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Old 18th July 2013, 05:14 PM   #1
Bigun is offline Bigun  Canada
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Default TGM7 - an amplifier based on Greg Ball SKA

[scroll down to post #8 for initial schematic, post #183 for as-built schematic, post #160 for photo of final pcb]

With the public release of the schematics for Greg's very well regarded power amplifier "Simple Killer Amp" !... I am going to start a new build.

I have dubbed this the 'TGM7', it's how I number my projects. This is the 7th one. Each project I try to do something different to learn something new. The new elements for this project that interest me are i) MOSFET outputs (never used them before), ii) double LTP input (never used this before), iii) Greg's novel topology for high PSRR and iv) I want to use a commercial pcb vendor instead of making my own pcb. I plan to use Freeware Eagle to design the pcb and I'd welcome help.


What I plan to do differently from Greg's original GB150 ?
- since Greg is not releasing his pcb layout I'd like to develop a new one, possibly using a different form factor (Eagle Freeware is limited to 100mm x 80mm anyway).
- component choices will be looked at, some of the original devices are not easily found. I am also not shy to use surface mount parts as there are many more choices of good components in this format. Many surface mount parts are relatively large and do not need to be a chore to use. It allows for a compact pcb design with less solder and less risk of bad joints.

If anyone else is interested feel free to weigh in with your advice etc.


EDIT: some things changed after I started this thread so I have edited this opening post:

1) I'm building with two pairs of FETs, same as Greg
2) Modified current sources.

EDIT: since the first version suffered from oscillations I have done a new layout for the pcb. This time I have included the option for a new topology - replacing the front end dual LTPs with complementary singleton inputs (some folk call this a current feedback amplifier or 'CFA') but this option has lower PSRR

...and Paulo built his own version with through-hole parts :-)
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Last edited by Bigun; 29th April 2014 at 02:55 PM.
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Old 18th July 2013, 06:00 PM   #2
PauloPT is offline PauloPT  Portugal
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I hate SMDs...

A sugestion: design a cd servo instead of the NFB cap. DC coupled the amp sounds fabulous!
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Old 18th July 2013, 06:33 PM   #3
mlloyd1 is online now mlloyd1  United States
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sounds like fun.
i'll be watching.
btw, if you are not wed to Eagle, you might have more options using Diptrace free versions.

mlloyd1
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Old 18th July 2013, 07:17 PM   #4
CBS240 is offline CBS240  United States
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With Eagle lite, you can draw outside that limit and include it within your PCB boarder, you just cannot place parts from the schematic page there. SMDs are the new way to go, just be aware of Pd derating. I use tiny SMD extensively without issue.

Q-fet is Fairchild's designation for the planer stripe type vertical mosfets. This style of fet is produced by other companies now but I think Fairchild's Q-fets were some of the first ones produced. Minor differences compared to cellular hex type devices is a sharper increase in Cgd at Vds saturation, and a greater dependence of Gm upon Vds for a given Vgs. This may or may not be an issue depending on the circuit. They do seem to be able to take more abuse though. I like them quite a bit but I have only used them in class A or with some sort of local error correction such as HEC. I have noticed that the only thing different between FQA designation and FQP is the package size. Some of them are available in both packages and the only difference in the datasheet is Pd and Pd derating. They are the same die. Also, I have found that if you can find a way to eliminate the mica under the tab and mount them directly to a larger hunk of metal like in this amp, the TO-220 devices will surprise you at the amount of power they can handle.
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Old 18th July 2013, 09:41 PM   #5
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by PauloPT View Post
I hate SMDs...
Cheer up Paulo, it really isn't that bad, honestly. Besides, not all the parts would be SMD, it will depend on what is the best choice and we haven't starting to look at the parts yet.

Quote:
Originally Posted by PauloPT View Post
A sugestion: design a cd servo instead of the NFB cap. DC coupled the amp sounds fabulous!
It sounds exciting but I think it's for a phase 2 project, first build should be close to the original design. Let's choose some nice caps for the feedback so as not to have to worry about them.
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Old 19th July 2013, 01:46 PM   #6
Bigun is offline Bigun  Canada
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UPDATE: Greg has given permission for us to clone his pcb so I'm going to take a look at this direction first since it will capture all the benefits of what Greg has found works best.
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Old 21st July 2013, 09:05 AM   #7
AKSA is offline AKSA  Australia
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Gareth,

How is the project coming along?

Cheers,

Hugh
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Old 22nd July 2013, 12:31 AM   #8
Bigun is offline Bigun  Canada
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Hi Hugh,

Well it's slow - on accounts of nice weather, a good movie and the local open air Jazz festival (followed by a business trip next week).

Anyhow, I went ahead and plugged the baseline schematic into Eagle. This of course means choosing part sizes. I decided to stick with through-hole parts - partly as I like the idea of reproducing the essence of the original. It's fun - reminds me of when I started out by cloning AKSA (it's still playing).

I may also keep room for two pairs of output devices if it doesn't get too crowded, but the intent for my build is to use just the one pair and with +/-50V rails still get full power. I did find reference somewhere to a GB75 pcb design that Greg was working on some time ago with a single pair but it seems he never finished it.

I see a couple of tweaks that will have to be incorporated yet - the first being to split the resistor in the tails of the current source/sink and move the end of rail cap to the junction between these resistors - a classic and popular approach to improving the PSRR of the current source for nearly zero cost and effort.

The other change I'm mulling over is to take out the temperature sensing diode-wired-transistors. It seems to me that the master device in the current source/sink can be used as the temperature sensing element all by itself (simulations seem to show it works) with some emitter degeneration to tune the temperature sensitivity - simplifies the circuit a little, but I wonder if there is some nasty consequence I'm overlooking ?

Any tweaks and mods you'd suggest ?
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"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

Last edited by Bigun; 22nd July 2013 at 12:56 AM.
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Old 22nd July 2013, 02:50 AM   #9
fab is offline fab  Canada
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Hi Bigun

I will be following this thread closely since I do intend to listen to an SKA GB150 amp

Regarding the output mosfet I am interested in suggestions as long as the VGSoff is high enough for the SKA amp to work properly at full power.

As for mods I would put separate source resistor for each mosfet allowing less need for VGS close match...

If you are able to take out the temperature sensing diode-wired-transistors I would try it also in my CFB version. However, if Greg Ball has inlcuded it I suppose that it was to obtain the proper compensation for the mosfet VGS drift where VBE drift is different...

Fab

Last edited by fab; 22nd July 2013 at 02:56 AM.
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Old 22nd July 2013, 05:54 AM   #10
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I had a look at the FQP12P20 QFET and it has -ve Vgs temperature coefficient all the way up to 10A. This means that there is very real risk of hot spots on the die. Modern devices push this temperature coefficient threshold higher and higher and have made SOA a headache in linear use. Tempco at the 100mA mark is about 5~6mV/C
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