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Old 26th Jun 2020, 6:31 pm   #21
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Default Re: Ceramic cartridge preamps revisited

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Originally Posted by ThePillenwerfer View Post
Cleaned-up PCB details for anybody who wants them.
Much clearer than the original - thanks - but missing the short track between the input ground pin and R2 ( it would still work but maybe introduce noise?)
I don't like the (original) ground loop round the edge of the board and would have broken it between the input ground pin and the power ground pin along the top edge.
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Old 26th Jun 2020, 7:09 pm   #22
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Default Re: Ceramic cartridge preamps revisited

Rats. Well spotted and thanks.
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Old 27th Jun 2020, 9:25 am   #23
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Default Re: Ceramic cartridge preamps revisited

I wonder if it can be made to work at lower than 12Vd.c.? I ask this because it would be useful if it could be powered in some battery record players.
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Old 27th Jun 2020, 5:43 pm   #24
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Default Re: Ceramic cartridge preamps revisited

Quite possible, but easier with a bootstrapped bipolar version.

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Old 27th Jun 2020, 10:01 pm   #25
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Default Re: Ceramic cartridge preamps revisited

Well, I breadboarded the circuit from #1 incorporating the gain mod in #13. It does work, but I'm not sure how well it's handling the input signal - I guess I need to dig the scope out. It's only pulling about half a mA, which seems suspiciously low - maybe I'm not measuring it properly, though I can't imagine what I'm doing wrong. I'll poke around a bit more tomorrow.

I suppose I'd better put it into LTSpice and compare simulation with reality.
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Old 27th Jun 2020, 11:04 pm   #26
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Default Re: Ceramic cartridge preamps revisited

I've had reasonable results from my breadboard version. I built it as per #14, using a load resistor (collector to ground) of 4k7.

I used a generic J112 JFET, which is not exactly optimised for audio. This example has a Vgs of about 3.7V.

First step is DC conditions, of course. For this, look to R3 - this sets the current in the JFET channel and the resistors below it. They use 2k2, which gives around 270uA. This current flows in R4 back to the output (in my version), so you end up with a corresponding voltage drop across that, which subtracts from the FET Vgs.

If you make R4 high, it results in too high a DC voltage drop across it, meaning the DC output voltage is too low to handle a larger signal. It's a balance...

So I changed R3 to 10k, and set R4 to 10k. As a result, there's about 0.6V dropped across R4, meaning that the DC voltage at the collector of TR2 is Vgs minus 0.6V. About 3V in my case. I'm using a 10V DC supply at the moment, so would much prefer that to be 5V, but that's fixed by the JFET.

That gave good results at unity gain, so I added 1k and 47uF to ground. The gain in theory should be 11; the measured gain was 10.3. That's because of the non-infinite open-loop gain, of course. But, it's greater than 10 - note the "1+" that I mentioned earlier.

With an output voltage of 1V RMS, the THD was just over 1%. Not stellar...

Reduced to unity gain and the input increased to again give 1V at the output, the THD is just over 0.1%

I haven't looked at the spectrum of the distortion, so can't say if it's "nice" second-harmonic. I could do that tomorrow.

I would expect better results from a BJT, and can try that tomorrow as well. Biasing it to be closer to the centre of the rails comes as default, and that makes this thing more useful for a wide range of supply voltages. I don't know what voltage you need to drive a UL84, but what I have in front of me only manages 1.3V RMS before the negative-going half starts to clip.

To get the DC mid-point up, I added a 10k resistor in parallel with the 47u cap. This, of course, doesn't alter the AC gain, but means that the output voltage is now roughly double Vgs, at just over 6V. The exact value could be tweaked if hitting the mid-point was important.

More tomorrow...
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Old 27th Jun 2020, 11:15 pm   #27
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Default Re: Ceramic cartridge preamps revisited

That current sounds about right..... T1 only needs to pass about 0.25mA in order to turn on T2, another 0.25mA there would set the output voltage at T2 collector at around 3V and T1 source at around half a volt. With a 2N3819 nearer the lower end of their Idss range that seems reasonable.

