Are you going to adjust the front end current source?
Your measurements show a slight unbalance in the diff pair.
btw. No other theory but the zener. Which is no longer a
possibilty to verify since they are destroyed.
Few years ago something similar happened to me. In that
particular case a few poor 9,1V zeners caused two smoking
zen's. Binned the remaining ones.
Your measurements show a slight unbalance in the diff pair.
btw. No other theory but the zener. Which is no longer a
possibilty to verify since they are destroyed.
Few years ago something similar happened to me. In that
particular case a few poor 9,1V zeners caused two smoking
zen's. Binned the remaining ones.
rtirion said:
For the time being, no. I think a small offset may be regarded as normal operation. When I hooked up the diff pair as a true current mirror the balance was close to perfect, so if there is a DC offset now, it could just as well be a response to correct the output.
I'm going to leave it for now, though I have a small doubt on the 45mV DC offset on the left channel
<brag mode>
For R11 and R14, I have used
Vishay RNC90Y Ultra-precise low-drift metal film resistors with an initial tolerance of 0.05%
They were excess from one of the projects 
</brag mode>
Please tell me why!
Why on earth does it seem that each time I post success in this forum on my A2 amps, reality catches up the day after...
While listening to some CD's, about an hour and a half after switching the amps on, number 2 starts to produce a noise again... Not a ground loop hum, more like white noise.
I quickly disconnected my speaker and got my multimeter downstairs. There is no consistent DC on the output. The reading changes between -10mV and -25mV which is still less than the -45mV on the number 1 amp. AC readings are below 5mV and below the reading capability of my DMM. The noise is audible, but very soft.
So, whatever theory is left, I don't know. I'm afraid some component may have had a jab, jolt or whatever, and may be prone to fail. I wouldn't like to see a catastrophic failure you understand....
I'm frustrated and disappointed, and I'm thinking to rebuild the front-end with brand new components for both amps, hoping to remove the failure.....
Why on earth does it seem that each time I post success in this forum on my A2 amps, reality catches up the day after...
While listening to some CD's, about an hour and a half after switching the amps on, number 2 starts to produce a noise again... Not a ground loop hum, more like white noise.
I quickly disconnected my speaker and got my multimeter downstairs. There is no consistent DC on the output. The reading changes between -10mV and -25mV which is still less than the -45mV on the number 1 amp. AC readings are below 5mV and below the reading capability of my DMM. The noise is audible, but very soft.
So, whatever theory is left, I don't know. I'm afraid some component may have had a jab, jolt or whatever, and may be prone to fail. I wouldn't like to see a catastrophic failure you understand....
I'm frustrated and disappointed, and I'm thinking to rebuild the front-end with brand new components for both amps, hoping to remove the failure.....
noise
Did you check for oscillations? You might want to poke around
your amp with a scope, before hooking them up to really
nice speakers. Check the Aleph Current Source around the BJT.
Check stability with a square wave input to see if there
is overshoot/ ringing on the edges.
If possible:
Check the phase margin. I.e. measure the frequency were your
amp reaches unity gain. Measure phase at given frequency.
Otherwise:
Measure -3dB bandwith. If larger then 100kHz, consider
increasing the cap accross the feedback resistor.
Do not choose the capacitor larger then neccessary.
In all cases it is smart to have the amps idle for at least 30
minutes before taking measurements.
Please do not be frustrated/ dissapointed. The "problems" are
(should be) part of the fun in the journey of constructing an amp.
Every problem solved means you (we) learned something
of value.
You could even take a break of two days away from the amps.
Then start measurements/ testing in the weekend?
Did you check for oscillations? You might want to poke around
your amp with a scope, before hooking them up to really
nice speakers. Check the Aleph Current Source around the BJT.
Check stability with a square wave input to see if there
is overshoot/ ringing on the edges.
If possible:
Check the phase margin. I.e. measure the frequency were your
amp reaches unity gain. Measure phase at given frequency.
