PLZ pre-preamp

[Richard]

Originally Posted by Black Stuart

Here is one picture
problem miniaturising the PCB picture?

Yes Q for transistor,
https://electronics.stackexchange.co...r-transistor-q

[Black Stuart]

Greg,
thanks for that, we spent at least 1.5 hours trying to get the pics small enough for WD.

Richard - time to order the caps, I've got so much work to do on my system but can't get away from ter garden, serious pruning of fruit trees and now making a potage of a chicken run before the weather turns. Then i can work on my speakers/PLZ/h/amp - it never ends.

[Black Stuart]

Hi, PCB picture.

[bob orbell]

I would try taking pictures of landscapes Stuart, . Bob

[Black Stuart]

Ultra high definition Bob.

[bikerhifinut]

I think I may have stumbled on the circuit.
I had asked a friend who is very knowledgeable on amplification and apparently the large power devices are used because of their base spreading resistance properties.
I was given a name of a designer, W Marshall Leach and I looked it up on the web.
I have attached the link.
All is revealed I think its a common base amplifier to this design. An examination of the PCB suggests this with the emitters commoned and the preset pot is configured as a variable resistance which could be in place of R2 in the published circuit to adjust the very low bias current quoted here as 125uA (0.125 mA).
I'd be loth to fiddle about with the 470uF electrolytics unless you are absolutely sure they havent upset the biasing. The 100 uF as you can see from the circuit are simply DC blocking coupling caps to the output.
Interesting comment too about certain designers grinding off the transistor markings as if their type was some sort of industrial secret when its been lifted from a design in the public domain!
OK stuart I think you've got something to go on there.
Try it with the existing caps in, you won't blow anything up with 3V across it. You've already prodded about with your DVM so its had a few volts bunged in places already.

http://leachlegacy.ece.gatech.edu/headamp/

Andy.

[Black Stuart]

Andy,
thanks for that. Yes very frustrating not having any clue as to the transistors. You've explained why I measured such low current on those devices.

It will be interesting to hear what a LOMM sounds like via this dedicated pre-preamp.

Apparently the stereohedron stylus was developed to avoid patent problems with the shibata design AND is supposed to be better.

There are those who swear that this type of cartrdige combined the best of MM and MC and betters MC. Nice to know that it can be re-tipped by Soundsmith if it is that good.

Let's see!

[bikerhifinut]

All cartridges, generally speaking, can be retipped by a competent technician. As well as Soundsmith there are people like Len Gregory and Dominic harper in the UK that offer cartridge overhaul and retipping services.
That's quite useful as at a price, you can have your stylus not only replaced but possibly upgraded to a better/different profile.
There are many opinions on stylus profile, and the most advanced and expensive don't necesarrily guarntee a better audible performance.
That stereohedron is simply a brand name for a line contact variation I think possibly a Microline. The Shibata was initially developed to play Qudraphonic LPs where the decoding information etc was up beyond the 20 khz region and so a stylus and suspension had to be developed to be able to trace those high frequencies.
Other really effective profiles are, for example, the Fritz Gyger types and the GygerS and II amongst others are used by Goldring in theoir cartridges. I happen to like the Gyger profile a lot as well as the Ortofon Fine Line which for me are every bit as good as the way more expensive Shibata profile which can be a bit edgy to my ears.
Other factors are things like tip and cantilever mass and stiffness. This seems obvious of course. And brings me to why the output of moving coils is so low. In order to have a manageably low mass and thus low inertia, the coil on the end of the cantilever has to be light and constructed of extremely fine wire with very few turns. This means that the magnet assembly around the coil has to be incredibly strong and the field highly focussed in order to produce a useable output voltage, which is still in the order of a couple of hundred microvolts or so. Moving coils have really only taken off since the development of very strong magnets using rare earth metal alloys etc, making them easier to design and manufacture.
So this brings us to the good old Moving magnet, for which I shall include variations such as moving iron and variable reluctance which are all basically variations on a theme. As you will know, a moving coil works by waggling a coil in a magnetic field to generate a voltage, and therefore by the same token if you hold the coil steady and "waggle" the magnetic field around the coil you will again get a voltage generated. The advantage here is that you can wind a nice big coil to get a more effiecient generation of volts at the cost of a smaller magnetic flux density as the magnet on the end of the cantilever has to be tiny in order to keep mass down, or the magnetic alloy in a moving iron design that alters the field from an external magnet and then induces the coil voltage. On balance however the advantage is an output some 10 to 20 times that achievable by a moving coil. And there are many very good cartridges indeed with high outputs.
The issue here though is the DC resistance and inductance of a larger coil will cause its own problems with matching and frequency responce, especially at the higher frequencies, this is why most magnetic cartdiges need a 47k loading and are very susceptible to capacity effects from circuitry and screened cables due to the RC and LC low pass filters created thus.
So there are a few very low output moving iron and moving magnet cartridges, basically all that is, is fewer turns on the pickup coil which brings the advantage of MC in terms of matching and less susceptibility to capacitance effects.
Magnet technolgy has benefited these designs to the same degree in that the inertia from tip and cantilever mass can be reduced which allows the fancy line contact stylus to actually respond at the high speed (ie frequency) demanded.
I've heard a Grado low output moving iron and very nice it was but I felt the responce was skewed at the frequency extremes. There are articles out there that suggest we should alter the RIAA response filtration for MM and MC/LOMM because of the effects of electrical loading and ability to trace higher frequencies more effectively. It's more than just that simple step up which really only has to be linear from Lf to HF.
Loading in MC is also a very contentious subject, for the most part a MC cartridge is unaffected by capacitance issues, which is a good thing and also usually anything above 25 ohms depending on the DC resistance etc of the coils, which is usually never more than 4 ohms or so on the better carts, will not have any material effect. I havent noticed my Ortofon MC cartridges to ever be fussy about DC loading. I can't speak for other brands. I suspect the improvement I hear with MC is down to the reduced masses being accelerated in the groove and so we hear less mistracking and a better detail, combined with a much reduced interaction with the various capacity, resistance, and inductance related effects in the electronics and wiring. The same result may be reasonably expected with low output magnetic types for the same reasons.
A long post Stuart but maybe explains the reasoning?


