Weird Amega Mike Pre....

So I got this Amega mic pre/record head pre/speaker out thing.

It had UTC A-21's for mic input and thes little drawer modules with the mic pres in them.

I reversed the circuit board in the drawer and this is what I came up with:

EDIT: picture in post below>>>>>

The transistors are a mystery. they are unmarked in TO39 style cases.
I don't know the supply voltage, just that it is negative. so I'm guessing PNP.

Anyone have any ideas?

Man that's weird, why do you need that 50uF cap at the input trafo? Multiple feedback paths.... Wow, that's kinda crazy!

Peace!
Charlie

Yea if I could just figure out the supply voltage I could test it out. Maybe -15V? -20V?

I would like to know what the output impedance is to.

Anyone?

Bluebird;
Thanks kind of cool.
Any voltages, transistor#'s or photos would be great.

The 39K has voltage across it and the transformer winding would short out that bias DC voltage. The Xformer would not like it also.

It is happier with the cap in.

Hey chunky......get that Staines filth outa your avatar :thumb:

Ok Butta....I'll be ca re ful.

adriana,
Thats the problem, the transistors are not marked. theres some 2N307/306 in some of the rest of the circuitry in the main chassis, but as for the preamp card I just don't know.

heres some pictures:





Maybe I'll just give -15 volts a shot.

If you have a variable supply, bring the voltage up slowly with the current limit LOW! See how hot she gets and test the ckt as you go. Prolly -15 isn't going to hurt anything. It might not bias right and have some funky clip, clip, clipping.

Good eye on that 39k (36k?) on "Q1" base to the ground, Adrian! Negative supply stuff like this drives me NUTTY! :twisted:

Peace!
Charlie

So I got this thing working with -15V. Sounds pretty good on my bench.

I was just wondering if someone could tell me what the output impedance might be with this: would a 600:600 output TX work?

heres the schemo again:

It might be worth you having another look at the circuit diagram, just in case there are errors....

.... for example, is there anything else connected to the base of the first transistor. I cannot see how it is biased into any kind of linear region if it really is like that.

... also, I can't make sense of why the 100uF/6V cap (top left) is connected like it is, particularly with the 50uF/15V there like that.

... I also feel unhappy about the 100uF/6V top centre. It would make more sense to me if there was an additional resistor between the collector of the 3rd transistor and the 3k9/100uF junction (ie. a bootstrap collector load).

... a number of other aspects of this schematic look kinda odd, too (although some parts are quite straight-forward - the output configuration, and some of the -ve fb setups).

My feeling is that you could have another look at the pcb and double check the layout.

Alan

In fact, looking at this schematic with the view of turning it into something that would *work* (is this a new and compelling game-show possibility?)

... I would (numbering transistors left to right)

1) Move the LH end of the 18K between Q1 and Q2 so that it connects to the base of Q1 instead of the emitter.

2) Move the LH end of the 100uF/16V away from where it is, and connect it to 0V (and turn it around so that it is correctly polarised).

... so then we have effectively turned Q2 into a common emitter amplifier, but taken DC f/b from its emitter to set an operating point for Q1 (and hence Q2 itself). The DC conditions then look pretty credible, and the overall AC -ve f/b will produce a gain of x60 (roughly 36dB) for this first stage.

The alternative might be that the 100uF is connected as-is, but the 50uF/15 is just a -ve rail decoupler (ie. not actually connected to the 5k6/4k junction), so the 100uF is bootstrapping the collector load of Q1. But then the 18k will provide lots of AC -ve f/b as well as DC, which will kill the gain of Q1, leaving all the work to be done by Q2 - and I don't think much benefit will be seen by the bootstrapping, so I will put my money on my first suggestion.

I'd very much suspect that the collector load on Q3 and emitter configuration on Q4 will end up looking very much like the Q1/Q2 arrangement (whatever that turns out to be...)

Alan

(PS. I suggest nobody tries to build this circuit as it is shown here...)

Sorry (answering original question)...

I was just wondering if someone could tell me what the output impedance might be with this: would a 600:600 output TX work?

Click to expand...

Output impedence will be pretty low, and should be fine to drive a 600R 1:1 trafo.

Alan

(PS. the jpegs disappeard for me a few seconds ago - site crashed? or original schematic being edited???)

apparently the site is down...

Sorry guys I use my band website for server space. I guess we couldn't pay the bill this month.....

times are tough. :?

Oh by the way there IS something else connetcted to the first base. the 1 watt 18K....I get a revised schemo up....

Oh by the way there IS something else connetcted to the first base. the 1 watt 18K

Click to expand...

!hey-hey!

Alan

OK I revised the schematic. Closer?

I actually tested it out (just parts loose in a cardboard box) today with a drum mic up.

sounded cool. I set the input TX at 1:1 and it seemed to handle a condenser 8 feet away from the kit but the output was pretty low. If I turned the gain pot up for high output the signal would distort.

maybe it needs a step up TX on the output.

Maybe it needs -20V?

Thanks for the input Alen. I appreciate the help.

Ok, it now has a certain believable consistency about it. Also, I would expect it to work now.

Personally, I think it is a bit crazy what has been done with the C's on the emitters of Q2 and Q4 - why they haven't just been returned to ground, I don't know.... And the R's in the same position - why not to rail? ... But I guess the guy had something in mind when he did it... (although it looks a bit as if he changed his mind in mid-design).

Rgrds
Alan

Hey Alen,

would controling the gain with this be better by manipulating the 680 feed back resistor and just fixing an interstage cap between the two stages instead of the pot?

also could a 20:600 TX work on the output?

One more thing, how large could the 820 be made in the first stage to control the gain of the first stage?

would controling the gain with this be better by manipulating the 680 feed back resistor and just fixing an interstage cap between the two stages instead of the pot?

Click to expand...

Well, controlling gain via the feedback gets difficult if you want a wide gain variation, as apart from the gain, you often seem to go from an inadequate frequency response at one end, to the verge of instability at the other.

In fact, this type of arrangement (as here) is a reasonable compromise in many ways - there should be plenty of gain overall, and it is taken in two easy chunks. If it is stable to start with, it will stay stable.

The only concern might be in overloading the input stage if the signal is too hot. In which case the usual answer is a pad (although you could contrive to reduce the gain of that stage, although the low values of resistors there might well make it difficult to do without loading the output).

Although, looking at the values, the overall actual gain there seems rather low - a tad over 10dB in the first stage, and not-quite 20dB in the second stage. I guess there is some voltage gain in the input transformer, although it can't be too much or else the input impedance will bee too low... Reduces the opportunity for overload, I guess...

Alan

PS. I still feel this is a pretty peculiar circuit configuration... 'Unique' you might say (but not in the kindest way...)