Problem with offset adjustment
Hi,
Got my amp (Dacz PCB) up and running.
Set the bias voltage at 61mV. However I can get offset voltage down to 393 mV only.
Using a 200 Ohm trimpot on VR1, 1N4148 diodes on D9, D10, and 10 KOhm resistor on R6.
Any suggestions on how to get the offset voltage down?
Hi,
Got my amp (Dacz PCB) up and running.
Set the bias voltage at 61mV. However I can get offset voltage down to 393 mV only.
Using a 200 Ohm trimpot on VR1, 1N4148 diodes on D9, D10, and 10 KOhm resistor on R6.
Any suggestions on how to get the offset voltage down?
Hi X,
Thanks for the encouragement.
I only listened for a brief period and on a different set of speakers (not on my "daily driver") - but I would say that the initial impressions are positive. Strong bass, clear mids, pleasant but distinct upper frequencies... there was a richness in tonal texture of stringed instruments like the violin and also nicely fleshed out vocals... overall the sound had a nice foot tapping quality about it.
Thanks for the encouragement.
I only listened for a brief period and on a different set of speakers (not on my "daily driver") - but I would say that the initial impressions are positive. Strong bass, clear mids, pleasant but distinct upper frequencies... there was a richness in tonal texture of stringed instruments like the violin and also nicely fleshed out vocals... overall the sound had a nice foot tapping quality about it.
Come on X, you shouldnt even be asking this question... you know who designed it.. It must sound awesome. let me get some commitments and projects out of the way, I will let everyone know exactly how it sounds.
Come on X, you shouldnt even be asking this question... you know who designed it.. It must sound awesome. let me get some commitments and projects out of the way, I will let everyone know exactly how it sounds.
Prasi,
Given the reputation Mr. Dean has, asking me how this amp sounds is rather a test of my hearing or just to confirm there is nothing very wrong with the build.
Btw, guess what I am using in the power supply - a CRC power supply board from your last GB.
Hugh, guys,
The LED bias did the trick; after installing the green LED, the bias voltage fell into the minus range and brought it up to 0 mV. Thank you for the guidance on getting this fixed so easily.
I have been listening to the amp for more than an hour now and really enjoying what I hear.
Thank you Hugh for a great design!
The LED bias did the trick; after installing the green LED, the bias voltage fell into the minus range and brought it up to 0 mV. Thank you for the guidance on getting this fixed so easily.
I have been listening to the amp for more than an hour now and really enjoying what I hear.
Thank you Hugh for a great design!
Zia,
Much credit must go to Ranchu. He did the hard yards, including the original schematic, the ongoing build, and the critical listening. But thank you for your comments.
The issue of setting a reasonable narrow DC offset on any amp amp comes back to the bias resistor on the base of the first transistor, and of course, the beta of the first transistor. You need to know exactly the collector current and a guestimate of the beta. Since transistor betas can move quite a bit, this is a bit like blindly pinning the tail on the donkey!
Very small currents, typically 10-50uA, flow into the first transistor on the Quasi. I try to design for beta around 100, not too high, much less than many like the K1845/A992, which have beta up to 600 and usually around 250. This base current must pass through the bias resistor, 33k here, and if it's a gain of 100 for the 2N5401, you divide the collector current (1mA here) by 100 to get 10uA. Multiply this by the 33k bias resistor, add the 1.8V dropped across the series 1k8 feedback resistor, add again the 0.65V pn voltage of T1 and you get a total voltage at the top of the 33k of ([1.8Kx1mA] + [1mA/100]33k + 0.65) = 1.8V + 0.33V + 0.65V = 2.78V.
Since were are aiming for 0mV at the output, and we are using a pnp T1 and the current is passing from output along the chain to the 33k bias resistor, it must be negative.
So this voltage at the TOP of the 33k bias resistor should be -2.78V. Any increment up or down on this point will be passed identically to the output; so if we set -2.80V, the output offset will be LOWER than 0mV, and be at -20mV. At -2.76V, the output offset is +20mV, and so on..... I try to have a range of less than 100mV plus and minus so that if the amp is set at either extreme ends it would not destroy the loudspeaker!
Cheers,
Hugh
Much credit must go to Ranchu. He did the hard yards, including the original schematic, the ongoing build, and the critical listening. But thank you for your comments.
