Copyright © 1998
Cadman Enterprises Ltd
Welcome to the Graham Slee Audio Products Owners Forum Open to all owners plus those contemplating the purchase of a Graham Slee HiFi System Components audio product and wishing to use this forum's loaner program: join here (Rules on posting can be found here) This website along with trade marks Graham Slee and HiFi System Components are owned by Cadman Enterprises Ltd |
1970s Design Indulgence |
Post Reply | Page <1 151152153154155 345> |
Author | |
Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
Post Options
Thanks(0)
|
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.
|
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
peterb
Senior Member Joined: 02 Feb 2017 Location: Cornwall Status: Offline Points: 332 |
Post Options
Thanks(0)
|
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 |
|
Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
Post Options
Thanks(0)
|
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. |
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
Post Options
Thanks(1)
|
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)
|
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
BackinBlack
Senior Member Joined: 05 Feb 2012 Location: Hinton, N'hants Status: Offline Points: 2020 |
Post Options
Thanks(1)
|
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.
|
|
Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
Post Options
Thanks(1)
|
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. |
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
Fatmangolf
Moderator Group Joined: 23 Dec 2009 Location: Middlesbrough Status: Offline Points: 8998 |
Post Options
Thanks(0)
|
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. |
|
Post Reply | Page <1 151152153154155 345> |
Tweet
|
Forum Jump | Forum Permissions You cannot post new topics in this forum You cannot reply to topics in this forum You cannot delete your posts in this forum You cannot edit your posts in this forum You cannot create polls in this forum You can vote in polls in this forum |