First of all, I wish you a nice good day!
I bought two of these cheap AIYIMA 1969 clones for crafting.
In contrast to the standard circuit, MOSFETs are used here (IRFP448).
The variant with MOSFET doesn't seem to be widespread, I couldn't find much via the search function.
On the seller's supply side, a static current of 30mA is recommended. This confuses me a bit.
With this setting, the amp works, but it feels like it doesn't develop much power and it doesn't get warm.
Can the value of 30mA be correct? If not, what values are recommended?
Operating voltage is 20V.
Thank you
Lenny
I bought two of these cheap AIYIMA 1969 clones for crafting.
In contrast to the standard circuit, MOSFETs are used here (IRFP448).
The variant with MOSFET doesn't seem to be widespread, I couldn't find much via the search function.
On the seller's supply side, a static current of 30mA is recommended. This confuses me a bit.
With this setting, the amp works, but it feels like it doesn't develop much power and it doesn't get warm.
Can the value of 30mA be correct? If not, what values are recommended?
Operating voltage is 20V.
Thank you
Lenny
So, it is not class A then... I found a site where the recommended quiescent current range is 30-60mA, for the same amp with the same MOSFETs.
What do you want/expect from this amp?
What do you want/expect from this amp?
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I wanted to build a Class A amp as a small project and I loved the simple 1969 design.
I still had a power supply and large heatsinks here anyway.
But back to the AIYIMA board. If I understand this correctly, a Claas A operation is not possible with MOSFET or only in my case?
The components would be able to withstand a higher current.
Could it be that the problem is the changing resistance value when heating MOSFETs, which virtually rules out CLaas A operation?
//Edit: I have traced a circuit diagram of the board and the component arrangements correspond to the org. design. The position of the power amplifiers is even marked with 2SC5198. I'm thinking about swapping the MOSFET for normal transitors.
On the other hand, maybe I should buy another, more suitable CHINA board. Can you recommend one?
I still had a power supply and large heatsinks here anyway.
But back to the AIYIMA board. If I understand this correctly, a Claas A operation is not possible with MOSFET or only in my case?
The components would be able to withstand a higher current.
Could it be that the problem is the changing resistance value when heating MOSFETs, which virtually rules out CLaas A operation?
//Edit: I have traced a circuit diagram of the board and the component arrangements correspond to the org. design. The position of the power amplifiers is even marked with 2SC5198. I'm thinking about swapping the MOSFET for normal transitors.
On the other hand, maybe I should buy another, more suitable CHINA board. Can you recommend one?
MOSFET's are fine for Class A provided the circuit is designed from the outset to use them.
If this AIYMA 69 is meant to be some kind of copy of the JLH69 then MOSFET's are not suitable as direct replacements, there would need to be considerable redesign.
Without seeing an actual circuit diagram its impossible to advise more... the 30ma bias current sounds bizarre and more suited to a preamp or headphone stage if Class A is intended.
If this AIYMA 69 is meant to be some kind of copy of the JLH69 then MOSFET's are not suitable as direct replacements, there would need to be considerable redesign.
Without seeing an actual circuit diagram its impossible to advise more... the 30ma bias current sounds bizarre and more suited to a preamp or headphone stage if Class A is intended.
I quickly identified and traced the components. it looks like the Chinese just replaced the transistors with mosfets and kept all other values. It is strange. I am new in amplifiers, but what do you think?
Interesting. So the 'secret' to the FET design is replacing the preset with a 10k to allow a lower bias current to be set. This allows the stage to run in a poor sort of push pull Class B stage
Here is the distortion plot of the original Class A and the FET version.
This is the FET and the distortion is poor and has lost that 'magical' profile of a typical Class A output stage. This is reminiscent of Class B amps of the 70's and 80's.
And this is the regular JLH running in correctly biased Class A.
Also the FET version compromises stability and has a poor peaky response.
And the original even with using modern faster parts is still well controlled.
So this FET version is imo a poor design and is nothing like the original in concept unfortunately.
Here is the distortion plot of the original Class A and the FET version.
This is the FET and the distortion is poor and has lost that 'magical' profile of a typical Class A output stage. This is reminiscent of Class B amps of the 70's and 80's.
And this is the regular JLH running in correctly biased Class A.
Also the FET version compromises stability and has a poor peaky response.
And the original even with using modern faster parts is still well controlled.
So this FET version is imo a poor design and is nothing like the original in concept unfortunately.
I should add that you can bias the FET version on to a higher bias and regain much of what is missing.
Wow, you modeled and simulated it. Very cool, thank you!
