Nuuk's buffer issues

valleyman

Member
2008-02-07 7:21 pm
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
 

valleyman

Member
2008-02-07 7:21 pm
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.
 

valleyman

Member
2008-02-07 7:21 pm
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
 

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valleyman

Member
2008-02-07 7:21 pm
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
 

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valleyman

Member
2008-02-07 7:21 pm
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
 

valleyman

Member
2008-02-07 7:21 pm
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.