William,
The improvements in mid and high are what you should expect.
The loss in bass is due to your loss in open loop gain. To get that back you need to get rid of the source resistors of the diff pair altogether, and increase the drain resistors to 900 ohm to 1k. Your open loop gain would then be comparable to before. Only now you have a lower open loop bandwidth for the second stage. But that may not be a problem once the loop is closed.
As to the bootstrap cap, put a 2200uF cap in and try to see whether it makes any difference.
Don't give up as yet.
Patrick
The improvements in mid and high are what you should expect.
The loss in bass is due to your loss in open loop gain. To get that back you need to get rid of the source resistors of the diff pair altogether, and increase the drain resistors to 900 ohm to 1k. Your open loop gain would then be comparable to before. Only now you have a lower open loop bandwidth for the second stage. But that may not be a problem once the loop is closed.
As to the bootstrap cap, put a 2200uF cap in and try to see whether it makes any difference.
Don't give up as yet.
Patrick
Hi Patrick,
I said the mids and highs were good to very good. Before the change the mids and highs were good to very good too.......
It is always difficult to tell the difference if some part (like the bass) is lacking.
I tried without the source resistors and got 132mV of offset wich is a bit too much for my taste. I will have to search a bit further for some better matched pair with high enough Idss. The 30 pieces I´ve got are a lot further apart as I´m used to with IRF parts
At the moment the drain resistors are 820R and bias is 4,7V/820 = 5,7mA. If I use a 900-1000R drain resistor bias will be 4,7-5,2mA so I could use a diff pair wit an Idss between 9 and 10mA. I´ve got 7 of those so maybe I´ll find some that work without source resistors. (2SJ109 would be welcome.......)
Could you explain why the open loop gain of the first stage has an influence on the bass performance? I must admit that I only looked at 1kHz signals and above cause I didn´t expect any difficulties here.
As for the cap in the active current source. Going to 2200uF would probably change the time from 1 to 10 seconds before distortion will be setting in. I don´t think this behaviour is important for normal listening and I will have a look if my Aleph 5 has the same "problem". It would just be nice to understand.
William
I said the mids and highs were good to very good. Before the change the mids and highs were good to very good too.......
It is always difficult to tell the difference if some part (like the bass) is lacking.
I tried without the source resistors and got 132mV of offset wich is a bit too much for my taste. I will have to search a bit further for some better matched pair with high enough Idss. The 30 pieces I´ve got are a lot further apart as I´m used to with IRF parts
At the moment the drain resistors are 820R and bias is 4,7V/820 = 5,7mA. If I use a 900-1000R drain resistor bias will be 4,7-5,2mA so I could use a diff pair wit an Idss between 9 and 10mA. I´ve got 7 of those so maybe I´ll find some that work without source resistors. (2SJ109 would be welcome.......)
Could you explain why the open loop gain of the first stage has an influence on the bass performance? I must admit that I only looked at 1kHz signals and above cause I didn´t expect any difficulties here.
As for the cap in the active current source. Going to 2200uF would probably change the time from 1 to 10 seconds before distortion will be setting in. I don´t think this behaviour is important for normal listening and I will have a look if my Aleph 5 has the same "problem". It would just be nice to understand.
William
> Could you explain why the open loop gain of the first stage has an influence on the bass performance?
It has been a long time since I did this, so I am only recalling from memory.
If I remember correctly, the standard AlephX-100 has about 30-40dB NFB on 8 ohms. This drops proportionally with load impedance. If your TOTAL loop gain is further reduced (since you have changed nothing for the second stage, this can only be a result of the first stage changes), AND you have a very reactive load which drops to say 1 ohm during large amplitude transients, your amplifier will run out of gain very soon.
Anything below 20dB NFB is asking a lot from the JFETs, as you would be putting something like +/- 0.2V at their gates. I think you would be driving one of them over Idss, and the other completely off (and hence deeply into its non-linear region.
Also, your damping factor suffers as the amount of NFB drops, if I am not wrong.
