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Old 11th November 2012, 12:03 PM   #1
GregH2 is offline GregH2  Australia
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Default Simple 85W amplifier

Hi All,

This is my latest little project. A "Fetzilla" derived design which uses no exotic parts and delivers about 85W RMS (42V rails). It sounds pretty good and does a near perfect rail to rail 20kHz square wave.

Although it's a rather conventional circuit with nothing new, the more unique elements are:

1) H2 heavy singleton input with current feedback.

2) CFP VAS with bootstrap CCS, run at nearly 40mA to drive the output fets directly with minimal loading.

3) Vgs multiplier for temperature compensation. Works perfectly.

The amp can deliver oodles of current and has powerful bass, a forward mid range and clean treble. All in all, quite a nice little amplifier with a bigger and better sound than the schematic might suggest. The amp is thermally stable and stable into capacitive loads. It simulates quite well too, as the figure below shows.

Thanks to Hugh and Paul for their help and inspiration with this project.
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File Type: jpg Sherrin_FFT.jpg (316.6 KB, 1325 views)
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File Type: pdf Sherrin_rev2a_as_built_sch.pdf (21.5 KB, 955 views)

Last edited by GregH2; 11th November 2012 at 12:20 PM.
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Old 11th November 2012, 12:27 PM   #2
mikelm is offline mikelm  England
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Nice one Greg - can't wait to hear this one . . .
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Old 11th November 2012, 01:55 PM   #3
Jay is offline Jay  Indonesia
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Any chance to see a lateral version?
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Old 11th November 2012, 02:04 PM   #4
mikelm is offline mikelm  England
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I guess it depends on the device pin outs but if they are the same a lateral version would be simple - just replace the Vgs multiplier with a resistor - but since you have asked the question I'm guessing that the pins are different.
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Old 11th November 2012, 02:09 PM   #5
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Hi Greg.

I think a Darlington VAS with the driver collector at ground would perform better than a CFP. Use only two resistors for bias current, one across the driver BE and another across the VAS BE, this way your VAS will have much less distortion as both transistors are NPN and the driver does not experience the collector swing of the VAS. The best driver has low Cbe and high Hfe - BC550C/2N5089/MPSA18. You will also have less potential problems with local CFP parasitics and positive feedback loops acting up.

In your configuration there is no local feedback at the CFP to speak of. This is because the impedance at the rail is orders of magnitude less than the impedance of T2's emitter, and so any potential feedback current sinks into the rail instead. So it acts basically the same as my Darlington above, except with much less current gain and more distortion because of both Early effect and Miller effect.
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Old 11th November 2012, 03:09 PM   #6
lineup is offline lineup  Sweden
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Quote:
Originally Posted by GregH2 View Post
1) H2 heavy singleton input with current feedback.

2) CFP VAS with bootstrap CCS, run at nearly 40mA to drive the output fets directly with minimal loading.

3) Vgs multiplier for temperature compensation. Works perfectly.
You have a very nice circuit there, Greg.
I had a look at schematic.
As you say nothing fancy, but only the transistors needed.
I am impressed with your results, squarewave and all.

Hope your thread here can make some more build a Fetzilla alike amplifier.
Because this topology works just fine.
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Old 11th November 2012, 03:32 PM   #7
mikelm is offline mikelm  England
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Quote:
Originally Posted by keantoken View Post
I think a Darlington VAS with the driver collector at ground would perform better than a CFP. Use only two resistors for bias current, one across the driver BE and another across the VAS BE, this way your VAS will have much less distortion as both transistors are NPN and the driver does not experience the collector swing of the VAS.
Wow - I just simulated ur suggestion KT . . . . .

all HD is less than -110db in spice @ about 10W

I never saw anything like it before.

and rise time is much faster - neat idea ! . . .

. . . but 2nd at -115db is lower than 3rd at -110db - so will it sound musical ?
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Old 11th November 2012, 03:36 PM   #8
Boscoe is offline Boscoe  United Kingdom
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Originally Posted by mikelm View Post
Wow - I just simulated ur suggestion KT . . . . .

all HD is less than -110db in spice @ about 10W

I never saw anything like it before.

and rise time is much faster - neat idea ! . . .

. . . but 2nd at -115db is lower than 3rd at -110db - so will it sound musical ?
I may sound different but those magnitudes of distortion are too low to be audible.
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Old 11th November 2012, 04:02 PM   #9
Jay is offline Jay  Indonesia
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Originally Posted by mikelm View Post
Wow - I just simulated ur suggestion KT . . . . .
Look at also Mooly's M1 HEXFET. Mooly took the feedback from the VAS and use CCS instead of bootstrap. By taking the feedback from output and running bootstrap instead of CCS, performance is also good.
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Old 11th November 2012, 04:03 PM   #10
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Mikel, by changing the resistors I think you can change the harmonic profile.

At most frequencies I think the miller cap will be the dominant source of loading for the input stage, and this cap responds to the distortion of the output stage, which is why I think after eliminating VAS distortion the 3rd harmonic begins to dominate. Try a resistor from the output of the amp to the VAS input, this should add purely monotonic distortion. The only alternative to adding distortion is to reduce the distortion of the output stage, which would mean redesigning the circuit.

Last edited by keantoken; 11th November 2012 at 04:05 PM.
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