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Old 19th May 2013, 06:02 PM   #21
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by gootee View Post
Mcd99uk,

The point was that you are using bogus diode models, there. You must use a diode that can actually be purchased and installed.

So, initially at least, you have to right-click on each semiconductor that has no part number and select a real part from the list of built-in models. If you don't like any of the parts that already have models provided, then you will need to download a model for a different part.
Your are of course correct. I know the diode choice is very significant. I have yet to research what is available and what spice models are available.

Am i right in thinking ltspice default is an ideal diode?
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Old 19th May 2013, 06:17 PM   #22
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by gootee View Post
Mcd99uk,

The point was that you are using bogus diode models, there. You must use a diode that can actually be purchased and installed.

So, initially at least, you have to right-click on each semiconductor that has no part number and select a real part from the list of built-in models. If you don't like any of the parts that already have models provided, then you will need to download a model for a different part.
Like the MMSD4148 diode. Its SMT but i don't mind SMT soldering. And available from farnell @ around 10p each. Have you any recommendations for good diode for this prurpose SMT or through hole?

Many thanks

Paul
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Old 20th May 2013, 06:18 AM   #23
gootee is offline gootee  United States
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Quote:
Originally Posted by mcd99uk View Post
Your are of course correct. I know the diode choice is very significant. I have yet to research what is available and what spice models are available.

Am i right in thinking ltspice default is an ideal diode?
I don't think it's ideal, based on some quick simulation testing. I'm not sure but it might just be a type that resembles a generic diode on some particular type of IC die, which is what spice was originally developed to simulate.
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Old 20th May 2013, 06:22 AM   #24
gootee is offline gootee  United States
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Quote:
Originally Posted by mcd99uk View Post
Like the MMSD4148 diode. Its SMT but i don't mind SMT soldering. And available from farnell @ around 10p each. Have you any recommendations for good diode for this prurpose SMT or through hole?

Many thanks

Paul
Whatever you use, it might be wise to do some testing that specifically tries to investigate the model itself. Some of them are extremely unrealistic. In particular, I remember using a 4148 model from somewhere that was "quite optimistic". There should be several models available from different manufacturers. You should probably at least compare them, in a text editor, to see if any stand out as having parameters that are a lot different than most of the others.

I don't know enough to recommend any particular diode. Sorry.
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Old 20th May 2013, 11:17 AM   #25
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Strange I didnt get email notification from this thread which I happen to be interested in as I primarily focus on CFB topologies. I see some peaking in closed loop response which might mean problems. Have you tried to use lag compensation, just RC from vas to ground or RC from vas to the rails from each vas. Usually lag comp means wider bandwith with this topology. I have never used a emitter enhanced vas in this topology so Im rusty here but using baxandall vas and no error correction I manage about the same THD figures.
Can you post the .asc file.
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Old 20th May 2013, 12:09 PM   #26
mcd99uk is offline mcd99uk  United Kingdom
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Hello Manso,

The peaking response was caused by adding the pure capacitive load to test stability. The peaking appears to come from the HEC when driving capacitive loads with no L//R filter. It can be corrected easily but slows the HEC down.

I have not used lag comp on the vas as in Self's book he says that it is a brutal way to treat a VAS. That's why I went for a resistive load back to the IPS output. This seems to reduce the harmonics at the VAS.

The distortion figures can be made much better with the removal of the clipping diodes. I am currently looking for an alternative way to stop VAS saturation. An alternative method is (which worked very well but has a quite a few extra parts) detailed in Cordell's book and can be seem in my first amp attempt that turned into a monster.

No problem posting the .asc file. Will do it after work tonight. Would be good having a another brain on this.

Paul
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Old 20th May 2013, 02:39 PM   #27
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This is a CFB topology, its quite different to VFB. Self´s view is not of any consequence here youll find. Self actually shows this topology and claims its sub optimal, little does he know that many of the best amps actually use this topology. The way he wrote about it I have doubts in my mind if he is aware of the difference between CFB and VFB, see what he calls it. I doubt youll find a CFB opamp that is not compensated in this way. As the beta enhanced vas has much higher gain it could be of some value here but Id have to look at simulations. Without the beta enhancement I doubt it would be.
I havent got much experience with hawksford error correction, have you tried ONDF error correction, I get better results.
This topology clips simmetrical and cleanly, I wouldnt bother too much with clipping diodes.
A good example of this topology is the british Cyrus amp range, from model 3 upwards about all models use this topology.
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Old 20th May 2013, 03:03 PM   #28
mcd99uk is offline mcd99uk  United Kingdom
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Thank you for taking interest in my little project. I am just beginning in the field of amp design and have much to learn.

I had to use some form of clipping protection with this amp as when the VAS saturates the output completely collapses, as you will see when you try simulating my design once I have posted the .asc file.

My main design goals are to have a very stable amp, well behaved into clipping and lastly to have reasonably low distortion.

would you be kind enough to post a link to an example of this ONDF correction? I have done a quick google (Can't use internet too much during work time) and found luxman references??? Or even a link to a schematic of one of your amps?

I will be trying different compensation as you suggest but to the power rails as I would like to keep ground connections to a minimum to make PCB layout easier.

Shame the only Cyrus amp I have access to is a Cyrus 2.
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Old 20th May 2013, 04:42 PM   #29
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Yes Luxman is about the only manufacturer that I know that uses it. ONDF has some advantages over hawksford. I havent used it on a global feeback amp before but I dont see any problems. Maybe better I send you a paper as not a lot of info is available on the net. Pm me with your email.
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Old 20th May 2013, 06:13 PM   #30
mcd99uk is offline mcd99uk  United Kingdom
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ASC file as requested...
Attached Files
File Type: asc compdiff 4.asc (20.1 KB, 19 views)
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