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Old 6th Mar 2016, 11:18 am   #21
Radio Wrangler
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Default Re: Solid state preamp for EL84

5mV from a magnetic cartridge to the several volts needed at the grid of the EL84 is going to need a gain of well over a thousand. It's beyond the capabilities of a single transistor.

The original query related to the amplification of the output of a low-output ceramic piezo-electric cartridge to replace that of a high output crystal cartridge, where a gain of 10 or 20 would do.

I think Chris is on the right track.

Anyway, there's plenty of room in most record players, so a little kit for a small 2-stage amplifier with a small mains transformer PSU would find a number of takers, methinks. Another variant with a magnetic input/RIAA characteristic would allow the fitment of one of the DJ cartridges to any player with a gentle enough deck.

David
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Old 6th Mar 2016, 12:04 pm   #22
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Default Re: Solid state preamp for EL84

Quote:
Originally Posted by Herald1360 View Post
Quote:
Originally Posted by paulsherwin View Post
There is absolutely no problem with stealing power from the valve cathode to power the preamp.
Not normally for a multistage amp, but in this case with only the EL84, a simple class A preamp fed from the cathode volts is, as has been mentioned, physically incapable of delivering enough output swing to the grid of the same valve to deliver full output which requires a p-p grid input of nearly twice the cathode voltage.

In this case, simplest is still a single transistor, but it needs to be fed from the HT at a voltage of somewhat higher than twice the cathode voltage.

Something like an MPSA42 could use HT directly and with 150K collector resistor and unbypassed 15K emitter resistor and bootstrapped biassing via 10Meg + 47K resistor from collector to base will give a gain of about 10 with an input impedance of about 1Meg.
Your circuit is certainly a better solution technically.

But why does the EL84 drive need to be so high? We're not trying to drive the EL84 to clipping, just emulate the output it produced with the original high output ceramic or crystal cartridge. That only needs a couple of volts. Or am I missing something?
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Old 6th Mar 2016, 12:28 pm   #23
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Default Re: Solid state preamp for EL84

A couple of volts output from a couple of hundred millivolts from a ceramic cartridge means a voltage gain of about 10. If the impedances were the same that would be 20dB gain, but the impedances are not, the higher voltage is at a higher impedance so that the power gain is substantially less than 20dB. Theoretically, a transformer would do the job, but practically it would be a nightmare at the impedances involved. Primary inductance would get you at one end and stray-C at the other.

A single bipolar or J-FET would do the job, but the cathode voltage of the EL84 wouldn't be high enough for the transistor to do enough swing to use the EL84 fully.

Stealing a few mA off the HT isn't going to end the world.

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Old 6th Mar 2016, 12:52 pm   #24
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Default Re: Solid state preamp for EL84

The OP stated that the unit is a Dansette Bermuda, according to a schematic I'm looking at it's a single stage EL84 job, according to the EL84 data sheet for single ended operation about 4.5 volts RMS is needed to give a power output of around 5 watts or so, I don't think the original PU cartridge would have shoved that out?

If that's the case then the grid drive can be scaled down accordingly, eg: not a lot of supply voltage needed for the transistor(s)

Lawrence.
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Old 6th Mar 2016, 1:47 pm   #25
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Default Re: Solid state preamp for EL84

This Thread seems to be going round in circles. In the Dansette "Bermuda" the original high-output cartridge (a BSR TC8H) delivered 1volt to the EL84 control grid. My reading of all this is that the OP is trying to use a more readily available (Stereo-Compatible) medium-output cartridge - typically giving 200mV - boosted x 5 to replicate the performance of the original cartridge. I don't know where magnetic cartridges crept in to this. You wouldn't use these in a Dansette. Edward
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Old 6th Mar 2016, 3:24 pm   #26
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Default Re: Solid state preamp for EL84

Slightly OT, but if a high output crystal cartridge gives say 1V rms at the groove modulation normally used for these figures, wouldn't it give considerably more from a very loud passage? So a watt or so of normal listening level on a moderate passage could easily peak up to 5W on a loud bit.
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Old 6th Mar 2016, 5:35 pm   #27
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Default Re: Solid state preamp for EL84

Well Yes, because it's analogue and it will always vary with amplitude. On these Dansettes, a 45 or a 78 would sound considerably louder than an LP simply because of the higher (groove) modulation of such recordings. Edward
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Old 7th Mar 2016, 11:56 am   #28
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Default Re: Solid state preamp for EL84

Hey guys,

I managed to find the original design.

It's very simple.

It almost sounds like it needs a little attenuating like it's overdriving the EL84 but it also sounds like serious impedance mismatching.

I would really like to get my head around what's actually going on.

I'm trying to use readily available modern, stereo, ceramic cartridges albeit lower output than the originals. I have needed a solution for this for some time but have never been able to get this design to work.

I'm happy to abandon the BC109 altogether and use a F.E.T or any other mass produced solid state device. So please don't feel bound by the BC109 if there's a better solution please feel free to recommend it.

Thanks again all.
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Old 7th Mar 2016, 12:38 pm   #29
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Default Re: Solid state preamp for EL84

You may get some clues from this: http://www.r-type.org/articles/art-127.htm. The things that immediately strike me are that your volume control is in a different place and the published circuit has an extra capacitor (C5) across the transistor's supply.

I build an amplifier with a 741 as the preamp. To power that I used the heater supply.