The issue of setting a reasonable narrow DC offset on any amp amp comes back to the bias resistor on the base of the first transistor, and of course, the beta of the first transistor. You need to know exactly the collector current and a guestimate of the beta. Since transistor betas can move quite a bit, this is a bit like blindly pinning the tail on the donkey!
Very small currents, typically 10-50uA, flow into the first transistor on the Quasi. I try to design for beta around 100, not too high, much less than many like the K1845/A992, which have beta up to 600 and usually around 250. This base current must pass through the bias resistor, 33k here, and if it's a gain of 100 for the 2N5401, you divide the collector current (1mA here) by 100 to get 10uA. Multiply this by the 33k bias resistor, add the 1.8V dropped across the series 1k8 feedback resistor, add again the 0.65V pn voltage of T1 and you get a total voltage at the top of the 33k of ([1.8Kx1mA] + [1mA/100]33k + 0.65) = 1.8V + 0.33V + 0.65V = 2.78V.
Since were are aiming for 0mV at the output, and we are using a pnp T1 and the current is passing from output along the chain to the 33k bias resistor, it must be negative.
So this voltage at the TOP of the 33k bias resistor should be -2.78V. Any increment up or down on this point will be passed identically to the output; so if we set -2.80V, the output offset will be LOWER than 0mV, and be at -20mV. At -2.76V, the output offset is +20mV, and so on..... I try to have a range of less than 100mV plus and minus so that if the amp is set at either extreme ends it would not destroy the loudspeaker!
Cheers,
Hugh
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Sorry for the stupid question: where are the directions to set bias?
Should I follow post 1795?
http://www.diyaudio.com/forums/soli...mentary-mosfet-amplifier-180.html#post5117044
I think I missed that class.
I finally decided to place the last parts: output coil and 10R resistor!!
Thanks.
Should I follow post 1795?
http://www.diyaudio.com/forums/soli...mentary-mosfet-amplifier-180.html#post5117044
I think I missed that class.
I finally decided to place the last parts: output coil and 10R resistor!!
Thanks.
Last edited:
Hugh,
I did my build with IRFP240 and running of +/- 34V rails.
Bias is set at 60mV for now - should I set to 50mV?
Zia,
This is a moot point. I tend to set them lower rather than higher because I seek efficiency, particularly in a hot climate like yours and mine. But if you set the quiescent at say 60mV, which corresponds to 180mA/6.1W on each output device, you might notice it actually sounds a bit better because when you do the modelling you see that the harmonic profile changes.
I do not have a quasi here to test with, so I can't be sure that at 180mV it sounds better than at 100mA, but you are invited to try some critical listening and see if you can hear a difference. The other issue is that the quality of the mains changes during different times of the day; one guy who may know definitely might be Terry, Stillforgiven. He has done a LOT of critical testing and knows it all - he is about my age (I understand, hope I'm correct!).
Cheers,
Hugh
This is a moot point. I tend to set them lower rather than higher because I seek efficiency, particularly in a hot climate like yours and mine. But if you set the quiescent at say 60mV, which corresponds to 180mA/6.1W on each output device, you might notice it actually sounds a bit better because when you do the modelling you see that the harmonic profile changes.
I do not have a quasi here to test with, so I can't be sure that at 180mV it sounds better than at 100mA, but you are invited to try some critical listening and see if you can hear a difference. The other issue is that the quality of the mains changes during different times of the day; one guy who may know definitely might be Terry, Stillforgiven. He has done a LOT of critical testing and knows it all - he is about my age (I understand, hope I'm correct!).
Cheers,
Hugh
Zia,
You might also try 250mA and 350mA too - just to see how it sounds! Check the offset after changing..... it should be no problem at all and even at 350mA the dissipation with 34V rails is only 0.35 x 34 = 11.9W on each device, will within the output devices with good heatsinks.
Good luck, you might find a sonic sweet spot!!
Cheers,
Hugh
You might also try 250mA and 350mA too - just to see how it sounds! Check the offset after changing..... it should be no problem at all and even at 350mA the dissipation with 34V rails is only 0.35 x 34 = 11.9W on each device, will within the output devices with good heatsinks.
Good luck, you might find a sonic sweet spot!!
Cheers,
Hugh