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Old 7th Mar 2016, 10:50 pm   #41
paulsherwin
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Default Re: Solid state preamp for EL84

I agree that a TL071 design is the one to go for if you're going for an op-amp. Chinese eBay sellers will sell you 20 second sourced TL071s for £1, and if you use a socket you can sub dozens of alternatives.

The OP did want a single transistor design though.
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Old 8th Mar 2016, 9:24 pm   #42
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Default Re: Solid state preamp for EL84

Hello guys,

I've decided to abandon the BC109 and have ordered some of the recommended op-amps and some of the high voltage transistors. I'm going to try a few of the ideas on this thread.

I will let you know in a few days.

Thanks again,
Tim
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Old 10th Mar 2016, 6:30 pm   #43
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Default Re: Solid state preamp for EL84

Unusually for me, as I usually shy away from innovation, I decided to build a ceramic cartridge preamp to a circuit I found online, as per Fig.8 attached.

I was pleased to find that having obtained the 2N3904 transistors and constructed the same that it worked and sounded quite good (at least to my ageing ears!). I ran it from a pair of used PP3's giving 17.8v on load.

I used one of the generic red stylus cartridges to run into a single UL84 record player amp but the volume would not go above comfortable listening level even when flat out.

The circuit states a supply voltage of 12v to 22v but Philips literature for the 2N3904 gives a max voltage of 40v.

Would increasing the voltage to nearer the 40v improve the volume output or can anyone suggest any circuit changes that might help?

Thanks

Alex
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Old 10th Mar 2016, 6:59 pm   #44
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Default Re: Solid state preamp for EL84

I found this and the circuit interesting, but confusing. Surely the whole point of using a ceramic/crystal cartridge is that it needs no RIAA equalisation? Also such cartridges ideally need to work into a 1 Meg load? Edward
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Old 10th Mar 2016, 8:26 pm   #45
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Default Re: Solid state preamp for EL84

Making Q2 a (voltage) gain stage rather than an emitter-follower would do it but I'm sure an op-amp circuit would be simpler. If you had a small driver transformer of say 1:4 ratio then that could go between the output and the valve stage.
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Old 10th Mar 2016, 8:41 pm   #46
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Default Re: Solid state preamp for EL84

OK, looking at the attached circuit it uses the 'charge amplifier' approach.

The second 2N3904 is an emitter follower, it gives only unity voltage gain, but it gives plenty of current gain to drive the low impedance volume control

The first 2N3904 does the real work. With its emitter grounded and the input directly to its base, it is going to impose a load of only a few hundred ohms on the ceramic cartridge, which loads down the output voltage of the cartridge quite a lot. The gain of the transistor is used to compensate for this, and this is controlled by the two capacitors C1 and C2.

In fact the 'gain' of the circuit in terms of the ratio of output voltage to cartridge EMF (what its output would be off load) is set by the ratio of the cartridge capacitance to the capacitive network C2, C2 and R2 to an extent.

This arrangement works, but there are three things not right for your use.

1) It puts DC voltage on the cartridge. The cartridge is piezo electric and that works both ways, so the 700mV of DC creates a bit of stress in the cartridge. This is just bad practice rather than a show-stopper.

2) There is DC across the volume control, which should make it nice and crackly. More bad practice. R4 should be a fixed resistor, and the volume control should be after the DC block capacitor.

3) The overall gain is going to be a bit low for what your UL84 needs. ThE gain is set by the C1 C2, R2 R3 network AND the capacitance of the cartridge. Changing this network isn't the answer, because it can't get more gain than Q1 can give alone. You need another stage of gain.

4) (no-one expects the spanish....) I just don't like series resistor base bias like that. It makes the collector voltage too dependent on the transistor current gain, so sometimes transistor or resistor selection has to be done to get best results.

