Thanks for the responses, I’ve read post’s 90 & 93 as suggested, way over my head unfortunately! If I return the unit to Richer Sounds, I assume they would just replace the failed component, any idea if that component has been upgraded in the intervening years ? Also any idea how much it would cost to send it to someone who could/would do the “reliability” upgrades etc as outlined in x-pro’s post #93 ? Or am I better off to replace it with a new Amp ?
Thanks,
KTMKev
Thanks,
KTMKev
Do you know the headline of the article and the issue (I think it was a magazine that was published monthly or every two months) ?
Maybe it is possible to download the article anywhere under
https://www.worldradiohistory.com/index.htm
Thank you very much for your advice.
For A3i owner's, which want to use MC cartridges instead MM versions, this thread could be of interest:
https://www.diyaudio.com/community/...8-to-mc-cartridges.151527/page-2#post-4577074
P.S.: in the closed thread under
https://www.hi-fi.ru/forum/forum48/topic39954/?PAGEN_3=3&PAGEN_1=8
are to find some discussions concerning this circuit topology.
hello
I just got an A3i.
looks like it has never been used.
tested with mission mk 70 mkII, I love it.
infinity reference 21i, I like it a lot less.
I just find that the power supply makes a purring noise.
I will have to dismantle the pcb and check the solders.
Is there an order to perform to access it?
I just got an A3i.
looks like it has never been used.
tested with mission mk 70 mkII, I love it.
infinity reference 21i, I like it a lot less.
I just find that the power supply makes a purring noise.
I will have to dismantle the pcb and check the solders.
Is there an order to perform to access it?
Hi All.
I've just re-joined, I was on here a little some years ago.
I have an A3i in bits at the moment, and I've just read the whole of this thread.
Thanks to everyone who's put in the time to help one another.
My amp has the LED fault, which I think has been well covered in this thread. New transistors and higher power resistors on order!
I think I undrstand that.
What I don't understand is that the two resistors next to R114 and Q102/2 are burned.
These are R4, R5 in the schematic (unless I'm sadly mistaken!), the 100R emitter resistors for the input pair.
I can't see how these ever saw enough power to get in that state?
Somebody well up the thread did mention seeing the same, but years ago.
Do I presume this can only be 'death by oscillation'?
I do have a scope and some experience in electronics but haven't worked in audio.
I am considering reducing the LED current.
I am also considering down-rating the amp, as I also have here a Cambridge A300 which AIUI is 50W/ch, so ought to have a suitable transformer in it.
I believe the A300 is seriously traumatised inside although the case is fairly tidy. (for a used 25 year old unit!).
I have a few other amps to play with, a few years a go I started collecting cheap or junk amps with a plan to build some active crossover bi-amp system.
Too many projects, not enough time! We're actually using an Arcam Alpha 7 and I have a Creek 4040 which I've got working for my small office room.
I'll let people know how it goes, but any insight on the emitter resistors would be gratefully received.
Thanks!
I've just re-joined, I was on here a little some years ago.
I have an A3i in bits at the moment, and I've just read the whole of this thread.
Thanks to everyone who's put in the time to help one another.
My amp has the LED fault, which I think has been well covered in this thread. New transistors and higher power resistors on order!
I think I undrstand that.
What I don't understand is that the two resistors next to R114 and Q102/2 are burned.
These are R4, R5 in the schematic (unless I'm sadly mistaken!), the 100R emitter resistors for the input pair.
I can't see how these ever saw enough power to get in that state?
Somebody well up the thread did mention seeing the same, but years ago.
Do I presume this can only be 'death by oscillation'?
I do have a scope and some experience in electronics but haven't worked in audio.
I am considering reducing the LED current.
I am also considering down-rating the amp, as I also have here a Cambridge A300 which AIUI is 50W/ch, so ought to have a suitable transformer in it.
I believe the A300 is seriously traumatised inside although the case is fairly tidy. (for a used 25 year old unit!).
I have a few other amps to play with, a few years a go I started collecting cheap or junk amps with a plan to build some active crossover bi-amp system.
Too many projects, not enough time! We're actually using an Arcam Alpha 7 and I have a Creek 4040 which I've got working for my small office room.
I'll let people know how it goes, but any insight on the emitter resistors would be gratefully received.
Thanks!
Thanks for replying.
I would have expected an input overload to be limited by the pre-amp, and also attenuated by R3 being 10k in series?
If enough amps have gone through Q1 and Q2 to wreck 100ohm resistors, then the current sources top and bottom probably aren't working either?
I don't fully understand the FET Q7, is this a popular circuit configuration, or is there a good explanation of it anywhere?
I was also confused by looking at a schrmstic I got from the 'net which seems to be the A3 (carburettor version?) not the A3i, this has some differences around the FET Q7.
I've just realised I was thinking completely wrong about Q7 and Q8.
One version of the A3 schematic I'm looking at (on electrotayna) shows the Q8 and Q10 collector currents as 12mA, this drops a clue as to what the FET current should be!
The other channel looks OK and may even be working.
It's been helpful to write this down as I think it through again.
i
I would have expected an input overload to be limited by the pre-amp, and also attenuated by R3 being 10k in series?
If enough amps have gone through Q1 and Q2 to wreck 100ohm resistors, then the current sources top and bottom probably aren't working either?
I don't fully understand the FET Q7, is this a popular circuit configuration, or is there a good explanation of it anywhere?
I was also confused by looking at a schrmstic I got from the 'net which seems to be the A3 (carburettor version?) not the A3i, this has some differences around the FET Q7.
I've just realised I was thinking completely wrong about Q7 and Q8.
One version of the A3 schematic I'm looking at (on electrotayna) shows the Q8 and Q10 collector currents as 12mA, this drops a clue as to what the FET current should be!
