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Old 29th Oct 2018, 8:49 pm   #1
plaka78
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Default Cartridge preamp

Experimenting with fitting a modern cartridge to replace a high output crystal cartridge. The original cartridge is very harsh. The out put from the modern cartridges is far to low so I designed a one transistor amp and the results were excellent, Managed to make the final circuit postage stamp size,
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Old 29th Oct 2018, 11:39 pm   #2
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Default Re: Cartridge preamp

How about a circuit?

Interesting to see how you got both enough voltage gain and a suitably high input resistance from one transistor (unless it was a FET).
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Old 30th Oct 2018, 10:26 pm   #3
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Default Re: Cartridge preamp

There was a Mullard ic TAA 320? ; a combined Fet and bipolar in a TO-92 ? case.

I used this successfully as a pre amp on both valve and transistor amps.

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Old 30th Oct 2018, 11:05 pm   #4
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Default Re: Cartridge preamp

I just used a pair of 2N3819s with a ceramic pick up with good results back in the day.
Transistor amplifiers just did not have enough input impedance.
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Old 31st Oct 2018, 1:07 pm   #5
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Default Re: Cartridge preamp

These days a simple op amp circuit will suffice. A TL071/2 or similar, and easily set input resistance and gain.
Simples.

A.
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Old 31st Oct 2018, 7:23 pm   #6
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Default Re: Cartridge preamp

This is the circuit I came up with and it works very well giving out about 2.5 v of audio
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Old 31st Oct 2018, 7:32 pm   #7
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Default Re: Cartridge preamp

There is quite an impedance mismatch there, though it may be acceptable if the player doesn't have any bass anyway. A 470k or 1M resistor will improve the matching if there's enough gain.
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Old 1st Nov 2018, 1:30 am   #8
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Default Re: Cartridge preamp

Input impedance for that circuit is only about 25k. With 470k in series with the input, gain is about -7.5dB!

Even fed from a low impedance source, max gain is about 18dB.

If you simulate it with the cartridge equivalent circuit of a voltage source with about 600pF in series, (suggested here: http://www.keith-snook.info/wireless...ualisation.pdf ) the gain to the output is about -28dB at 50Hz rising by 20dB per decade to 14dB or so, but this isn't reached until about 20kHz!

All I can say, really, is if it works, fine, but theoretically it's not a lot of use as a ceramic cartridge preamp.

If you darlington the BC547 with another one, change the bias resistors to 10M and 3.3M and get rid of the 330p capacitor you end up with something with an ultimate gain of about 18dB, but it's still 3dB down at about 300Hz and down to no gain by 40Hz, so still not wonderful.

A simple opamp circuit will work much better.
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Old 1st Nov 2018, 11:47 am   #9
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Default Re: Cartridge preamp

Chris, is what you're suggesting an emitter follower DC coupled to a common emitter?
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Old 1st Nov 2018, 1:35 pm   #10
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Default Re: Cartridge preamp

Quote:
Originally Posted by Herald1360 View Post
All I can say, really, is if it works, fine, but theoretically it's not a lot of use as a ceramic cartridge preamp.
Unless I've missed something, the OP hasn't said that the "modern cartridge" is ceramic.

The theory is well beyond me but could be that this circuit works better than it should in theory because the cartridge is in fact magnetic?
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Old 1st Nov 2018, 9:23 pm   #11
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Default Re: Cartridge preamp

Quote:
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Chris, is what you're suggesting an emitter follower DC coupled to a common emitter?
Sort of. A Darlington pair is two transistors with commoned collectors and emitter of the first connected to base of the second. It behaves simplistically as a single transistor with a current gain of the product of the individual current gains. These gains won't necessarily be the same even for identical transistors since each operates under very different dc conditions.
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Old 1st Nov 2018, 9:24 pm   #12
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Default Re: Cartridge preamp

Quote:
Originally Posted by ThePillenwerfer View Post
Quote:
Originally Posted by Herald1360 View Post
All I can say, really, is if it works, fine, but theoretically it's not a lot of use as a ceramic cartridge preamp.
Unless I've missed something, the OP hasn't said that the "modern cartridge" is ceramic.

The theory is well beyond me but could be that this circuit works better than it should in theory because the cartridge is in fact magnetic?
A very insightful comment!

Perhaps the OP would elucidate......
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Old 1st Nov 2018, 9:37 pm   #13
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Default Re: Cartridge preamp

Looking at the circuit, 330pf shunt, straight across the input would be very wasteful of the signal from a piezo-electric cartridge, but could be considered to be a load on a magnetic cartridge. The input impedance will not be friendly to piezo cartridges either.

So that starts me thinking it may be for magnetic cartridges.

But if it is, the 47k in the collector and 4.7k in the emitter say that the max open loop gain is only ten before the load the circuit drives and that 330k bias resistor in a feedback position come into play. With a typical magnetic cartridge giving around 5mV, I don't see there being enough gain. I also don't see the frequency response shaping needed for velocity-sensitive pick-ups.

A mystery. It doesn't seem to suit either common type of cartridge, and moving coil would be even further from its capabilities.

I can't say anything further until it's revealed what it is for.

David
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Old 2nd Nov 2018, 1:39 pm   #14
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Default Re: Cartridge preamp

If he is getting 2.5V of output from it then it must be a ceramic cartridge. I doubt if any magnetic cartridge would be happy in an arm like that. If he likes the sound then he obviously is not a reggae or organ fan, as there will be little coming out below 1kHz. The ceramic coupling capacitors may add a little 'warmth'.
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Old 2nd Nov 2018, 6:24 pm   #15
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Default Re: Cartridge preamp

Thanks for all the interesting comments on this thread, I am using a cheap 1.50 ceramic cartridge from china.
I must admit I was amazed how well it actually sounded. The record player is very basic using an UL84

I will have a go at designing something more complex next week with 2 transistors if I got time to see what improvement I can make, Or if someone could post a circuit would be good.
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Old 2nd Nov 2018, 6:43 pm   #16
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Default Re: Cartridge preamp

If you have any FETs like 2N3819s to hand, you could build a simple common source FET preamp. The component count will be similar to your bipolar design but the input impedance will be much higher. I have no idea if it will actually sound better, but it should do.

https://www.nutsvolts.com/magazine/a...ircuits_part_2

Alternatively if you have an opamp like an LM833 or even an LM358 then you could build something with that. There are lots of circuits if you google, some very simple.
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Old 3rd Nov 2018, 12:09 pm   #17
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Default Re: Cartridge preamp

If you fancy playing with 2N3819s, I've got a few hundred (courtesy of a purchase of some secondhand storage drawers complete with interesting contents). Let me know and I'll pop a few in the post.

Similarly for some sort of general purpose opamp.
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