Is Gainclone Direct Coupled? Appropriate Preamp? - Page 3 - diyAudio
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Old 9th April 2011, 11:31 AM   #21
AndrewT is offline AndrewT  Scotland
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I build every amp input like that. For me it is standard.
Quote:
This is the way to connect every AC coupled amplifier.
What is sad is that National do not show this as typical and instead show a stripped down version that can suffer instability as the preferred implementation.

I failed to mention that that input is the start of a star of stars. No loops.
The Signal Ground star can be physically located in a number of positions and the schematic is still identical.
Slide the SG star over to touch j1-1 and you have Leach's/ Self's option.
Slide the star over to the right to keep everything very close and compact next to the -IN pin and it works great for opamps.
Slide the star down and set R3 to 0r0 and you have a monoblock amplifier where Power and Signal are starred on the PCB.
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Old 9th April 2011, 11:56 AM   #22
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In my design I moved my star, so that everything has the shortest possible traces, but also similar length, except the trace with the 10R to ground is quite long but very thick... I suppose a picture is worth a 1000. I see the first resistor's first function as being a cable termination, thats why I place it at the last point any wire connects to, rather than say at the input socket, and then have a long piece of wire to the input cap, which combines as a secondary function of high-passing the signal.
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Last edited by digits; 9th April 2011 at 12:00 PM.
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Old 9th April 2011, 12:43 PM   #23
AndrewT is offline AndrewT  Scotland
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your input layout looks good.
Very small loop around +IN, R2 & R1.
Are there two SMD resistors under the PCB? 1M0 at j1-1 and 1k0 at -IN?

HF decoupling C5 & C6 is good.
MF decoupling is good.
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Last edited by AndrewT; 9th April 2011 at 12:45 PM.
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Old 9th April 2011, 01:01 PM   #24
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Yes its a bit of a hybrid a sort of compromise if you will between available parts, knowledge of best practice, etc...

I'm trying to understand the R2 R3 thing, are you saying it would be more correct to return R2 with the trace from R1 to ground, or that each should have its own return like in the schematic?

Last edited by digits; 9th April 2011 at 01:12 PM.
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Old 9th April 2011, 01:58 PM   #25
AndrewT is offline AndrewT  Scotland
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It might help understanding the input schematic if you temporarily remove R3.

You are left with J1-1 & 1-2 where Signal Hot and Signal Return come in and the two input filters.

The amplifier with it's NFB loop measures the voltage across R2. That signal is magnified and sent to the output.

The signal across R2 must not be contaminated by any non audio signal nor interference.

This is why the Signal Ground must not be directly connected to the Power Ground nor to the PSU Zero Volts point.

Now take that same input circuit and imagine where the current flows from the source, around the input circuit, out through Signal Return back to the source. That LOOP must be signal only. No other signal can be allowed to share any part of that loop sending a non signal current through even a small portion of that LOOP.

Now put back R3. It provides the voltage reference to allow the output to match/follow the input.
Inserting R3 and connecting it to the PSU Zero Volts or to the Power Ground does not allow those other (non signal) currents to enter that LOOP and SHARE a length of that signal loop. The PSU or Power Ground interference arrives at the top of R3 but cannot go anywhere.The contaminating current would need it's own entry and return points in the signal loop. There is a potential "entry" point but there is no exit/return. No contaminating current can flow from Power Ground into the Signal LOOP.
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Last edited by AndrewT; 9th April 2011 at 02:00 PM.
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Old 9th April 2011, 02:31 PM   #26
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Thanks for your patience, I see a few more approaches after reading your post. I will try and see if I can change it closer to what I think you said
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Old 9th April 2011, 03:48 PM   #27
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Ok here it is, hope its better now.
The fat trace below the star is not ground it is just the trace to R3 which is the 10R to ground.
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Last edited by digits; 9th April 2011 at 03:51 PM.
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Old 9th April 2011, 05:22 PM   #28
AndrewT is offline AndrewT  Scotland
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I prefer the previous input layout. I liked the compact +IN, -IN, R2, R4.

Consider the possibility that C7 may need to be physically bigger. 150uF to 220uF electrolytic with a diode to prevent reverse charge >0.7Vdc.

Also consider adding inverse parallel diodes across the 10r to the main audio ground. And maybe a tiny ceramic cap as well.

A slight improvement in operational flexibility can be obtained by adding a signal trace to the previous layout.
Instead of a 0.2" pin pitch screw down terminal block, insert a 3pin single in line 0.1" pin pitch in the same location. Pin 1 remains as you have shown. Pin 2 moves to the cross in between your two existing pins. Pin 3 goes where you currently have Pin 2.

Pin 1 is Signal Return.
Pin 2 is Signal Hot AC coupled.
Pin 3 is alternative Signal Hot DC coupled. This pin is connected to the far side of C8 to bypass the cap and connect direct to R2 & R4.

Jens showed this gem in his Leach clone.
I attached the Signal Hot and Signal Return to the bottom of the PCB where the pins are soldered in.

On the top side I use a 0.1" computer shorting socket.
Park it on Pin 1 and your amp is AC coupled.
Short across PIN 1 to Pin 2 and you have the "test amplifier set up" with shorted input.
Short across Pin 2 to Pin 3 and your amp is DC coupled at the input only. This allows a source item that already has a DC blocking cap that cannot be easily bypassed to become the only input DC block. You don't need two series connected DC blocking caps.

Jens was brilliant providing us with this extra third pin. Worth copying on every amplifier you make.

Two final points.
Orient all your electrolytics in the same direction. Makes installation and checking later much easier.
Provide optional pin pitch pads for as many capacitors as you have space for, without altering your layout.
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Last edited by AndrewT; 9th April 2011 at 05:24 PM.
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Old 9th April 2011, 08:16 PM   #29
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Wow, thats alot of data... I hope I got it right, appart from the size of the NP caps which will be scaled to real world parts I will use in the end... saw some pretty film caps in one of the threads this afternoon which I can still fit on my board. But I want to ge this grounding and coupling thing perfected.
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Old 9th April 2011, 11:44 PM   #30
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Hmm i could put this filter in my lm3886 from chipamp.com if i air wire it.
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