Tertiary feedback circuit gain bump

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peter purpose

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Joined
Jun 3, 2004
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Gents,
I picked up an old bit of gear for the transformers nailed to it. It has discrete circuitry and a tertiary feedback winding on the output transformer.
I fired it up and it works nicely, but would like to see if I can get more gain out of it before I just rob the transformers.
I tried replacing the feedback network with an ordinary op-amp type network, but the negative rail would not come up to spec (as standard, the neg rail takes 30 seconds or so to come up, but this stayed at -5 or so volts) .
Nothing got warm or damaged and standard circuitry was put back in.
The tertiary winding is roughly 22dB down from the secondary output.
RGV gives me +&- 5dB, but I'd like to shoot for more.
So gentlemen... Can it be done?

Cheers
peter

lgli.jpg
 
Looks like DC passes through that tertiary winding... is it a gapped core?

For overall greater gain, I'd wonder if T1 has a good excuse to stay in there...

Keith
 
Keef,
Got about 600mVdc on the tert. No idea if gapped, but no dc on pri or sec.
Was looking at doubling the variable gain, but seems harder than at first thought.

peter
 
Is RGV anything more than a simple signal shunt? And can the ratios be changed to alter gain? It reminds me of an early General Electric SS program amp that uses a shunt volume control. I'm probably totally wrong on that.
 
After a quick look I'd have said that R11 needs lower value for higher gain. But the design is too complex to be sure after a minute if that really works well. Shorting R5 may be another option.

Samuel
 
[quote author="rodabod"]How much gain would be a useful amount of gain?[/quote]

20dB variable would be nice. Desk line inputs.
 
[quote author="emrr"]Is RGV anything more than a simple signal shunt? And can the ratios be changed to alter gain? [/quote]

Slightly more complex than a simple shunt I'd say.
Ratios are fixed.
 
[quote author="Samuel Groner"]After a quick look I'd have said that R11 needs lower value for higher gain.

Samuel[/quote]

Overall gain maybe Sam, but I'm looking to double the variable gain.
 
I could lose the attenuator and surrounding parts alltogether, but how could I then get any variable gain. The usual method bears no fruit.
 
General question - is it bad form to pad levels when dealing with line levels in order to change gain. Ie. - say have fixed 10dB gain and pad 20dB to achive -10dB?
 
When I something like this, with so many parts and reflecting what had to have been a protracted development in likely pre-simulation days, there's a tendency to just start from scratch. I.e., give me the transistor budget and the performance target and away we go.

However, that would quite possibly miss the merits of what may be a good design for what the creator set out to do.

Is there a known inception date for this thing? Judging from the transistor types it could be pretty old, say mid-70s or before.
 
Having said that, to get more gain variation by manipulating the principal feedback levels may court disaster. From the looks of things, with the 150uH inductors and R-C network across the feedback R, the piece is carefully optimized against oscillation while preserving various tweaks for distortion and bandwidth, given the properties of the output transformer.

The issue you had with the -12V rail is worrisome by itself---what kind of a power supply would be so sensitive to loading, yet you don't see any device running hot?

EDIT: Probably making the gain a bit higher is the easiest, although distortion may well get worse in a hurry. R11 getting variably smaller is probably the first thing to try.

C5 looks backwards, if the circuit output at T2 is really zero, but the reverse bias is small enough that it's likely not worth changing.
 
Haha... I've flummoxed the brainiacs.

<<Is there a known inception date for this thing?>>
I've got an 83 test sticker, but nothing much else.

I think I'm just going to nick the transformers and put a 2520 between em.

Thanks all... :thumb:
 
Looks a lot like an opamp to me.

...In which...

...case...

[Ian Dury... Reasons to be cheerful (part III)]

Ratio of R11 to R12 would have a big gain effect.

Try it before you chop it.

Keef
 
If you can see the sides or bottom of the lam stack you can easily tell whether it is a gapped output transformer or not (assuming standard E-I core). If it's potted or in an extruded can you're SOL...

If each layer of laminations is oriented the same way, it is gapped and from the side you will see a clear join where the ends of the "E"s butt against the "I"s. This will appear near the top OR the bottom, but not both.

If it's interleaved (not gapped) you will see alternating orientations of "E" and "I" laminations. The pattern could be 1-1 (shown below) or 1-2 or 2-2, etc. From the side you will see the ends of the "I"s at the top and the bottom of the stack which will show as "dashed" gaps at BOTH ends.

interleaved gapped
E-I E-I
I-3 E-I
E-I E-I
I-3 E-I
E-I E-I
I-3 E-I
E-I E-I
I-3 E-I
E-I E-I
I-3 E-I
E-I E-I
I-3 E-I
...

A P
 
[quote author="SSLtech"]Looks like DC passes through that tertiary winding... is it a gapped core?
Keith[/quote]

Not a lot of dc though---about half a milliampere it looks like.

When PP says he replaced things with a more standard feedback network I'm not sure what is meant, since there's one there already (R11, R12 etc.). If the tertiary winding is removedthen the free ends of L1 and L2 need to be tied together of course.

Oh, also, it looks like C3 is reversed, again not too seriously.
 

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