Hi,
designed this amp, it sound very good but would like to understand its strength and weakness.
The intent was to design a class A amp using a constant current source (LM317) to compensate for a cheap voltage source.
Appreciate any help.

If anyone else builds it would love to hear their feedback.
Thanks

Edited on 12/9/2017:
After some good feedback by members (especially Bordodynov) replaced the IRF530 mosfet with 2N7000 and the freq response is now very flat from 40 - 20KHz. If some is interested in building this amp please use 2n7000 or some other mosfet with low input capacitance. It works with a BJT too. Please find the an updated screen shot from LTSpice.
Thanks.

Edited on Mar 3 2017:
Please see the updated version in post - https://forum.allaboutcircuits.com/...-amp-sound-so-good.142409/page-3#post-1244982

 

Ditch the mosfet, and replace with a MPF102 jfet, don't know why you need a constant current in the drain load...??

 

Ditch the mosfet, and replace with a MPF102 jfet, don't know why you need a constant current in the drain load...??

Thanks, when I remove the constant current source the output drops dramatically along with the audio quality. I will try the jfet.

 

(Note: rhetoric that might seem abrasive removed by moderator)

You've done a good research or found a great researcher
Or you're born with an eye for best
With an attitude of less is more
If you're not any one of above (thats hard to digest lol)
Then,
Class A deserves its A status always as it operates on full sine wave..not just like compromising classes, b ab d e any many more in future!
To operate on entire signal it needs constant current, as there are no compromising way out like lower classes
Even if you see complex high watt schemes, Jlh or any, there are always a cc stuff sitting atop.
Oh how did you got your cc source? Solenoid, NPass like lamp, high watt resistor, pnp like bd140, smarty?
There are ways not using any of the above just said, though limited to 4om drivers, and less supply voltage
Mosfet sounds tubey, personal choice, beyond any logic )
Tip stuff are power trans..
So everything works fine
Looking forward to read your next smart post
)))

 

Hi Indr,
appreciate your post, enjoyed it
Yes I was trying to design with the least components as possible. Yes MOSFET sounded tuby, and I like that. The LM317 is used as the constant current source, I did not show the full LM317 CC source on the diagram.
Seemed more efficient (not sure about the calculations) but It does not generate as much heat some of the other class A amps I have for similar output ?
Any tweaks that may improve this ?

thanks

 

Hi again
You are just fine with that scheme
I repeat, just fine, bcoz after all these making, I'm using 5, 9 & 12v only, with very good room filling ambience
I too dislike those big schmes, jlh, hiraga..all with harsh sounding trannis...and made for commercial success,
No one will buy one or two elemnt simple stuff..to cash on convoluted world, you gotta be convoluted !!!!
Ok,
After some time when your creative and get-the-best soul itching for more, try two schemes....
ZCA by mark houston of australia
Power follower class A by Andrea Ciuffoli
But both need large around 6a transformer, but chief components are few....and all are dear mosfets
I cant get tubes atleast now..but I can get tubey..))
For some unknown reason I dont like voltage regulatorss like 317, so this is a bd140 pnp based cc source..enough amp
If you ever consider transistors, go for Bd ones, made for audio purpose only, with very high hfe or beta...tips are good for pumping watts, I sometimes listen tip 142 class A a darlington trany..but too much harsh pitch
You can tinker with different mosfets, z44, 530, 640 740..my fav is 740 as of now..till I get k1058 or j121 but chances are slim as of now..
And tinker with speakers..to flo the bass
And??? The real hero....the songs )))

 

Hi Indr,
appreciate your post, enjoyed it
Yes I was trying to design with the least components as possible. Yes MOSFET sounded tuby, and I like that. The LM317 is used as the constant current source, I did not show the full LM317 CC source on the diagram.
Seemed more efficient (not sure about the calculations) but It does not generate as much heat some of the other class A amps I have for similar output ?
Any tweaks that may improve this ?

thanks

It might even work without the TIP122, but I don't get the MOSFET gate bias - the gate isn't current controlled like a BJT base.

 

Hi again
You are just fine with that scheme
I repeat, just fine, bcoz after all these making, I'm using 5, 9 & 12v only, with very good room filling ambience
I too dislike those big schmes, jlh, hiraga..all with harsh sounding trannis...and made for commercial success,
No one will buy one or two elemnt simple stuff..to cash on convoluted world, you gotta be convoluted !!!!
Ok,
After some time when your creative and get-the-best soul itching for more, try two schemes....
ZCA by mark houston of australia
Power follower class A by Andrea Ciuffoli
But both need large around 6a transformer, but chief components are few....and all are dear mosfets
I cant get tubes atleast now..but I can get tubey..))
For some unknown reason I dont like voltage regulatorss like 317, so this is a bd140 pnp based cc source..enough amp
If you ever consider transistors, go for Bd ones, made for audio purpose only, with very high hfe or beta...tips are good for pumping watts, I sometimes listen tip 142 class A a darlington trany..but too much harsh pitch
You can tinker with different mosfets, z44, 530, 640 740..my fav is 740 as of now..till I get k1058 or j121 but chances are slim as of now..
And tinker with speakers..to flo the bass
And??? The real hero....the songs )))

Thanks, will try those tweaks when I get some time.

