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Old 28th November 2010, 10:19 AM   #511
AndrewT is offline AndrewT  Scotland
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Hi,
do not put the "ground" on either cap pair.
These links carry very high pulse currents back and forth to the transformer via the rectifiers.
Take a short, or longer, lead from the last pair and place the Audio Ground on this lead. This isolates the Audio Ground from the voltage variations that accompany the pulsing currents.

The reason for not tapping into the first cap pair is that what initially looks like a C//C filter is actually an rC(l+r)C filter.
The first r is the resistance of the transformer windings +wiring +rectifiers.
The l+r is the resistance and inductance of the lines between the capacitors. These impedances can be very small but they are not zero.

If one compares that rClrC to a more conventional two stage smoothing using RCLC then it is obvious that one never taps off smoothed DC from the first capacitor.

Always tap off the last C and keep the wiring from transformer to rectifiers and from rectifiers to smoothing short and twisted.
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Last edited by AndrewT; 28th November 2010 at 10:27 AM.
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Old 28th November 2010, 12:15 PM   #512
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Are there any downsides on 'inverting' the frontend?
I mean flipping over the FE horizontally and using 2sk's in stead of 2sj's, leaving the CCS up to a 2sk and using an Nmos? The noise figures look the same for the sj and sk's. Or is this just a silly and bad idea ?
The reason why is that I won't be needing the hard-to-get 2sj74's-2sj109's.
I have intensively searched on this forum but i could not find any thoughts on this... I have attached a rough sketch.
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Old 28th November 2010, 07:58 PM   #513
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[The reason for not tapping into the first cap pair is that what initially looks like a C//C filter is actually an rC(l+r)C filter.
The first r is the resistance of the transformer windings +wiring +rectifiers.

Andrew,
Does this mean that Nelson's 2R2K 'bleed' resistors on the BA2 power design should be placed as in the circuit diagram, before the first capacitor rather than after the 10uF cap which is where I've put mine or are you referring to series resistance?
Regards,
Chris
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Old 28th November 2010, 08:29 PM   #514
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Backbones View Post
.......Does this mean that Nelson's 2R2K 'bleed' resistors on the BA2 power design should be placed as in the circuit diagram, before the first capacitor rather than after the 10uF cap which is where I've put mine or are you referring to series resistance?
I was referring to series resistance in the rCrLC example I gave.
The charge bleed down resistors are a different animal inserted for a completely different reason.
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Old 28th November 2010, 08:39 PM   #515
reytnz is offline reytnz  New Zealand
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[QUOTE=AndrewT;2379573]Hi,
do not put the "ground" on either cap pair.
These links carry very high pulse currents back and forth to the transformer via the rectifiers.
Take a short, or longer, lead from the last pair and place the Audio Ground on this lead. This isolates the Audio Ground from the voltage variations that accompany the pulsing currents.

Hi AndrewT,

From which junction is the best place to tap the 'psu ground' which connect to the Thermistor to AC 'Earth'?
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Old 28th November 2010, 09:01 PM   #516
AndrewT is offline AndrewT  Scotland
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Originally Posted by reytnz View Post
From which junction is the best place to tap the 'psu ground' which connect to the Thermistor to AC 'Earth'?
what a mix up of names/labels in the question.

The PE (third wire in the mains cable) connects to chassis. I call this Safety Earth. I deliberately never call anything else "earth"

The Main Audio Ground connects to chassis.

Do not confuse terms, you will confuse us and your self.
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Old 28th November 2010, 11:38 PM   #517
reytnz is offline reytnz  New Zealand
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Quote:
Originally Posted by AndrewT View Post
what a mix up of names/labels in the question.

The PE (third wire in the mains cable) connects to chassis. I call this Safety Earth. I deliberately never call anything else "earth"

The Main Audio Ground connects to chassis.

Do not confuse terms, you will confuse us and your self.
My apologies if my query caused confusion. You have now an idea where my electronics understanding lies, My english is not that good aswell. This grounding thing is very confusing to me, as far as I know, there are 3 separate ground inside an Amplifier ( not sure ). " Safety Earth", "Power Supply Unit ground" and the " Main Audio Ground ". This is my understanding, please correct me if I'm wrong. The "Safety Earth" wire from the mains cables is bolted to the chassis, The "PSU ground" is the junction in the capacitor bank ( last pair of caps ) and the "Main Audio Ground" is a point ( star ground ) somewhere connected by wire to the PSU Ground. My question is : Where is the optimum point to put the thermistor? this is the thermistor that the other end is connected to safety earth. I guess to the capacitor bank ( last pair of caps ), not to the " Main Audio Ground"

Hi Andrew,

You said, " The Main Audio Ground connects to chassis." Thats not what it says in Mr. Pass schematics. please enlightened us.

Cheers,
ReyT
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Old 28th November 2010, 11:52 PM   #518
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by reytnz View Post
.........This grounding thing is very confusing to me, as far as I know, there are 3 separate ground inside an Amplifier ( not sure ). " Safety Earth", "Power Supply Unit ground" and the " Main Audio Ground ". This is my understanding, please correct me if I'm wrong. The "Safety Earth" wire from the mains cables is bolted to the chassis, The "PSU ground" is the junction in the capacitor bank ( last pair of caps ) and the "Main Audio Ground" is a point ( star ground ) somewhere connected by wire to the PSU
this is all correct.

The Main Audio Ground, or any part of it that is capable of passing Fault current, should be connected to chassis.
This connection should never carry significant current. Leakage current to Earth is about all it will pass in operating conditions.
Download the grounding article on our Home page and read it thoroughly.
I think all the answers are in there. Well, I can't think of any that could be left unanswered.
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Old 29th November 2010, 12:36 AM   #519
reytnz is offline reytnz  New Zealand
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...Thanks
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Old 30th November 2010, 10:31 PM   #520
JimM is offline JimM  United States
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Default BA2 cascode front end

I have 3 pairs of cascode front end boards (requires one board per channel) that I will let go for what they cost me $10.00 per board plus shipping. The boards were laid out with 7.5mm spacing for Caps 205,206, and 208, which allow for higher voltage caps. Resistors R205,208,209,210,211 laid out for 3watt. The cascode transistors are laid out for ZTX550 PNP, with a voltage divider circuit for ZTX550 the remainder of the circuit is BA2 front end. The boards aree 3.9 in by 3.8 in.
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