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 151152153154155 157>
Author
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: 11130
Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 14 Sep 2021 at 9:15pm
The input stage is well decoupled, or is it?

If the HT drops for long enough, the IPS current falls and the emitter voltage with it. The increase in base-emitter voltage causes the transistor to turn on harder, and the VAS turns off a little. The result at the output is an increased voltage, and not the expected fall in voltage.

I guess the more than 100x NFB blinded my imagination, thinking wrongly as the cure all. True, it is "dead" stable, but it isn't magic.

Talking about voltage regulation, it might make sense for the IPS supply to be regulated - at least its current cannot change once it is. Doing so might reduce the DC step sufficiently.
Not simple enough for Google-Bot to understand...
Back to Top
peterb View Drop Down
Senior Member
Senior Member
Avatar

Joined: 02 Feb 2017
Location: Cornwall
Status: Offline
Points: 218
Post Options Post Options   Thanks (0) Thanks(0)   Quote peterb Quote  Post ReplyReply Direct Link To This Post Posted: 14 Sep 2021 at 10:46pm
That sounds familiar, the 1968 Bailey 30W Amp used a transistor to do something similar.
Peter
---------------------------------------
Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2
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: 11130
Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 2021 at 12:58pm
The IPS is now regulated in the simplest way possible: resistor and Zener.

The DC operating point step is now in the expected direction (downward on-load), and only moves by two volts, min to max load.

The DC operating point is now set at 40V off load, falling to 38V on load. The off-load HT is about 79V, so at low output (normal domestic listening levels) the output is more symmetrical. On load, it is slightly less symmetrical, but at the high levels that would give, the change to asymmetry won't be so noticeable. The transients are very short anyway.

THD1 (1kHz) is better than 0.03% ref 1W, and THD16 (16kHz) 0.045%.

Maximum output test for 0.1% gave 44WPC into 8 ohms. Still using the 160VA transformer, 230V primary on 248V 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: 11130
Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 2021 at 1:25pm
Originally posted by peterb peterb wrote:

That sounds familiar, the 1968 Bailey 30W Amp used a transistor to do something similar.

The 1968 Bailey 30W Amp used a shunt regulator in the IPS base bias circuit, and instead of being fixed like a Zener, it was a "variable transistor Zener" able to "slide" with the time constant of the RC network on the transistor's bias. Whether  or not this allowed the output to maintain full DC output stability due to the signal conditions I explained earlier, is debatable. A commercial version of the design was sold by Radford (Series 3?). Radford had the advantage of knowing how to make good transformers! (see http://www.audiomisc.co.uk/HFN/Radford/revisited.html)
Not simple enough for Google-Bot to understand...
Back to Top
BackinBlack View Drop Down
Senior Member
Senior Member
Avatar

Joined: 05 Feb 2012
Location: Hinton, N'hants
Status: Offline
Points: 1715
Post Options Post Options   Thanks (1) Thanks(1)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 2021 at 2:45pm
Graham,
This has been a long forensic examination of the seventies design obviously requiring dedication and single mindedness. I'm pleased to see that the end appears to be in sight, although I'll miss reading each new chapter as investigations progressed.
I don't doubt that at the time of the original design quite a lot of investigation and head scratching went on in "perfecting" the design.
Do you think that many of the now apparent shortcomings might have been identified with the aid of today's test/measuring equipment and circuit analysis software, or was development time curtailed to allow production to proceed?

Ian
Just listen, if it sounds good to you, enjoy it.
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: 11130
Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 2021 at 6:44pm
Originally posted by BackinBlack BackinBlack wrote:

Do you think that many of the now apparent shortcomings might have been identified with the aid of today's test/measuring equipment and circuit analysis software, or was development time curtailed to allow production to proceed?

Ian

Some things maybe, but in the transition from valves to transistors there must have been some misunderstandings. An example being Dinsdale's admission that he'd not considered the heavier ground current, which he found to be a problem when converting his amp to stereo. You can see the same in the Quad 303 (R115), and also the use of a regulated supply. Decca research "solved" the grounding problem, but not as completely as Ed Cherry saw it. Even Doug Self made the comment that after 40 years of amplifier development, nobody could claim it perfected.

There were solutions for the unmeasurable from John Linsley Hood, who was sometimes lambasted in letters to the (WW) editor, but Mr Vereker took note and it did well for him. I'm pretty sure everybody was learning, and not just from measurements. Then there is the discredited Matti Otala and his "overshoot" explanation. I still think he was right in some ways, just that he presented the "proof" with shaky evidence. I don't think anybody can claim to fully understand everything about amplifiers, simply because we can't measure the thing it's supposed to do - the music.

Obvious faults were not all that obvious for the designers of the day, and you can see how I've missed plenty over the last 150 plus pages. Learning is an upward spiral where we often coming back to a previously discarded solution, finding it useful next time, because the reasons gradually reveal themselves.

Op-amp similarities with modern power amplifiers firmly shoved earlier circuits out of the way. And I guess the reason was, as you suggest, to curtail development time when things got too problematic. Even so, designers like Bob Cordell have developed the mathematics which also work to analyse and correct these older designs - in many cases allowing simple calculator sums with which the simulator agrees.

Even with seemingly endless development time, we cannot give every answer, and I think the Radford - Bailey story is just one of many to show that. We can only conjecture that if it works, it works like this... until we discover otherwise.

Not simple enough for Google-Bot to understand...
Back to Top
Fatmangolf View Drop Down
Admin Group
Admin Group
Avatar

Joined: 23 Dec 2009
Location: Middlesbrough
Status: Offline
Points: 6150
Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 15 Sep 2021 at 8:00pm
Excellent work.
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
Back to Top
 Post Reply Post Reply Page  <1 151152153154155 157>
  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 0.188 seconds.