New Posts New Posts RSS Feed - 1970s Design Indulgence
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Welcome to the Graham Slee Audio Products Owners Club

 

Open to all owners plus those contemplating the purchase of a Graham Slee audio product wishing to use our loaner program: join here
Subscribe to our newsletter here (Rules on posting can be found here)


1970s Design Indulgence

 Post Reply Post Reply Page  <1 545556
Author
DogBox View Drop Down
New Member
New Member


Joined: 10 Feb 2019
Status: Offline
Points: 32
Post Options Post Options   Thanks (0) Thanks(0)   Quote DogBox Quote  Post ReplyReply Direct Link To This Post Posted: 16 Apr 2019 at 12:54pm
Originally posted by BackinBlack BackinBlack wrote:

Steve, have you seen this thread - https://www.hifisystemcomponents.com/forum/phono-preamp-pt2-mc_topic1459_page1.html ?

 

Nah, didn't get to hone my knowledge on that one..! Was right into it when I heard some replies had been writ. 

I hope I described the Theil circuit properly..?! No, it doesn't oscillate. I probably screwed up in my description as some other oscillations come flooding back! 

I can only think of a reason why someone might want things BRIGHT was if they had ears that were dull. 
I wanted to hear for myself what a certain frequency 'tone' sounded like and "if" I had the pitch where I thought it would be... Embarrassed What a revelation THAT turned out to be! Then I found an Increasing Tone which started so low it took a while for my desktop computer speakers to start giving me something to listen to! Then, right before my very ears the sound vanished where I thought I had "good hearing" in that range! Nothing. Zip. And I had only just been for a hearing test a week previous. Damn! Well, soon went looking for something else to listen to - something that didn't make me feel so bad! 
Back to Phono Preamp - Part the two. 

DogBox   
Back to Top
Graham Slee View Drop Down
Admin Group
Admin Group
Avatar
Telling it as it is

Joined: 11 Jan 2008
Location: South Yorkshire
Status: Offline
Points: 8140
Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Apr 2019 at 8:05pm
So, in serious tuning mode I examined the bootstrap.

Initially I thought about replacing it with a current source, but it can be seen that there is a DC bias problem using one with this configuration.

Just in case you cannot see it, should the supply voltage vary, as it would with load and signal conditions, T2's emitter voltage would grow or shrink in sympathy, meaning its base voltage would vary, and if it were a low DC gain input stage such as a LTP or PNP "singleton", it would oblige - but not with the high DC voltage gain of this configuration.

(Hmmm, I wonder if a regulated supply to T1 might help in other respects?)

So, thinking about the "quality" of bootstrap "constant current", I thought about the amount of possible distortion being passed from the output back to the voltage amp output. If the signal isn't identical to that of the voltage amp, and it won't be, we have a kind of "oozlum" condition.

This self-reference really needs to be of the least distortion, and seeing as it's the upper EF output stage which gets the real benefit (or not), taking the bootstrap origin from the driver emitter rather than from the output, should see much less distortion.

At least, if it's going to be bright, it ought to sound clean.

So, at the same time I thought I'd take a look at the zero crossing LF phase, and decreasing the bootstrap value from 100uF to 47uF afforded a few extra degrees in bass phase margin. A constant current source (if it were to work) would result in much flatter bass phase.

Another trick often used is to bypass the bias generator with a large-ish value capacitor, so whilst I had the 47uF box open, in went those.

Regarding thoughts on a stabilised supply, I quote John L Linsley Hood from his book Audio Electronics: "The principal sonic advantages which this characteristic brings are a more 'solid' bass response"
Not simple enough for Google-Bot to understand...
Back to Top
Graham Slee View Drop Down
Admin Group
Admin Group
Avatar
Telling it as it is

Joined: 11 Jan 2008
Location: South Yorkshire
Status: Offline
Points: 8140
Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Apr 2019 at 9:47pm
A thought occurred about if this was modified to a fully complimentary output stage, what would the differences be?

It was found that a 22pF T2 collector to T1 emitter stability capacitor would be required instead of the 10pF with the quazi-complimentary stage, for a near 90 degrees phase margin.

Now, this is according to the simulator, and by now my trust in simulation is wearing thin. So is the quazi stage behaving like a complimentary stage in the real world?

If it were, then it's only got 56 degrees phase margin, which tends to sound bright. I suppose there's only one way to find out. And in went a 22pF, so now we have to wait, again.
Not simple enough for Google-Bot to understand...
Back to Top
Graham Slee View Drop Down
Admin Group
Admin Group
Avatar
Telling it as it is

Joined: 11 Jan 2008
Location: South Yorkshire
Status: Offline
Points: 8140
Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Apr 2019 at 11:55am
The collected wisdoms, and then some

Research, not bordering, but exceeding the obsessive in designing an amplifier (any solid-state will do!), led me to studying the works of the masters: Self; Cordell; Hood; and even the occasional thought of Vereker.

