How can I adapt a ceramic preamp for magnetic carts?

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scrapjack+
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How can I adapt a ceramic preamp for magnetic carts?

Post by scrapjack+ » 12 Jun 2019 05:38

I've been wanting a tiny, as in no larger than a vcr tape, amp/preamp with headphone output for testing various electronics during repair. I'm not looking for hifi, just something that:
1) Takes as little space on my work bench as possible, no larger than a VHS tape.
2) No hardwired cords.
3) Can preferably have battery power option for when not near a power outlet.
4) Be able to conveniently show me if a cassette player or turntable is outputting stereo audio, easy with earphones.
5) Be clear enough that I can tell if the audio is music or electrical noise, and if it is heavily distorted, such as by a motor running at half speed.

Since I gutted one Crosley to fix another for someone, I have some parts laying around. I have the circuit board No. st14012 from the Victrolica model number vsc 550bt, a Crosly clone. Since it draws 5v1A DC I could mod this to be powered off a usb bank anywhere. It has a bluetooth reciever, a 3.5mm line in, RCA line out, and a headphone out, and of course the phono leads. Also there is unused usb and mic solder points I might fill. It has the right connections to be able to test microphones, speakers, and any stereo component. It seems like it would be an ideal circuit to make test equipment with. There is just one problem, Crosleys have ceramic cartridges. Some record changers I work on are from the 50's and 60's, and so they too have ceramic cartridges. However, I also look at more modern turntables from the 70/80's with magnetic cartridges occasionally. I can get a micro and "B" usb panel leads for less than $3, and I have some scrap material for the housing, as well as spare RCA sockets, so with a solution this could be a $3 piece of test equipment.


I don't know if the p188 crosley cart has the highest voltage of all the ceramics, past and present, or If I need to add a resistor to ensure any ceramic cartridge will work without burning the preamp. Do I need to add a a resistor, and what would be the lowest safe ohm value? I suspect the ceramic cartridge with the highest output is probably infamous in certain crowds.

Magnetic cartridges, as I understand it, are standardized at a voltage much lower than any ceramic cart. It would seem that would likely prevent such a presumably low power amp from outputting anything by the line out, or headphone port. Is there a simple way around this, perhaps by adding a switch to allow alternate leads to bypass some internal resistors? I know there is probably a step up transformer, but those can distort signals, and I already have extra switches and capacitors on hand.

vanakaru
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Re: How can I adapt a ceramic preamp for magnetic carts?

Post by vanakaru » 12 Jun 2019 17:38

I would buy cheapest MM Phono headphone amp that runs off 9V(or 12V) and use battery to power it. If you test ceramic carts just add simple circuit(one cap and two resistors - search this forum for ceramic carts) on the line. MM carts put out 2,5mV to 9mV, ceramics up to 2V.

scrapjack+
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Re: How can I adapt a ceramic preamp for magnetic carts?

Post by scrapjack+ » 14 Jun 2019 04:28

I'm not seeing many that are cheap, and have both the phono preamp input and a line input for non turntable components. I've worked on plenty of circuits that couldn't drive headphones, so I need both inputs. In fact allot ot the cheap ones I was seeing were only the preamp, needing a separate main amp so I'd probably still need to build a housing for this circuit.

Perhaps I'm just over thinking it. I only have two magnetic carts on universal head shells, the rest being filled with some extra ceramic carts. I'm thinking when I find the circuit boards power transformer I'll connect my Technics output to the phono leads and see how the circuit handles the two magnetics. With luck I'll hear something by the time the volume is maxed, if not I'll measure the voltage from the internal speaker terminals.

vanakaru
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Re: How can I adapt a ceramic preamp for magnetic carts?

Post by vanakaru » 14 Jun 2019 09:38

I think you need to compile the one yourself. Get Phono preamp, simple headphone amp and wire as you need.
https://www.ebay.co.uk/itm/Stereo-Phono ... %7Ciid%3A1

https://www.ebay.co.uk/itm/TPA6120-Head ... SwphhcsPLE

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Re: How can I adapt a ceramic preamp for magnetic carts?

Post by scrapjack+ » 16 Jun 2019 00:55

vanakaru wrote:
14 Jun 2019 09:38
I think you need to compile the one yourself. Get Phono preamp, simple headphone amp and wire as you need.
https://www.ebay.co.uk/itm/Stereo-Phono ... %7Ciid%3A1

https://www.ebay.co.uk/itm/TPA6120-Head ... SwphhcsPLE
That may be my fall back plan. The circuit I have already has the headphone amp, I could always wire a secondary magnetic preamp in like a tape input, without having to trace the circuit. That may increase the current draw and prevent me from running it off usb however.

