Hi Eric,
yes I can do that but it can take a few days (weeks?) before I open it up again.
William
yes I can do that but it can take a few days (weeks?) before I open it up again.
William
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Thanks William! A few weeks is not a problem as there is no way that I'll have my amps up and running by then - I think my time scale is best measured in months! Certainly no rush...
Eric
Eric
What is the source of the offset at the ouput terminals? Does Aleph 30 have the same problem? Thanks.
Hi,
do you mean the offset between the two terminals or the offset relative to ground?
The first is also present in the Aleph 30 but not really a problem as long as you match the differential pair nicely (below 100mV is OK)
The second can only exist in bridges amps like the Aleph-X and isn´t reall a problem either cause the speaker won´t see it.
William
do you mean the offset between the two terminals or the offset relative to ground?
The first is also present in the Aleph 30 but not really a problem as long as you match the differential pair nicely (below 100mV is OK)
The second can only exist in bridges amps like the Aleph-X and isn´t reall a problem either cause the speaker won´t see it.
William
What I mean is that the DC voltage difference between the two halves (sometimes a few volts). From what I read in this thread that it changes as the amp warms up. I cannot imagine that this would also be the case for Aleph 30.
I see that some people use a resister (30 ohms or so) between the output and the ground to ``reduce'' the offset. If a 30 ohm can do the trick, a speaker should also do it (most current will go through it anyway as the DC resistence of a speaker is very small).
I see that some people use a resister (30 ohms or so) between the output and the ground to ``reduce'' the offset. If a 30 ohm can do the trick, a speaker should also do it (most current will go through it anyway as the DC resistence of a speaker is very small).
Hi,
the 30 ohm resistor is between outputs and ground, not between outputs so the speaker will have no effect here as it is not connected to ground! The X is a BRIDGED amp.
The DC- Offset between the two halves is the relative DC-Offset and is normally just a few millivolts even when cold. The offset that changes during warm up is the absolute offset between the outputs and ground. There is certainly a way to avoid this but I haven´t found it yet. It would be interesting to see what a factory build X does............
William
the 30 ohm resistor is between outputs and ground, not between outputs so the speaker will have no effect here as it is not connected to ground! The X is a BRIDGED amp.
The DC- Offset between the two halves is the relative DC-Offset and is normally just a few millivolts even when cold. The offset that changes during warm up is the absolute offset between the outputs and ground. There is certainly a way to avoid this but I haven´t found it yet. It would be interesting to see what a factory build X does............
William
Wuffwaff,
You are assuming the two halves will warm up exactly the same way, resulting in the same offsets. Maybe some one who actually built on can comment on how close they are in an actual amp.
Aleph 30's offset probably does not change as much duing warm up-- As a commercial product I cannot image it does. There must be a way to control the offset.
You are assuming the two halves will warm up exactly the same way, resulting in the same offsets. Maybe some one who actually built on can comment on how close they are in an actual amp.
Aleph 30's offset probably does not change as much duing warm up-- As a commercial product I cannot image it does. There must be a way to control the offset.
Hi Ctong,
since I build both an Aleph5 and a pair of Aleph-X I am not talking about theoretical things.
One channel starts at 18mV and eventually gets to 16mV, the other channel starts at 3mV and is near zero after two hours and reaching around 55°C. It helps to connect the diff pair mechanically.
My Aleph 5 starts at around 28mV and gets to 20mV after warm up, the other channel is a bit lower but shows the same behaviour.
William
since I build both an Aleph5 and a pair of Aleph-X I am not talking about theoretical things.
One channel starts at 18mV and eventually gets to 16mV, the other channel starts at 3mV and is near zero after two hours and reaching around 55°C. It helps to connect the diff pair mechanically.
My Aleph 5 starts at around 28mV and gets to 20mV after warm up, the other channel is a bit lower but shows the same behaviour.
William
Hi William,
Great! So the two halves are nearly identical. Did you build the 30 or 60? Where did you post your schematic? Thanks.
