but if the series parts are in the "wrong" order it makes it harder for anyone to try the bootstrap for themselves - just swap the pot and R
Hugh
It is quite possible when using a SMPS and a metal case , that if the owner rubs his finger across the metalwork of the case, he will feel a funny vibrating sensation. This is due to the previously mentioned 100nF capacitor between the primary and secondary sides of the SMPS. You may wish to consider fitting an earth terminal to the rear of the case for this reason. It also often leads to a small but noticeable increase in SQ if used.
SandyK
It is quite possible when using a SMPS and a metal case , that if the owner rubs his finger across the metalwork of the case, he will feel a funny vibrating sensation. This is due to the previously mentioned 100nF capacitor between the primary and secondary sides of the SMPS. You may wish to consider fitting an earth terminal to the rear of the case for this reason. It also often leads to a small but noticeable increase in SQ if used.
SandyK
Member
Joined 2009
Paid Member
Has the turn-on been modeled yet ? - and the turn-off ?
If you're expecting a thump at turn on until the amp offset has settled it may be enough to cause discomfort to the headphone user who may not have engaged the mute switch ?
Aspen Headphone Amplifier
Hugh
The voltage between chassis of the SMPS powered equipment and earth, can exceed 100VAC when measured using a typical DMM with it's 10megohm input impedance.
This caused a lot of problems for Graham Slee, but luckily most of it was picked up by those on his list of testers. I was one of those testers.
The earth point mentioned was a result of these findings.
SandyK
Hugh
The voltage between chassis of the SMPS powered equipment and earth, can exceed 100VAC when measured using a typical DMM with it's 10megohm input impedance.
This caused a lot of problems for Graham Slee, but luckily most of it was picked up by those on his list of testers. I was one of those testers.
The earth point mentioned was a result of these findings.
SandyK
KT,
again you are spot on with all the assumptions you made following my previous post. With regard the minimum capacitance in the Virtual earth, it needs to pass the lowest frequency to the headphones at the very least.
Two resistors are fine as long as you swamp the bias current which in our case is a few hundred micro-amp. Thus between 4.7K - 10K would be sufficient.
Nico
again you are spot on with all the assumptions you made following my previous post. With regard the minimum capacitance in the Virtual earth, it needs to pass the lowest frequency to the headphones at the very least.
Two resistors are fine as long as you swamp the bias current which in our case is a few hundred micro-amp. Thus between 4.7K - 10K would be sufficient.
Nico
Hugh,
regarding the cap multiplier used with switching power supplies, you are absolutely right, there is no point with it. The switching noise is bets reduced with inductors, but if the SMPS is of at least medium standard there will be an inductor in the output path.
Capacitors in the range 47 - 100 nF normally specified for EMI suppression is normally quite sufficient accross the input terminals, either a MOV or two back to back zeners slightly above the power supply voltage can be used to short any spikes comming through.
Bypassing the large virtual earth capacitors with 47 - 100 nF EMI suppression caps can also help along.
regarding the cap multiplier used with switching power supplies, you are absolutely right, there is no point with it. The switching noise is bets reduced with inductors, but if the SMPS is of at least medium standard there will be an inductor in the output path.
Capacitors in the range 47 - 100 nF normally specified for EMI suppression is normally quite sufficient accross the input terminals, either a MOV or two back to back zeners slightly above the power supply voltage can be used to short any spikes comming through.
Bypassing the large virtual earth capacitors with 47 - 100 nF EMI suppression caps can also help along.
Hi Gareth,
with the virtual earth there should theoretically be no thump as the two capacitors will charge at equal rate and the V/2 or firtual earth point will remain V/2 and the output of the amp actually follows this V/2 as well. It is just at very low voltages where the transistors has not started conducting yet that there may be some very slight thump.
with the virtual earth there should theoretically be no thump as the two capacitors will charge at equal rate and the V/2 or firtual earth point will remain V/2 and the output of the amp actually follows this V/2 as well. It is just at very low voltages where the transistors has not started conducting yet that there may be some very slight thump.
