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Old 16th May 2016, 3:25 pm   #141
peter_sol
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Default Re: Solid state preamp for EL84

Perhaps your frequency is 1.075
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Old 16th May 2016, 3:37 pm   #142
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Default Re: Solid state preamp for EL84

Quote:
Originally Posted by vampyretim View Post
I've measured peak to peak voltage in the transistor preamp only.

I have .4 V with a 10.75khz frequency coming out of my signal generator. I have measured this with nothing connected to the signal generator output.

I get with the vol pot 3 quarters of the way up 2.1 V at the collector. The frequency stays the same.

Now once I go beyond 3 quarters on the vol pot the top of the waveform starts to clip quite harshly and I measured this in the preamp output so the clipping seems to be happening in the preamp even though the pot is at the output.
As it's been a while it might be worth posting the current schematic that pertains to the above tests.

Lawrence.
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Old 16th May 2016, 4:35 pm   #143
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Default Re: Solid state preamp for EL84

Hi guys,

Below is the circuit.
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Old 16th May 2016, 4:49 pm   #144
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Default Re: Solid state preamp for EL84

What voltage is the HT shown? you show a 350 volt decoupler - so are you running from the main HT of the record player or a lower voltage derived from the cathode?
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Old 16th May 2016, 5:31 pm   #145
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Default Re: Solid state preamp for EL84

So its clipping at roughly 2V?
Might be insufficient supply volts so there's insufficient headroom?

I have to confess Tim that by now I'd have given up and built something with a TL071 (072 for stereo) and KNOW that it would work and be stable.

Although I would likely build a 2 stage preamp using the first half of a dual opamp as a unity gain impedance buffer into a gain stage feeding the input of the EL84 amp. If you want to know how I can give you a circuit, or any of the other guys can too. Did I send you a PCB with a simple Maplins preamp kit on it? If its the stereo version its easy to knock up a 2 stage mono preamp using it by cascading one channel output into the input of the other channel and adjusting the input and feedback resistors to get a high impedance unity gain first stage feeding a gain stage.

A.
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Old 16th May 2016, 6:16 pm   #146
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Default Re: Solid state preamp for EL84

Hi guys,

Yes Andy it does seem to be clipping at the 2V mark.

I'm going to give the op amp a go next and will respond to your pm soon. Thanks a lot for all your help!

There are a few differences between my diagram and the circuit I've built. In the circuit I haven't got 2.7M in the input the signal is injected straight into the .1uf cap and the output cap is .01 not .001uf

Have I made the correct frequency calculations? (Post 140)

My H.T is 200V and it's coming from the H.T rail of the El84.

Many thanks guys.
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Old 16th May 2016, 7:58 pm   #147
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Default Re: Solid state preamp for EL84

No worries Tim, I wasnt having a "go". I have to admit an admiration for your tenacity.
I'll have to trawl back through the messages, but if you need any ideas on component values I'd be happy to help, as will the other more experienced guys I'm sure
Regards.
A.
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Old 16th May 2016, 9:43 pm   #148
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Default Re: Solid state preamp for EL84

How about rebuilding the EL84 single stage amplifier as the later two stage used in the Conquest using an ECL82? It is a simple enough circuit to copy with no extra major components. It would overcome the problem of needing a high output cartridge keep the amp "all valve" and be a copy of what Dansette did on later players, so should work ok?
Just a thought?
Rich.
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Old 17th May 2016, 9:05 am   #149
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Default Re: Solid state preamp for EL84

Hi Rich,

I have done this in the past and I found it to be quite time consuming and a bit of a pain.
I would rather have some preamps that can be built in quiet business times and easily installed without hassle.

