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Gainclone using the LM4780 based on Cordell
Gainclone using the LM4780 based on Cordell
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Old 22nd March 2018, 03:21 AM   #1
mexmike is offline mexmike  Mexico
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Default Gainclone using the LM4780 based on Cordell

Hi,

I'm new to the forum.

Please find see the attached schematic of my effort to reproduce the Cordell Super Gainclone using the LM4780 as opposed to the LM3886.

I have simulated the circuit in Proteus with some success and plan to see if it works in reality. Probably due to my lack of knowledge, the current mute circuit works in reverse, open circuit gives amplification. The output initially went rail to rail with SW1 circuit closed. Installing output caps C22 and C23 seems to have fixed that but I'm still unsure as to why.

I know it is probably futile to try to simulate the circuit as reality is always very different. I did this because it helps me to prevent catastrophic failure and wasted time.

I am not an electronics expert, and realise there are probably many errors in the circuit, therefore, I was hoping for any criticisms prior to embarking on silk-screening a pcb. I believe I need to separate the signal grounds from the power grounds.

Any input is more than welcomed.
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Old 22nd March 2018, 05:28 AM   #2
PRR is offline PRR  United States
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Quote:
Originally Posted by mexmike View Post
...Any input is more than welcomed.
Don't the "+IN" pins need a DC reference?

Pretty sure they do.
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Old 22nd March 2018, 06:25 AM   #3
mexmike is offline mexmike  Mexico
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Thanks PRR.
Could you give me a hint on how to implement that? Would that be like a high value, say 47k, resistor going to the signal ground?
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Old 22nd March 2018, 09:19 AM   #4
Tibouchina is offline Tibouchina  Australia
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Default Look at the schematic on page 540 of Bob's book.

Quote:
Originally Posted by mexmike View Post
Thanks PRR.
Could you give me a hint on how to implement that?
It's meant to be connected to the quiet ground. Pin 5 of U1:B and U4:B are also meant to be connected to the quiet ground.

I don't think the development of the Super Gain Clone was ever completed? Do you have access a Cordell PCB layout?

It might be easier to build one of composite amplifiers discussed and marketed on this forum. (My_Ref Fremen Edition and/or Modulus-86/286) Then at least you can get a decent PCB as a starting point, proven design, build instructions and plenty of support if things go wrong.

Regards ...

Last edited by Tibouchina; 22nd March 2018 at 09:34 AM. Reason: A couple of extra comments
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Old 22nd March 2018, 01:05 PM   #5
jackinnj is offline jackinnj  United States
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Gainclone using the LM4780 based on Cordell
Bob showcased the Super GC at the NJ Audio Society and it sounded fantastic.

Pins 16 and 22, IN+A and IN+B should be grounded.
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Old 22nd March 2018, 01:28 PM   #6
Tibouchina is offline Tibouchina  Australia
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Default About the earthing?

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Originally Posted by jackinnj View Post
Bob showcased the Super GC at the NJ Audio Society and it sounded fantastic.

Pins 16 and 22, IN+A and IN+B should be grounded.
Thats good to hear it progressed. I assume the schematic is the same as documented in his book? Was it using the Klever Klipper?

Are you suggesting a single common ground? Bob's schematic has a quiet ground and a circuit ground. Quiet ground is connected to circuit ground via a 4.7 ohm resistor.
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Old 22nd March 2018, 08:27 PM   #7
mexmike is offline mexmike  Mexico
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Many thanks for the kind input.


I really don't want to build an amp from a premade board, instead, I like to simulate in Proteus and tinker and fiddle with what comes out using the oscilloscope and listen to the audio out.


No I don't have a Cordell PCB, I'll get a silkscreen made, diy the PCB with ferric chloride using the most basic of principles, using other boards for reference only regarding layout.


I tried the Myref amp in Proteus but most probably due to my lack of knowledge, it would go bananas at every opportunity, so I won't even attempt a build despite the fantastic reputation of the amp (RIP Mauro and sorry I couldn't do you justice).


Regarding the quiet 4.7 Ohm resistor, everything works great except the Mute switch, which previously worked in the oposite sense. With 4.7 Ohms, the amp will not mute. With any resistor smaller than 10M, either the amp stays unmuted or gives a strange square wave or similar. I need to research the math on the datasheet more. I'm not sure if this has anything to do with the inverted configuration of the amp. I've used the mute on a standard non-inverted GC in Proteus with no reverse mute operation, so know the simulation model works.


I've attached the DC Servo page from the Cordell schematic.
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File Type: jpg Fig 27.3 Cordell.jpg (424.6 KB, 227 views)
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Old 22nd March 2018, 08:38 PM   #8
jackinnj is offline jackinnj  United States
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Gainclone using the LM4780 based on Cordell
FWIW, the proteus model is probably cobbled together from the schematic of the LM3886. Texas Instruments does have a working model for the LM3886 but it is encrypted and difficult to find ever since their Overture Series chips were re-branded and the single chip amplifiers EOL.
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Old 22nd March 2018, 09:38 PM   #9
mexmike is offline mexmike  Mexico
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Yes that is quite correct. The schematic I posted would look exactly like two 3886's if the child circuit was visible.
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Old 22nd March 2018, 09:48 PM   #10
jackinnj is offline jackinnj  United States
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Gainclone using the LM4780 based on Cordell
A la:
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File Type: png LM4780_Inverting.png (16.3 KB, 205 views)
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