I have relatively efficient speakers (B&W 804s), and I do not need a lot of bass for my taste. So for me it was never an issue. I can recall (this has been years) comparing the Rollins Aleph-X with 4x 044s and 2x 9610s with the 100W AJX and for me it was definitely an improvement in terms of bass.
Compared with the AlephX100 with 9610s, I found highs much cleaner and sweeter. But that is only my observation with my own setup. AND I was (am) using 2SK1529s and NOT IRFP240s.
JFET was 2SJ109, no source degeneration, drain 910R.
Patrick
It has been a long time since I did this, so I am only recalling from memory.
If I remember correctly, the standard AlephX-100 has about 30-40dB NFB on 8 ohms. This drops proportionally with load impedance. If your TOTAL loop gain is further reduced (since you have changed nothing for the second stage, this can only be a result of the first stage changes), AND you have a very reactive load which drops to say 1 ohm during large amplitude transients, your amplifier will run out of gain very soon.
Anything below 20dB NFB is asking a lot from the JFETs, as you would be putting something like +/- 0.2V at their gates. I think you would be driving one of them over Idss, and the other completely off (and hence deeply into its non-linear region.
Also, your damping factor suffers as the amount of NFB drops, if I am not wrong.
I have relatively efficient speakers (B&W 804s), and I do not need a lot of bass for my taste. So for me it was never an issue. I can recall (this has been years) comparing the Rollins Aleph-X with 4x 044s and 2x 9610s with the 100W AJX and for me it was definitely an improvement in terms of bass.
Compared with the AlephX100 with 9610s, I found highs much cleaner and sweeter. But that is only my observation with my own setup. AND I was (am) using 2SK1529s and NOT IRFP240s.
JFET was 2SJ109, no source degeneration, drain 910R.
Patrick
Zen Mod,
ac-current-gain: checked and i.O. (51%)
bias: checked and i.O (6,9A)
Nothing was changed at the active current sources so this is as expected.
Patrick,
don´t think my speakers (Thiel CS6) are so difficult to drive at lower frequencies. The lowest impedance (3 Ohms) occurs at higher frequencies.
I will try the 910R / no Rs version when I´m back from holiday.
Do you know how much the gain will change from 820R / 8R?
Do the 2sk1529 have a lower input capacitance? Are they still available?
William
ac-current-gain: checked and i.O. (51%)
bias: checked and i.O (6,9A)
Nothing was changed at the active current sources so this is as expected.
Patrick,
don´t think my speakers (Thiel CS6) are so difficult to drive at lower frequencies. The lowest impedance (3 Ohms) occurs at higher frequencies.
I will try the 910R / no Rs version when I´m back from holiday.
Do you know how much the gain will change from 820R / 8R?
Do the 2sk1529 have a lower input capacitance? Are they still available?
William
The gain for the first stage with Toshiba JFETs (J74s or J109s) is roughly :
R drain / (30ohm + R source).
You can then work out the difference between different versions easily. The 30 ohm comes from 1/Yfs at 5mA bias, which is about 33mS.
Yes 2SK1529 is still widely available. And they have much lower capacitances. Datasheet is available on the internet (too large to post).
If you get 100 pieces, they are only slightly more expensive than IRFs (in Germany). But this was 3 years ago, I have to say.
Patrick
R drain / (30ohm + R source).
You can then work out the difference between different versions easily. The 30 ohm comes from 1/Yfs at 5mA bias, which is about 33mS.
Yes 2SK1529 is still widely available. And they have much lower capacitances. Datasheet is available on the internet (too large to post).
If you get 100 pieces, they are only slightly more expensive than IRFs (in Germany). But this was 3 years ago, I have to say.
Patrick
Hi,
only found some 1529´s at 4,57€ a piece at Kessler.......
So gain now is 820/40 = 20,5 (26dB)
Gain 910/40 = 22,75 (27dB)
Gain 910/30 = 30,33 (29,6dB)
so I could raise the open loop gain by 4dB, and loose some in the second stage.
I´ll just try. BTW the bass loss is not only appearant on high level listening but also at lower levels.
William
only found some 1529´s at 4,57€ a piece at Kessler.......