So this circuit's output is being kept low by the feedback system, I think you have enough supply volts with two 9v batteries. Increasing the supply won't do a lot. Just measure the DC voltage on Q2 emitter and check that it's about half-way up your supply voltage just in case R3 does need fiddling to give things enough elbow-room.

Edit: Just seen Llama's post. Yup, prime territory for an opamp. A much better approach.

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Old 10th Mar 2016, 10:29 pm   #47
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Default Re: Solid state preamp for EL84

Well, I must thank Radio Wrangler in particular for his very comprehensive reply. The circuit came from a article called "Transistor Tutorial Audio Amplifiers Part 5" by Tony Van Roon (VA3AVR) and Ray Marston.

So the way forward is with a TL071 based design then. There seem to be a lot online. Can anyone point me to one that would do the job of boosting the signal from a generic medium output caridge to a level siutable for direct input into a UL84?

Thanks

Alex
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Old 10th Mar 2016, 11:45 pm   #48
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Default Re: Solid state preamp for EL84

Could a blocking capacitorof maybe 10nf or so not be fitted in series with the cartridge, which would isolate it from DC? If it was used with a UL84 or any similar valve, class 'Y' isolators - I've recently used 2n2 caps for this purpose - would be essential anyway, since most amps. built around UL84s are of the 'live chassis' type. If I were building that circuit, I, too, would make R4 (4k7) a fixed resistor , feeding a volume control via the coupling cap. at Q2 emitter, as Radio Wrangler suggests
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Old 10th Mar 2016, 11:56 pm   #49
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Default Re: Solid state preamp for EL84

I don't understand why that circuit doesn't have a cap at the input. Maybe I've missed something?

It is better suited to driving a transistor amp than a valve amp. The emitter follower stage isn't doing anything with a high impedence load.
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Old 11th Mar 2016, 9:40 am   #50
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Default Re: Solid state preamp for EL84

Just to confirm, although the amp I'm working on uses a UY85 and UL84, the power supply is from a mains transformer as per the Regentone AHG8 circuit.

Like the OP, what I really need is a circuit for a suitable preamp that uses cheap easily available parts and which, with suitable modification, can be used with both AC only and AC/DC amps. Does such a thing exist?

Thanks again

Alex
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Old 11th Mar 2016, 10:21 am   #51
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Default Re: Solid state preamp for EL84

There are so many variations of powering in record player amps that any circuit attempting to be universal would probably need to include its own little mains transformer, and once you've bitten that bullet, things become a lot easier.

In all fairness, where do you stop? Use a bit bigger transformer and do a full solid state amplifier? (contrary to some opinions, rather nice ones can be designed) How much originality needs to be kept varies from one person's opinion to another. A lot of people just want a box which makes noises, but want to avoid the poor sound and naff styling of the modern tat.

In all such mucking around, there is the issue of safety and liability which has been done to death in other threads (intended as figure of speech, not a pun) But even so, it is always something which should be considered.

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Old 11th Mar 2016, 10:32 am   #52
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Default Re: Solid state preamp for EL84

I'm not sure after all this if the orginal problem has been addressed here. For those seizing the opportuity, there must be a real market of several hundred units for a self-powered, very basic op-amp, as a "plug in" solution between cartridge and volume (usually 1meg) control. Edward
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Old 11th Mar 2016, 11:17 am   #53
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Default Re: Solid state preamp for EL84

My philosophy in these matters is that any modification I do to make an otherwise inoperative piece of vintage technology usable again, is that such modification should be completely reversible at a later date should a subsequent owner desire to do so.

I had in mind therefore a separately mains powered preamp which could be secreted somewhere suitable in the usually cavernous innards of a record player or radiogram and which didn't require any modification to the exterior of the player.

As Edward says the demand for such a beast is only likely to increase as the supply of high output cartridges dries up.

