advertisement


Phono board options

How do you want the functionality split up

  • Four small boards

    Votes: 6 18.2%

  • Two boards, each a complete channel

    Votes: 13 39.4%

  • Two boards, stereo front end and stereo EQ

    Votes: 1 3.0%

  • One big board

    Votes: 11 33.3%

  • Don't care

    Votes: 2 6.1%
  • Total voters
    33
  • Poll closed .
As promised, schematic of JFET circuit for MM cartridges.


head3a on Flickr

This is pretty quiet, but does not begin to compare with the bipolar versions for MC cartridges. For an MM source, it is limited by the intrinsic Johnson noise of the source.

Distortion is low, overload margin high, and PSRR very good, because of the differential pair second stage.
 
Hi PD

Thanks for the updates, they look great. What determines the value for the electrolytics?

Regards
Peter
 
PL135 said:
Hi PD

Thanks for the updates, they look great. What determines the value for the electrolytics?

Regards
Peter
Click to expand...

In the low output version? Trying to keep the impedance low in the feedback path, for noise reasons. By simulation, 220u only gives a minor rise at the very lowest bass frequencies. In effect, all 4 are in parallel for noise estimation purposes. At 20Hz, 1,000uf (close enough to 4x220) has an impedance of 8Ohms, which is comparable to the sum of the various equivalent noise impedances (3.3 Ohms for the feedback resistor, 5 Ohms or so for each transistor (all in parallel), a smidge extra for 1/f, a few ohms for the cartridge source resistance).

I guess you could go down to 100uF or even 47uF with only a modest impact, other than a rise in low frequency noise, which is not very audible. The wacky feedback arrangement means the bass response is very extended.

My thought, which may be wrong, is that people would only build the low output variant if they really needed it, as it cannot be adjusted for MM use, unlike the main version. In this case, they probably want to scrape every last bit of S/N ratio they can.
 
Going to 47uF would make smaller polypropylenes just feasible, e.g. the ones that valab sell on ebay. Would this be worthwhile - they would rather dominate a board ? If using electroytics then what types would be recommended for this sort of use ? I am sure I would be tempted to put a russian ft-1 22n teflon bypass on any electrolytic in the feedback and they are not that big at around 25*10mm
 
PigletsDad said:
My thought, which may be wrong, is that people would only build the low output variant if they really needed it, as it cannot be adjusted for MM use, unlike the main version. In this case, they probably want to scrape every last bit of S/N ratio they can.
Click to expand...

I'm quite sure there's a few people who would only be interested in the low output MC version, myself included.
 
Hi PD

Correct, the electrolytics in the Low Output version. Thank you for the detailed explanation, very informative and insightful.

A single board version with interchangeable input modules will enable the phono stage to cater for all cartridge types and one will be able to compare different input stages to boot!

Regards
Peter
 
The standard version is optimal for most low output moving coils, it avoids the large value capacitors entirely.

The Kontrapunkt C is 470uV, so would be fine with the standard stage; turn a record mastered from analog tape up loud and you will hear the tape hiss very clearly when the tracks start, just before you eardrums get blown out by the first note!

The sort of things that need the stage with the capacitors are weird stuff like the old Ortofon MC2000, the Kondo cartridge and the like; the standard stage is significantly quieter than say Naim K board and is my recommendation for anything above 200uV.

Using film caps would be possible in the stage with capacitors, and would sound better than electrolytics; the down side is the increased physical size makes it much more prone to picking up magnetic interference. If you put it in a mumetal case, the problem would go away; alternatively you may be able to arrange a spot far from mains transformers, fans and motors where the fields are low enough.

The electrolytics run with 0.7V one them, enough so that the insulating layer should form properly, but I would expect noticeable warm up and run-in effects. You certainly want a type with very low leakage, but ESR isn't in itself such a big deal. I only know a few types, so can't really make suggestions on the basis of experience. I would choose Panasonic FC myself, as they are cheap, good and readily available, but others know more.
 
Thanks for the updates and for giving me something else to research :)
the down side is the increased physical size makes it much more prone to picking up magnetic interference
Click to expand...

Are you asuming we would / could put the transformer within the same enclosure ?
 
Would the standard version have enough gain for a Denon DL103 @ 300uV ? My stand alone Naim boards certainly didn't. I've halved the value of R13 to up the gain but could do with a touch more tbh.
 
The gain is easily adjustable over a fairly wide range, and for 300uV I would use the standard one.
 
Currently I am using Ortofon cartrige although I am unsure what the output is(I'll have to take a look). On some of the other boards the physical layout allowed for different capacitors(electrolytics) as well as films. Could this be factored in with the layout or would this be a hindrance to sound quality?
 
Whoever does the layout (probably not me), will have some hard choices to make.

In high level circuits, size is a secondary consideration most of the time.

If you make the board big enough to accommodate 4 giant film caps the area that will pick up hum is much increased; the tradeoff is then between better sound from the cap choice, versus the risk of an annoying drone in the background. A middle way is to us electrolytics, and make room for modest size film cap bypasses. There is no "right" choice, but each way brings with it some pros and cons.

The standard circuit doesn't need big caps, and this is why it is reccommended for almost everything except the very lowest output cartridges.
 
Thanks for that PD, would this layout be beter with a solid fill (ground plane) for 0v or individual tracks (star earth)
 
Ground plane for most of it; every bit of shielding is good.

The hand made prototypes, and the production variant, were both ground planed.

The way I did it was to have a separate ground plane for each module (L head, R Head, L eq, R Eq), and wire them together in a star ground fashion, using shielded cable for the signal links. The production version was broadly similar, but had no point to point wiring.
 
I've been looking at the PSRR of the EQ stage, and simulated some different variants, about 8 so far.

To get a big improvement, you have to go to a much, much more complex design, and I am very wary about that - the opportunities for trouble outweigh the gains. The stock design has an adequate PSRR, and is nicely simple. If anybody wants the gory details, I can explain what I looked at, but the upshot is that the transistor count goes way up to get a big win.

So my engineering choice is to keep the signal path simplish, and accept that the power supply has to be vaguely quite; it needs noise in the audio band to be below a few mV. You can't quite do that practically with a purely passive design, but even very simple regulation can deliver it; something like an LM317/337 would be way more than good enough, or a simple VBE or whatever.

If the +/-24V supply is not completely horrible, a VBE style divider (amplified capacitor) simulates as doing very well for the head-amps, in all 3 variants. Depending on which head board(s) you have, the total draw for a stereo setup is round about 0.1A per rail, say 5W in total.

My suggestion for power supply (unless you have another device running on +/-24 V that you want to piggy back off) is as follows:
1) Remote box containing moderate size (say 30VA) toroidal transformer with 18-0-18 secondaries, rectifiers and a CRCRC filter made up of 2,200uF caps (35V or higher please) and 4.7Ohm resistors. The resistors will drop about 0.5V each, giving a smoothed output at about +/-28V, with perhaps 50mV peak to peak of ripple. This gets fed, via a 3 wire cable, to the actual phono unit.
2) The EQ board has the 317/337 pair on it, and a couple of holes to allow people to feed the +/- 24V to the head board(s).
3) The head boards contain their own local VBE circuits.

What do people think of this as a plan?
 
I like a simple approach. Those of us who want more can still try to tweak the linear supply with STRs or the like.
 
You must log in or register to reply here.


advertisement


Back
Top