 

It might even work without the TIP122, but I don't get the MOSFET gate bias - the gate isn't current controlled like a BJT base.

It is negative feedback. When the drain voltage exceeds the gate threshold voltage, the MOSFET begins to conduct and hold the drain voltage from getting much higher. AC coupling in current from a signal source would cause this to act as a possible very linear transimpedance amplifier provided the source impedance is not very low. In the case of a very low source impedance the MOSFET is a self-biased class A amplifier.

 

See

 

It might even work without the TIP122, but I don't get the MOSFET gate bias - the gate isn't current controlled like a BJT base.

The ideas was to form a darlington like pair using a MOSFET and a BJT. Then I assumed it to behave more along the lines of a BJT ? and biased it as such.

 

It is negative feedback. When the drain voltage exceeds the gate threshold voltage, the MOSFET begins to conduct and hold the drain voltage from getting much higher. AC coupling in current from a signal source would cause this to act as a possible very linear transimpedance amplifier provided the source impedance is not very low. In the case of a very low source impedance the MOSFET is a self-biased class A amplifier.

It sometimes works - but discharging the gate capacitance doesn't always.

 

The ideas was to form a darlington like pair using a MOSFET and a BJT. Then I assumed it to behave more along the lines of a BJT ? and biased it as such.

MOSFET overheating is possible if you don't include a G/S bleed resistor.

 

MOSFET overheating is possible if you don't include a G/S bleed resistor.

Thanks. The amp currently running MOSFET with no heatsink, it shows no warming, its temp around 72F. I asked this question earlier too, about it being a little more efficient? The LM317 delivers around 250ma. LM317 and TIP122 have heatsink.

 

iinselt.
What is the source of the signal. I'm interested in the output impedance of the signal source and the maximum amplitude of the signal. And also the load resistance (4, 8 or 32 ohms).

 

iinselt.
What is the source of the signal. I'm interested in the output impedance of the signal source and the maximum amplitude of the signal. And also the load resistance (4, 8 or 32 ohms).

Hi,

pc sound card -----> Tube preamp ( Little bear BLUE P5-1) ---> This Amp ---> 6 ohm modified Pioneer SP‑BS22‑LR speakers.

hope that helps.

 

I considered another scheme. On one transistor and with negative feedback. Due to the presence of a signal in series with the source, the frequency response turned out to be poor. A field effect transistor has a large (relatively) input capacitance. So I asked about the source of the signal. If this source of the signal is low-resistance, the bandwidth will be sufficiently wide. I tried to find the parameters of your amplifier or find its circuit. So far I have not succeeded. I simulated your circuit and was surprised at its quality with such a simple implementation and current of 250 mA.

 

I considered another scheme. On one transistor and with negative feedback. Due to the presence of a signal in series with the source, the frequency response turned out to be poor. A field effect transistor has a large (relatively) input capacitance. So I asked about the source of the signal. If this source of the signal is low-resistance, the bandwidth will be sufficiently wide. I tried to find the parameters of your amplifier or find its circuit. So far I have not succeeded. I simulated your circuit and was surprised at its quality with such a simple implementation and current of 250 mA.

Hi Bordodynov,
please find the circuit diagram of the pre-amp I am using, hope you find that helpful.
Is it possible to share the simulation file - did you use LTSpice or similar?
Would be interested in any variations that you might make to this circuit, please keep us posted. Thanks

 

Hi,
designed this amp, it sound very good but would like to understand its strength and weakness.

And I really like this schematic.
It is a good scheme.

Strength:
The LM317 protects the power circuit limiting to a constant current
Even if you do short circuit between E and C of TIP122 do not burn.

Weakness:
No reaction(feedback). Larger distortions.
Slew rate not very good.

If you like how it sounds it is the most important.


what you can improve:
Take the IFR530 aside which is useless there and comparable as a price with TIP122.
TIP122 is darlington and has enough amplification.
The IRF530 added linearity and clarity so little that I think is useless

 

Hi iinself.
The output impedance of your preamp is 4.7kOhm. This affects the overall frequency response.
I advise you to determine the frequency response for a complete set of circuits (tube preamplifier + your amplifier). Or use only your amplifier + additional series resistor 4.7kΩ.