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Messages - GB

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Amplifier Discussion / Re: FET Self Biasing Method
« on: March 30, 2021, 10:44:51 PM »
Hi. Edvard is correct about FETs having a wide range of specs... that is why they are not favoured in mass-produced equipment. However it is possible to allow / design for this.
There are simple circuits that will allow you to measure Idss and Vgs cut-off of FETs using a few components and a multi-meter. Once measured a precise design can then be done... and component values can be calculated. The Drain resistance value you have mentioned seems way high.
There are multiple articles on the net re biasing calculations. There is one in particular by Dimitri Danyuk to set up bias so that an FET works like a triode vacuum tube... Google it. I have built many circuits using this principle. They work very well.
For your (and others) info I have attached something that I wrote for a magazine a couple of years back. It is copyright... but I would welcome your feedback and comments.
Cheers,
GB

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Hi Joe... here's a further suggestion following (broadly) your Option 2... but using FETs rather than TL072's.
It follows the VOX AC-30 structure including Normal / Boost selection and the VOX Bass / Treble tone stack.
Notably... it follows the "Danyuk" FET biasing method to get them to perform like (valve) triodes. The schematic shows 2N series FETs... but I have also built versions using Toshiba 2SK series... which sound better to me... but are harder to get hold of.
Let me know what you think. I have a circuit description... and even a (FET and Transformer) based power amp to go with it... but that's a whole other story!  ;)

3
A schematic might be revealing just a little too much at this point. How about a brief sound clip instead?  :)

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Amplifier Discussion / Re: Using a choke for a TDA7293 amp
« on: August 12, 2018, 05:28:34 AM »
Yeah... agree with JM. Not worth it.
The TDA will inherently reject power supply ripple in any event.
The TL072 should be de-coupled from the PS with a couple of (say) 100 ohm resistors in series and 100uF caps behind them on each of the + and - supply rails... as OP Amps normally are.

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Hi people... I'm aware that this a very old thread but I just wanted to acknowledge the very many good thoughts in it... particularly from Phatt and Teemuk (whose publication I have read from to cover!) Many thanks for that.  <3)
I have also read Danyuk's paper on FET triode emulation... along with a lot other related stuff.
I could now probably write a book of my own!  ;) ... but I built an Amp instead... also emabracing many of the thoughts in related posts.

(Very) Breif Summary:-
* The signal chain is very closely modelled on VOX AC-30
* It uses the Danyuk FET biasing method - done correctly it works
* It uses Power MOSFETs AND an Output Transformer.

I am NOT a musician... but several who are... have used it - and like it! I have sound clips if anyone is interested?

Below is a pic... I'll wait to see if any interest / questions???

Cheers... GB

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Amplifier Discussion / Re: Heatsinks for your poweramp
« on: August 09, 2018, 06:38:37 AM »
Hi Guys... notwithstanding the (very valid) point about them failing... Cooling Fans can be very effective... potentially reducing the size of a given heatsink by 50% - or more.
Attached is a pic of a 100 watt with a relatively small heatsink - but with two 40mm fans directed down the cooling fins. The amp runs of a nominal 50 volt supply so these are two 24 volt DC units - with a little bit of additional circuitry to ensure that the voltage is equally distributed between them.
Bottom line it works very well.
PS. I have contemplated adding some sort of thermal protection (fuse or breaker) as a last line of protection - but haven't got that far yet.