So gain now is 820/40 = 20,5 (26dB)
Gain 910/40 = 22,75 (27dB)
Gain 910/30 = 30,33 (29,6dB)
so I could raise the open loop gain by 4dB, and loose some in the second stage.
I´ll just try. BTW the bass loss is not only appearant on high level listening but also at lower levels.
William
> only found some 1529´s at 4,57€ a piece at Kessler.......
http://www.fibra-brandt.com/
List price 3 Euros + VAT. But ask him how much for 100.
Anything less than 100 you will not get good match (from experience); so I suggest you try to find out the cause to the bass loss first, which I admit is a bit of a puzzle to me as well.
Or maybe you should wait until Nelson publishes the Aleph J circuit for some hints.
Patrick
http://www.fibra-brandt.com/
List price 3 Euros + VAT. But ask him how much for 100.
Anything less than 100 you will not get good match (from experience); so I suggest you try to find out the cause to the bass loss first, which I admit is a bit of a puzzle to me as well.
Or maybe you should wait until Nelson publishes the Aleph J circuit for some hints.
Patrick
Hi Patrick,
did you compare the 1529 to the IRFP240´s (I´ve got plenty of the latter.....)
One hint concerning the Aleph J could be the use of 2 output fets instead of 3 (6?)
Let´s wait until I´ve tried the 910R plus no source resistors to see if this helps. If not I can return to the 9610´s and see if the dissappeared bass returns.
I will build a pair of Aleph 30´s for a friend in the near future. Will do further experiments with the JFets in these.......
William
did you compare the 1529 to the IRFP240´s (I´ve got plenty of the latter.....)
One hint concerning the Aleph J could be the use of 2 output fets instead of 3 (6?)
Let´s wait until I´ve tried the 910R plus no source resistors to see if this helps. If not I can return to the 9610´s and see if the dissappeared bass returns.
I will build a pair of Aleph 30´s for a friend in the near future. Will do further experiments with the JFets in these.......
William
jupiterjune said:
My post was in reference to post #3, wherein the poster references another circuit. However, the same principles apply.
There's a reason that you don't see the "big guys" using circuits that drive devices past their limits: it's not a good idea.
Let's begin with the transfer curves that you can find in the datasheets. You'll note that for any device that we would describe as "linear," the chart stops when the control element--in this case, the Gate--hits the axis. True, you can find charts that go past the axis...and they're usually in textbooks as examples as to why you don't want to overdo it. In a nutshell, the behavior of the device becomes grotesquely nonlinear. Going a little bit over the line gives you distortion. Going a little bit further drives the device into catastrophic breakdown.
The whole point of John Curl's complementary JFET follower (which is what the poster was referring to, as I recall) is that each part--the N-ch and the P-ch--is matched against the other. The further you get from that ideal, the more you have to look at ways to rein in the stronger device. The typical way to do this is to add resistance under the Source of the part with the higher Idss in order to bring the pair back into balance.
To allow the Idss of one part to completely override the other is not a particularly good idea. Let's assume for the moment that you're not driving the underdog part so hard that it's drawing Gate current (this is bad), you're pretty much guaranteed that the JFET is in a very nonlinear state; not something I'd call a resistor.
This is DIY, you're pretty much free to do what you want. I choose not to do this particular thing.
Just for giggles, test the circuit for distortion, particularly with a strong input signal.
And yes, using the CCS to overdrive the gain devices in a differential gives you pretty much the same sort of problem. Depletion mode JFETs are not meant to be operated as though they are enhancement mode devices.
Grey
GRollins said:
I fully agree that using the complementary matching against the other could be the best. But, this can not stop new way of experiment and fun. The overriding does not necessarily mean that it passes the Idss axis as you say. I’d like to think that the overriding means vertically shifting the Q-point keeping the original transfer curve’s slope. The overriding might not improve its own narrow span of current swing though.
The narrow span of current swing is not a big deal in my specific case. For example, Babo Zen needs very small current swing there. And, I don’t see any symptom of actual problem from my Babo Zen yet, except the very good sound. I will see further if I meet any problem, for your info.
