Hi all,
I have browsed the forum but did not find answer to my concern. I just have finished one channel for a Citation 12 (Mosfet). Everything is OK I am very happy with the sound.
I played with it by making some investigations with a scope and found that the clipping is not symetrical; negative swing clips first at 28 Volts; the positive swing clips at 30 volts.
- Is this situation normal? Both supply rails are perfectly symetrical (36 Volts)
- I have at output a DC offset about 40 mV, can I reduce it by better matching of output FETs? or matching input symetrical pair ?
Thanks for your help,
Vince
I have browsed the forum but did not find answer to my concern. I just have finished one channel for a Citation 12 (Mosfet). Everything is OK I am very happy with the sound.
I played with it by making some investigations with a scope and found that the clipping is not symetrical; negative swing clips first at 28 Volts; the positive swing clips at 30 volts.
- Is this situation normal? Both supply rails are perfectly symetrical (36 Volts)
- I have at output a DC offset about 40 mV, can I reduce it by better matching of output FETs? or matching input symetrical pair ?
Thanks for your help,
Vince
I think that's normal.
For offset, I would adjust the value of the resistor off the
Collector of the input transistor which drives the base of the
npn voltage gain transistor.
😎
For offset, I would adjust the value of the resistor off the
Collector of the input transistor which drives the base of the
npn voltage gain transistor.
😎
Hi Mr. Pass,
I am very pleased to receive a reply from N.P himself 🙂
So there is no way to make the clipping symmetrical on both positive and negative swing. I will make a try to reduce the offset.
Thank you for your help and also all your actions for DIYers.
I have all the parts but could not find yet heatsink to build an Aleph 5. Parts are in a box somewhere in my garage for many years. Your reply encourages me to start a new search for heatsink and build this Aleph.
All my best,
Vince
I am very pleased to receive a reply from N.P himself 🙂
So there is no way to make the clipping symmetrical on both positive and negative swing. I will make a try to reduce the offset.
Thank you for your help and also all your actions for DIYers.
I have all the parts but could not find yet heatsink to build an Aleph 5. Parts are in a box somewhere in my garage for many years. Your reply encourages me to start a new search for heatsink and build this Aleph.
All my best,
Vince
Hi Mr. Pass,
As you said, I adjusted the value of the resistor of the
Collector of the input transistor which drives the base of the
npn voltage gain transistor ( 680 R replaced by 560 R) and the offset is null now.
About the clipping, I have noticed that when I reduced the bias current (thru IRFP 240) down to 30 mA the clipping is more symmetrical (without load).
In your document related to Mosfet Citation 12 it is said that "D3 ensures symmetrical clipping" (see attached screen capture). 😕
As you said, I adjusted the value of the resistor of the
Collector of the input transistor which drives the base of the
npn voltage gain transistor ( 680 R replaced by 560 R) and the offset is null now.
About the clipping, I have noticed that when I reduced the bias current (thru IRFP 240) down to 30 mA the clipping is more symmetrical (without load).
In your document related to Mosfet Citation 12 it is said that "D3 ensures symmetrical clipping" (see attached screen capture). 😕
Attachments
Hi NP,
That's what I did and still trying to do because I replaced MPSL51 by BC55C and MPSL01 by BD139, those transistors are easy to find overhere. The amp is singing but I am not sure that I didn't mess the bias somewhere. In my case, must I concentrate my effort in adjusting the bias resistors of the 1st and 2nd gain stages only?
By the way I use for output devices IRFP240 and I have increased supply voltage up to +-40 Volts without seing
.
Vince
That's what I did and still trying to do because I replaced MPSL51 by BC55C and MPSL01 by BD139, those transistors are easy to find overhere. The amp is singing but I am not sure that I didn't mess the bias somewhere. In my case, must I concentrate my effort in adjusting the bias resistors of the 1st and 2nd gain stages only?
By the way I use for output devices IRFP240 and I have increased supply voltage up to +-40 Volts without seing
.Vince
Hi NP,
I worked around the amp, it is doing fine I am very very happy with the sound.