I might have a play with one of my hundred or so remaining stock of 2N3819s!
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Old 27th Jun 2020, 11:25 pm   #28
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Default Re: Ceramic cartridge preamps revisited

Thanks for the reassurance about the current. I haven't tried to set the output voltage yet so that may improve things. I can also increase the supply voltage - I should be able to take it to 30V without anything going pop.
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Old 28th Jun 2020, 12:43 pm   #29
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Default Re: Ceramic cartridge preamps revisited

The DC output voltage is set by the FET's Vgs, so altering the supply voltage won't shift it substantially. Though if your circuit is sitting close to the positive rail, then lifting the supply is obviously a good thing.

With mine, sweeping the supply voltage from 10 to 30V only changes the collector voltage by about 100mV.

I've attached a sketch of my version, showing the differences around the output and feedback side. I prefer my arrangement because it separates out AC and DC conditions as much as possible, and allows for a higher standing current in the second transistor, which translates to higher gm and better drive capability. That might be academic when using this in front of a valve, but not everyone will want to do that.

I've just replaced the JFET with a transistor - a generic BC337. To bias, a pair of 100k resistors make a mid-point that is decoupled to ground, then the 1M resistor connects to that rather than ground.

In practice, one of those 100k resistors needs to change, as the small base current pulls the midpoint slightly. Not a problem. But it's a mid-point that follows the supply voltage, so more volts gives you more headroom, as you'd expect but don't get with the FET version. Round 1 to the BJT

Distortion is improved to 0.08% at unity gain and 0.3% at x10-and-a-bit. I'm sure that could be improved by increasing the current in both transistors - I might try that in a bit. Still, a worthwhile improvement over 0.1% and 1%. Round 2 also to the BJT.

To claw back some points, how about input impedance? To measure that, I simply found the LF -3dB point, which was 17Hz. That implies an input impedance of 940k, and most of that is because of the 1M input resistor. Hence, the input impedance of the BJT itself is around 15M. That's pretty good - better than some might expect, and plenty good enough for the application.

So - as is so often the case for audio amplification - I see no benefit to using a FET here, and plenty of drawbacks. There are places where they are essential - like condenser mics - but that's not the case here.
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Old 28th Jun 2020, 1:14 pm   #30
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Default Re: Ceramic cartridge preamps revisited

Your distortion figures are interesting, as I don't have any way of measuring that with any precision. They are higher than I'd expect - is that normal for these sorts of circuits? Presumably an audio op-amp would give better results.

Could you post a quick sketch of your bipolar mods? I'd like to try them, and don't want to misinterpret anything you've said.
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Old 28th Jun 2020, 2:39 pm   #31
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Default Re: Ceramic cartridge preamps revisited

Yes, that's fairly normal - the circuit is just too simple to expect much gain and low distortion.

As a buffer, it can be quite respectable. If you replace Rf with a short the distortion will fall a lot - I've got 0.007% here, and the true value might be a bit lower if built properly. Adding a current source as a collector load would help, but that's moving away from the simplicity.

I'll try the original circuit as published in a moment. In the meantime, mine is attached. I don't recommend anyone builds it, though. A TL071 or similar would be easier and perform much better.

By coincidence, a few months back I was playing with these sorts of circuits because I needed to make some DI boxes for live sound. The requirements are a high input impedance to avoid loading the piezo guitar pickups - 1M is a convenient value - and a balanced output stage that can drive a mic input stage with an input impedance that might be as low as 1.2k. No voltage gain is required, and noise is rarely a problem as the signal levels are about a volt. Power comes from the desk in the form of phantom power.