> did you compare the 1529 to the IRFP240´s (I´ve got plenty of the latter.....)
Yes. I have published the curve somewhere. They have similar transconductance. It is well known that I prefer 2SK1529s. But that is a matter of taste. Nelson does not.
One hint concerning the Aleph J could be the use of 2 output fets instead of 3 (6?)
How you tell !!!!!
I use 12 Power FETs per channel in total.
Don't be so Dutch. IRFPs cost nothing !!
PS I am not German, onl live here. But you can still hate me nevertheless. (Joke)
Patrick
Yes. I have published the curve somewhere. They have similar transconductance. It is well known that I prefer 2SK1529s. But that is a matter of taste. Nelson does not.
One hint concerning the Aleph J could be the use of 2 output fets instead of 3 (6?)
How you tell !!!!!
I use 12 Power FETs per channel in total.
Don't be so Dutch. IRFPs cost nothing !!
PS I am not German, onl live here. But you can still hate me nevertheless. (Joke)
Patrick
found some bass
Hi,
this morning I wanted to check the cold startup abs. dc offset after setting it to zero yesterday evening.
It´around 2,75 Volts wich means that I can easily raise the McMillan resistors to 20k or so.
I also checked the rel. dc offset wich was 4mV on the right and around 5V on the left......
Somehow while carrying the amp downstairs a source resistor had lost contact and there was a slight imbalance
This probably put some tension on the left woofer causing it not to woof very nice anymore.
So after resoldering and reseting the missing bass found it´s way back into my living room
The sound is quite nice now.
When I´m back from holiday I will first raise the McMillans to 20k, this had quite some influence on the sound with the 9610´s and probably will with the 2sj74´s
William
Hi,
this morning I wanted to check the cold startup abs. dc offset after setting it to zero yesterday evening.
It´around 2,75 Volts wich means that I can easily raise the McMillan resistors to 20k or so.
I also checked the rel. dc offset wich was 4mV on the right and around 5V
on the left......Somehow while carrying the amp downstairs a source resistor had lost contact and there was a slight imbalance
This probably put some tension on the left woofer causing it not to woof very nice anymore.
So after resoldering and reseting the missing bass found it´s way back into my living room
The sound is quite nice now.
When I´m back from holiday I will first raise the McMillans to 20k, this had quite some influence on the sound with the 9610´s and probably will with the 2sj74´s
William
Re: found some bass
bingo
wuffwaff said:Hi,
this morning I wanted to check the cold startup abs. dc offset after setting it to zero yesterday evening.
It´around 2,75 Volts wich means that I can easily raise the McMillan resistors to 20k or so.
I also checked the rel. dc offset wich was 4mV on the right and around 5V
Somehow while carrying the amp downstairs a source resistor had lost contact and there was a slight imbalance
This probably put some tension on the left woofer causing it not to woof very nice anymore.
So after resoldering and reseting the missing bass found it´s way back into my living room
The sound is quite nice now.
When I´m back from holiday I will first raise the McMillans to 20k, this had quite some influence on the sound with the 9610´s and probably will with the 2sj74´s
William
bingo
Hi Patrick,
yes I will. I will probably use some 5R source resistors with a 50R trimmer in parallel though to avoid having to buy another 50 2sj74´s (yes I know I´m dutch)
I will also change the McMillan resistors to a value as high as possible.
I´m already using 12Fets per channnel (IRFP240). Changing them to 2sk1529 would really be a bit of work as they are all hard wired and not so easy to change. It is probably better to sell mine and build some new ones.....this way I could make them a bit wider to fit a non humming transformer.
William
yes I will. I will probably use some 5R source resistors with a 50R trimmer in parallel though to avoid having to buy another 50 2sj74´s (yes I know I´m dutch)
I will also change the McMillan resistors to a value as high as possible.
I´m already using 12Fets per channnel (IRFP240). Changing them to 2sk1529 would really be a bit of work as they are all hard wired and not so easy to change. It is probably better to sell mine and build some new ones.....this way I could make them a bit wider to fit a non humming transformer.
William
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