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Old 25th Jun 2020, 2:29 pm   #1
paulsherwin
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Default Ceramic cartridge preamps revisited

I came across this anonymous preamp design on the net today. It's very simple and uses cheap commodity parts, and I thought it would be ideal to match a low output ceramic cartridge to a low sensitivity UL84 type record player amp.

https://www.electroschematics.com/lo...-preamplifier/

I'm planning to breadboard it in the next few days. Does anybody have any comments before I do?
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Old 25th Jun 2020, 3:25 pm   #2
Edward Huggins
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Default Re: Ceramic cartridge preamps revisited

Very useful. Whilst not your intention small batches of these would sell I'm sure. The 1meg input impedance will be ideal. But with an amplifiying factor of 100, should not a small trimpot be added to match its output to the UL84/EL84 grid? Assume you will power from the valve's Cathode. Looking forward to your report!
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Old 25th Jun 2020, 3:35 pm   #3
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Default Re: Ceramic cartridge preamps revisited

It should do the job. The (demanded) gain is over 40dB which seems rather optimistic and the dc conditions are dependent on the idss of the fet but it should be fine. The basic circuit is well known.

Ken

Last edited by Ambientnoise; 25th Jun 2020 at 3:36 pm. Reason: Typo
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Old 25th Jun 2020, 3:41 pm   #4
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Default Re: Ceramic cartridge preamps revisited

C1 and C3 seem rather larger than they need to be given the impedances involved, so I'll have a tinker with those. Presumably R1 and R7 are stoppers to improve stability.
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Old 25th Jun 2020, 3:43 pm   #5
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Default Re: Ceramic cartridge preamps revisited

I didn't sim, honest !

ac gain R4/R5
dc gain R4/(R5+R6)

T2 could be anything with lots of gain as its doing most of the work, if you ignore the the input impedance! (which probably could be a lot more that 1M with suitable R2)

It looks quite neat to me.
Is 1M a standard ?

dc
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Old 25th Jun 2020, 3:46 pm   #6
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Default Re: Ceramic cartridge preamps revisited

Most FET preamps seem to use 1M on the gate. I might try 2M2. Of course, originally the cartridge would be across a 1M volume control.

I've got lots of BC214s which should be suitable for T2.

Most of you lot would do all this in a Spice simulation, but I'm old school
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Old 25th Jun 2020, 3:53 pm   #7
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Default Re: Ceramic cartridge preamps revisited

It's got quite high gain, so with a medium output ceramic cartridge there could be enough output swing to mean that you either need to select your 2N3819 for high-ish Vp or else play with resistor values to get the quiescent voltage at the output of the bipolar reasonably close to the middle.

Designing with JFETS gets difficult because of the very wide spreads of some (well, all) parameters. and the 2N3819 was especially wide.

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Old 25th Jun 2020, 3:56 pm   #8
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Default Re: Ceramic cartridge preamps revisited

I could possibly add a preset to set the voltage midpoint.
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Old 25th Jun 2020, 7:04 pm   #9
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Default Re: Ceramic cartridge preamps revisited

It's worth giving it a stab with a meter to see where it's sitting and go from there. A stab with a scope while playing something to see how far away from clipping it's running might also add comfort factor.

Designing with JFETs and MOSFETs in todays world where adjusters have to be avoided at all costs is a bit of a challenge.

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Old 25th Jun 2020, 7:14 pm   #10
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Default Re: Ceramic cartridge preamps revisited

I should make it clear that I'm certainly not interested in building lots of these as a commercial product. Anybody interested in doing so is quite welcome to incorporate any insights that my amateurish tinkering may produce.

That said, anybody with the most basic electronics skills could build one as a one off on a backplane or a bit of Veroboard.

I'm not going to do anything until it cools down a bit
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Old 25th Jun 2020, 8:46 pm   #11
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Default Re: Ceramic cartridge preamps revisited

This circuit works;
I would recommend the circuit is well screened, e.g. metal box
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Old 25th Jun 2020, 9:17 pm   #12
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Default Re: Ceramic cartridge preamps revisited

I think that one has unity gain and an enormous input impedance due to bootstrapping. You can also get low frequency humps or instability, depending on the source.
The original will prob be fine, just be aware of the high gain and worth a check on the dc level on the output
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Old 26th Jun 2020, 12:30 am   #13
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Default Re: Ceramic cartridge preamps revisited

Quote:
Originally Posted by dave cox View Post
I didn't sim, honest !

ac gain R4/R5
dc gain R4/(R5+R6)

T2 could be anything with lots of gain as its doing most of the work, if you ignore the the input impedance! (which probably could be a lot more that 1M with suitable R2)

It looks quite neat to me.
Is 1M a standard ?

dc

You can set the gain (within reason, around 10x is probably Ok for simple low output ceramic cartridge preamp) by choosing R5 using the formula above. For 10x use 1k. The output dc level can then be set by playing with R6- increase it to drop the output dc, reduce it to increase. A useful starting point is around the value where the sum of R5+R6 is around 2.2k (about what it was with the original values for R5, R6).