The other channel looks OK and may even be working.
It's been helpful to write this down as I think it through again.
i
I'm noticing that in this version of the A3 schematic, the gate of the FET has a snubber(?) of 43k in series with 220pF, absent on the A3i?
https://elektrotanya.com/cambridge_a3_sch.pdf/download.html
The snubber if that's what it is, goes from the gate to the local +ve rail.
The A3i has 100nF from the Drain to the +ve rail.
If the FET is prone to oscillating, which might explain a few things, might it be an idea to add that snubber?
Am I right in thinking the 100nF C6 doesn't play a big role in the response, it's 'just' decoupling?
https://elektrotanya.com/cambridge_a3_sch.pdf/download.html
The snubber if that's what it is, goes from the gate to the local +ve rail.
The A3i has 100nF from the Drain to the +ve rail.
If the FET is prone to oscillating, which might explain a few things, might it be an idea to add that snubber?
Am I right in thinking the 100nF C6 doesn't play a big role in the response, it's 'just' decoupling?
I is true that Q1, Q2 should be safe in the given environment.
In post 90 I reported my own repair experience of different units.
Q1, Q2 probably can be overloaded by a defective output stage.
Q7 is a p-channel Mosfet source follower and can be substituted
by a p-JFet if you can find one. This variant is not seen frequently.
I refer to the circuit in post 1, from the designer of the amp, and
will not look at other schematics. Please consider we talk about the
same amp or not. Probably Alex X-pro can report about the changes
you ask for and you can find other useful posts from him here.
I just asked moderators to correct a small error in post 90.
In post 90 I reported my own repair experience of different units.
Q1, Q2 probably can be overloaded by a defective output stage.
Q7 is a p-channel Mosfet source follower and can be substituted
by a p-JFet if you can find one. This variant is not seen frequently.
I refer to the circuit in post 1, from the designer of the amp, and
will not look at other schematics. Please consider we talk about the
same amp or not. Probably Alex X-pro can report about the changes
you ask for and you can find other useful posts from him here.
I just asked moderators to correct a small error in post 90.
Hi as_audio,
Yup, the diff pair is overloaded by an output stage railed to either rail. This isn't normally a problem with J-Fets, but one of the BJT transistors in the average amplifier will become reverse biased and that can change its characteristics or even damage more severely.
Whether the diff pair is damaged or not depends on the gate-source breakdown voltage rating and what it sees. Now, maybe I'm thick, but I don't see any Fets in the A3. But with a capacitor to ground in the feedback, the pair will see the full offset voltage.
Yup, the diff pair is overloaded by an output stage railed to either rail. This isn't normally a problem with J-Fets, but one of the BJT transistors in the average amplifier will become reverse biased and that can change its characteristics or even damage more severely.
Whether the diff pair is damaged or not depends on the gate-source breakdown voltage rating and what it sees. Now, maybe I'm thick, but I don't see any Fets in the A3. But with a capacitor to ground in the feedback, the pair will see the full offset voltage.
I was wrong about the snubber not being there, it's just drawn over to the left in the diagram.
Sorry for any effort that caused!
The snubber is a long way from the FET on the PCB by some standards.
The PCB I hve here is labelled 'A Sereies Power Amp Version 1.0'
It has the 470ohm resistor designated R11 in the schematic at the start of the thread, labelled R13/R113 on the silk screen.
The serial number on the back panel of the case is 1058-A31-0798-0147
The PCB looks like FR4 to me, it is translucent when I put a desklamp behind it, apart from around the small signal transistors on both channels, where it's more brown and opaque.
There are a lot of small 'solder balls' in the flux residue.
Build quality is terrible, it looks like one power transistor got a lot more heat sink compound than the other 3!
I was intending to order some new green LEDs, but many modern ones have a Vf of around 2.9 V even at low current, I'm pretty sure the LEDs here should be 2V.
To be fair to this amplifier, it was working well when we got it, we probably left it powered up and idle for a long time.
Sorry for any effort that caused!
The snubber is a long way from the FET on the PCB by some standards.
The PCB I hve here is labelled 'A Sereies Power Amp Version 1.0'
It has the 470ohm resistor designated R11 in the schematic at the start of the thread, labelled R13/R113 on the silk screen.
The serial number on the back panel of the case is 1058-A31-0798-0147
The PCB looks like FR4 to me, it is translucent when I put a desklamp behind it, apart from around the small signal transistors on both channels, where it's more brown and opaque.
There are a lot of small 'solder balls' in the flux residue.
Build quality is terrible, it looks like one power transistor got a lot more heat sink compound than the other 3!
I was intending to order some new green LEDs, but many modern ones have a Vf of around 2.9 V even at low current, I'm pretty sure the LEDs here should be 2V.
To be fair to this amplifier, it was working well when we got it, we probably left it powered up and idle for a long time.
The schematic in your link is the wrong one check this one below.
Edit found the complete service manual for the A3i but I think it was posted by me earlier in this thread
Last edited:
Hello, my A3i crackles intermittently (frying noise) in the right channel. It’s not a crackly pot issue and it does this at whatever volume, also with the pre-amp disconnected. Except for the cracking, all is working fine. All LEDS light up (I changed a couple just in case). I changed both BC640, all resistors that had changed colour due to heat, and most electrolytic capacitors. I have ordered ZVP3306A which I haven’t replaced yet. Could this be the culprit? I am running out of ideas…
Votage across R23 reads 12mV, but across R123 the reading shows large fluctuations between 1800mV & 900mV.
thanks for your help & suggestions.
Votage across R23 reads 12mV, but across R123 the reading shows large fluctuations between 1800mV & 900mV.
thanks for your help & suggestions.