Partial Feedback Amplifiers

[audiousername]

Re: Re: Fun With Feedback

dry resistance??

I think he might mean unbypassed resistance in the cathode circuit, thus causing degenerative feedback.

 

[SY]

Alex, if the diversion is offensive to you, I can split it out into a separate thread. Just say the word.

Choky, I mean time to recover bias after overload- but thinking it through further, that's probably irrelevant for a battery. I wonder about how overloads might affect the life, though.

 

[Brett]

Alex, if the diversion is offensive to you, I can split it out into a separate thread. Just say the word.

How about he just gets over it? Threads are organic and tend to take on a bit of a life of their own and meander about. It just makes life more interesting. So, are you going to split every thread when someone whinges that it doesn't stay on-topic enough for them?

 

[SY]

When requested. And especially when I'm the naughty person who took it off topic.

 

[markanica]

Re: Partial Feedback issues

greetings all,

Yves and Alex thanks for your constructive replies,

I've played too with various implementations of this design with more or less success.
Interresting results where obtained in using a somewhat classic SRPP configuration for the driver, feeding the grid of the output stage from the upper cathode and returning the feedback resistor to the plate of the lower anode.
The two cathode resistors of this SRPP may be made unequal, since the FB resistor adds current in the lower portion alone.
[/B]

Nice idea of using an SRPP here, but also I see the feedback resistor is returned to a lowish impedance, wich will fight the I to V conversion mechanism to a point. JB states in his article on partial feedback: "The key point is that the impedance looking into the convertor must be high, so as to give the grid the freedom of movement required to convert the incoming current to voltage."

For now, it's what I plan to check using a penthode as driver and a -100v auxiliary supply.[/B]

This will probably better meet the required conditions of an I to V convertor. I want to try something like that as well.

And ... euh ... the huge voltage drive needed by the 6AS7 complicates the problem more than a lot but I understand this choice is to cope with your speaker.
I beleive that the higest the Mu, the better for the power tube.
[/B]

Yeah, I see your point. I play with whatever I have on hand, so 6AS7 it will be for now.
In the current setup the drivertube EC8010 can supply the grid up to about 38V input, before distortion due to grid current starts. On its own, the driver can deliver well over 70 Volts!
The 6AS7 is biased at -56V, so more power can be had, but at a price. A much more complicated circuit, that can easily undo the magic that comes through as it is now! But I will know when I try.
Exploring possibilities of circuits is a big part of the fun here!

Mark,

I think your schematic showing a cathode follower between the driver and the output valve, and a feedback loop from the anode of the output valve to the grid of the cathode follower would work. It's the only solution to the conflicting requirements of low output impedance for wide frequency response and high output impedance for high levels of local feedback I've seen. But now we're wrapping feedback around two stages instead of one, so we have to be careful about possible phase shifts and things because we have more components in the feedback loop.

Thanks for your support . It's sometimes difficult to 'see' the workings of a circuit...so I will set it up and see how it goes.....
I'll post when I have a result...


OT : I still can't figure out how to include images in a posting;
When I click the IMG button, a 'scriptprompt' appears. How to proceed from there?? Anyone?

 

[markanica]

Re: Re: Partial Feedback issues

OT : I still can't figure out how to include images in a posting;
When I click the IMG button, a 'scriptprompt' appears. How to proceed from there?? Anyone?

Never mind, found it by just clicking the help button and read text

 

[markanica]

Hi All,

Hi Mark

I've played too with various implementations of this design with more or less success.
Interresting results where obtained in using a somewhat classic SRPP configuration for the driver, feeding the grid of the output stage from the upper cathode and returning the feedback resistor to the plate of the lower anode.
The two cathode resistors of this SRPP may be made unequal, since the FB resistor adds current in the lower portion alone.

Yves, Alex and everyone else,
Can't stop thinking about alternative I to V convertors and since Yves started about SRPP I came up with another one... I'm not sure if it can work, but don't see anything against it
Sorry, I didn't do the math, never was good at that, altough I can read formulae... I mostly use my imagination...

Well, check it out and let hear what you think or

greetings

 

[Yvesm]

Hi All,
Yves, Alex and everyone else,
Can't stop thinking about alternative I to V convertors and since Yves started about SRPP I came up with another one... I'm not sure if it can work, but don't see anything against it
Sorry, I didn't do the math, never was good at that, altough I can read formulae... I mostly use my imagination...

Well, check it out and let hear what you think or

greetings

It's exactly what I've done ! With other tubes indeed
Read back each world of my last post

Yves.