I thought it might be interesting to play with JFETs, and built a very similar circuit to this, using a nice audio transformer to balance up the signal. The eventual circuit was like this one, but with a current source acting as the load, and working in push-pull to double the current output for a given standing current. The distortion was "high, but musical". In the end, I dropped all that and used a TL062. Not even an 072 - the 062 was considerably better than the discrete circuit, and draws much less current. The first op-amp section is a unity gain buffer, the other inverts to provide the balanced drive, so no transformer needed. I've since built several of them, and am glad I didn't have to faff about building the complicated discrete version. I used the nice transformers to make an isolating line-level DI to kill earth loops from laptops, keyboards, etc, and that's got me out of trouble more times than I care to mention!
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Old 28th Jun 2020, 4:56 pm   #32
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Default Re: Ceramic cartridge preamps revisited

Did anyone spot the mistake in my last sketch? I even managed to label the transistors incorrectly. The first transistor should have been BC337, and the emitter should have been at the bottom. Oops!

OK I so built up the original circuit. It doesn't bias properly with my JFET unless you increase the supply voltage to about 20V - the collector wants to sit at about 16V.

The minimum gain (C2 not installed) is about x5.3 - that fits, as it's 1 + (R4 / (R5+R6)) (x5.4).

At first glance, the distortion at this gain is very good indeed - only 0.018%. But don't be fooled - that's because the collector is only driving about 12k. Adding a 4k7 load (via a 47u cap) to make the conditions the same as my circuits brings the distortion up to 0.25%

With C2 installed, the gain jumps up to x112.

Distortion at this gain is actually quite reasonable at around 0.2%, though if you load up the output with 4k7 to make it comparable to my earlier versions, then this rises to 1%.

Changing the gain is fun.

First, the DC gain is too high, which is why the standing DC voltage is so high. To reduce that, you could reduce R4 from 10k, but if you do, the loading distortion rises. So perhaps increase R5+R6? For my JFET, something like 5k6 is a good compromise, resulting in a DC voltage in the 7V region.

Having done that, you can set the AC gain by varying the exact values of R5 and R6, while keeping the total about the same to maintain DC conditions. If we want a gain of 10, then the AC resistance needs to be 1.1k. Sticking with E12 values, let's make R5 1k and R6 4k7. This gives an AC gain of x10.3. The THD now measures 0.45%, which is worse than my BJT circuit, but better than my first JFET+BJT effort. But that'll be because of the loading. With 4k7 at the output (to make the conditions the same as mine before), the THD shoots up to 5%+!

So all in all, it's a whole bag of compromises that is hard to design because of the way the AC and DC conditions have been joined - to vary the AC gain, you have to change two resistors to keep the DC conditions the same. And it's only really suitable for driving high-impedance loads.

Any of these variations could be improved if you were willing to throw another transistor or two at it. Or just bung in an op-amp! It's boring, but will have much better performance and a lower component count.
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Old 28th Jun 2020, 6:01 pm   #33
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Default Re: Ceramic cartridge preamps revisited

It's good to see that even you confuse the BC327 and BC337! For some reason they decided to reverse the convention that PNP transistor part numbers are 10 higher than their NPN complements.

Thanks for all your input, I'll continue to tinker.

As it happens I do have some cheap TL072s on their way from China, so will lash something up when they arrive.
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Old 28th Jun 2020, 7:14 pm   #34
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Default Re: Ceramic cartridge preamps revisited

It's good fun playing around with these things, and quite educational. At some time in the past a couple of Japanese companies got a bee in their bonnets about FETs and did some amplifiers with JFETs in every position they could put them in. Their marketing departments then had a field day, pointing out the JFET's inherent square law characteristic and how this was so much more musical than bipolar transistors etc. etc. The brochures and magazine advertisements were full of it.... well, something ending in 'it' and beginning with 'bull'

Hard-nosed, experienced designers just sniggered. They'd been round the loop and found the problems with trying to extract decant performance from JFETs, found out how to do so, and then let that large variations in parameters from device to device without tight selection dissuade them from using the things wherever possible.

In a lot of ways, they look quite tempting, and it's awfully easy to say that they are inherently high impedance devices and therefore well suited to handling high impedance transducers. There is some truth in this, but it's not automatic and is sometimes overshadowed by other drawbacks.

The TL072 should be fine. That's a JFET input, but they are used carefully in it and a matched pair are used to cancel some of the uncertainties of the things.

