Lorenz Mic-Pre

[abbey road d enfer]

There are too many ways of describing negative feedback, many of which are inaccurate and some of which are downright misleading (like Schade or anode to anode). My personal preference is to state how the feedback is derived and applied - there is no need to refer to voltage or current. In this case, the NFB is shunt derived and shunt applied. I prefer this description because it immediately tells you two important effects of the NFB. Any feedback that is shunt derived lowers the output impedance, Any NFB that is shunt applied lowers input impedance. The only other way to derive and apply NFB is in series and series derived NFB increases output impedance and derived applied NFB increases input impedance. So,

shunt derived NFB reduces output impedance
Series derived NFB increases output
Shunt applied NFB reduces input impedance
Series applied NFB increases input impedance.

Cheers

Ian

Again, language barrier. We Gauls have appropriated the word "shunt" in a way that makes it unsuitable there. Shunt really means connecting a low-resistance element in parallels to a higher resistance one, in order to derive most of the current: typically, that's what we use to measure 10A with a 200uA needle meter.
That's why I used the term "voltage-to-current", which is the way it is taught here.

 

[ruffrecords]

Again, language barrier. We Gauls have appropriated the word "shunt" in a way that makes it unsuitable there. Shunt really means connecting a low-resistance element in parallels to a higher resistance one, in order to derive most of the current: typically, that's what we use to measure 10A with a 200uA needle meter.
That's why I used the term "voltage-to-current", which is the way it is taught here.

Interesting. In English, shunt can mean either a low resistance as in a meter shunt to measure current, or it can mean to route a railway carriage into a siding, or it can mean to hit the car directly in front of you or to tap off a fraction of a signal as in negative feedback. No wonder people find it confusing.

Voltage to current is of course perfectly valid, it just does not help you rememeber anything about the properties of that type of feedback. I am a great believer in simple rules that help explain apparently complex things,

Cheers

Ian

 

[PRR]

> to hit the car directly in front of you

Not used that way in America.

I suspect your use comes from shunting railroad cars, and your railroad car bumpers/buffers which are not used on American railroads.

 

[abbey road d enfer]

Voltage to current is of course perfectly valid, it just does not help you rememeber anything about the properties of that type of feedback. I am a great believer in simple rules that help explain apparently complex things,

I do have a very simple set of rules: when NFB is derived from voltage, it's the voltage that's regulated, which is for me synonymous with low-Z; when it's derived from current (series), it's the current that's regulated, which means high-Z.
When NFB is applied as a voltage (series) it decreases the sensitivity without changing the input current so it increases Z, when it's applied as a current, this current acts against the source current, so it decreases the input Z. It may seem tedious, but in fact,  I don't even think about it anymore.

 

[ruffrecords]

I do have a very simple set of rules: when NFB is derived from voltage, it's the voltage that's regulated, which is for me synonymous with low-Z; when it's derived from current (series), it's the current that's regulated, which means high-Z.
When NFB is applied as a voltage (series) it decreases the sensitivity without changing the input current so it increases Z, when it's applied as a current, this current acts against the source current, so it decreases the input Z. It may seem tedious, but in fact,  I don't even think about it anymore.

I can see how that works. I guess it is simply a matter of what works best for the individual.

Cheers

Ian

 

[DaveP]

I have translated the German magazine article posted by Rock into English and attached it below.  A measurement company tested it and produced their results.

German makes me smile, this example gives you an idea of what you might have to translate.

"I on every Saturday night to the cinema with friends  except the last Saturday in the month  go."
You have to get to the very end to find out what's happening!

German also has a tendency, like most eastern European languages (but not so bad) to dispense with the definite article.

"Do you want collect amplifier today or tomorrow?"

I take my hat off to all of you who have such good English as a second language, it must seem just as weird to you, but you do it so well.

