Something I've never really done is headphone amplifiers, and I don't know how well I can apply my power amplifier knowledge in terms of power supplies to the topic.
A friend of mine gave me his Behringer powerplay pro-8 headphone amplifier to see if I could fix it. It's an 8 channel headphone amplifier with a whole lot of input/output options. I think it's used mainly in a recording studio setting.
I have found that the primary of the power transformer is burnt out. I can obviously just replace it with another one, and that's pretty simple, but I was wondering if I could replace it with a switching supply?
The transformer (as far as I can tell) is a 15-0-15 VAC, 30 VA. How consumer electronics' transformers burn out is completely beyond me (this is the third I've seen personally at home), but the fact is it has.
I was thinking of replacing it with a Meanwell PD-2515, which is a 25 W +15 V, -15 V switching power supply, and then I'll bypass the regulators, and just use some smoothing capacitors (the ones in there will probably do at 1000uF each). Would this work well, or would it make the amplifiers noisy?
I wondered about doing this because it's reliable, a bit cheaper, and more efficient. Switching supplies are also much easier to get here these days.
A friend of mine gave me his Behringer powerplay pro-8 headphone amplifier to see if I could fix it. It's an 8 channel headphone amplifier with a whole lot of input/output options. I think it's used mainly in a recording studio setting.
I have found that the primary of the power transformer is burnt out. I can obviously just replace it with another one, and that's pretty simple, but I was wondering if I could replace it with a switching supply?
The transformer (as far as I can tell) is a 15-0-15 VAC, 30 VA. How consumer electronics' transformers burn out is completely beyond me (this is the third I've seen personally at home), but the fact is it has.
I was thinking of replacing it with a Meanwell PD-2515, which is a 25 W +15 V, -15 V switching power supply, and then I'll bypass the regulators, and just use some smoothing capacitors (the ones in there will probably do at 1000uF each). Would this work well, or would it make the amplifiers noisy?
I wondered about doing this because it's reliable, a bit cheaper, and more efficient. Switching supplies are also much easier to get here these days.
It's not that the primaries "burn out", it's generally the thermal fuse that fails, and not necessarily through overheating either (possibly turn-in current peaks causing fatigue in the long run?). Unwinding the transformer enough to be able to replace it is not for the faint of heart but not generally impossible, although difficulty may vary substantially depending on where it is. In some cases you will even find two outside terminals that allow bridging said thermal fuse and attaching an external one to the outside (ca. 72°C is recommended in this case, the internal ones tend to be 115°C or 125°C or thereabouts).
The type of power supply may have implications far beyond the power section, although that depends on general device construction. What may be of interest to you is that the current V2 version of this unit employs a SMPS.
Since this unit also features unbalanced inputs, it would be interesting to know how exactly they have wired the power supply inlet. Is the PE conductor going straight to chassis? Is the audio ground referenced to chassis? If yes to both, that would make a SMPS conversion quite straightforward (although you should/may find the unbalanced inputs at the front riddled with ground loop issues). That's a plain Jane IEC Type I device.
In this case hookup of the PD-2515 should be quite easy. You will need some brass standoffs and matching screws, as that's how the board gets its PE connection. Making a drill template should be easy once you have the board, tapping depending on what kind of thread your standoffs need. If you have done this sort of casework before, you probably know all of this.
I'm not sure whether this supply qualifies as "audio grade" out of the box though. You can't just attach big caps to a SMPS, it may not want to even start up like that. You could try making some RLC filtering by combining e.g. the existing 1000µ capacitors with series (toroidal) chokes, the latter in parallel with 10-22 ohms to get the Q down if the choke isn't quite lossy enough as-is. The choke would have to handle about 1 A comfortably without going into saturation.
Practical designs often combine SMPS with LDO regulators and a bit of passive filtering in between (as linear regulators eventually lose PSRR at high frequencies). This gives good results while keeping parts cost low. In your case the passive route may be the easier one, even if the chokes should make things 50% more expensive.
Further fancifying may be provided by installing an IEC inlet with integrated mains filter. The Meanwell wil have enough filtering to pass emissions but it is unlikely to be overdesigned in this respect.
A switch-mode supply, properly designed, may very well give lower noise than a traditional linear one, but it's not going to be cheaper at this point. There's also more parts to wear out, and more heavily loaded ones as well. (If you want the Meanwell to last as long as possible, consider recapping the secondary side with legit Nichicon or Panasonic caps right away.)