 

[fdegrove]

Hi,

Well, check it out and let hear what you think or

No need to smash me but I think_as a matter of fact I know_you can do much better than that in every single department:

Use an Aikido circuit for I/V conversion: you'll have much reduced cost, better PSRR, less distortion, no OPT to mess up the BW and
Zout will pretty much depend on the tube used for the WCF.

I'd still pay attention to the PS, there's no such thing as a free lunch even if mathematical models would have you believe otherwise.

Have a look:

AIKIDO

Cheers,

 

[sanford1]

That whole AIKIDO stream of consciousness is a real gem, there is a LOT in there! Broskie strikes again.

 

[markanica]

Hi All,

It's exactly what I've done ! With other tubes indeed
Read back each world of my last post

Hi Yves,
So I did reinvent the wheel!

Actually what you wrote was the following:
'Interresting results where obtained in using a somewhat classic SRPP configuration for the driver, feeding the grid of the output stage from the upper cathode and returning the feedback resistor to the plate of the lower anode.'

Whereas I suggest to return the feedback resistor to the grid of the upper triode. You can check that in the image.
The difference is just a single connection but it makes a big difference in the workings of the circuit....
Of which I am really curious what it will do, or not do at all. But that's just me marvelling at my own 'inventions'
(or has this been done 50 years ago?
I soldered it up tonight, spend some time getting oscillation out of the way. Oops, o..yeah, it's a grid with a 1Meg connected, quite sensitive. But haven't finished yet, still have to lift the 5687heater and it's getting late....
I'll get some more benchtime after tomorrow......


Hi Frank

Originally posted by fdegrove [/i]

No need to smash me but I think_as a matter of fact I know_you can do much better than that in every single department:

Well Frank, You give me smashing! ideas
It might help....

Originally posted by fdegrove [/i]
Use an Aikido circuit for I/V conversion: you'll have much reduced cost, better PSRR, less distortion, no OPT to mess up the BW and
Zout will pretty much depend on the tube used for the WCF.:

I did read JB's Aikido articles, but sofar missed the I to V convertor in that circuit. Of course I'm not an experienced EE, so I maybe I didn't see it, when it was right in front of me...

The thing, that I want to do is make an amp for an 800 Ohm speaker (Philips AD3800AM or 9710AM)
Since I very much like OTL's, I chose to go SE OTL, using one parallelled 6AS7 tube per channel.
I think, that must be doable, in fact I got it running already, getting 4.5 Watt out of it. pls.check att. schematic.
But now I am exploring how to use I to V in that circuit, which is a new adventure to me....

Originally posted by fdegrove [/i]
I'd still pay attention to the PS, there's no such thing as a free lunch even if mathematical models would have you believe otherwise.

Yeah, I started off doing everything chipregulated for convenience, but I noticed that whatever the tube does, the supply must counteract to hold the B+ steady. That is audible as a transsisstor sound. Well... As i'm not done changeing the circuit, the ps has to wait for improvement

Originally posted by fdegrove [/i]
Have a look:

AIKIDO

Great reading, I love it!

Bye for now

 

[gingertube]

More Examples for your possible interest

There are a couple of examples of the SRPP ish method here (for your possible interest).

http://www.turneraudio.com.au/htmlwebpgs02/schematicsimages.htm

In particular look at the two schematics:
"basic balanced shunt feedback"
"sophisticated balanced shunt feedback"

If you don't want to fuss with a floated heater supply for the top tube use a MOSFET with the gate fixed biased to about half the supply rail.

If you are a tube purist and don't want solid state stuff in the design then a pentode for the top tube will give superior sonic results to a triode - the pentode sonic results will be almost as good as you will get from using the FET.

Cheers,
Ian

 

[fdegrove]

Hi,

If you are a tube purist and don't want solid state stuff in the design then a pentode for the top tube will give superior sonic results to a triode

Keep in mind that these modified SRPP circuits will not work with the Aikido topology.
They'll work alright but you'll lose some of the advantages of the Aikido.

As for superior sonic results of the penthode/triode totems, I'm not that convinced at all.
I often find the simple SRPP the more convincing as far as musicality goes so pick your poison; figures or music....

Cheers,

 

[Yvesm]

Hi Mark !

You'r right ! I didn't notice that the resistor from the lower grid to the upper plate was 1M Looked too fast and beleived it was just a stopper, and this makes difference of course

Yves.

 

[Andrewbee]

Alexsander

hello Alexsander,
I sent you an email at the addrerss listed at the diyaudio.8 site.
Did you get it?

Thanks

Andrew

 

[markanica]

Partial feedback issues

Hi all,

Partial feedback to the grid of the CF part of a SRPP driver looks promising !

I checked today on the bench and it works just fine.
Have to see what its limitations are lateron.

