[Garbz]

So i built the multi hybrid headphone amp. It's powered by 24v and is single ended so naturally there's quite the few volts of offset on the output. Hence the design has an electrolytic output cap, should match the ****** source perfectly.

Currently on the output there's a 220uf 16v cap, followed by a 4.7k bleeder resistor, and a 120ohm output series resistor. The load is 300ohms (Sennheisers). The problem is when I simulate this load with dummy resistors and do DC offset measurments I get a power-on thump of 2-3v for about half a second, and power-off thumps of 800mV for a smaller fraction of a second. Since I intend to use the amp in an area where power failures are frequent I need to get these solved. It's not a punishment I want to expose my headphones to too frequently.

So is there something I don't understand about using such output caps or is this common behaviour for single ended amp designs?

 

[Budgie]

Caps have a low resistance when DC is first applied to them which increases as the charge in the cap builds up the the applied voltage. When it reaches the applied voltage, the cap will then block DC. In your circuit the charging current flows through the cap into a resistive voltage divider formed by your headphones and bleeder resistor to ground. The only fix I have found is to unplug the phones when turning on or off, or to add a relay to disconnect the phones during power up/down cycles. You could reduce the size of the pop by using a lower resistance bleeder resistor, but you will also lose amp output power in the resistor.

You could use a toggle switch to select between the phones and an equal value resistor when the phones arent in use.

 

[rickcr42]

you need a full mute relay on the output with a time delay that matches the charging time of the capacitor.

I like to go with 30 seconds as a standard time but that would flat out drive me nuts if my AC power kept going off so in your case just make the delay the same time as the output cap charging time

 

[Garbz]

Ok thanks I was experimenting with something like that, however I'm still trying to get the amp to hold longer then the relay during a power outage situation. My guess is big cap after the regulator and plug the relay in before the regulator.

 

[rickcr42]

Quote:

My guess is big cap after the regulator and plug the relay in before the regulator. __________________

the theory sounds OK but the reality of big caps is they are not always a good sound solution.Better to have the amp sound right while working even if it goes out occasionally than to have it always sound poor.Big caps in power supplies are very overated as an "upgrade" in many cases and smaller is sometimes not just better but WAY better.
Ever consider a straight battery power supply to correct your AC system problems ?
A simple battery supply/charger system when "off" would solve all your troubles

 

[Garbz]

It can't get too hard. It's a tube amp so wired point to point inside a computer.

That said while it probably does aversly affect the sound, it's a very simple tube hybrid amp, the mosfet isn't even audio grade, there's a big electrolytic and cheap really directly in the signa, the entire thing has a noise floor of only -70dB, and on top of everything it's driven by a computer's onboard sound. I think it's safe to say any drop in quality fit in perfectly. Now if only 2 bads made a good

I'll simply have to modify the current circuit to hold the relay long and try that cap idea. I'll let u know how I go in 2 weeks when I actually have time for this work again.

 

[rickcr42]

the power supply IS in the signal path and that is why there is so much press on the audibility of power supplies and what suits what circuit topology best.Folks need to get away from the idea that DC power is DC power and inconsequential.

The mosfet is not audio quality ? As far as I know none as designed as such but simply chosen by the specs for the intended application so has little to do with it unless you chose one with such a high gate capacitiance as to be really bad for audio use.

If you insist on the "cap hold" method look to car audio for the caps

 

[Garbz]

ok let me rephrase. I don't care how this amp sounds. I'm not really a tube man, I built this purely for proof of concept. I wanted to have something with tubes in it. It will get used very rarely and when it does it will be in an area where sound quality will not be paramount, i.e. lots of background noise.

I'm well aware that a PSU can make or break an amp, this just seems to me to be the easiest way to do this seeing as I already have a timedelayed relay using a darlington pair of 2n2222 rigged up to this.

 

[rickcr42]

heh,I was serious man

Car audio "lightening" caps.Big huge mamsuckers for storing energy

 

[Garbz]

The scary thing is I know you were serious

. I've seen thoes nasty 10 frad behmoths. Big voltage metres bolted to their top etc. Let's say it doesn't fit the portable nature of the amp

 

[rickcr42]

well you just lost me man.

You said it was point-to-point hybrid amp inside a computer three posts up and now it is a portable amp ?

 

[Garbz]

Lol no ok let me be specific.

It's point to point multi hybrid designed by our minituriser extrodinaire sojise (sorry about spelling). The amp is mounted in the top of a medium tower computer case. The computer itself needs to remain portable. I.e. It'll get carried around to LAN parties a lot, hence the need for the amp to not produce DC with frequent sudden loss of powers.

If we're still on the same page regarding capacitors I'm thinking thoes large heavy tube looking thingies. Not something I wish to carry around additionally to a large chunk of aluminium attached to an antiquated monitor.

 

[rickcr42]

Quote:

If we're still on the same page regarding capacitors I'm thinking thoes large heavy tube looking thingies. Not something I wish to carry around additionally to a large chunk of aluminium attached to an antiquated monitor.

Definately not for the squeamish man.Those puppies will play a song for days at headphone amp currents

touching one by accident during a move will give a nice little burn and will weld any metal parts they come in contact with if the + and - are shorted

You want compact man ? rip open one of those mini disposable Kodak cameras and take the cap out that stores the flash charge

 

[Garbz]

Did I mention I just wanted to eliminate the power-off thump, and not keep the amp going after the holocaust

 

[rickcr42]

http://www.partsexpress.com/resources/capsetup.html