Finally, check XLR input pin 1 hookup for AES48-2005 compliance in order to eliminate the potential for Pin 1 Problems (look it up if you don't know what this is). In a nutshell, AES48 says that pin 1 shall connect to chassis in the shortest possible way, and shall not connect to input circuitry audio ground. The PRO-8 / HA8000 is old enough to predate AES48, and other models like the A500 amplifier have been found to not be compliant.
I found a schematic for the HA4700, and sure enough there is no indication for compliance, as everything seems to connect to "A" GND. No idea how involved a mod would be; I imagine the XLRs are board-mounted and some cutting may be necessary to free the pin 1s. The more adventurous may even attempt to completely unsolder them and then sleeve the pin in the PCB through-hole with heatshrink or whatever and solder a wire to its end.
The type of power supply may have implications far beyond the power section, although that depends on general device construction. What may be of interest to you is that the current V2 version of this unit employs a SMPS.
Since this unit also features unbalanced inputs, it would be interesting to know how exactly they have wired the power supply inlet. Is the PE conductor going straight to chassis? Is the audio ground referenced to chassis? If yes to both, that would make a SMPS conversion quite straightforward (although you should/may find the unbalanced inputs at the front riddled with ground loop issues). That's a plain Jane IEC Type I device.
In this case hookup of the PD-2515 should be quite easy. You will need some brass standoffs and matching screws, as that's how the board gets its PE connection. Making a drill template should be easy once you have the board, tapping depending on what kind of thread your standoffs need. If you have done this sort of casework before, you probably know all of this.
I'm not sure whether this supply qualifies as "audio grade" out of the box though. You can't just attach big caps to a SMPS, it may not want to even start up like that. You could try making some RLC filtering by combining e.g. the existing 1000µ capacitors with series (toroidal) chokes, the latter in parallel with 10-22 ohms to get the Q down if the choke isn't quite lossy enough as-is. The choke would have to handle about 1 A comfortably without going into saturation.
Practical designs often combine SMPS with LDO regulators and a bit of passive filtering in between (as linear regulators eventually lose PSRR at high frequencies). This gives good results while keeping parts cost low. In your case the passive route may be the easier one, even if the chokes should make things 50% more expensive.
Further fancifying may be provided by installing an IEC inlet with integrated mains filter. The Meanwell wil have enough filtering to pass emissions but it is unlikely to be overdesigned in this respect.
A switch-mode supply, properly designed, may very well give lower noise than a traditional linear one, but it's not going to be cheaper at this point. There's also more parts to wear out, and more heavily loaded ones as well. (If you want the Meanwell to last as long as possible, consider recapping the secondary side with legit Nichicon or Panasonic caps right away.)
Finally, check XLR input pin 1 hookup for AES48-2005 compliance in order to eliminate the potential for Pin 1 Problems (look it up if you don't know what this is). In a nutshell, AES48 says that pin 1 shall connect to chassis in the shortest possible way, and shall not connect to input circuitry audio ground. The PRO-8 / HA8000 is old enough to predate AES48, and other models like the A500 amplifier have been found to not be compliant.
I found a schematic for the HA4700, and sure enough there is no indication for compliance, as everything seems to connect to "A" GND. No idea how involved a mod would be; I imagine the XLRs are board-mounted and some cutting may be necessary to free the pin 1s. The more adventurous may even attempt to completely unsolder them and then sleeve the pin in the PCB through-hole with heatshrink or whatever and solder a wire to its end.
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That's exactly it, but I'm not going through the pain of unwinding until I find it. I've tried that before, and I'm not keen on wasting my time like that. 🙂 What gets me is why it fails on consumer goods. Even some of the power resistors show signs of working too hard in their short life.
Anyway, thank you for the advice. I assume the inputs and outputs are chassis-grounded. The power supply is as simple as it gets. 4 1N400x diodes in a full bridge rectifier, each bypassed with a ceramic capacitor, and a pair of electrolytics on the other side of the regulators.
If it is likely that the SMPS will work, I will (if I go ahead with it) use one, and then see if additional filtering is needed. I do like the advice of improving the capacitors, but I'm not sure Meanwell would approve of it, in the case of a guarantee/warranty.
There are no XLR inputs or outputs on this amplifier.
Thank you, once again, there is a lot you say to put my mind at rest.