I decided to play with the circuit board and ended up reassembling the broken case as best I could to make it easier, and this meant I might as well reconnect the internal speakers. I learned that there is some sort of protective circuit in this. When I plugged my mp3 player in at max volume on the player as is needed for my surround sound system, and max volume on the circuit no sound came from the speakers as I turned the circuit volume knob down the sound played. The bluetooth receiver and line in work fine. Both the line in and the bluetooth in can output to the line out, the headphones, and the internal speakers. The circuit as is, appears to be a suitable audio amp for all my non turntable testing needs, VCR's, Tape decks ect. It will be even more useful as a multi-tool If I add a direct tap to the speakers to test the speaker output of amps/receivers. Then I won't have to keep a scrap speaker cone on my work bench collecting metal dust.

Testing the phono input was actually fun. If I can figure out what went wrong, I may be better able to trouble shoot more expensive gear. The motor and phono leads leave the main board and plug into a small board built into the motor. I have a stripped down board that was part of a damaged motor I used instead of cutting and stripping the wires. The output wire points on the secondary board are labeled R, L, GND, -S, +V. The wire points for the tone arm line up perfectly to RCA brackets from VCR's. I used alligator leads to connect the two but I'll solder it in later. I started with my Technics SL-Q2, with a QLM 32 MK3 magnetic cartridge. I could not get any audio from the internal speakers or the headphone port at any volume, but oddly, the line out produced a signal usable by my stereo at volume 25, a common listening level. I then connected to my Technics a spare head shell equipped with a P188 ceramic Crosley cartridge, as this is the same model cart as the circuit came with. To my surprise, the results were the same as with the magnetic cart. Since the tone arm leads connect to the same secondary board that is built into the motor, I thought that maybe I need a motors resistance to allow the circuit to work, like a computer psu needs a dummy load to replace the motherboard to make a benchtop psu in order to stabilize the power ripples. Sadly I do not have any spare motors to test this with only the partial board mentioned prior. I do however have a second example of this table intact. I may use the alligator jumpers to drive that motor with the Technics phono input and do a ohms/continuity test to see if I can simply bridge a few contacts together to complete a circuit, or if I need to add a dummy load in the form of a resistor(s). I also tried an original Crosley headshell with it's stock wiring, same result as with the Technics, so I am fairly certain the issue isn't the tone arm wiring, but related to the amp circuit, perhaps the disconnected -S, and +V points.

Because I got a similar results with both magnetic carts and ceramic carts I think this circuit may be able to naively support magnetic carts if I figure out what has disabled the phono headphone amplification. Aside from the missing motor the over all circuit is stock, and the table did run when dismantled, it was the housing that was broken.

I not convinced basic trouble shooting will yield a solution. I wish I knew more about circuit design, but that hasn't been the easiest subject for me to learn. The whole circuit board has the combined surface area of a credit card, so I doubt I'll ever see a simpler or more compact design. I did find the FCC's analysis of this circuit, but it seems to only pertain to the bluetooth radio.
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scrapjack+
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Re: How can I adapt a ceramic preamp for magnetic carts?

Post by scrapjack+ » 16 Jun 2019 02:19

Also I was testing with one broken speaker lead so only one speaker was outputting sound. Bridging the unlabeled red lead to the negative point allows quiet output to the headphone port with a loud buzz, and even quieter output to the speaker with the QLM magnetic cart with the phono leads wired for mono. Quiet but usable with standard Iphone earphones. Since I can't find the rest of my alligators I used forceps to hold the power lines connected, so that I could connect it as stereo. I got much louder volume through head phones and the speaker, enough to make the speaker usable. I had to hold one end of the yellow rca to the Technics Ground and the other to the RCA audio ground. When I did that the buzz stopped. There are 3 wires that go from the circuit board to the motor, maybe this circuit does with transistors what the 60's record changers did mechanically. Bridging the -S and +V produces a spark and no sound, but bridging the third voltage wire to the point labeled +v allows the headphone/internal_speaker amp to run. I did not expect to find that kind of auto shut off circuitry on a modern device.

I was definitely over thinking this. Again if anyone else would find a LoFi compact test amp useful, this circuit board came from the Victrolica model #VSC-550BT. So, a $20 used Crosley can actually handle a magnetic cart, or any ceramic cart with an output voltage less than or equal to the output of the Pfanstiehl P-188D at 0.35v. It has BT in, a pre-amp, and a line input, as well as line output, headphone out, and speaker out if you choose to keep them involved.

Thank you everyone for your time.

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