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Old 7th January 2013, 09:27 AM   #1
smatteo is offline smatteo  Italy
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Default HELP: new W1 mosfet amplifier 150w c.feedback

Hello

Me and Davide have strong passion for audio and decided (2 years ago) to design an audio amplifier with high performances and cost appropriate our possibilities.

Here you may find more schemes and first pcb prototype layout. We need your help to make stable this concept/amplifier, something is wrong and needing more than our expertise to identify and solve the problem.

goals:

- current feedback topology
- based on mosfet hitachi 2sk1058 and 2SJ162
- speed :-)

Davide report:

I don't remember the steps we went through to get to such design, but I'll try to recover.

As far as I remember you came to me with some Toshiba audio Hitachi J50 saying:" ...it would be fantastic to build our own ampli..."
The you came also with the story of the Current Feedback Amplifier and the famous Analog Devices Appnote, saying:" ...it would be fantastic to use this topology with the mosfet..."

We had a look at the Analog appnote and some other papers/docs on the web, but I needed to make some changes to the schematics in order to put my fingerprint on it (in the bad and the good ;-) ).

The overall design phylosophy was to implement a Current feedback amplifier with an open loop gain lower than an opamp and so with open loop input compliance that can more easy handle the transients without saturation. Such design will lead to non perfect linearity, so distortion levels well above the 0.000000...1% of the "opamp" audio amplifiers, that should give... more naturality to the listening (but that's all speculation). The design closed loop bandwidth was as high as 1MHz, with a low pass filter at the input to limit anyway the amplifier BW.

The input stage was designed fullly symmetric to work from DC. An auxiliary input node was provided to connect to an integrator in order to have 0V DC (so the overall amplifier is not DC). POSITIVE integrator!!

The VAS stage was as well designed in a symmetrical way using a non balanced current mirror to have also some current gain. A cascode topology was used here.

The driver stage is a classical voltage follower and was designed to supply all three parallel branches of the power stage.

Everything was ready to start; a preliminary release was built using one couple of cheap IRF530/IRF9530 to assess the correct behavior of the amplifier. Results were good both in terms of BW, stability, distorsion... so the Hitachi were installed and...we got a very good 65MHz oscillator.

:-)


Don't forget to ignore the scheme of power-on button that you may find in the lower left corner of General_SCH_Ampli.pdf. It's not part of amplifier scheme.

I included also the scheme of amplifier without feedback for spice simulation.

Any help and suggestion will be appreciated.
Thanks in advance.

MatteoSantinato
Attached Files
File Type: pdf SCH_spice_Ampli .pdf (37.7 KB, 105 views)
File Type: pdf General_SCH_Ampli.pdf (68.4 KB, 94 views)
File Type: pdf SCH_Power Supply.pdf (44.4 KB, 46 views)
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Old 7th January 2013, 12:34 PM   #2
djk is offline djk
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"the Hitachi were installed and...we got a very good 65MHz oscillator. "

Try 470Ω gate resistors, see if that helps.
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Old 7th January 2013, 12:50 PM   #3
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Shurely R16 (Rf = 8.2K) is too big for a CFA.!
In CFA, Cf compensation in // with Rf is forbidden, but Rf matters for stability:
The more Rf, the more is the bandwide and the more is instability.
Try lower Rf ( dont worry for banwith: you reach Mega Hertz anyway).
After that you will need some other compensations for the Hitachi Mosfets...
Francesco Balena
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Old 7th January 2013, 01:05 PM   #4
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There is missing a capacitor (about 100p) from the collector of Q3 to GND. This is normally needed in CFPs for compensation.
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Old 7th January 2013, 04:32 PM   #5
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I second Franceco, R16 is too large, decrease its value to around 3k, but also R25 offcourse. There is no compensation present, moschfets idea might be good enough although there is better compensation schemes for this amp. Where I dont agree with Francesco is the compensation in // with cF, this is a discrete design not a 8 pin opamp, this type of compensation works very well with this type of amp, just ask NAD, Cyrus, Accuphase and many others. I have a amp pretty much identical to this front end here except I use hawksford version of the cascode and triple BJT output, sounds very good. I think your oscillation started because laterals have about half the ciss of verticals, the large value keeping it stable. Surely you can get better transistors than those BDs, they are hopeless as cascode transistors.
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