Topic Options
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 153154155156157 345> |
| Author | |
Graham Slee 
Admin Group 
Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
 Post Options
 Thanks(1)
 Quote Reply
 Posted: 19 Sep 2021 at 3:08pm |
|
Return of the bootstrap. 
Edited by Graham Slee - 19 Sep 2021 at 3:10pm |
|
|
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(0)
Quote Reply
Posted: 19 Sep 2021 at 7:01pm |
|
It is getting considerably harder to explain what's happening dynamically. R5 being much larger than R6 seems to be the "shock absorber" here. At least T1's current is being maintained close to the wanted 2.3mA, falling by only 3% at full power supply load, so the slew-rate doesn't seem affected from minimum to maximum load.
|
|
|
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(0)
Quote Reply
Posted: 20 Sep 2021 at 9:50am |
|
It is at this moment where I think we have discovered the real Achilles heel of single supply power amplifiers. That is to say, where the power supply is unregulated.
There are two commercial power amplifiers using this self-same configuration, and they both use regulated power supplies. The problem is in making the DC at the output stay the same independent of load fluctuations. Regulation of the IPS plus the VAS current source almost pulls the trick off, where the output stage DC keeps within two or three volts of where it should be. That being from no-load to maximum load. It renders the DC stability of the two transistor feedback pair useless, but there is the DC coupled global negative feedback in there, which is the only mechanism left that can contribute to DC stability. So why have the two transistor feedback pair? You need two transistors for IPS and VAS, but can it be done differently? Well, yes, and many did. So perhaps I've been taught a hard lesson. In my defence, and as I've said throughout, nobody ever documented the reasons why things were done one way or another - not until the LTP differential input came along (which Bailey could have used, but didn't - we are individuals after all). But, even if we use a different voltage stage, such as Leak (Dinsdale), would there be any difference? They all use decoupling electrolytics, which have long time constants, during which the DC must depart from its ideal. The dynamics here need to be understood as relative rather than absolute. Modern design establishes absolutes which only (fully) work in a dual-rail domain. To establish the real relativity between single supply IPS and power supply becomes quite tricky. |
|
|
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(0)
Quote Reply
Posted: 20 Sep 2021 at 12:14pm |
|
There seems to be only one way to prevent the movement of the output DC, and that is to regulate the supply to the voltage amplifier (IPS and VAS), otherwise we are left with a 6.5V step between no load and maximum load.
|
|
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
|
|
|
Sylvain
Senior Member 
Joined: 18 Jan 2010 Status: Offline Points: 481 |
Post Options
Thanks(0)
Quote Reply
Posted: 20 Sep 2021 at 7:00pm |
|
I have been quietly reading and trying to understand and following the posts of the last weeks but now ...!!!!!!! '' single power supply '' or dual or regulated or Not ........
|
|
|
|
|
Graham Slee
Admin Group
Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
Post Options
Thanks(0)
Quote Reply
Posted: 21 Sep 2021 at 10:24am |
|
Signal dynamics will cause the power supply voltage to fluctuate.
What we (often?) don't see is the effect this has on the positive end of the output capacitor. What happens on the positive end lasts for a while on its negative end, that "while" being the time constant of the cap and speaker impedance. Within that time constant, lots of other frequencies happen - high frequencies. It is therefore important that the DC voltage on the negative end of the output capacitor stays at 0V. Unfortunately it doesn't! You might expect that with on to off load power supply variations of say 8 volts, for the positive end of the output cap to see a difference of 4 volts - because it's set to roughly half of the supply. Would you expect to see 6.5 volts? The only way of coming to terms with that, is to understand that the DC biases shift, and in doing so, it might register that at DC there is an equivalence to instability - because the biases are constantly adjusting through NFB to suit the changing power supply voltage. These biases, are the two biases in the voltage amplifier - the IPS and the VAS. If these were supplied by a regulated supply, there would be no change, and therefore, the output stage would simply follow that, meaning no change to the output capacitor. At present, that is not the case. The low volume we listen at domestically is insufficient to make the power supply voltage vary by a large degree, but the change is still there, but on a much smaller scale. Therefore, the distortion heard is small, but not insignificant. By messing about with a VAS constant current source and rudimentary IPS voltage regulation, the audible distortion fell away. Measurably, the third harmonic did also. Therefore, I have to come up with a method of regulating the voltage amplifier side of the HT. |
|
|
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)
Quote Reply
Posted: 22 Sep 2021 at 10:14am |
|
There's a member asking if the Proprius is bright. One day I hope he'll see why I spend so much time and effort - because I can't stand bright either.
I guess there are no reviewers reading these pages. They don't want to learn why "two measurably identical amps can sound different." It could lead to the end!  Brightness here refers to a distortion, that if you solely rely on state of the art analysis, you will never find. I have rock-steady intermodulation - the best yet! It means nothing if it leads to tinnitus. We're told we can just about hear 60dB S/N - this is 70dB S/B (signal to brightness) - but I told you why the reading is false the other day. Today I will order some TL783 voltage regulators to use in regulating the voltage stage. |
|
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
|
|
|
Post Reply
|
Page <1 153154155156157 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 |
Forum Software by Web Wiz Forums® version 12.01
Copyright ©2001-2018 Web Wiz Ltd.
This page was generated in 0.143 seconds.
Copyright ©2001-2018 Web Wiz Ltd.
This page was generated in 0.143 seconds.