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KBK 2nd August 2006 07:52 PM

Output section impdeance change query
Hello dudes!

My MFA preamp has 'artifical' output impedance levels created by a resistor being in series with,and just after the ouptut coupling caps.

In a solid state unit, I know what to do.I remove them and run on dangerous ground. Screw up with a RCA cable for one second ,and the output section is dead shorted. No big deal, I like the sound I get with a direct connection. I'll take that chance. Even with preamps with unreplaceable and unobtainable outputs. (like the pure fet Acoustat TNP)

In tube circuit, neh.. not a good idea to futz around without really knowng what you are doing. Evidently, by this query, it may be obvious that this is a question for me.

I'm trying to figure out if I should lower it by much, if at all. The resistors in both the phono box and line out are both about 337-325ohms (two 6xx 1/4 watters in parallel). Likely, that number can be moved about a bit, and with differeing results.

I don't expect I'll be attempting to punish it with ridiculous loading, any time soon, so artifical boost is not required. I'd like to lower the effect of the resistor, but still have it there, if nessessary. Preferrably, I'd like the resistor to go away, but the original a circuit has it in there for a reason. Clue me in on the possibilities, iffin you can.

I'm askling as I am two seconds away from completing a parts Connexion order, and wanna get a bunch of resistors to finish up my order. The ones for this particular purpose will be expensive, ie, bulk foil.

I don't want to buy them twice. I'm thinking of going from the 337ohm, to 220ohm.

ErikdeBest 2nd August 2006 08:02 PM


From what I understand you have an all tube line stage (phono and line)!? With those it is, most of the times, no problem to shorten the output...look at the heretical and other examples, where a switch short circuits the output to gnd.

Those resistors, as far as I know, are there to avoid oscillation. Very low impedance output stages (cathode followers) need them (look at Morgan Jones third edition) for stability. probably you can reduce the value, but I am sure there is no way to calculate...a bit like a grid stopper, depends on valve, circuit, etc.


KBK 2nd August 2006 08:10 PM

The usual. cooking. A little of this, a little of that. etc. Is that what you mean?

Output is octal tubes with about 12-15uf of bypass. It is the same unit I have photos of in the 'grounding MFA luminesence' thread, on page two.

Brian Beck 2nd August 2006 10:52 PM

I think Erik is right, assuming that the MFA uses cathode followers. For CFs, it is not unusual to use resistors of about the same value as the cathode output impedance (~1/gm) to protect the CF from reactive loads such as very capacitive cables that might cause instability. I wouldn't worry about them. The output resistance might be in the 700 or 800 ohm range with these resistors, and that's not bad.

KBK 3rd August 2006 01:21 AM

Well, I erred of the conservative side, IMHO (SS throwback behaviour :p ), and ordered a set of bulk foil 1 watt 220 ohm resistors, the stock value being ~337 ohm. We shall see if grief ensues..or not. I tend to use very simplistic, non-audiophile cable designs. I don't like high C cables. Won't touch em, no sir. Right now, it's a set of the eichman 6 cables. Good performance for the money. (Got a show price on those.....!)

Brian Beck 3rd August 2006 12:33 PM

That can't hurt, but it may not be your highest pay-back tweak. I'd be tempted to put some Mills wire-wound resistors in place of what appear to be large plate load resistors. And of course, you could have a long-term avocation just rolling caps in this beast.

Heresy: As undeniably cool and rare as this thing is, as a DIYer I'd be tempted to see if it might be worth a huge sum to some buyer on eBay or Audiogon in stock, but working, condition. I'd run with the money and build my own preamp with recent audio-grade goodies. You could even clone the MFA if you suffer from separation anxiety and can find a schematic. You might even have money left over to build a small power amp and buy some SACDs; who knows?

KBK 3rd August 2006 03:59 PM

The estimated re-sale value, due to what it actually is, on audiogon, right now, is somewhere in the range of $7 to $10k. All I need to do, is clean it up,and be stubborn about the value in resale, wait anywhere between 1 day and 6 months, and I WILL get that sum. If I put it on audiogon, with the right header in the advert, I'd wager I'd get 3k views in less than a week.

It's a question of it's value to me, personally. I've never liked the buyer/seller, never listen to it-see it as a piece of meat guys, who sell great equipment purely for the cash and live for money. I hate those guys. I don't desire to even vestigally brush up against them. That's my dilemma, in a nutshell.

I'm aware it is not the highest payback tweak. Just a hair below that of the plate resistors and insane matching of tubes. They are a series connected bottleneck point, though. Caps are fun, yes, an avenue I'm exploring.

As a pure, stock collectors item, this particular item is right up there with the top of the heap in the audiophile world. That it came my way for such a princeless sum, is simply a bit of icing. It only assumes the value of resale, if I choose to see it that way.

I'd like to see it as something I could mod slightly in terms of wiring basics, etc (largely stock other than wiring upgrades),and have it maintain it's value or increase slightly-due to me (I,I,I!!) owning it, but I look into the pond, and I don't quite fall into it yet. :p

Here's something that needs be explored: When I was connecting up the phono box the othr day, disconnecting the RCA cables that connect it to the M.A. Cotter transformer I'm using ($NIB,$15 at a yard sale!). When the tip of the RCA (positive) is touching the inside of the RCA jack (no ground) the phono amp runs at full clip..and the draw on the PS box goes through the roof and oscillates at about a 1/2hz rate. Boing..oing..oing....(draw-starve-recover-draw-starve-recover-etc) goes the PS trannies and rectifiers, for a few seconds. Only the phono box does this. This means that there is obviously a need to address power draw stability in there - somewhere. That the phono box's gain and corresponding capacity to dump Big V signal is obscene, is not questioned.

Here's one that will make the deal, for those who missed it, just a bit more sad: The thing was so, uhm, unused and NEW, that me unhooking and playing with it about 4-5 times, put more wear on the RCA jacks than they had, by a factor of 2-3. No-one had ever even really used this thing, before I got it.

KBK 5th August 2006 01:47 AM

Oh yeah. The real reason to not build my own, and why I almost always mod gear instead of creating from scratch. Learning what effect the different parts have in a given circuit, one that is already up and working.

and nowadays..the fact that I simply don't have the time to start futzing with an entire build from scratch. You have no idea how bad I can get. For me to build my own preamp, for example, I'd start by going on down to the metals factory and slapping the engineers around and redesigning the base metallurgy of the chassis. Then it needs to be poured right, rolled right, made right, etc. Then it gets crazy from there! When I start from scratch I really start from scratch. It's very unlikely I'd use metal for the chassis, at all.

It's safer to work from someone else's finished product, otherwise I'll never get it done, I'll still be working on the details, which will be sooooooo deep....what I'm saying is that I know my own potential to drive myself mad-whilst seeking perfection. I've even built the same piece of gear 4 times...each with different solders, to find out what the heck solder is all about.

It's far safer for me to start with Bruce's best, and take it from there.

Build a REAL chassis. Stop screwing around!!!!!!!

And another needed component for that 'ultimate' chassis:

Take all those metal chassis, put them in the trebuchet, and launch those buggers into space. Got no use for metal chassis. Waste of time.

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