Hi Redjr,
So sorry to hear about your FO amp. I have heard that these amps have known reliability issues and sometimes spontaneously go out. You can see many of these complaints in the FO thread - and unfortunately, the OP/designer is not responding to requests for help.
This is the first time I have heard the Yarra/Aksa Lender mentioned as part of an amp output stage failure. If you never heard a catastrophic pop from your speakers and just had silence - sounds like it had an issue with sudden onset oscillation before the speakers were even connected? These events can destroy MOSFETs instantly. You might be able to repair them with new outputs - I think they used double die Exicon laterals? Note that it works fine with the V4H, an amp with similar CFA topology.
So sorry to hear about your FO amp. I have heard that these amps have known reliability issues and sometimes spontaneously go out. You can see many of these complaints in the FO thread - and unfortunately, the OP/designer is not responding to requests for help.
This is the first time I have heard the Yarra/Aksa Lender mentioned as part of an amp output stage failure. If you never heard a catastrophic pop from your speakers and just had silence - sounds like it had an issue with sudden onset oscillation before the speakers were even connected? These events can destroy MOSFETs instantly. You might be able to repair them with new outputs - I think they used double die Exicon laterals? Note that it works fine with the V4H, an amp with similar CFA topology.
Thanks X. I never did hear a loud pop, just silence. Once I pull one of the modules I'll see if it's the MOSFETs. If so, maybe I'll attempt a repair. I follow the FO thread, so I guess I've been really lucky - until today!
Am I correct on R118 to adjust the gain down a bit on the Mel DBs?
Well I'm 2 for 2 now!
Hi all,
My amp travails continue. I just had the same experience with my new V4H amp too. Something is seriously wrong either in the way I constructed the Yarra, or the way I'm powering up equipment. This afternoon, I turned the pre off first, immediately heard a 2 sec tone from my speakers and then the smell of fresh incinerated electronics with more crackling. I immediately pulled the plug on the amp. Two amps! Now I know it's not unlucky timing. What's going on. I'm concerned.
Before the Yarra, I've been using a little Parasound pre using a trigger to turn on my amp(FO) from the pre without incident - ever. When I first powered up the Yarra a couple weeks ago I did notice a loud pop in my speakers. No damage was done. Since that initial power up I've been careful of the sequence - powering upstream devices first and the power amp last. Is there a critical sequence to powering up and powering down using the Yarra that I need to carefully observe?
Is it possible I unwittingly omitted something in the Yarra, or DB builds, or left something disconnected that should be. If so, not sure what it could be. Is this a sequence issue, grounding? The Yarra survived and is playing just fine on my bench feeding a little class D amp as I write this. But before I connect up another amp I need to ensure this won't happen again. Something is obviously causing this to happen, that I've never encountered with other equipment.
I could see in my V4H amp the same resistor in both channels was blown. Hopefully they protected the output devices. I hope it can be repaired by replacing the 2 resistors. Have not investigated further yet.
Advice observations anyone.
Hi all,
My amp travails continue. I just had the same experience with my new V4H amp too. Something is seriously wrong either in the way I constructed the Yarra, or the way I'm powering up equipment. This afternoon, I turned the pre off first, immediately heard a 2 sec tone from my speakers and then the smell of fresh incinerated electronics with more crackling. I immediately pulled the plug on the amp. Two amps! Now I know it's not unlucky timing. What's going on. I'm concerned.
Before the Yarra, I've been using a little Parasound pre using a trigger to turn on my amp(FO) from the pre without incident - ever. When I first powered up the Yarra a couple weeks ago I did notice a loud pop in my speakers. No damage was done. Since that initial power up I've been careful of the sequence - powering upstream devices first and the power amp last. Is there a critical sequence to powering up and powering down using the Yarra that I need to carefully observe?
Is it possible I unwittingly omitted something in the Yarra, or DB builds, or left something disconnected that should be. If so, not sure what it could be. Is this a sequence issue, grounding? The Yarra survived and is playing just fine on my bench feeding a little class D amp as I write this. But before I connect up another amp I need to ensure this won't happen again. Something is obviously causing this to happen, that I've never encountered with other equipment.
