My amp is an old Rega Mira II. It sounds fine at low volume, but at moderate to high volume there is significant distortion in both channels, which is especially noticeable on speech (sounds like a mic being overloaded).
Pre-amp output is clean; so it's a definitely power amp. issue.
However, I cannot find anything obviously wrong. Voltages around the power amp circuit all seem quite normal as far as I can tell; there is no DC offset; QC is fine.
I've attached the circuit (power amp section is same in 2000 and Mk II).
Can anyone give me a clue as to where the problem might be?
Thanks!😕
Pre-amp output is clean; so it's a definitely power amp. issue.
However, I cannot find anything obviously wrong. Voltages around the power amp circuit all seem quite normal as far as I can tell; there is no DC offset; QC is fine.
I've attached the circuit (power amp section is same in 2000 and Mk II).
Can anyone give me a clue as to where the problem might be?
Thanks!😕
Attachments
Is this an amp that used to work OK, and now is playing up; or a newly acquired unit?
Sounds to me like there may be a power supply issue, the voltage rails are sagging/modulating badly when called upon to supply significant current. Try monitoring these rails while the amp is working under significant load ...
Sounds to me like there may be a power supply issue, the voltage rails are sagging/modulating badly when called upon to supply significant current. Try monitoring these rails while the amp is working under significant load ...
A collapsing PSU would have been on my my first suggestions too. Get a measurement under load, both rails.
Thanks for your suggestions, but power supply voltage (+, -) is looking just fine under load.
fas42: yes this amp has been working just fine.
fas42: yes this amp has been working just fine.
You'll need a 'scope to look at power supply modulation properly. DMMs typically give average readings within the refresh cycle, so may not tell you much.
You could simplify things by just temporarily adding extra main electrolytics of say, half to equal the present value to verify whether there is something amiss with the supply.
You could simplify things by just temporarily adding extra main electrolytics of say, half to equal the present value to verify whether there is something amiss with the supply.
Rafael L: yes, it's the same in both channels.
Ian Finch: I guess this is the next step ... will have to install 'scope software first ...
Ian Finch: I guess this is the next step ... will have to install 'scope software first ...
In this case your problem is of pre-amp or power supply (diodes probably). Try connecting only the power amplifier without pre-amp for know where is the problem.
Rafael L: The Mira has pre-amp outputs, and their output is clean, so definitely not a pre-amp issue.
Another thought - haven't looked at the posted pdf - could there be some protection circuitry triggering at a lower level than it normally should?
It might sound stupid ...there is a remote chance that the mains switch cannot handle the current any more Bypass it for a second to see how it will go
Kind regards
Sakis
Kind regards
Sakis
As the problem affects both channels, it is likely to be in the main power supply. I suspect either the electrolytics have dried out or one of the rectifier diodes has gone open (unusual). Either of these would give correct dc, but excessive ripple under load
fas42: good thought ... there's an IC (U3) that seems to handle protection but looks like it's all about DC offset detection; doesn't seem to be anything level-related (like you see in NAD's soft clipping for example).
It has Impedance-detecting VI limiting (freely translating from Spanish) built around TR15/17 and surrounding components (and equivalent ones in the other channel).
Will limit current for resistive loads below 2r2 or real world (reactive) speakers below, say, 3 ohms or 3A into shorts.
So it's easy to trigger on complex loads; did you switch speakers or something?
Will limit current for resistive loads below 2r2 or real world (reactive) speakers below, say, 3 ohms or 3A into shorts.
So it's easy to trigger on complex loads; did you switch speakers or something?
JMFahey: thanks very much for that info. I have not changed the speakers (Infinity Reference 20s). I'm going to recap the power amp and power supply ... will be a few weeks before I can get to that.
Check for fused tracks and dry joints in the power supply area. This is far more likely than a rectifier diode going open, they usually fail short
Have you measured the distortion at the higher levels into a dummy load? fft?
Just thinking laterally have you ruled out the speakers? Is the distortion across the board or at higher frequencies? Getting an fft showing the spectrum would be interesting. Reason I asked about the speakers is because I had this problem and it was because I had too low a slope on the crossover. Increasing it to 4th order fixed it. Of course in your case if it used to be fine, then it is not the design of the crossover, but it could be degraded components...
Tony.
Just thinking laterally have you ruled out the speakers? Is the distortion across the board or at higher frequencies? Getting an fft showing the spectrum would be interesting. Reason I asked about the speakers is because I had this problem and it was because I had too low a slope on the crossover. Increasing it to 4th order fixed it. Of course in your case if it used to be fine, then it is not the design of the crossover, but it could be degraded components...
Tony.
wintermute: interesting thought; but speakers have been fine, and are 100% OK with my temprary replacement amp.
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