Otherwise:
Measure -3dB bandwith. If larger then 100kHz, consider
increasing the cap accross the feedback resistor.
Do not choose the capacitor larger then neccessary.
In all cases it is smart to have the amps idle for at least 30
minutes before taking measurements.
Please do not be frustrated/ dissapointed. The "problems" are
(should be) part of the fun in the journey of constructing an amp.
Every problem solved means you (we) learned something
of value.
You could even take a break of two days away from the amps.
Then start measurements/ testing in the weekend?
Having lost some favorite speakers to an amp project turned R.F. oscillator I can heartily agree with the last post . And I have no on to blame but my own impatience, as I own several scopes.
If you don't have a scope let it idle with no signal for several hours loaded with a 4-16 or so ohm 1/4 watt resistor as a load and see if it becomes warm. Smoked copper is irreversable.
I am finally starting on my own Aleph 2 Where are all of you getting your mosfets Digikey has increased the price quite a bit on IRFP240PBF. Nobody seems to stock the Fairchild FQA19N20
But I found that I can buy 100 from Fairchild directly for US$1.27 each. Am I likey to get enough matched devices for an Aleph2 from that quantity?.
If you don't have a scope let it idle with no signal for several hours loaded with a 4-16 or so ohm 1/4 watt resistor as a load and see if it becomes warm. Smoked copper is irreversable.
I am finally starting on my own Aleph 2 Where are all of you getting your mosfets Digikey has increased the price quite a bit on IRFP240PBF. Nobody seems to stock the Fairchild FQA19N20
But I found that I can buy 100 from Fairchild directly for US$1.27 each. Am I likey to get enough matched devices for an Aleph2 from that quantity?.
amp-guy said:Sounds like the fairchilds are maybe the better choice . I can't figure why no one stocks them. Actually there is a fairchild fab plant a rocks throw away from my home, don't know which devices they make there. so looks like I will just order direct from the website.
Maybe you should start going through the trash bins.
Can't you get them from IR anymore? Or was I just lucky enough to order them from IR overnight 4 years ago?
-- edit --
Anyway, just for an update on the process so far:
- I've swapped the front end of the "faulty" amp with the "healhy" one and it appears the problem moved with it.
- I checked the power supply with a resistor load and it remains symmetrical, so I guess the rectifier is ok and the problem was not caused by an assymetrical supply
I'll see if I can get the problem isolated with the scale model. I'd hate to blow my IRFP240's, because I haven't got any matches left.
Same applies for the IR9610's... If they appear to be the broken one's I'll need to get new ones and match them.
-- edit --
Anyway, just for an update on the process so far:
- I've swapped the front end of the "faulty" amp with the "healhy" one and it appears the problem moved with it.
- I checked the power supply with a resistor load and it remains symmetrical, so I guess the rectifier is ok and the problem was not caused by an assymetrical supply
I'll see if I can get the problem isolated with the scale model. I'd hate to blow my IRFP240's, because I haven't got any matches left.
Same applies for the IR9610's... If they appear to be the broken one's I'll need to get new ones and match them.
Component Tolerances
Question for Mr Pass (or anyone willing to answer):
About the Aleph 2 design:
Some fellow forumers seem to be quite keen on making sure the constant current source sinks exactly 0.5A, and are willing to tweak the differential pair such, that any DC offset is removed. On the other hand, the original design doesn't mention anything of making these values fit precisely, and the service manual clearly states the Aleph 2 does not need any adjustments.
Q1: To be sure: This means that -for example- the constant current source output is actually dependant on the initial tolerances of the componenents used, as well as the typical junction voltages?
Q2: It is only mentioned that the mosfets in the differential pair are matched, as well as the fets in the output stage. Are the bias transistors also supposed to be matched?
Q3: Assuming that the above two answers are yes, There must be a spread in -for example- current source output between different amplifiers. Does this have a noticeable impact on sound?
Q4: The differential pair will work to compensate any DC imbalance between the constant current source and the lower mosfets. To which extent can this compensation be regarded as normal operation, and from where may one expect audible influence of mismatching. (In other words, how important is it to have upper and lower fets closely matched?)
