advertisement


Does anyone know of a good DIY MC phono section?

Maybe a bit of progress.

With the board as was, nothing connected other than three wires from my bench supply and an oscilloscope probe and just sitting on the table, the hum varied from 25mv to huge, between 50Hz and 100Hz according to mood and everything was very sensitive to vibration, positioning, being breathed upon etc.

A bit of tinkering really changed nothing.

I added a pair of 47u electrolytics across the supplies where they reach the board. The result was much quieter rails before the regs and more or less silence on the output. I then realised that no signal was getting through. The eq cap had failed to short circuit. But one of the stages was hum free. The input stage was still noisy though.

So I removed the gyrator pass transistors and bodged LM317/LM337 regulators in their place. Now with no wires connected to the input the result was pretty hum free.

Adding an input connector, a spare pickup arm and an ancient Supex SD900, all draped across the table, I can get the hum down to 5mv, against a noise of 1-2mv. Digital stimulation of the input device shows it's working.

So the next spare moment I will try it with loudspeakers and a proper turntable.

I'm tempted to build four gain modules on Veroboard and see what happens.

Paul
 
Paul Ranson said:
...So I removed the gyrator pass transistors and bodged LM317/LM337 regulators in their place. Now with no wires connected to the input the result was pretty hum free...l
Click to expand...

So does that point the finger at some sort of incompatibility with the gyrator stage of the TeddyReg? But I have used the on board regs and 4 full off board TRs - all with the same result.
 
I hesitate to draw firm conclusions. I looked at the supply rails and they looked clean. No hum. So 'don't know' at the moment. I didn't try bigger conventional caps at the 'TeddyReg' output, for example.

Paul
 
A little more meddling between football games:
  1. Disabled the on board gyrators and inserted lashed up LM317/337 12v regs in their place on one channel only and - still get the hum.
  2. Hooked up the second channel with batteries - that channel operates silently...
...so does that mean it's coming through the power supply path?
 
I've had a go with a real turntable into a real speaker. When positioned far enough from transformers the hum is more or less on a par with the background hiss. So that's good.

I think the key thing so far was the pair of 47u electrolytics on the power input. If I can clear the nozzle of my solder sucker I may reinstate the 'Teddy Reg' arrangement for the front end and see whether that was a real result.

FWIW I don't like the star earth arrangement. I would distribute the stars about a bit, I 'd like the input loading cap and resistor, for example, to be very closely connected to the bottom end of the feedback resistor, and then the whole lot to the common for the local regs. There are essentially no currents flowing in these connections, other than the one between the regulators.

This is intuition rather than physics.

Paul
 
I'll add some decoupling caps later this morning and report back...

...PD vetted the layout, but I did wonder if the mixing of the TR grounds and signal grounds was in some way contributing to the hum...
 
Paul Ranson said:
I've had a go with a real turntable into a real speaker. When positioned far enough from transformers the hum is more or less on a par with the background hiss. So that's good.

<snip>

Paul
Click to expand...

Big sigh of relief; it can be quiet for somebody else then!

The hiss level should be pretty low, so I hope this means it is very quiet. In my system, on most discs, you can here the tape hiss from the master when the track starts if you listen carefully.
 
OK, so I added a pair of 100uF/63v caps across the power inputs and it definitely helps. But the hum is still there - even if it's now at or below the level of the background hiss and the microphonics from the (rather crude) auto return mechanism of my workbench turntable are still there even if damped a little. So I can't help feeling that this is masking the problem rather than solving it. Especially since the battery operated channel is completely silent. But it is now listenable.

I think I may go back to the drawing board on the circuits and see if I can't separate out the TR, Gain and RIAA stage grounds and prep some more prototypes.
 
I does indeed though it has so far only debuted on my bench system and I haven't really tried to listen - just to see if the hum has gone and of course the channels are horribly mismatched as one is running from the power supply's raw 30v rails and the other at +/- 9 and 18v or less as those batteries must be getting a little tired by now.