Unfortunately I have neither the skill nor equipment to knock up any thing other than the odd one or two to get the players in my collection fully working, but surely there must be an entrepreneur out there who could do so?
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Old 12th Mar 2016, 12:32 pm   #54
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Default Re: Solid state preamp for EL84

Hi guys,

I now have all the parts that I need to build Chris's design in post 19 and thanks to Andy I now have all the parts to build an op-amp preamp.

I will probably have a go at building the op-amp on Monday and will of course let you all know how it goes.

Thanks again for all your help.
Tim.
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Old 13th Mar 2016, 11:25 am   #55
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Default Re: Solid state preamp for EL84

Just a quick question about Chris's design post 19. What value of cap shall I use to couple to the valve stage? .01?
Thanks again.
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Old 13th Mar 2016, 11:36 am   #56
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Default Re: Solid state preamp for EL84

It's not at all critical. 0.01uF will be fine.
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Old 14th Mar 2016, 7:09 pm   #57
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Default Re: Solid state preamp for EL84

Hi all,

I didn't get a chance to do much today.

I did though get a spare hour to build Chris's design. I had really bad hum and quite a bit of distortion.

I have uploaded a video.

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

I had plenty of gain and plenty of bass. I don't know if there's anything that can be done to fix the hum. The hum increased as with the volume. But then the preamp was before the volume pot.

Any thoughts?
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Old 14th Mar 2016, 7:35 pm   #58
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Default Re: Solid state preamp for EL84

Hi Tim,
My feeling is insufficient smoothing if it's the design using a bit of Ht stolen from the record player amp. So you could look at a bit of extra filtering. I'll have a gander at the circuit.
Also if there's too much gain you'll get hum picked up because of the preamps sensitivity inside a cabinet stuffed full of hum fields from transformers and motors?
So.
1) check the power supply in for hum.
2) Check your earthing and screening of the signal circuit, you may inadvertently have a hum loop.
3) Check you haven't got too much gain, 4x is more than enough i think?
4) you may need to put your preamp in a metal box to screen it from hum fields?

5) consider a standalone preamp with separate PSU fed from a battery or it's own Transformer.

Other wiser heads than mine will be able to confirm or discount my suggestions.

Andy
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Old 14th Mar 2016, 7:52 pm   #59
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Default Re: Solid state preamp for EL84

had A listen on youtube.
Sounds like its overloading and causing the signal to clip and distort badly.
Too much gain!
If the input isnt overloading you could try attenuating the output with a potential divider. I am not entirely sure if it works the same for transistors as valves, but if it does simply make a PD using the 150k collector resistor, start with say 47k +100k and have the 47 k at the HT end and take your output from the connection between the 2 resistors. Experts can you say if this will work?
This may also have the effect of reducing your hum to an acceptable level.
However if its being fed to the volume control then the distortion could be an input overload from the cartridge.
I also think your hum could be partly induced by the wires floating about on your lash up rig, plenty of scope for hum pick up which then gets amplified by the estimated 10x gain (Chris' estimation).
I'd still be inclined to decouple/smooth the HT coming in also. Trouble is you need 250V capacitors and high value ones at that working voltage are not cheap! Even low value 47uf or so ones are pounds each so maybe that aint the answer.

Andy.
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Old 14th Mar 2016, 8:14 pm   #60
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Default Re: Solid state preamp for EL84

Hello Andy,

I'm going to have a go at building the op amp preamp asap.

I had someone come and help me in the workshop today and I didn't get a chance.

I agree that I think it's bad smoothing and too much gain. The valve amp hums from the half wave rectification so why wouldn't the preamp?
The excessive gain only exacerbates things.

I really do think it has potential though and want to see if there's anything that can be reasonably done to improve it.

I must say I have no experience with op amps and am really going to need help in getting these TL071s working. I agree that the preamp having its own mains transformer a bridge rectifier and a voltage regulator is the way to do it properly.

I will have a go at your recommendations and get back tomorrow.

I will get cracking with the op amps once I've seen Chris's design through.

Thanks again.
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