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Old 28th Jun 2020, 7:30 pm   #35
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Default Re: Ceramic cartridge preamps revisited

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it's also feasible to run a piezo-electric cartridge into a low-Z amplifier, treating it as a current source and to equalise the resulting slope.
In the past I have just stuck one into a moving magnet RIAA input, it worked well.
 
Old 28th Jun 2020, 9:45 pm   #36
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Default Re: Ceramic cartridge preamps revisited

Last one for now - probably academic interest only, but look at what you can get for the price of just 1 transistor and 1 resistor:

x1 THD: 0.002%
x10 THD: 0.006%

This is rather surprising - I have actually spent more time validating these measurements than I did coming up with the circuit. The measurement conditions were the same as above, so are directly comparable.

Just to remind you, the earlier results from the 2-BJT circuit were 0.08% and 0.3% respectively. It goes to show how sensitive the basic circuit is to loading.

Note that the 3rd transistor - a simple EF - is included in the feedback loop. That's risky, but it does seem to be stable on this breadboard lash-up. The Miller capacitor on the second transistor wasn't needed to make it stable, but it did bring the distortion down slightly, and is recommended.

Apart from the EF, the main change was to increase the load resistor for the first transistor to 33k. At 10k, the x10 THD is 0.02%. That was a bit surprising - I have a couple of theories, but it's getting late now...

In terms of supply voltage, it's happy from about 9V up. Currently running at 15V, taking 2-3mA. It'll go to 20V and above if needed (check transistor and cap ratings!).

A quick check shows that the input impedance hasn't changed, and is very close to 1M (and could be made closer by increasing the 1M resistor and re-biasing to cancel the loss caused by the base current of the first transistor).

As it stands, it's basically a (very!) small audio power amplifier - single-ended (rather than LTP) input stage, VAS, and an output stage. The output stage could be improved by replacing the 3k3 load resistor with a current source. Or go the whole hog and add a PNP follower (and bootstrap the 20k collector resistor - actually, that does reduce the distortion slightly with just the basic circuit). The hi-fi crowd love discrete op-amps, and they're quite simple to make.

Before recommending this, I'd want to build a couple and make sure there's no instability or other nasties. A TL071 would still be better and require fewer components, but it's quite satisfying to play around with "real" components once in a while
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Old 28th Jun 2020, 10:03 pm   #37
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Default Re: Ceramic cartridge preamps revisited

That certainly looks worth trying. I'll build it tomorrow if I have time.

Is the 1k stopper on the input really necessary?
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Old 28th Jun 2020, 10:25 pm   #38
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Default Re: Ceramic cartridge preamps revisited

It'll need a DC block capacitor on the output, if the subsequent stage doesn't already have one.

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Old 28th Jun 2020, 10:26 pm   #39
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Default Re: Ceramic cartridge preamps revisited

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Is the 1k stopper on the input really necessary?
Perhaps not...

However, most of the time you want ultrasonic filtering at the input, so it forms part of that, plus it offers a bit of protection (usually in conjunction with a pair of diodes from input to each rail). Whether it's needed to ensure stability with this circuit is debatable, but the circuit is still built, so I'll have a play when I get the chance, hopefully tomorrow.

BTW, as the issue of stability has been raised more than once in this thread, the attachment might be of interest. It comes from ("Small Signal Design", by Douglas Self)
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Old 28th Jun 2020, 10:36 pm   #40
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Default Re: Ceramic cartridge preamps revisited

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It'll need a DC block capacitor on the output, if the subsequent stage doesn't already have one.
I've omitted that on all my sketches for brevity - as we're not in "absolute beginners" territory I've assumed that anyone contemplated building this would realise that. Likewise, I haven't drawn a rail decoupler, but there is one on the breadboard. A few hundred ohms of isolation resistance at the output wouldn't go amiss either...

Once something that could be called a "final design" emerges, then those practical details that make it possible for anyone to build can be included. I'd better find some larger Post-its
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