(I did cheat and SPICE it )
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Old 26th Jun 2020, 12:17 pm   #14
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Default Re: Ceramic cartridge preamps revisited

The AC gain is 1+(R4/R5). That holds for low to medium values of gain, after that you have to consider the non-infinite open-loop gain, naturally.

That "1+" is quite important - with R4 replaced with a short, you have a unity-gain follower - aka "CFP" (compound/complementary feedback/follower pair). Commonly done when you want something a bit better than a standard emitter follower. The configuration was used in the BBC LS5/8 crossover, for example. You also see this configuration used for audio output stages. Most often with unity gain, but you do sometimes see examples where there is modest gain (perhaps x3 or x4), which usefully enables the designer to run the front end of the power amp at lower supply voltages.

Another place where that configuration works well is video amplifiers, where you often need a gain of x2 - in which case you just make R4 and R5 equal. There's more than enough bandwidth for that application - it's surprisingly "fast" for such a simple circuit. This is something to keep in mind when experimenting - speed can bring problems.

Given that most ceramic cartridges are high output - in the ballpark of 0.5V, perhaps - I'd suspect that even x10 (20dB) is more than required.

I'm not convinced a JFET is needed here. Using a BJT instead will still result in a high input impedance. For audio circuits, I much prefer BJTs because of their higher gm and more predictable results, and mostly only use them for controlling signals (e.g. power-on mute). In really simple terms, if we assume a follower circuit made from a pair of BJTs with a hfe of 200 each, that's a combined current gain for 40,000. If there is a 1k load resistor to ground at the output of the follower, the input impedance should be 40,000 times 1k, which is 40G! Obviously, the reality won't be anything like as simple, but either way, you'll comfortably get into the low M-Ohms region.

Having said all that, some ceramic cartridges need a lower impedance load for equalisation. Syncrodyne has written a lot about this on this forum and others - for example, see his posts in this thread: https://www.vintage-radio.net/forum/...ad.php?t=60957

For the sake of balance, I admit the JFET does simplify the biasing slightly, even if the results vary according to Vgs. If using a BJT at the input, you'd need another resistor from the rail, and that means the rail needs to be somewhat cleaner than is required as it stands. But against that, Vgs is quite variable...

To conveniently separate the DC and DC conditions, I would change the arrangement around the output side. As it stands, the load for the follower is the sum of R4 and R5 (and R6 for DC). Instead, drop a resistor from the collector of TR2 to ground - choose a value that gives a nominal 1-2mA in the collector (so for a 10V rail and 5V at TR2 "output", pick anything between 2k7 and 4k7, perhaps). Having done that, omit R6, but retain R5 and C2 to ground, and R4. R4 and R5 now set the gain, but don't alter the DC conditions, because the DC gain is unity (1+R4/infinity). Keep R4 the same - as its value sets the current in TR1 - and alter R5 as needed.

Last edited by mhennessy; 26th Jun 2020 at 12:24 pm.
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Old 26th Jun 2020, 12:53 pm   #15
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Default Re: Ceramic cartridge preamps revisited

At too much feedback it might become an RF oscillator?
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Old 26th Jun 2020, 1:04 pm   #16
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Default Re: Ceramic cartridge preamps revisited

With the solid bootstrapping in that circuit, a bipolar would work a treat, but need a couple more resistors to set the input bias. In return, operation would be much more consistent.

JFETs used to turn up in more places than they really suited because some people were more comfortable with them than with bipolar devices. There was a 'Gee, they're just like little valves!' factor which gave some people warm and fuzzy feelings -Pat Hawker was a bit of a sucker for them

With the bipolar operating as a series-feedback arrangement and with the input bias chain bootstrapped, the input impedance can easily be made high enough that you can add a shunt resistor to set any other value of Zin to make the cartridge happy.

As an alternative approach, 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. The Wireless World Bailey-Burrows preamp does this properly. It's quite a rare technique.

David
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Old 26th Jun 2020, 1:09 pm   #17
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Default Re: Ceramic cartridge preamps revisited

Quote:
Originally Posted by PJL View Post
At too much feedback it might become an RF oscillator?
Yes. They are usually unity-gain stable, but perhaps slightly more prone to oscillation than a standard emitter follower. As I alluded to, definitely something to watch for. Stopper resistors are normally sufficient IME.

Once the gain is x2 or more, they are pretty well behaved in my experience, but again, be alert
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Old 26th Jun 2020, 1:44 pm   #18
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Default Re: Ceramic cartridge preamps revisited

Thanks for all suggestions. I'll play around with bootstrapped bipolars too.
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Old 26th Jun 2020, 2:07 pm   #19
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Default Re: Ceramic cartridge preamps revisited

Cleaned-up PCB details for anybody who wants them.
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File Type: zip low-noise-preamp-pcb.zip (114.4 KB, 36 views)
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Old 26th Jun 2020, 3:05 pm   #20
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Default Re: Ceramic cartridge preamps revisited

I would be tempted to use a UCL82. It seems a lot less messing about! Not so much fun perhaps.
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