I could see in my V4H amp the same resistor in both channels was blown. Hopefully they protected the output devices. I hope it can be repaired by replacing the 2 resistors. Have not investigated further yet.
Advice observations anyone.
Yeah, you're telling me. That's the same sequence I use, but I just wanted to see if there would be any negative unintended consequences by power off the pre first, and yep there was. Big consequences! However, most gear should not be this sensitive to the order of powering up and down. At this point, I'm almost afraid to hook another one of my better amps to it. At least until I have some empirical evidence as to the cause.
However, most gear should not be this sensitive to the order of powering up and down. At this point, I'm almost afraid to hook another one of my better amps to it. At least until I have some empirical evidence as to the cause.
Yeah, you're telling me. That's the same sequence I use, but I just wanted to see if there would be any negative unintended consequences by power off the pre first, and yep there was. Big consequences!
If you really like the FO amp modules, try ordering replacement MOSFETs and swapping them. You will need to do the whole setup bias current zero DC using three simultaneous DVMs again though.
If you are looking for a new amp, you might consider the Alpha BBB (ABBB) or Alpha 20 or Alpha Nirvana (to be released). All of those amps are zero adjustment.
Again, sorry your amp went - but I highly doubt the Yarra/Melbourne caused it. I have used it with many different amps (Class AB, Class A, Class D, and even zero feedback MOSFET follower) and never had a problem. But it probably would be a good thing to power the amp up or down before the preamp is powered up or down.
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Hi Redjr,
I am looking over the schematic and now recall that the output capacitor bleed resistor R126/136 is default value of 47k. That’s probably too large a value and will result in a long decay period for the output cap DC offset at power on. This was not an issue if you are DC coupling the Yarra. However, to bleed the output coupling cap quickly, this resistor should be changed to about 680R to 1k. The power up sequence should still be followed with preamp turned on first then power amp. And power amp turned off first then preamp.
I will update the Post #1 to alert folks of this change.
I am looking over the schematic and now recall that the output capacitor bleed resistor R126/136 is default value of 47k. That’s probably too large a value and will result in a long decay period for the output cap DC offset at power on. This was not an issue if you are DC coupling the Yarra. However, to bleed the output coupling cap quickly, this resistor should be changed to about 680R to 1k. The power up sequence should still be followed with preamp turned on first then power amp. And power amp turned off first then preamp.
I will update the Post #1 to alert folks of this change.
Recently, my quest for absolute zero noise and hum for my USSA5 and Yarra combo, i changed many different way of groundings methods, to the point of frustration... I even pull out the RCA cables in and out of the system while they are still singing away.
And i have forgotten which amp or preamp i switched off first. So far nothing happens to the USSA5. It is direct dc couple, i bypass the coupling cap in the USSA5, as i will be using the stock in the Yarra.
Lets investigate further to the cause of your system issue.
And i have forgotten which amp or preamp i switched off first. So far nothing happens to the USSA5. It is direct dc couple, i bypass the coupling cap in the USSA5, as i will be using the stock in the Yarra.
Lets investigate further to the cause of your system issue.
What is the potential hazard of not making this change for capacitor coupled systems?
Have made the recommended changes to R126 and R136 on the Yarra MB. Also swapped R118 on Mel DBs for gain reduction. Will test tomorrow. I have to say, I'm a little hesitant to test it on anything but a chipamp of obvious concern as to the reliability. I can't afford to loose another amp - DIY or otherwise. Something like this has never happened before in all of the amps and pres I've built.
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On a DC coupled output, a 47k load resistor is fine, for a cap coupled output into a high impedance load, normally a smaller 220uF cap can work fine (like we used on Aksa Lender). but we were trying to make a low impedance headphone capable preamp, so one option was a rather large capacitor was spec’d. The larger combo should have a smaller value resistor to bleed of the DC that forms on the opposite side of a cap during power on. A 47k res and 2350uF cap combo has an RC time constant of 77 seconds vs 1.6s for 1k resistor. So you would need to wait over a minute before you could expect the DC to be close to zero after power up.
Thanks for making the changes, all I can say is that you can check the DC offset after power up with a DVM before using with another amp. Also, power up the Yarra first before the power amp. Wait at least 5 seconds after power up of Yarra.