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Old 12th May 2008, 08:45 AM   #21
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by G33/33
Hi Alex,

is there any alternatives for ZVP3306A ? I can't find it in Indonesia

can I use IRF 9610 / 9510 ?
ZVP3306A is available from Farnell. Possible alternatives are ZVP3310A, ZVP2110, ZVP4105, BSP254.

IRF9610/9510 are far too big for this position and would seriously impair performance and stability. I am considering now an "upside down" version of the amp, using PNP input and N-ch MOSFET instead of P-ch - as there is wider choice of suitable devices. Another possible P-ch alternative could be 2SJ76-79, I may try it in that position and see what happens.

Alex
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Old 12th May 2008, 09:09 AM   #22
G33/33 is offline G33/33  Indonesia
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Quote:
Originally posted by x-pro


ZVP3306A is available from Farnell. Possible alternatives are ZVP3310A, ZVP2110, ZVP4105, BSP254.

IRF9610/9510 are far too big for this position and would seriously impair performance and stabitity. I am considering now an "upside down" version of the amp, using PNP input and N-ch MOSFET instead of P-ch - as there is wider choice of suitable devices. Another possible P-ch alternative could be 2SJ76-79, I may try it in that position and see what happens.

Alex

Thanks Alex,
I will try to find BSP 254 and 2SJ76-79.


rgds,
G33/33
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Old 12th May 2008, 09:59 AM   #23
ppa is offline ppa  Italy
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Hi Alex and All,

for the power bjts I would like to suggest the a1943 and c5200 by Semelab , I have tried they I can say the Semelab works very fine more than the same bjts by other companies.
I have used they in a my amp. I have found they at Farnell.

For q9 I would like to suggest the bc327.

for q8 and q10 other changes don't improve the sound for their configurations.

change q9 I think it's important.

for q3, q4, q5,q6 other changes don't improve the sound for their configurations.



Pier Paolo Abbate.
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Old 12th May 2008, 01:06 PM   #24
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Default Re: Cambridge Audio A3i repairs and mods

Quote:
Originally posted by x-pro
I've designed that amp more than 12 years ago. As it happened, it was designed to be a 40W per channel amplifier but at the last moment it was promoted by the management into 60W just by boosting the supply voltage. That and also poor quality PCB and some components contributed to a common fault - as a result of a local board overheating one of the LED's (usually LED2) or Q8 transistor next to it (BC640) goes open circuit. Thankfully, protection circuit in this case just disables the output. It is useful, if you came across this amplifier, replace both LED2 and Q8, mounting them at least 10-12 mm from the PCB.
I think this is exactly the problem with my A3i, I've ordered new LED along with a BC640, hopefully I should have a working amp again. Thanks for posting this info and the schematic.
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Old 15th May 2008, 10:00 PM   #25
x-pro is offline x-pro  United Kingdom
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Another update.

After playing in LTSpice with A3i circuit I found an alternative way to compensate it, resulting in a substantial increase of OLG at high frequencies and better THD figures. Here are the results after I've tried the simulated network on a real A3i:

THD+N measured by Amber 3501 in 300 kHz measuring BW for full 50W into 8 Ohm are 0.006% for 10 kHz and for 0.008% for 20 kHz - . At lower frequencies or levels distortion are on the limit of my measuring equipment. Attached picture shows the spectrum of 10 kHz 10V RMS into 8 Ohm - in 48kHz bandwidth THD is 0.0004% .

Alex
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File Type: gif 10v_rms_10khz_8ohm.gif (15.6 KB, 1320 views)
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Old 15th May 2008, 10:05 PM   #26
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And here is another graph - for 1kHz 50W (20V RMS into 8Ohm) output. THD = 0.0004%.

Alex
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Old 12th June 2008, 07:39 PM   #27
r7aake is offline r7aake  Switzerland
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Hello,

my A3i amp died two weeks ago and I found the fault Q8 transistor that I already replaced.

I thought it would be all fine, but after listening to some music for a few minutes, loud DC noise (again) in same channel speaker + relay protection (outputs cut).

Turning the amp off then on again, seems to work for a few minutes, no more.

The LED2 seems to be fine (lighted). Can it still be damaged? SHould I replace it anyway? It's almost sticked to the PCB, but not warm at all and visibly working.

When turning to faulty state, I can measure about -30 Volts on the collector of Q8 instead of 1.1 Volts (that I can measure on Q108, or when starting the amp, on Q8).

Any help greatly appreciated.

Cheers,
Gilles
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Old 12th June 2008, 07:54 PM   #28
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by r7aake
Hello,

my A3i amp died two weeks ago and I found the fault Q8 transistor that I already replaced.

I thought it would be all fine, but after listening to some music for a few minutes, loud DC noise (again) in same channel speaker + relay protection (outputs cut).

Turning the amp off then on again, seems to work for a few minutes, no more.

The LED2 seems to be fine (lighted). Can it still be damaged? SHould I replace it anyway? It's almost sticked to the PCB, but not warm at all and visibly working.

When turning to faulty state, I can measure about -30 Volts on the collector of Q8 instead of 1.1 Volts (that I can measure on Q108, or when starting the amp, on Q8).

Any help greatly appreciated.

Cheers,
Gilles
Hi Giles,

yes, it is most likely that LED2 is faulty. Just replace it and lift the replacement at least 10-12 mm from the PCB.

Cheers

Alex
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Old 12th June 2008, 08:38 PM   #29
jaycee is offline jaycee  United Kingdom
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Just out of curiosity, why does it need to be away from the pcb? Is there some part nearby that dissipates a lot of heat or so ?

I've never actually seen the inside of the A3i, was just wondering why having seen only the schematic
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Old 12th June 2008, 08:50 PM   #30
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by jaycee
Just out of curiosity, why does it need to be away from the pcb? Is there some part nearby that dissipates a lot of heat or so
Yes, that is exactly the problem.

Alex
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