaters are referenced via a centre tap?

Otherwise I think the mini transformer PSU is the way to go.

Tim, if you do get up and running with that path I'd love to see how small you got it on a piece of veroboard as I reckon less than 3 sq inches would be needed. which would make it mountable very close to the arm base?

Andy.
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Old 16th Mar 2016, 10:09 pm   #87
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Default Re: Solid state preamp for EL84

Thanks Andy,

I'm still going to build the op amp preamp. I will make a start in a few weeks when I have some more time.

I will let everyone know how it goes.

Does anyone have any thoughts on the voltages?

I expected everyone to say that they were way off!

What voltages were you all expecting to see?

This thing really does want to work. The distortion you can hear in the video are not the cameras amp clipping, that's what the amp sounds like.

I'd be grateful for some feedback on the voltages.

Many thanks,
Tim.
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Old 16th Mar 2016, 11:16 pm   #88
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Default Re: Solid state preamp for EL84

You can reduce the voltages on the transistor by increasing the value of your 10k smoothing resistor to 47k or even 100k. I don't think the voltages are a problem though, as they are way under the maximum ratings of the transistor.

You may need to further attenuate the input, as clipping by the transistor is the most likely cause of the distortion.
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Old 16th Mar 2016, 11:35 pm   #89
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Default Re: Solid state preamp for EL84

My feeling too.
It sounded to me like you were overloading the input.
you could try a 1meg pot in front of the input, a preset would be ideal and then you could ease the attenuation until it sounded right.

A.
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Old 17th Mar 2016, 7:55 am   #90
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Default Re: Solid state preamp for EL84

With that many volts on the collector, clipping would be at a comparably high voltage swing. There is plenty of room to swing positive and plenty of room to swing negative. I wonder if the gain is higher than expected for some reason.

I'd be very reluctant to try to attenuate the input, firstly because attenuation at such high impedance levels involves either very high value series resistors or increases te loding on the cartridge, also because if an amplifier is giving too much gain, it would give better bandwidth and linearity with the gain turned down, and also because an attenuator ahead of a high gain amplifier is a poor thing to do in terms of noise figure.

Chris has a working simulation for it, so he could wind up the input voltage and see where things start to go wrong.

But we seem to be trying to fix the transistor amplifier before we know where the distortion is happening. Every home should have an oscilloscope!

David
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Old 17th Mar 2016, 12:28 pm   #91
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Default Re: Solid state preamp for EL84

Hi guys,

these two pictures are of a signal injected into the amp.

The first is the clean tone coming in before the 2.7M input resistor.

The second is at the output.

It seems pretty clean to me.

Any thoughts?
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Old 17th Mar 2016, 12:37 pm   #92
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Default Re: Solid state preamp for EL84

Can't see much wrong with that. Were those measurements taken while you were hearing audible distortion from the speaker?
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Old 17th Mar 2016, 12:37 pm   #93
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Default Re: Solid state preamp for EL84

They both look distorted

What's the signal source ?
dc
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Old 17th Mar 2016, 12:40 pm   #94
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Default Re: Solid state preamp for EL84

Quote:
Originally Posted by vampyretim View Post
It seems pretty clean to me.
Any thoughts?
Looks clean but PP input/output voltages needed to make any judgement as to wot's wot with the rest of the player.

Lawrence.
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Old 17th Mar 2016, 12:42 pm   #95
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Quote:
Originally Posted by paulsherwin View Post
Can't see much wrong with that. Were those measurements taken while you were hearing audible distortion from the speaker?
In all fairness no, the tone sounded fine.

Shall I inject some music into it?
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Old 17th Mar 2016, 12:44 pm   #96
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Quote:
Originally Posted by dave cox View Post
Whats the signal source ?
It's a Nombrex type 27 signal generator on the A.F setting.
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Old 17th Mar 2016, 12:45 pm   #97
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Quote:
Originally Posted by pwdrive View Post
Looks clean but PP input/output voltages needed to make any judgement as to wot's wot with the rest of the player
Ok, can someone talk me through this?
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Old 17th Mar 2016, 12:59 pm   #98
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Default Re: Solid state preamp for EL84

I haven't been following thread in detail but ref to what I said: What was the volts per division setting (in the cal position) on the 'scope when the waveforms were observed that will give the PP voltage (peak to peak) From that you can tell if the output is to high for the players original amp, having said that it would depend to some extent how the volume control is connected, if the volume control is connected as per original then an excessive signal output from your preamp might sound ok at a low volume setting but drive the EL84 into distortion as the volume control is advanced...Hope I've not confused...

Lawrence.
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Old 17th Mar 2016, 2:02 pm   #99
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Default Re: Solid state preamp for EL84

Quote:
Originally Posted by vampyretim View Post
Quote:
Originally Posted by paulsherwin View Post
Can't see much wrong with that. Were those measurements taken while you were hearing audible distortion from the speaker?
In all fairness no, the tone sounded fine.

Shall I inject some music into it?
Crank up the signal until it does sound distorted, then measure again.

We are trying to establish if the preamp is introducing the distortion or if it is coming from somewhere else, say a cartridge/stylus problem or a bad valve.
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Old 17th Mar 2016, 2:14 pm   #100
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Default Re: Solid state preamp for EL84

Hello Paul,

It's the preamp. It's unstable ever so slightly and the valve amp amplifies the unstable signal.

https://m.youtube.com/watch?v=XEAc33...ature=youtu.be

Bear with the bad camera work it's difficult to use a scope and camera at the same time.

It might not be very clear from the footage but the audio is bouncing up and down at the preamp. This is the ripple that I have been hearing.

Have a look and see what you think.
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