See Att. image how I done it

Bye for now

 

[Alex Kitic]

Andrewbee,
thank you for your e-mail, I did get it -- and will reply shortly, I am just a little over my head with work (I was down with the flu for almost a whole week, and my clients are "going bezerk".

Brett,
Why wouldn't you get over it? Like you sit with friends in a pub and there come some new guys who take over your table, changing conversation topic into something you are not at all interested in, and eventually your wife and kids are waiting for you at home?

SY,
If it was you who changed the topic, why did you do it? Never mind, this way I have a lot more time for myself: whoever wants to contact me, is free to do so. E-mails are more concrete and constructive.

Regards to all,
Aleksandar

 

[markanica]

Hi all,

Aleksander,

I wish you a quick recovery and good health.

I understand your objections, but still want to invite you to share knowhow, so all (me) can have a better understanding of what is going on in partial feedback circuits.

There are a couple of examples of the SRPP ish method here (for your possible interest).

http://www.turneraudio.com.au/htmlwebpgs02/schematicsimages.htm

In particular look at the two schematics:
"basic balanced shunt feedback"
"sophisticated balanced shunt feedback"

If you don't want to fuss with a floated heater supply for the top tube use a MOSFET with the gate fixed biased to about half the supply rail.

If you are a tube purist and don't want solid state stuff in the design then a pentode for the top tube will give superior sonic results to a triode - the pentode sonic results will be almost as good as you will get from using the FET.

Cheers,
Ian

Hello Ian,

I looked over at the Turner website, and yes interesting stuff.
Seems to me that this guy knows what he is doing. I'll keep the link for reference, thanks.

I just lifted the heaters to 70V= and probably won't use mosfets as I killed too many of them just by touching...
As for penthodes, got some in the parts box and will try them if necessary, for now i'll stick to triodes....


I am exploring possibilities with partial feedback and make lots of mistakes on the way. I learn something each time, great!

The last mistake I made was to trust the readings of my DMM.
It can do AC readings, but at 1khz it already deviates. So I used a 400Hz inputsignal and assumed that would be ok.

The DMM reads an output of 1.04 V AC @400Hz, on an input of 1V exact.

This morning I repeated the measurement, but used a Tube AC Voltmeter. And got a reading of 100 mV output.

Back to the DMM again: 1.04 V

Took a reading again, both connected to input : 1V AC on both meters!
Both to output: 1.04V on the DMM and 0.1V on the Tube Voltmeter!
Checked the signal on the O-scope, looks ok.

Repeated the procedure again, now including the O-scope.
The O-scope confirms the readings i get from the Tube Voltmeter, the DMM is way off, but only at the output side. At the input side all readings confirm each other.......

Connected a cap between DMM and output to isolate DC (140V).....Yeah, that's it; now the DMM reads 104 mV AC

Learned something here!


Bye for now

 

[Brett]

Brett,
Why wouldn't you get over it? Like you sit with friends in a pub and there come some new guys who take over your table, changing conversation topic into something you are not at all interested in

Well, it's not your pub, nor even your table in the pub. This is a public forum and members are allowed to post to any thread. Threads that are allowed to find their own way are much more like an active and animated discussion between a group of people where the participants feel free to contribute as they please and are interested, rather than having one person sit and determine what will or won't be discussed. Splitting off continually takes energy away from the flow of the discussion and usually the split or original discussion (thread) will wither, which is a great pity as some of the best stuff comes out from offshoots of the main discussion. If someone want's to rigidly determine the parameters for discussion,, then perhaps they should start their own forum.

This thread is not "Alex's Thoughts on Partial Feedback Amplifiers" but just "Partial Feedback Amplifiers", where I feel it's wholly appropriate to discuss variations in biassing as that is very pertinent to the whole design.

 

[Alex Kitic]

LAST POST

Brett,
you did it, and can take every possible pride out of it.

This thread is not "Alex's Thoughts on Partial Feedback Amplifiers" but just "Partial Feedback Amplifiers", where I feel it's wholly appropriate to discuss variations in biassing as that is very pertinent to the whole design.

Now you can feel whatever you want. The important thing is that you at least know what are you talking about.

Well, it's not your pub, nor even your table in the pub. This is a public forum and members are allowed to post to any thread.

Of course it's not my pub, nor my table -- but until I'm paying for my drink, it's as if it were mine. Ever heard of the maxime "you should not angry someone whom you'd like to ask something"?

Not to loose any more time, this is it. Now go find your answers elsewhere... no wonder JB (whom a lot of you seems to love and appreciate -- but scarcely understands his tought) is so much against forums.

My own forum? No way, man... it's difficult even to find the time to answer to all those good people who write with questions...

Buye, buye (farewell)
Aleksandar