DaveP

 

[L´Andratté]

I love the english language,...because the English love their language. In an interview with (the formidable guitarist) Martin Taylor, he points out how in every corner of the UK people breed their own ways of ´saying things´ (I had noticed -but never realized- that too), and they´re  often so striking and pittoresque, it makes me laugh. I don´t think english (or probably any language) can be really translated.  ;D
/on topic

 

[L´Andratté]

I do have a very simple set of rules: when NFB is derived from voltage, it's the voltage that's regulated, which is for me synonymous with low-Z; when it's derived from current (series), it's the current that's regulated, which means high-Z.
When NFB is applied as a voltage (series) it decreases the sensitivity without changing the input current so it increases Z, when it's applied as a current, this current acts against the source current, so it decreases the input Z. It may seem tedious, but in fact,  I don't even think about it anymore.

Well, I don´t call that exactly *a very simple set of rules*, but maybe that´s just me 
/on topic (I know I shouldn´t overplay this )

 

[alexc]

I admit to a certain 'schadenfreude' regarding the feedback when using 'schade' terminology  8)

Its very Freudian.

....

I have a tube build on my bench at the moment with a pentode first stage driving a strong triode second stage - I think I might play about a bit with the Lorenz Mic Pre feedback arrangements.

A little different in my app; hiZ input to grid with a 5963  'special red' octal metal pentode and a 6sn7 triode stage.

Be interesting to measure with/without the nfb(s) ...  and compare with my usual direct couple connection.

 

[abbey road d enfer]

- I think I might play about a bit with the Lorenz Mic Pre feedback arrangements.

As Ian mentioned often, controlling gain with NFB over a large range causes serious stability problems. I suspect the  Lorenz designers chose to apply local NFB to the output stage in order to reduce the Open-Loop Gain.
This is something that can be done with tubes because the distortion is acceptable with a much lower level of global NFB than would be necessary with transistors.

 

[moamps]

II've cleaned up the schematic and translated it but I've left the stuff in the drawing box as is. 

You did some errors, IMO.
R4 shoud be 1k6,
left switch,  unconnected positions should be connected to ground,
right switch, positions 20, 25 and 30 should be connected together

 

[DaveP]

You did some errors, IMO.
R4 shoud be 1k6,
left switch,  unconnected positions should be connected to ground,
right switch, positions 20, 25 and 30 should be connected together

Yes, you are right,! Thank you for taking the time to check it. I'll make the corrections later today and re-post.

It is very difficult to see decimal points on this old drawing, missing the switch connections was just an error on my part.

DaveP

 

[DaveP]

OK, I have corrected the original drawing post.

Apologies to those who copied the first one, you will need to do so again.

Thanks to Moamps for spotting the error, in fact the voltage on the cathode of V1 is decipherable now as 1.6V, I had not felt confident enough to include it before.

DaveP

 

[abbey road d enfer]

in fact the voltage on the cathode of V1 is decipherable now as 1.6V,

That would imply 1mA cath current, which is not consistent with Ia=0.62mA and Ig2=0.11mA, resulting from the other voltage measurements.  The plot thickens...  ???

 

[Ericbazaar]

That would be very helpful Eric

Please post as many pix as you like.

DaveP

Ok, i take pictures today and will post them soon. My two Preamps are modificated by "RFT Technicans" i think, will draw the Mod in Schematic and post here too.

 

[moamps]

@DaveP
You did a great job. Just one small correction. It looks that +B2 (320V) is connected to the ground.
@Abby
The voltage drop on  R18 (47k) is 50V so I think that 1mA for first tube's cathode current is a good assumption.
I agree that in later measurements first and second kirchhoffs are not so happy.

 

[Ericbazaar]

Front

 

[Ericbazaar]

Inside Topview, the Power Transformer is shielded by a big Mu Metal Can.

 

[Ericbazaar]

Input Transformer EÜ333 is shielded too, i've measured a ratio of 1:20!

 

[Ericbazaar]

The Gain Switch, is shielded again with a Hood normally. Reinstalled for make Pictures