— Joe

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Old 7th Mar 2016, 2:01 pm   #30
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Default Re: Solid state preamp for EL84

That circuit will work but it has much too much gain and a huge impedence mismatch. Reduce the 3.3M resistor to 2.2 or 1M, reduce the 33k resistor to 10k and add a 470k-2.2M resistor in series with the input. Experiment with the value until the output is correct and/or the overload distortion stops.
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Old 7th Mar 2016, 2:17 pm   #31
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Default Re: Solid state preamp for EL84

Looking at that circuit, it's unsuitable for various reasons. Firstly, its input resistance is far too low for a ceramic cartridge- only about 25K at best. It has a gain of about 100 inherently, but if you pad the input up to 1Meg (the right sort of load for a ceramic cartridge) that falls to about 2.5!

Actually that might just about do, with a 470K in series with the input, the gain is about 4.5, though the bass response may be a bit thin.

The other problem, though, is the same as already mentioned, lack of enough maximum output to fully drive the EL84 anyway if it's run from the cathode voltage.

One work round might be.....

Fit another equal value cathode resistor in series with the present one. Bypass both resistors with their own 470uF capacitor. Connect the bottom of the volume control to the mid point of the resistors. This will give you twice the DC for the preamp but leave the EL84 biasing the same at the expense of a small drop in EL84 anode-cathode voltage.

So, that will need another 470u, another cathode resistor and a 470K resistor over what you have now. Worth a go?
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Old 7th Mar 2016, 2:21 pm   #32
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Default Re: Solid state preamp for EL84

3k3 in series with the emitter would a) decrease the gain and b) increase the input impeadance. And change the biasing to voltage instead of current.
 
Old 7th Mar 2016, 2:25 pm   #33
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Default Re: Solid state preamp for EL84

I agree with all you have said Chris, but the OP clearly wants the simplest possible design, both to construct and comprehend. An overall gain of even 2.5 will still make a very significant difference and may be adequate. It's easy enough to try it.

Using current bias in a circuit like this won't cause any problems and simplifies things.
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Old 7th Mar 2016, 2:47 pm   #34
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Default Re: Solid state preamp for EL84

The Burrows and Quilter modified version of the Bailey preamp in Wireless World operated ceramic cartridges into low-Z loads, but handled this with frequency compensation and a highish amount of available gain. Essentially it was a charge-amplifier. Simple it was not.

I too agree with Chris. It might help, but it won't sound right.

A simple circuit isn't the one with the fewest parts, it's the one that does the job and works first time. Parts are cheap, hassle isn't.

David
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Old 7th Mar 2016, 3:57 pm   #35
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Default Re: Solid state preamp for EL84

I understand the OP doesn't want to use a separate power source.
However as david said, less parts doesnt always mean less hassle.
I still say a tiny mains transformer, full wave rectification, and a simple Voltage reg (a 78** would do and then you dont have to calculate setting resistors) would give you a reliable, steady hum free supply that won't interact with the valve amp.
As Pillenwerfer said, a 741 opamp would make a decent pre in this application although a TL071 wouldnt be any more expensive.
If Vampyre Tim wants to PM me I'll give him a board with one of these titchy pcb mount transformers on it and chuck in a TL072 op amp and an 8 pin DIL socket. Just use one channel of it. i'll check my ladybird book of circuits too as theres definitely a ceramic step up amp design on it using a simple op amp. I'm having a clear out due to a hopeful house move so you'll be doing me a favour really.

Andy.
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Old 7th Mar 2016, 4:05 pm   #36
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Default Re: Solid state preamp for EL84

And something just sprang to mind.
I had an old Wall wart battery eliminator that gave 9V. I took it apart as it was literally falling apart and inside was a very small mains transformer about 1 cubic inch with a bridge rect and a 470uF capacitor on a board about the size of my fingernail.
Could be a good source of these items for future mods.
As Edward Huggins says, The demand for a simple effective step up amp will only increase as high output cartridges disappear.

A.
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Old 7th Mar 2016, 7:38 pm   #37
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Default Re: Solid state preamp for EL84

Hello guys,

I have been tinkering around with this thing all day. I did as recommended and put 470k in the signal line. So the huge impedance mismatch has been fixed, however I now have very little gain or bass.

I feel like abandoning the bc109 altogether for this purpose. I will though give heralds suggestion a go as I'm curious.

And yes Andy I would like to try it with a small mains transformer too. I'll pm my address.

I should say I too agree with the sentiment that parts are cheap and hassle isn't. I don't mind it having more components so long as it works well.

Thanks again guys. I'll post progress tomorrow.

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Old 7th Mar 2016, 8:56 pm   #38
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Default Re: Solid state preamp for EL84

I think Chris's design back in #19 is the best overall compromise if you don't mind a bit of extra complexity. HV transistors are cheap from eBay sellers, and many of us will have something like a BF258 or a Japanese equivalent in the junk box, scavenged from a scrap portable TV or monitor.
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Old 7th Mar 2016, 9:36 pm   #39
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Default Re: Solid state preamp for EL84

Hi Gents, attached an outline of the TAA320 circuit from the Mullard FET booklet.

These chips are easily and cheaply available on e-bay and with a little extra decoupling work well. they draw about 6mA HT current and if gain is too high NFB can be introduced at the 10R/ 250uF junction.
This is used to replace the high output cartridges with lower output types, not for use with modern very low output types.

Ed
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Old 7th Mar 2016, 10:00 pm   #40
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Default Re: Solid state preamp for EL84

Ed, I think he may be safer sticking with the TL071 family of opamps mentioned earlier. There are many makers and hundreds of pin-compatible parts which would drop in and just work. It means that if he ever wants to do another or if he needs a spare far in the future, then something useable ought to be available. Also the gain can be changed easily and over a very wide range with just a pair of resistors.

David
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