Many thanks,
Tim.
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Old 17th May 2016, 9:41 am   #150
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Default Re: Solid state preamp for EL84

Oh I see sorry I thought it was just one record player you were working on I can see it would be long winded if you have a few to do.
I did try a Maplin preamp myself some years back I powered it from the heater supply rectified and smoothed. I got nowhere having problems with it sounding shrill and causing a slightly distorted sound on higher output records like older 45s. Having read through this thread I have learned a lot and now have more idea why it didn't work very well, I expected it to be simple!
To be honest I didn't try very hard I had an hour or two trying various things and then reverted to looking for a good high output cartridge which I managed to get eventually.
I will follow this thread with interest!
regards
Rich.
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Old 18th May 2016, 3:23 pm   #151
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Default Re: Solid state preamp for EL84

For what its worth:
I feel the way forward here is a "one stop" solution. i.e. a self contained step up preamp that can be simply connected to the mains via the on off switch of your record player. That way you arent worrying about what voltages to work with either via HT or heaters. Use a subminiature mains transformer and a simple PSU using a simple Voltage regulator cos its easy and works.
Likewise a 2 stage opamp based step up with the first stage acting as an impedance buffer into a gain stage that I'd set at about 5x which would be ok with say a 9 to 12V supply. If you use 24V then you can get more headroom but I can't see a need for more gain. Fine tune your output with a preset pot into the input stages, or you could put it between the buffer stage and gain stage which would be my preference, and I reckon you have a fairly universal design that can be retrofitted with relative ease in a spare space underneath, preferably as close to the tonearm lead outs as practicable.
The issue of Stridency/excessive treble can be approached by a simple Low pass filter at the output or input or a bit more sophisticated would be a compensation capacitor in the feedback loop of the gain stage. You'd not need anything fancy here as we aren't dealing with hi end hi fi so a ceramic would likely be adequate.
And for Tim, if you are making more than a couple of these it may be worth your while to design a simple pcb to build it on, although I'd be happy to knock one up on a bit of perforated board as and when needed.
I hope my ideas are sound (sic) and have given folks something to work with.

Andy.
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Old 19th May 2016, 10:01 am   #152
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Default Re: Solid state preamp for EL84

Strange that you get clipping when you wind the volume up, given than the control is AFTER the pre-amp. Also, with a couple of hundred volt rail, why does it clip at all ?

I must be missing something.

Is the circuit for the rest of player in the public domain ?

dc
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Old 19th May 2016, 5:14 pm   #153
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Default Re: Solid state preamp for EL84

Is the clipping measured at the preamp output?
just thought maybe 2V is the point where the EL84 in this circuit overloads and if Tim's measuring at the EL84 output that would explain it?

A.
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Old 19th May 2016, 5:42 pm   #154
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Default Re: Solid state preamp for EL84

Maybe the valve is pulling grid current.

Lawrence.
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Old 19th May 2016, 7:26 pm   #155
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Default Re: Solid state preamp for EL84

Quote:
Originally Posted by bikerhifinut View Post
Is the clipping measured at the preamp output?


I should have read back. The distortion is measured at the preamp output before the Power amp.

A.
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Old 20th May 2016, 9:12 am   #156
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Default Re: Solid state preamp for EL84

It might be worth isolating the grid connection from the preamp then 'scope the preamps output at the top of the volume control with the volume control turned to max.

To do this disconnect the connection from the volume control to the grid and connect a 470k to 1meg resistor (or nearish value) from the grid to chassis.

Lawrence.
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Old 20th May 2016, 10:57 am   #157
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Default Re: Solid state preamp for EL84

Hi guys,

Thanks for all your help.

I'm going to do as you suggest Laurence.

Here's the full circuit.

I will post my results later.

Thanks again all.
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Old 20th May 2016, 11:01 am   #158
peter_sol
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Default Re: Solid state preamp for EL84

On most amps of that kind The pickup goes to the top of volume control and the valve grid to the wiper.
Some were as you have drawn but I should rewire as the preamp won't like it.
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Old 20th May 2016, 11:19 am   #159
vampyretim
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Default Re: Solid state preamp for EL84

Sorry for the confusion guys.

I drew that part wrong pickup signal is at the top of the pot and wiper goes to grid!!
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Old 20th May 2016, 11:32 am   #160
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Default Re: Solid state preamp for EL84

No problem, what needs to be established is weather or not the clipping is occurring in the preamp or being caused by the valves grid, so disconnect the grid etc as per earlier post (don't forget to fit a substitute grid resistor) and 'scope the slider from min to max volume.

Lawrence.
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