From what I know now, it doesn't make sense for me to invest any more energy in this board with the MOSFET. I prefer to look around for other boards.
But last question - what else I can think of if I swapped out the irfp448 for 2sc5198, would that be an easy way?
Or just for fun against presumably completely unsuitable APT50GN60BG (Are these available in your program?)- I still have a few of them lying here. Thank you!
From what I know now, it doesn't make sense for me to invest any more energy in this board with the MOSFET. I prefer to look around for other boards.
But last question - what else I can think of if I swapped out the irfp448 for 2sc5198, would that be an easy way?
Or just for fun against presumably completely unsuitable APT50GN60BG (Are these available in your program?)- I still have a few of them lying here. Thank you!
The 2SC5198 should be OK if you alter the component values back to the original. That would mean the preset would have to be much lower in value and in simulation with modern transistors a value of around 250 ohms gives the correct bias current. One real issue is that the wiper of the preset carries quite a high current of around 35 milliamps and that is really to much for any small type preset or multiturn type. One option would be to tag a preset in temporarily, set the bias and then remove and measure the resistance and fit a fixed value.
Your other option is to use the FET's but bias the amp to the correct value which I think was around 1.2A for an 8 ohm load.
The APT50GN60BG is an IGBT which is a bit like a FET input at the gate (or base) but controlling a BJT transistor. All things, well most things, are doable but I wouldn't like to say what modifications would be needed or how it would ultimately perform using those.
Try biasing the FET's first to a higher current. They will get hot (just the same as the ordinary transistors) and you should increase the bias slowly and let the heatsink stabilise before increasing to a final value. Set the final value with the amp fully hot.
Your other option is to use the FET's but bias the amp to the correct value which I think was around 1.2A for an 8 ohm load.
The APT50GN60BG is an IGBT which is a bit like a FET input at the gate (or base) but controlling a BJT transistor. All things, well most things, are doable but I wouldn't like to say what modifications would be needed or how it would ultimately perform using those.
Try biasing the FET's first to a higher current. They will get hot (just the same as the ordinary transistors) and you should increase the bias slowly and let the heatsink stabilise before increasing to a final value. Set the final value with the amp fully hot.
Thank you for your detailed reply!
Unfortunately, I can't raise the current that much. With the current design, it starts humming from about ~ 0.5A. R10 is probably the cause.
I'm going to enlarge it a bit and test it again.
In a next step, I will rebuild to the org. design.
Thanks again for all the infos!
//Edit: Thought about it. Seems like R10 can`t cause the humming?!
Unfortunately, I can't raise the current that much. With the current design, it starts humming from about ~ 0.5A. R10 is probably the cause.
I'm going to enlarge it a bit and test it again.
In a next step, I will rebuild to the org. design.
Thanks again for all the infos!
//Edit: Thought about it. Seems like R10 can`t cause the humming?!
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It sounds like a power supply issue if the hum increases as you turn the bias up. Don't under estimate just how power hungry and hot class A is. Small switching supplies as used with ACA are the often the best option.
Have you seen the ACA? (which uses FET's)
https://www.diyaudio.com/community/threads/amp-camp-amp-aca.215392/
https://diyaudiostore.com/collections/kits/products/amp-camp-amp-kit
Have you seen the ACA? (which uses FET's)
https://www.diyaudio.com/community/threads/amp-camp-amp-aca.215392/
https://diyaudiostore.com/collections/kits/products/amp-camp-amp-kit
You were right. The hum came from a power supply that was too weak.
I still have an old one on a Dell notebook 19V 6.5A, which works quite well for the time being. I have now increased the current to 0.9A and it sounds much better.
Does the circuit work in Class A now? Is there a thumb value for how warm the transistors can get in continuous operation?
Thanks again for your support! I'll take a closer look at the amp you linked. 👍
I still have an old one on a Dell notebook 19V 6.5A, which works quite well for the time being. I have now increased the current to 0.9A and it sounds much better.
Does the circuit work in Class A now? Is there a thumb value for how warm the transistors can get in continuous operation?
Thanks again for your support! I'll take a closer look at the amp you linked. 👍
That will be running in Class A. The transistors will run hot, how hot depends on the thermal capacity of the heatsink and the airflow around it. If the centre leg of the transistor sizzles water (100C and over it is definitely to hot). If you can hold your fingers or hand on the heatsink for 5 seconds or so then its OK. The heatsink can be as hot as a hot radiator and that is OK.
For class A its the heatsinks thermal resistance/conductance to ambient that matters. The capacity doesn't come into steady-state heatflow.