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Magnum P100 preamp

Tony L

Tony L

Administrator
Thanks to a very kind site donation recently this landed on the doormat...

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I was aware of Magnum from back in the ‘80s and the connection to IncaTech and Colin Wonfor (@CJ14 here on pfm), but I knew nothing at all about this preamp. It describes itself on the back panel as a “Mains-Battery-Powered Stereo Preamp”, which kind of makes sense when one sees the huge capacitor banks inside. It also appears to have both a MM and MC stage on separate inputs, so a potentially useful bit of kit.

Here’s an inside pic:

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At present it isn’t working, not so much as a light on the front, but I’ve had a chat with Colin W via email and have a few ideas as to what to check. From my own diagnostics power is getting in, the bridge rectifier seems to be doing its thing, but the relay isn’t clicking open to send power to the rest of the preamp, i.e. the problem looks to be very close to the start. There is no sign of damage, the big caps all look good and in-circuit testing suggests very low ESR etc.

My Retro Chip Tester Pro (thread here) can test the 4000 series logic, and the 4027 near the relay passes without issue (as do the two 4052s elsewhere). The chips are all in nice posh turn-pin sockets. I also have some 555s from when I was fixing an Apple II, so I swapped a new one in and again no change. It’s not the chips! After exchanging emails with Colin it looks like the issue could well be the transistor, or diode next to the relay. I’ve got the transistor in order. I think the diode may be ok, but I’ll need to pop it out to test it properly.

Anyway, potentially an interesting preamp…

PS A Nytech CA252 came in with the same donation. That will be cleaned-up and moved on, probably as a restoration project for someone else at some point in the future. I’m going to focus on this one for a while.
 
A great find there - I used to run some original Tablettes bi-amped with Magnum Pre, 2 mono blocks & a stereo power amp - great sound. Poor man's Naim!
Regarding your problem, have you checked the relay itself? You could pop off it's plastic cover and manually operate it.
 
Oooooooo, i'm a sucker for Magnum amps. I have in my collection, Magnum A-40 power amplifier, a pair of MF-125 mono power amps and an MP-125 pre-amp. My real satisfaction though are the seriously heavy (for their size) Magnum A100 mono power amps, matching power supplies and a very tidy matching p100, the same as yours, Tony. Magnum made lovely amplifiers, well, certainly to my ears anyway, and never cease to make me smile when i box swap. I also remember an advert, back in the day, where the big Magnum A100 mono blocks, pre and power supplies were advertised with a pair of Shahinian Obelisks.
 
I know very little about Magnum beyond hearing an amp or two at the Music Room in Manchester back in the ‘80s. A shop that did their own thing rather than the Linn/Naim groupthink of the time, I remember they favoured The Source turntable, Magnum amps and I remember hearing large Magnepans there. I’m pretty sure I bought a pair of Gale 301s there, I certainly bought a pair somewhere!

rabidlistener said:
Regarding your problem, have you checked the relay itself? You could pop off it's plastic cover and manually operate it.
Click to expand...

I’ll wait until the transistor and diode turn up and then pull the board out for a better look. I’ve not fully disembowelled it yet. The relay certainly looks very clean, I can’t see any hint of arcing or anything. I don’t really want to risk breaking it until I’ve ruled-out the transistor and diode (which I can test once out of the board).

To be honest I have no idea how this preamp works! I know nothing about it, never even heard of it previously. All I know is what is sitting on the rug here. I suspect the clue is in the phrase “Mains-Battery-Powered Stereo Preamp” written on the back and the fact it has a relay and two (unlabelled) lights on the front panel. It doesn’t have an on/off switch. I assume, maybe wrongly, that it maintains a power reservoir by switching the charge supply to the capacitor banks on and off, and I’d therefore guess one lamp on the front is a mains indicator, the other a charge indicator, but I may be way off base here. If someone could confirm this I’d be grateful.
 
Thanks Colin, appreciated, though I’ve ordered some of the transistors and the diodes, should be here with me tomorrow or Wed!
 
I bought the mc 250 pre amp on the bay last year with the encouraging words " Serviced"..... and then less encouragingly bit in "1999 " !!
Seller also stated it had an amazing phono stage, and to be fair for an integrated it is pretty nice. Tied me over nicely until
bigger fish come along.
 
Tony L said:
I know very little about Magnum beyond hearing an amp or two at the Music Room in Manchester back in the ‘80s. A shop that did their own thing rather than the Linn/Naim groupthink of the time, I remember they favoured The Source turntable, Magnum amps and I remember hearing large Magnepans there. I’m pretty sure I bought a pair of Gale 301s there, I certainly bought a pair somewhere!
Click to expand...
I bought a Source/Zeta turntable from the Music Room in Manchester, after brining in my LP12/Ittok/Karma to compare to it....the Linn came a distant second, so it was traded in against the other Scottish TT.

The initial demo used a Magnum Pre/power and it sounded quite ordinary. Fortunately I had brought in my 42/Hicap which we hooked up (I can't remember into which power amp) and things immediately got much better.
 
rontoolsie said:
I bought a Source/Zeta turntable from the Music Room in Manchester, after brining in my LP12/Ittok/Karma to compare to it....the Linn came a distant second, so it was traded in against the other Scottish TT.

