Nuuk's buffer issues

[valleyman]

I currently putting together a pair of ZD5s actively driven by chipamps.
I am using BrianGTs kit and have assembled them without issue and they're working fine.

I have put together Nuuk's discrete transistor buffer which can be found here:
http://myweb.tiscali.co.uk/nuukspot/decdun/gainclone2.html#discrete

I have assembled it on Veroboard but am having issues. The amps are pretty quiet when run by themselves, but as soon as I connect the buffer I get a loud (easily audible throughout the room) hum on the test speaker although it does seem to work and music does play.

My first thoughts are a possible grounding issue but I'm really not sure.

I have added the buffer to the amp circuit as follows:

Instead of connecting the signal input to the "in" on BrianGTs amp I connected it to the "in" on the buffer
I then connected the "out" on the buffer to the "in" on the amp
I left the signal ground connected to the "sg" terminal on the amp and have not connected it to the buffer at all
I have left the chassis ground on the amp disconnected as the case is to be made from wood (inside the speakers).

Aside from this I have rebuilt the buffer circuit in case I made any mistakes.
The buffer PSU rails are fine reading +/-15V
I have triple checked the transistors are in the right way

I'd be very grateful to anyone who can shed some light on this.
Many thanks

Paul

 

[Onvinyl]

Hi Paul,
you must attach the buffer's ground to the common ground as well!
Rüdiger

 

[AndrewT]

Hi,
you have connected the send signal to the buffer but you appear to have omitted the return signal back to the source.
Remember circuits are loops. The currents flow and return.

 

[valleyman]

Many thanks, that makes sense. I hadn't really forgotten the return, I just wasn't properly regarding the buffer and amp as what they are: two completely separate circuits... aside from the interconnect of course, but anyway, I connected signal ground to buffer 0V and buffer 0V to the "sg" in on the amp and the problems have disappeared.

 

[psymo]

Paul, I started building building one of these a while ago but due to other commitments have not yet completed it. Out of interest, now that you have got it working, how does it sound? What transistors and PSU did you use?

Thanks

Pete

 

[cuibono]

I've had hum issues when I omitted enclosures (metal is the best). I recently had hum issues that were related to poor interconnect - I was using 4m radio shack line level interconnects, and getting hum. Once I made a nice IC, the hum totally vanished.

 

[valleyman]

To psymo:

Please have a look at my other thread here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=118064

I used a psu similar to the one Nuuk describes, and theres a decent close up shot of it in that thread. I have used a single rectifier bridge, 4700uF smoothing caps, .1uF bypass cap and dual regs per rail.

I haven't really experimented with transistors yet. I got ZTX1053a, ZTX653, BC637 and BC639 to try out. I did a quick comparison and the 1053a definitely sound very good but I will post back with more details when everythings put together and I do some proper listening tests.

Cuibono:

The interconnect is definitely not to blame, works fine in other applications and is some nice canare LV77S terminated with canare phono plugs.
The lack of a metal chassis is a problem I hadn't really considered. I suppose it's not so much a "problem" as a manageable issue which just needs to be dealt with.


Now onto another problem... I just connected the pot and am having problems when the pot is turned all the way down. I have attached a diagram showing my exact wiring which should make things much clearer.
Basically the pot works fine and music plays fine except when I turn the pot all the way down when there is a big hum, the speaker cone is forced all the way outwards and within a few moments it blows the speaker (thank god I bought some cheap £5 cones to test on).
Thanks for all your replies

 

[AndrewT]

DC blocking cap on both the input and the NFB loop.
Is Nuuk's implementation a DC coupled amplifier?

 

[valleyman]

Unfortunately I can't answer your question, but I've attached the schematic so you can see. It does have a DC blocking cap on it's output though so I thought there was no way it could output DC.

I have also attached a pic of a wiring attempt I made excluding the amp just to make sure the buffer was OK and it worked fine, I was able to turn the pot all the way down without any rubbish hurting the speaker

 

[valleyman]

And that wiring diagram

 

[Nuuk]

There is some DC on the input and if you use a stepped attenuator, you may hear a clicking noise as you change the volume.

I've used the buffer without a DC blocker on the input and apart from the attenuator clicking, it has been dead quiet.

 

[valleyman]

Thank's for weighing in Nuuk. I'm using a pot, not a stepped attenuator. I hope I have been clear, the problem ONLY occurs when the pot is turned down to minimum, ie when the wiper is at ground. If I have it at very, very low there is no problem and I can change the volume happily etc, but as soon as I turn the pot to minimum speakers go boom.

I haven't worked out why that was happening but decided trial and error might be quicker than working it out. I have now moved the pot to after the buffer which has resolved the issue. Now I just have lots of hum to contend with. I'm looking at gainphile and digi's posts in the hope they can help

 

[poynton]

Have you tried a different pot.?

The wiper may be going o/c at the minimum.


Andy

 

[valleyman]

OK, well after some fiddling I ended up bored and unable to work out where the noise was coming from... so I switched off my soldering iron for the day and hey presto the noise is gone.

So, you live and learn, the noise was just from having the iron switched on sitting next to the test setup. In future I'll always make sure it's switched off when I'm testing.

On the upside, the constant fiddling has left me with a decent grounding scheme so that amp is probably quieter than it would have been had I not been forced to spend ages trying to get rid of a grounding issue that wasnt a grounding issue.

Thanks to all for the help.

 

[Nuuk]

All's well that ends well as they say! The trouble is in this game - there is no end!