GB

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Hi HS... thanks for that! Just working on the updated version now. Will keep you posted.
Cheers,
GB

8
Hi guys... been of the grid for a bit... but there have been some developments.
Most notably I have "sold" (albeit only covering component costs) a couple of units and the feedback (forgive the pun) has been very encouraging.
One customer is a pro muso and also a guitar teacher - so quite critical of sound.
To address a couple of the point raised by others:-
1) The gate clipping of FETs will be different - and hard to accurately duplicate. It is what it is.
2) Nonetheless the FETs (PROPERLY set up) can do a good job of emulating a triode (12AX7). I have tested this using a DSO with FFT analysis.
3) The schematic of the output stage which is posted in this thread is mine - but a simplified view. Actual ccts need a bit more added.
4) Re the 1+1:1 transformer. That is one option. They can be salvaged from old PA amps but difficult (impossible) to buy new ones off the shelf.
5) I have used PA Line transformers. These have been applied to valve guitar amps - mainly using a (high) 100 volt version of same. I have used a (low) 25 volt version which, approximately, has a 3+3:1 primary / secondary ratio. A long and detailed story for another day - but with correct MOSFETs, and setting up currents and voltages correctly they work! - including the saturation effects mentioned by Katopan and Bulldozed in this thread.

Bottom line... I am now thinking about rolling all of this accumulated knowledge in to a kit and/or publishing details.
Would be most interested in any comments / interest on this possibility.

Cheers,
GJB

PS - there seems to be two threads going on here. Search "GB" for the other.

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The Newcomer's Forum / Re: SSTS Preamp build and schem
« on: March 28, 2018, 02:17:48 AM »
Hi... I have looked at the schematic you referenced. Although it is using JFET (presumably) to emulate tube sound it is not ideal. If you are interested I have a better circuit that you could use?
GJB

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Hi... the Schematic posted above is not the latest design... gotta keep some secrets! :-)
But shows the general layout... and is (obviously) the preamp only.
GB

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Hi people... been absent for a while... won't bore you with the details.
I have (mostly) read the comments since my last post... some good thoughts in there. Thanks.
I have got a little more active I the last month or so and being doing some more development... aided by a supply of Toshiba FET's that I have been able to secure... 2SK117, 170, 369, 30's... etc.
This has been very helpful as they are:-
a) Graded at manufacture (BL, V, G, etc. versions) albeit that I still INDIVIDUALLY test and select them, and,
b) Especially made for audio applications
Importantly they have helped me refine the design process of biasing in "triode" mode as per the article I referred to some time back
(Google "Dimitri-Danyuk FET Triode Emulator")
I have developed an XL sheet making it easy to get all the components values I need by simply specifying Vgs, Idss, and the supply voltage.
BTW - There are also some "wider" considerations (not mentioned in the above) that need to be considered.
While I could go in to detail... this process seems to work... I have looked at numerous FFT spectrums on my scope... and, much more importantly, had the critical (positive) evaluation of several (good) musicians... unlike myself! :-(
I have also refined the output stage further with (again) a good supply of 2SK1058 and 2SK2221 (audio) power MOSFETs...
I could continue... but perhaps I'll wait for any feedback / questions about specific items before continuing...

Cheers
GJB

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Amplifier Discussion / Solid State version of a VOX AC-30 Guitar Amp
« on: January 08, 2017, 03:53:10 AM »
Hi people... I would like to get a few comments please...
Some time ago I started building custom Guitar Amplifiers... not because I am a Muso... but rather because my (then teenage) son was.

Fast forward 15 years...

I have most recently completed a few different versions of a Solid State Guitar Amplifier modeled on the (classic) VOX AC-30. Its key attributes are:-
1.   Does not use ANY Integrated Circuits (in the signal path)
2.   Does not use ANY bi-polar transistors
3.   Uses FET semiconductors throughout ... JFET’s (small signal) and specialist MOSFET’s (power)
4.   Amp architecture / signal path / tone controls based on the (classic) VOX AC-30 amplifier
5.   Uses an Output Transformer... like ALL Valve Amps... which provides significantly tonal differences to direct-coupled circuits
6.   Output Damping factor of transformer is much “looser” than solid state amps
7.   Lower damping factor substantially changes the dynamic characteristics of the speakers
8.   Is switchable between Class A(?) (like the VOX AC-30) and Class B for more “conventional” operation. This provides some tonal differences liked by musicians (Clean / Crunch)

While I could write lots more I will perhaps wait for any specific questions and/or general interest first.

Thanks

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