So as for Paul, I think it's time to reinstate the gyrator half of the reg and match the cards up together...they are getting to look pretty hacked up by now :)
 
PigletsDad said:
The circuit supports MM operation; about 6 resistor values need to be swapped.

There is also an intermediate setting optimised for high output MCs, although the MM variant will work nicely for these, you can get an extra couple of dB less noise by tweaking for this case.

If you wanted to support MM and MC, the best bet is probably to build two front end stages, and switch between them. The circuit has a separate gain stage with flat frequency response, followed by an EQ stage.
Click to expand...

Hi PD, I'm going to pick up an AR XA turntable with (I suspect) an MM cartridge on it. It's my first deck and I don't have a phono stage, but I do have a rather extensive collection of bits and bobs with which to build one!

Could you give me a bit of guidance on what should be changed on your MC phono stage to make it suitable for use with a MM cart? This is the diagram I'm working from: http://i26.photobucket.com/albums/c114/neiljadman/PhonoStageSchematic-1.jpg

I don't have any 2sc2546 or 2sa1084 transistors, but I can get the equivalent KSC1845FTA and KSA992FBU pretty cheaply from Mouser, but would BC550 and BC560 be suitable in their place? I've got a crapload of those in the bits box.

Thanks,
Carl
 
hacker said:
Hi PD, I'm going to pick up an AR XA turntable with (I suspect) an MM cartridge on it. It's my first deck and I don't have a phono stage, but I do have a rather extensive collection of bits and bobs with which to build one!
Click to expand...

The AR XA is good enough to show you what vinyl is all about.

The arm is crude at best, but with a bit of fettling it'll "do". In my view it helps to disable the vertical damper, which is done by simply removing the little pin that connects the arm to the damper, then screwing the post down tight to lock up the damper barrel.

The foam mat will most likely either be long gone or disintegrated, the XA works well with a felt mat, cork, or the Ringmat.
 
This is the fella:

ARTurntable001resized.jpg

ARTurntable002resized.jpg
 
Looks like a Shure of some sort on there? I recommend staying away from Grado cartridges on the AR, they tend to be sensitive to the field around the AC motor and hum toward the end of the record.

Is that a weight lifting bench? Get the turntable offa there and do three sets of dumbell chest presses.
 
For MM BC550 and BC560 are just fine; arguably maybe even better than the exotics. There is little advantage running the frontend at 12V for MM, so simplify the PSU arrangement, and use 24(ish) V everywhere.

You need to make the following resistor changes:
R2 becomes 47K.
R8 and R9 become 39K (if supply is 24V) For 12V supply, use 20K.
R1 and R6 become 3.3K
R3 and R7 become 100
R4 becomes 1K5
R5 depends on the cartridge output, and the overall gain you need in your system, but 220 Ohms should be a good choice. If your cartridge is lower output, or your system needs more gain, try reducing R5. If you get down to say 100Ohms, and still need more gain, increase R4.

Two capacitor changes:
C1 is the load cap, and depends on cartridge and tonearm wiring. Try 220pF to start, but reduce if balance is dull, and increase if too bright. If you know the suggested load capacitance, and the wiring capacitance of the tonearm, you can subtract one from the other to get a soundly based starting point.

C3 can be adjusted depending on how bad RF pickup is your system. Start with 1nF, which gives extra rolloff above about 100kHz. Increase if RF is a problem. Don't go above 5(ish)nF, otherwise the treble will roll-off. If you increase R4 to get extra gain, reduce C3 in proportion.
 
hacker said:
This is the fella:
Click to expand...

Welcome to vinyl, Carl! That's a $20 first step down the road towards audio nirvana. :)

BTW, I might have some LP12 take-outs in the garage that you're welcome to – and some spare LPs in need of a new home.
 
You must log in or register to reply here.


advertisement


Back
Top