Problems with a dicrete buffer please help.

[justblair]

I hope that you can help.

I built a discrete buffer according to the Decibel dungeon site.

NUUK's discrete buffer

I'm feeding it with 15v supplied by my smps supply through a set of 317, 337 voltage regulators.

The 0v line is attached to power ground, The signal earth is not attached to the buffer.

When i feed it a signal through my soundcard I can only play very low volumes before I get distortion. When it plays without distortion the sound seems very good.

The other thing is that when no source is connected I get a pretty lound thump through the speakers which occurs every second or so.

I'm thinking my grounding is the problem can anyone suggest a cure??

[AndrewT]

Temporarily remove the SMPS and use a transformer /rectifier/ smoothing.

Are you using dual polarity?
or
single polarity with DC blocking in & out?

[justblair]

Originally posted by AndrewT
Temporarily remove the SMPS and use a transformer /rectifier/ smoothing.

Are you using dual polarity?
or
single polarity with DC blocking in & out?

Hi Andrew.

I dont have a tranny to try out with, well perhaps not, I will look at the sherwood amp I just picked up off the street. its a AX5010R that was being thrown out by someone moving house, no idea if it works.

Now the polarity thing, forgive my newbieness here. Are you referring to the voltage in ie +15 and -15 = dual polarity? That is what I am using.

As for DC blocking, I have tried the circuit with (and without) 4.7uf poly caps for the output as per the schematic, but not the input. It did cross my mind this morning that the soundcard may be producing some DC? I have gf related stuff to do tonight so I can try it till tomorrow. I tried the onboard sound AC97 and a SB live24bit. Neither works.

What concerns me is that the buffer produces this thumping noise when nothin is connected, it also does it when there is no power to soundcard. My MCE machine goes to sleep often. I am imagining that this is not normal behaviour for the buffer?

Any other suggestions, even educated guesses are welcome, I am running out of inspiration here....

[FastEddy]

" ... When i feed it a signal through my soundcard I can only play very low volumes before I get distortion. When it plays without distortion the sound seems very good. ... I get a pretty lound thump through the speakers which occurs every second or so. ... thinking my grounding is the problem ..."

Yes, a DC blocking cap on the output should be tried. That thump could also be a momentary discharge from a bad cap on the sound card, your buffer or further upstream.

" ... 317, 337 voltage regulators. ..." = Implies dual, split rail, +/- 15 VDC supply. Yes?

I have never appreciated the "sound card" scenario, having had to do too much work on computers with bum, noisey power supplies and all manner of related questions. I prefer an external "sound card" connected via the USB or FireWire interface with their own supply(s). And of course, there is lots of grounding questions when audio comes right out of or into the computer box.

[justblair]

Originally posted by FastEddy
" ... When i feed it a signal through my soundcard I can only play very low volumes before I get distortion. When it plays without distortion the sound seems very good. ... I get a pretty lound thump through the speakers which occurs every second or so. ... thinking my grounding is the problem ..."

Yes, a DC blocking cap on the output should be tried. That thump could also be a momentary discharge from a bad cap on the sound card, your buffer or further upstream.

" ... 317, 337 voltage regulators. ..." = Implies dual, split rail, +/- 15 VDC supply. Yes?

I have never appreciated the "sound card" scenario, having had to do too much work on computers with bum, noisey power supplies and all manner of related questions. I prefer an external "sound card" connected via the USB or FireWire interface with their own supply(s). And of course, there is lots of grounding questions when audio comes right out of or into the computer box.

The thump is coming when nothing is connected (or is connected but not powered) so I know its the buffer thats producing it, I just don't know why! I would rather that it didn't

I have tried the Blocking Cap on the output, makes no difference wether its in the circuit or not. I will try adding one to the input. I have been browsing sites that discuss modding the soundcard. It may (or may not be a high DC ofset at the soundcard) I think when I had a cd player working on it, i was getting higher volume. I was getting distortion, but my Cambridge CD4 aint got no volume control so it was giving full volume to the buffer. Though the distortion wasn't so bad.

Yep the regulators are giving +/-15v so this is dual I am pretty certain (as certain as a newb can be)

I am interested that you mention grounding problems from a pc source could you elaborate?

I do intend building a usb dac next, I think that it will be an interesting project, though I am lost at the moment in deciding what to build or how to start on it. I will tackle that in time.

If anyone has built this buffer before could you comment as to whether this thump thump thump is common when the source is not connected?

Thanks

B

[justblair]

I think i might have worked out the problem...

The soundcard is giving out headphone line out. This will be much higher than standard line out.

If that is the case is the buffer being overloaded by the signal. I measured the voltage on the soundcrd output. Its near 5v (dc on the multimeter) as opposed to mv on the cd player.

Now by my reconning, the base of the first transistor in the buffer will now be always on?

If this is the case I need to get a standard line out from the pc yes?

[G]

In general 2v out is pretty standard on most sources so yes I would say 5v out would be a problem for the buffer. You could use a voltage divider at the input to lower the input signal.

[davidallancole]

justblair, if you have 5VDC coming out of your computer, thats not good. Put an input capacitor in to block the dc from your soundcard from coming into the buffer. The 5VDC from the computer will be biasing Q1 higher than it should be causing the headroom to be limited (distorts at lower voltage than source without the dc offset problem i.e. your cd player).

You could use one of those capacitors you used for the output dc blocking cap as the input dc blocking cap.

This circuit as a buffer should be able to handle an audio source ~ up to +-14V or so. Seeing as how most cd players put out 1V or 2V peak, this thing should be far from distorting but if your computer volume is turned right up, it might be getting up there. I have no idea which rails they use in the computer for audio but I would guess the +-12V rails. So with the dc offset problem plus say a 10V peak audio signal, you will be hitting the voltage rails and distorting.

As for the thump thump problem, what do you have in the audio chain after this buffer? Could it be something past it thats causing this thump. What I understand from what you have said, with no source plugged in and the power turned off to this buffer, you are still getting a thump thump on your speakers?

[justblair]

The thump thump thump I appear to have solved for now. I replaced the transistors I had been using. I think I must have fried one at some point.

I tried adding a 10uf cap in the signal path (it was the only non-polar one I had. It had little effect. I think that I have to either stick in a voltage divider or look at my source.

My guess is that the soundcard believes its driving headphones for some reason. I could try modding the card to bypass the op amps.

I might try and resurect my cambridge audio dacmagic instead though... till I can build something a bit more hifi

Thanks for the suggestions

[peranders]

Just wondering: Why do you have a buffer in the first place when you have a powerful signal source?