(The above being responsible for more than 50% of commercially available solid-state hi-fi amplifiers - the big companies.)

But, although all their methods actually work, if the amplifier is left on for 60 - 100 hours, using it for say 30 - 50 hours, and just left on for the rest of the time, they always go bright, thin, etched, lifeless (add your own synonym).

To reach 100 hours an amplifier only used for a couple of hours a week, and then religiously switched off, will take a year. But does it really reach the same kind of 100 hour burn-in as just leaving it permanently on and playing music 50:50?

Perhaps it will take a few years of "normal use"? Then after a "re-cap" it will do another few years? (Am I onto something here?)

Is this why I always saw Quad 303's (regulated power supply) and Quad 405's (you don't get parasitics with current dumping) permanently on in the broadcast installations I witnessed?

OK, yes, I designed a broadcast studio power amplifier, which was sold, but I never received any feedback one way or another. I'm not as "famous" as Self et-al.

Bruce has owned a Dynaco for years which uses the same config as here, but uses a regulated power supply and he hasn't complained of the symptoms I find.

The Proprius stay on for months at a time - no problems here!

So, I have to wonder if the modern way of unregulated power supply designed solid-state power amplifiers is any good? And does their eventual state result in upgradeitis? And if so, it will always be a road to nowhere, no matter what garbage the review magazines spew.

So what next? Well, I guess it will have to be a regulated power supply.
Not simple enough for Google-Bot to understand...
Back to Top
Graham Slee View Drop Down
Admin Group
Admin Group
Avatar
Telling it as it is

Joined: 11 Jan 2008
Location: South Yorkshire
Status: Offline
Points: 8140
Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 20 Apr 2019 at 8:16pm
Giving it one more chance, probably because I'm just a tad tired and don't really want to build a regulated supply (OK, call me lazy if you want), I thought I'd have another simulation session.

I wanted to see the effect altering the output stage bias would have. The problem I had was in getting the right voltages across the emitter resistors. I was down to just two diode junction drops and still had nearly 100mV across them.

A quazi-complimentary output stage requires 3 diode junction drops - or 1.8V - to be just on. 2 diode drops should have it just about turned off with virtually no voltage across the emitter resistors.

I measured the base emitter voltages of the output transistors and found them close to 0.6V which is correct for silicon transistors. Then I measured the BD139 and BD140, which to my amazement were functioning normally with only 0.3V bias.

Good job I'm the only one working in here as I shouted f*** off! The expletive is meant for the n*b at ST who wrote the simulation model, leaving out the bias voltage!

OK, maybe I should have checked, but I trusted the manufacturer. What's that? "Cursed is he who trusteth in man"? Too true!

I have to thank Bob Cordell for making available to all, his versions of the simulation models I require through his website. All other devices were his versions, and now the BD139 and BD140 are also his.

Did things change? Well, yes. I was able to set up the output stage quiescent current (Iq) by adjusting for the real world emitter resistor voltage drops.

There will be a set-point of between perhaps 15 - 25 mV which equates to 45 - 75 mA, which 1. gives the best third harmonic on test; and 2. at that setting the phase and gain margins will differ from those at other settings.

At that point I can see how the output behaves into different capacitive loads, seeing that a capacitive load sets a response pole which can upset stability due to negative feedback.

By the looks of things the most recent stabilisation method - the JLH type - has not been providing sufficient phase margin due to the values indicated in simulation not working out in practice. And maybe I now know why?
Not simple enough for Google-Bot to understand...
Back to Top
Farmer View Drop Down
New Member
New Member
Avatar

Joined: 24 Dec 2010
Location: Soggy NW of UK
Status: Offline
Points: 12
Post Options Post Options   Thanks (0) Thanks(0)   Quote Farmer Quote  Post ReplyReply Direct Link To This Post Posted: 20 Apr 2019 at 10:10pm
Sounds like you had a bit of a “eureka” moment there..
It might just turn out to be the discovery that explains a lot.

I had a similar moment today having arranged to borrow a friends much more powerful tractor - because we want to “mole” in some pipe about 2’ down (so it doesn’t get ploughed up). 
Went round to collect tractor, and found keys where they’d been left for me. Then spent 25 minutes trying to get the thing to start, in neutral, foot on brake, turn key - loads of lights, but nothing doing..
Was about to admit defeat and make the call of shame to the friend, when I realised the PTO switch was engaged... 
My eureka moment..
Tractor borrowed, pipe ‘moled in’ and tractor returned. 
Did reactivate the PTO switch on leaving it - a bloody good theft deterrent!!
Back to Top
 Post Reply Post Reply Page  <1 545556
  Share Topic   

Forum Jump Forum Permissions View Drop Down

Forum Software by Web Wiz Forums® version 12.01
Copyright ©2001-2018 Web Wiz Ltd.

This page was generated in 0.063 seconds.