Great! So the two halves are nearly identical. Did you build the 30 or 60? Where did you post your schematic? Thanks.
A question about the bias and the maximum current output. It is my understanding that with the type of current sources in the Alephs the maximum output is twice the bias current. Yet the service manuals often give a larger ouput current. For example, Aleph 30's bias is "slghtly less than 2amps" but the max output current is 5 amps. Any explinations? Thanks.
ctong,
Your understanding is correct IF the AC current gain is set at 50%. the Aleph 30 must have more than 50% AC current gain.
Your understanding is correct IF the AC current gain is set at 50%. the Aleph 30 must have more than 50% AC current gain.
Take some time to review the Aleph current source patent.
As used here, 50% AC current gain means that at maximum output half the current comes from the Aleph Curretn source's AC current gain, half is the static bias. 67% gain would mean that at max output, 67% of the current comes from the AC gain, so a bias of 1 amp could produce an ouput current of 3 amps (1 amp bias, 2 from AC gain)
As shown in the Aleph 30 service manual, the Ratios of R114 and R115 and the parallel combinations of R120,121 and R124-7 set the current gain. Since the output resistors add up to half the value of the emitter resistors in parallel, the AC gain would be 50% if R114=R115. Reducing R114 increases the AC gain. Since the Aleph 30 has R114 at 750R and R115 at 1K, the gain is greater than 50%
Hope this helps
As used here, 50% AC current gain means that at maximum output half the current comes from the Aleph Curretn source's AC current gain, half is the static bias. 67% gain would mean that at max output, 67% of the current comes from the AC gain, so a bias of 1 amp could produce an ouput current of 3 amps (1 amp bias, 2 from AC gain)
As shown in the Aleph 30 service manual, the Ratios of R114 and R115 and the parallel combinations of R120,121 and R124-7 set the current gain. Since the output resistors add up to half the value of the emitter resistors in parallel, the AC gain would be 50% if R114=R115. Reducing R114 increases the AC gain. Since the Aleph 30 has R114 at 750R and R115 at 1K, the gain is greater than 50%
Hope this helps
Here is my explincation. It is probably releted to the nonlinearity of the transconductance curve: upswings have a larger ac current than down swings for both the output transisters and the current source, therefore for both positive and negative signals the max output current can be somewhat larger than twice the bias current. This, however, might result in high levels of distortion.
Bob,
Thanks for the information. I now understand the ac gain issue. But it seems that what you said is true before Q106-7 shut off during a down swing. After that point the output current cannot increase unless the current through Q108-9 keeps increasing.
The nonlinear transconductance curve provides a way for that to happen.
Thanks for the information. I now understand the ac gain issue. But it seems that what you said is true before Q106-7 shut off during a down swing. After that point the output current cannot increase unless the current through Q108-9 keeps increasing.
The nonlinear transconductance curve provides a way for that to happen.
Ctong,
when setting the amp up for more than 50% ac-current-gain the upper half (current source) will shutt off during negative current peaks causing the amp to get in some sort of AB mode. I don´t think this has anything to do with the nonlinear transconductance curve. You can set the ac-current gain for 4 times the bias or more if you want although it won´t sound very nice. An ac-current-gain of around 60-65% is acceptable though and always better than a clipping amp.
Like Bob said, try to read the patent or the articles on the Zen variations (here the active current source is explained in part 3 or 4 I think)
William
when setting the amp up for more than 50% ac-current-gain the upper half (current source) will shutt off during negative current peaks causing the amp to get in some sort of AB mode. I don´t think this has anything to do with the nonlinear transconductance curve. You can set the ac-current gain for 4 times the bias or more if you want although it won´t sound very nice. An ac-current-gain of around 60-65% is acceptable though and always better than a clipping amp.
Like Bob said, try to read the patent or the articles on the Zen variations (here the active current source is explained in part 3 or 4 I think)
William
wuffwaff said:
It's worth noting that if you bias the amplifier at the expected
peak output current, you won't get this effect, and you can set
the current gain arbitrarily.
😎