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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 22nd Jul 2017, 5:30 pm   #1
RojDW48
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Default Mk1 Decca Deccalian - inexplicable problem!

I replaced the 78rpm deck in this with a Garrard 2025, so I could give the beautiful amp something nice to reproduce but I am getting virtually nothing from a variety of cartridges. I have tried an X5M and a couple of brand new red Chinese things - all to no avail. BUT if I plug in a CD player to the same input - attenuated even - the old Decca belts it out in a most rewarding way! Continuity checks out and I can't find any evidence of attenuation anywhere on the 2025. Suggestions very welcome!
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Old 22nd Jul 2017, 6:05 pm   #2
Edward Huggins
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Default Re: Mk1 Decca Deccalian - inexplicable problem!

If it's the 1947model, I think the original Decca cartridge was transformer coupled to the AF stage. There's probbaly an impedance mismatch. the BSR X5M is a very sensible choice but you will need to bypass the transformer so as ato acheive the c.1 meg ohm input impedence needed.
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Old 23rd Jul 2017, 12:52 am   #3
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Default Re: Mk1 Decca Deccalian - inexplicable problem!

Quite right - but isn't that a step up transformer? I attenuated the output from the CD player. Surely the X5M should have overloaded the PU input with the tranny in place - but it's barely audible.
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Old 23rd Jul 2017, 6:04 am   #4
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Default Re: Mk1 Decca Deccalian - inexplicable problem!

If you put a few ohms across a crystal pickup it is like a short circuit to it so you will get next to nothing out of it. The CD player will have something like a 50 ohm output impedance so will not mind the load so much.
Just connect a resistor of between 470k and 1 megohm in series with the cartridge and take it from there as I think your amp may be the one that is balanced throughout and bypassing the input transformer may be tricky, but I stand to be corrected.
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Old 23rd Jul 2017, 6:16 am   #5
Radio Wrangler
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Default Re: Mk1 Decca Deccalian - inexplicable problem!

A transformer not only transforms the impedances of the things connected to it, it imposes its own magnetising inductance in parallel.

The inductance of the transformer windings required to work well with a magnetic pick-up is far lower than is needed to handle the same frequency range with a high impedance source like a piezo-electric cartridge. From the cartridge's point of view it will see the transformer as almost a dead short. You'll get massive attenuation, increasing at lower frequencies.

The tiny voltage left by the primary inductance shunting the cartridge does indeed get stepped up in voltage by transformer action but this is far too little to compensate.

The impedance level of a piezo cartridge suits valve amplification well and doesn't need a transformer. The impedance level of a magnetic cartridge suits transistor amplification well. The availability of good crystal and ceramic cartridges made life a lot easier than using the early magnetic pick-ups. When the transistor came to dominate amplification it encouraged a move back to magnetics.

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Old 23rd Jul 2017, 7:34 am   #6
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Default Re: Mk1 Decca Deccalian - inexplicable problem!

Quote:
Originally Posted by peter_sol View Post
If you put a few ohms across a crystal pickup it is like a short circuit to it so you will get next to nothing out of it. The CD player will have something like a 50 ohm output impedance so will not mind the load so much.
Just connect a resistor of between 470k and 1 megohm in series with the cartridge and take it from there as I think your amp may be the one that is balanced throughout and bypassing the input transformer may be tricky, but I stand to be corrected.
It was the more elaborate Decola that was balanced throughout. The transformer in this Deccalian is simply there to step up the output of the low impedance magnetic cartridge. As David indicates, you'll need to disconnect it so that its parallel inductance doesn't upset the ceramic cartridge.

You also need to disconnect any resistor-capacitor equaliser circuit on the secondary which is designed for the magnetic pickup. The ceramic cartidge is self-equalising if loaded with about 1 Mohm.

A similar project was discussed in an earlier thread: https://www.vintage-radio.net/forum/...d.php?t=122808

Best of luck - it should work well.

Martin
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Old 23rd Jul 2017, 12:33 pm   #7
RojDW48
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Default Re: Mk1 Decca Deccalian - inexplicable problem!

Martin is far too polite to point out that that previous thread was posted by...er...me It all makes sense again now. Thank you all for your patience. The first one of these I refurbished is still doing sterling service in the living room. And I have a third! Hopefully I won't have forgotten everything I learned by the time I come to restore that one....and post again!
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