I have a question about output devices bias current. I set arbitrary bias current at 100 mA about 10 mn after power up . After couple of hours of operation I measured the bias current (heatsink is warm not really hot) it was reduced to 60 mA. I think that the situation is normal and I wonder why not set the bias current directly at 60 mA at power up because it will stabilize itself at 60 mA after a while. At 100 mA bias or 60 mA bias the negative swing always clips first at the same voltage, I noticed no difference in sound or amp behavior.
Thanks a lot for your info,
Vince
I worked around the amp, it is doing fine I am very very happy with the sound.
I have a question about output devices bias current. I set arbitrary bias current at 100 mA about 10 mn after power up . After couple of hours of operation I measured the bias current (heatsink is warm not really hot) it was reduced to 60 mA. I think that the situation is normal and I wonder why not set the bias current directly at 60 mA at power up because it will stabilize itself at 60 mA after a while. At 100 mA bias or 60 mA bias the negative swing always clips first at the same voltage, I noticed no difference in sound or amp behavior.
Thanks a lot for your info,
Vince
No it won't. If you set the bias at 60mA cold then when warm it will be around 36mA.
The negative clipping first is due to the topology of the amp, there is nothing much you can do about it. I wouldn't worry about it unless you plan to drive the amp really hard all the time.
Hi ,
Thanks for your reply.
The temperature compensation circuit acts too strong right? I have added an another insulating pad between transistor (temperature compensation) and heatsink. I hope that it will smooth down the temperature compensation.
Thanks for your reply.
The temperature compensation circuit acts too strong right? I have added an another insulating pad between transistor (temperature compensation) and heatsink. I hope that it will smooth down the temperature compensation.
sorry to come back late I was very busy.
My bias current is always reducing when the amp is warm. If the amp is getting hotter will I expect crossover distortion or not?
On my circuit I have mounted the bias transistor on the same heatsink than output transistors is it good like this or must I put it appart? In his paper NP did not say anything on this point.
Thanks for your info,
My bias current is always reducing when the amp is warm. If the amp is getting hotter will I expect crossover distortion or not?
On my circuit I have mounted the bias transistor on the same heatsink than output transistors is it good like this or must I put it appart? In his paper NP did not say anything on this point.
Thanks for your info,
Yes the sound is nice but I didn't push the amp very hard yet.
This very strange because on other amps (not Mosfet, this is my first Mosfet amp) I own the bias current stays stable.
Thanks,
This very strange because on other amps (not Mosfet, this is my first Mosfet amp) I own the bias current stays stable.
Thanks,
You mean if ouput transistors are matched bias current stability will improve?
I was given 2 pairs of IRFP240, they are supposed matched so I used them as is, should I check if they are really matched?
I was given 2 pairs of IRFP240, they are supposed matched so I used them as is, should I check if they are really matched?
No I mean that in order to use n-channel devices rather than complementary, the characteristics of the upper and lower halfs are different.
While I am at it I will try also the Citation 12 version with complementary output devices (as I see in the schematic it is very easy to switch to complementary version from n-channel-devices version) to see which version is best. (sonically and to build).
I am facing buzz sound now in my speaker, can't get rid of it.
When the circuit was mounted on a wood board I had almost no noise, I had to stick my ear to the speaker to hear very very tiny buzz sound.
I mounted the circuit in a chassis and the buzz appeared, not very loud but it can be heard 1 meter from the speaker. I have checked for ground loops, took the transformer outside of the box, then put the transformer in a PC case PSU in order to put a shield around the transformer. The noise level has reduced significantly but some buzz still remain.
Something else that I could try?
When the circuit was mounted on a wood board I had almost no noise, I had to stick my ear to the speaker to hear very very tiny buzz sound.
I mounted the circuit in a chassis and the buzz appeared, not very loud but it can be heard 1 meter from the speaker. I have checked for ground loops, took the transformer outside of the box, then put the transformer in a PC case PSU in order to put a shield around the transformer. The noise level has reduced significantly but some buzz still remain.
Something else that I could try?
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