Sort of like the vibrating sensation you get when you rub your finger across your CRT screen? (hey, possible market spin - "our product sends pulses of electricity throughout your body and fries all harmful bacteria!")
Unfortunately, I do not have an IEC ground in my house, due to minimalistic safety procedures. I live in an old army base/airport.
- keantoken
To fix thump, could we use a relay at the output that is engaged by a delayed signal? Say it switches after a second or so, giving the amp time to warm up.
- keantoken
Member
Joined 2009
Paid Member
I don't know if this is relevant to the turn on/off quesiton...My TGM amp turns on with no audible thump and it has a similar topology to this headamp. I think this just means that the rails both come up fairly quickly and at the same time (and the error amplifier comes alive quickly enough to allow the negative feedback to get a grip on the output). However, my TGM does make a small audible signature on power-off. I think this is because the power draw of the two rails by the amp is unequal so the psu filter caps drain at different rates so the amp finds itself with one rail collapsing before the other one. I would guess the Virtual Earth will avoid this altogether ?
Aspen Headphone Amplifier
It seems to me that too many changes want to be incorporated in the redesign of Hugh's original elegant design.
I don't know about other people, but these days I am doing more and more headphone listening due to domestic requirements, where the main system can't be used to full advantage. I have got into the habit of turning my headphone amplifier on shortly before a listening session, which in many cases will also result in a small improvement in SQ, then on completion,I take off my headphones , then switch off the power. When using the 120 ohm output resistor as per the IEC specifications, even a reasonable turn off thump will not do any damage. In any case, the headphones are already off my head.
It is very easy to get into such a routine. Perhaps we are also worrying about something that may not even happen ? My main Class A amplifier , also has a small "sigh" on turnoff, but no turn-on thump. I don't see that a major problem, however I have incorporated a delay circuit in my main system's preamplifier.
SandyK
It seems to me that too many changes want to be incorporated in the redesign of Hugh's original elegant design.
I don't know about other people, but these days I am doing more and more headphone listening due to domestic requirements, where the main system can't be used to full advantage. I have got into the habit of turning my headphone amplifier on shortly before a listening session, which in many cases will also result in a small improvement in SQ, then on completion,I take off my headphones , then switch off the power. When using the 120 ohm output resistor as per the IEC specifications, even a reasonable turn off thump will not do any damage. In any case, the headphones are already off my head.
It is very easy to get into such a routine. Perhaps we are also worrying about something that may not even happen ? My main Class A amplifier , also has a small "sigh" on turnoff, but no turn-on thump. I don't see that a major problem, however I have incorporated a delay circuit in my main system's preamplifier.
SandyK
We have nothing else to do than make up hypothetical problems with the circuit, simply so that we can fill up forum space debating how we are going to solve them! 
Now, what would be very helpful is if someone were to build the circuit, and test. Hook it up to the computer to record, turn on, play some music, then turn off, and then post the .wav (or FLAC, etc.) file here. That would give us something nice to go from.
- keantoken
Now, what would be very helpful is if someone were to build the circuit, and test. Hook it up to the computer to record, turn on, play some music, then turn off, and then post the .wav (or FLAC, etc.) file here. That would give us something nice to go from.
- keantoken
Most of us had problems with nits going through school, KT, but we certainly got over it..... Do carry on nitting, this is interesting. Nearly there....
Incidentally, while I agree turn on thumps could be an issue, it's really significant that we won't know what this thing sounds like or how we can live with it until someone builds it. Nico has come closest, but I won't be far behind.... certainly, it won't be built until it's road tested, that seems sure.
Cheers,
Hugh
Incidentally, while I agree turn on thumps could be an issue, it's really significant that we won't know what this thing sounds like or how we can live with it until someone builds it. Nico has come closest, but I won't be far behind.... certainly, it won't be built until it's road tested, that seems sure.
Cheers,
Hugh
Hi KT, who's amp are you referring too. If it is mine then there must be a mistake somewhere.
I have used the VT in all my designs since around 1983 and I do not have thumps, nor have I ever added any "output" protetion to any of my products.
But this should not stop anyone from adding or ommiting anything that they want. It is Do-It-Yourself audio after all.
Nico