Thanks for answers ;-)
Question for Mr Pass (or anyone willing to answer):
About the Aleph 2 design:
Some fellow forumers seem to be quite keen on making sure the constant current source sinks exactly 0.5A, and are willing to tweak the differential pair such, that any DC offset is removed. On the other hand, the original design doesn't mention anything of making these values fit precisely, and the service manual clearly states the Aleph 2 does not need any adjustments.
Q1: To be sure: This means that -for example- the constant current source output is actually dependant on the initial tolerances of the componenents used, as well as the typical junction voltages?
Q2: It is only mentioned that the mosfets in the differential pair are matched, as well as the fets in the output stage. Are the bias transistors also supposed to be matched?
Q3: Assuming that the above two answers are yes, There must be a spread in -for example- current source output between different amplifiers. Does this have a noticeable impact on sound?
Q4: The differential pair will work to compensate any DC imbalance between the constant current source and the lower mosfets. To which extent can this compensation be regarded as normal operation, and from where may one expect audible influence of mismatching. (In other words, how important is it to have upper and lower fets closely matched?)
Thanks for answers ;-)
Re: Component Tolerances
The diff pair current source operates at 20mA. Close matching of these will minimize DC offset. Unless you might be confusing the current source in the output stage, the supposed golden figure is that it contributes 50% AC gain.
Q1 - not sure on this one
Q2 - I suppose you can spend the time matching resistors, might as well since you've put so much effort into the build. I just use 1% source resistors and be done with it.
Q3 - I think that if you used different trannies in one amp versus the other you'd notice a difference. Thats not the case here, as you've used the same devices throughout, as long as critical voltages are relatively close, its not likely there will be perceived difference.
Q4 - Its not necessary to match upper with lower fets. Match upper and lower separately. Close matching here will ensure proper current sharing. If not matched properly, distortion will rise. In one of my previous builds, I was running an Aleph 5 output stage around 5.5 amps. I apparently didn't match the output stage properly and one of the fets died a firey death as it was hogging current. As long as you've matched to 0.01 mV, I wouldn't lose any sleep.
hope that helps
Bakmeel said:
The diff pair current source operates at 20mA. Close matching of these will minimize DC offset. Unless you might be confusing the current source in the output stage, the supposed golden figure is that it contributes 50% AC gain.
Q1 - not sure on this one
Q2 - I suppose you can spend the time matching resistors, might as well since you've put so much effort into the build. I just use 1% source resistors and be done with it.
Q3 - I think that if you used different trannies in one amp versus the other you'd notice a difference. Thats not the case here, as you've used the same devices throughout, as long as critical voltages are relatively close, its not likely there will be perceived difference.
Q4 - Its not necessary to match upper with lower fets. Match upper and lower separately. Close matching here will ensure proper current sharing. If not matched properly, distortion will rise. In one of my previous builds, I was running an Aleph 5 output stage around 5.5 amps. I apparently didn't match the output stage properly and one of the fets died a firey death as it was hogging current. As long as you've matched to 0.01 mV, I wouldn't lose any sleep.
hope that helps
Re: Component Tolerances
1 The bias is not critical, but is pretty well locked by the npn
transistor on the current source sensing the current through the
output Source resistor, so no.
2 The N channel Mosfets in parallel should be matched.
3 (1) was no.
4 It's normal. If you want to tweak it, you can adjust the amount
of current going into the diff pair or the resistor value off the Drain
of the input Mosfet. It does not have a large effect.
Bakmeel said:
1 The bias is not critical, but is pretty well locked by the npn
transistor on the current source sensing the current through the
output Source resistor, so no.
2 The N channel Mosfets in parallel should be matched.
3 (1) was no.
4 It's normal. If you want to tweak it, you can adjust the amount
of current going into the diff pair or the resistor value off the Drain
of the input Mosfet. It does not have a large effect.
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