Andy.

[Black Stuart]

Andy,
quite by chance I read something very similar on Aduigon which triggered my memory that I had bought a Pickering LOMM years ago and though I had liked it, see a post on WAD I had never bothered to use the PLZ that came with it.

All cartridges need a good phono stage to really shine and a good compatible arm, get this combo right and vinyl is worth the work you have to put into to get the optimum performance.

I wish I could go back about 30 years with the knowledge i have now minus the 'received wisdom', which mostly turned out to be b/s.

The signal from a cartridge, MM,MI or MC is very low level, any interference, any magnetic field which is stronger will alter that signal conductance. Long ago I rejected using any kind of shield on phono or any i/connect cable. The only cable that should be shielded is a power cable, I f that is done I have never noticed any interference.

I was still using commercial i/connects when I last listened to the Pickering 7500S, which all totally disregarded the stone cold fact that air is the best dialectric.

My Kenwood KD990 was designed from the ground up by engineers with open minds and they determined that the arm cable should be very high purity solid silver. This cable is solderded to a piece of tag board and the phono out cables are soldered to this.

As I have 2 x KD990s and the first only cost me £120 (it was dropped in transit but still worked) I intend to open up the arm on this deck so that there will be one continuous run of cable from cartridge tags to the PLZ or if I don't like what I hear to my MC3 phono stage, which I really do like.

What a shame these LOMM only came along at the wrong time Andy. The Ortofon/Soundsmith design has a huge advantage over MC which you have mentioned. The advantage of the Soundsmith cartrdiges also has a very real economic advantage they can be 'renewed' for a very reasonable price unlike MC.

I was very wary of MCs since production has been very variable. The ART 9 when properly made has a very good reputation but just a few years ago there were many being sold that were far from accurate in their alignment.

I never really liked the Goldring sound, I have an NOS G1000, forgot all about it. I have heard that Ortofon have very strict production protocols, hence no bad reviews on quality.

I shall give the PLZ a try and if I like what I hear, then around £8 for new caps to the same specs is a small investment.

There was an American who worked specifically on stylus research on V/E and he did a whole thread ob the various stylus profiles, their effect on vinyl wear and how well they tracked and relayed the information.

[bikerhifinut]

Further down the line and I have now got the new living room up and running on the first floor.
So I put a pair of AA cells into the PLZ that bob O kindly lent me to evaluate and plugged it into the MM input of the Rega Cursa preamp, I will try it in the WD Phono3 at some point too.
Well, it's fine as one would expect. My observations were that its susceptible to hum fields so keeping it well away from mains transformers is essential, as one would anyway when dealing with 300uV signals. The other observation is that it's just as noisy, if not a tad noisier than the Valve Phono3 and considerably noisier than my off board dedicated rega MC stage. I'm not surprised really as we are talking designs more than 30 years apart here. That said, at normal to loud listening levels it's inaudible under the Vinyl Roar etc.
Sound quality wise it's a tad brighter than using a pair of Ortofon T5 step up transformers, (Made by Sony and jolly useful little inline devices) and I'd be happy using it if it was all I had, there's nothing really not to like about it. I wasnt worried about leaving the original electrolytics in place, the worst that could happen is that they might have dried out over time but I think its unlikely and their service life in view of the never more than ambient temperature and weedy 3V Battery supply will likely be decades before any measurable drop in performance. Switching it off revealed a quite long period where it was still amplifying as the 470uF decoupling caps discharged so again these are good signs.

Andy