The initial demo used a Magnum Pre/power and it sounded quite ordinary. Fortunately I had brought in my 42/Hicap which we hooked up (I can't remember into which power amp) and things immediately got much better.
Click to expand...
Given I’m the proud owner of a Source/Zeta (as well as a high spec LP12) I’d love to know what vintage amps I should try out. I’m reliably told Mike Moore himself liked Audio Research amplification - and Koetsu cartridges given that current thread - but I’d like to know if other options.
 
Best sound I got out of the Source was in a dealership showroom..
Audio Research SP10 preamp with matching D250 power amp into Acoustat 2+2 speakers.

At home I still got great sound with a MF MVT preamp into Audio Research D70 power amp. The Zeta had a Koetsu Black which was a phenomenal match.

I also had a custom DC power supply built for the Source the replaced its puny wall wart one.

The zeta was rewired with VDH silver arm wire too.
 
Big fan of Magnum amps. Back in the early 80’s I was an avid reader of mags like HFA, duly went to the local Linn/Naim dealership to buy the obligatory holy grail LP12/Naim system, the dealer was the most arrogant unpleasant guy so we walked out without buying anything, ended up visiting another shop in town later on and walked out with a Pink Triangle/Magnum amp. Still have both some 40 years later.

Was pointed to a Gumtree advert last week, popped down to the seller about an hours drive on my Birthday and picked up an Inca Tech Claymore for £75 - original box and paperwork, working great with a Rega P1.

Magnum untouched still working fine, PT been upgraded/repaired a few times, probably once a decade.




​
 
No doubt a surprise to few here, but it turns out I’m an idiot. That thing where I said there are two LEDs on the front panel turns out to be one LED and a tiny, tiny power switch. Press that, and guess what, the relay clicks and it powers up...

PEBCAK.

Obviously I only realised this when I’d got most of it apart and was trying to figure out how to disconnect the “LEDs” from the front panel. I noticed one was kind of on a spring…

I’ll plug it in properly and see if it makes sound over the weekend, but I think it is working.

PS Nice IA125 above, I remember that one from Music Room. Looks very much like a Claymore.
 
Iain Docherty said:
I’m reliably told Mike Moore himself liked Audio Research amplification - and Koetsu cartridges given that current thread - but I’d like to know if other options.
Click to expand...
I tried quite a few cartridges in the Zeta/Source.
Koetsu Black (two of them as I denuded the tip of the first one)
Kikseki Blue
Monster Alpha
VdH MC 10
VdH MC One
Linn Karma

By FAR the best was the Koetsu Black that had an prodigious bass extension and pitch control. The Karma sounded quite ordinary by comparison.
Until..... (read on)

But the guy who built the custom DC supply for my Source as well as rewiring the Zeta (Steve McCormack at the Mod Squad in California) recommended I try his 'Cartridge Tip Toes'....basically a thin plate (I cannot remember if it was plastic or metal) with three small bumps that interfaced between the cartridge and the headshell. It only cost like $15 or so back in 1985, so I gave it a try. The Mod Squad was pretty much the first company that offered after market spikes to go under speakers (the original Tip Toes, not to be comfused with the cartridge Tip Toes).

(Clarification....I think it was his wife at the time, Joyce that rewired the tonearm)

With the cartridge Tiptoes in place the Karma gained another octave in LF extension and had greatly enhanced clarity and dynamics, and sounded almost identical to the Koetsu. Almost 40 years later I can still vividly remember the first track I played once this was installed....'This Masquerade' from George Benson's Breezin' album. And the electric bass that had been almost AWOL before stepped into the room, and the dynamics were explosive!

I have not heard of anyone else using the Cartridge Tip Toes, or any other company that makes anything similar. You would have thought that an extreme arm like the Zeta would not need these add-ons.

And yes, my Zeta eventually had frozen bearings as a result of the lubrication oil drying out into a gummy paste. The US distributor did a CLA on it for a very reasonable price, especially since I had imported the Zeta from the UK instead of buying it through a US dealer for $1000 of 1980s USD.
 
The P100 is very sick!


Nothing comes out aside from a weird whining noise. I assume something is oscillating. Volume control makes no difference, input selector makes no difference, I tried shoving a line signal through it and nothing there either. Just the weird whine, and it is out of both channels.

Anyone any ideas?
 
Hi Tony,

Yep it was sick, Tant Caps stressed to Max volts, oh just silly tip Tant Caps in Audio very bad idea, in signal path as DC block very very silly idea, they act like crap diodes.
Any PSU had died Tants S/C one smoked, Lin Regs dead.
OP Amp chips used as reg gone to hell was FET OP amp in PSU no way replaced with a Low Noise standard Bi-Polar OP27, not OP37, ok BW better but unstable a gain lower than 10. These power the MC stage like Buck Shunt Reg design. and have a good CMRR.
The MC stage is my patent using Kirchhoff Law, which I was paid royalty for after legal action with Relph.
The Start OFF ON Tony is OK yipee.
4700uF 25V caps running at 24V a cheap not to my original design Relph save pennies game.
The above caps are going to be changed for 4700uF at 35V and 105C no 85C MTBF will be huge
 
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