GC passive pre-amp dissapointment

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filholder

Member
Joined 2005
Hi ,

Yeah i will probably tie the large caps down once i mount the board in its chassis.

As for the input, there are actually the small pins between the transistors and the large caps. the signal pah is about as small as i could make it, however there is a slighty larger than ideal space between the second stage power reg and the transistors, but i had to do this since i need an area in the chassis there for the XLR socket i am using for the power in.

Have tested all the power supplu and that work might get to hear it on the weekend or monday once some big projects have finishes at work.

Fil
 
F

filholder

Member
Joined 2005
Hi All,

have almost finished my Nuuk buffered gainclone pre-amp. ALl working fine except the fact that with volume at minium i get 'normal listening volume with the 10k pot i put in. How can i reduce the volume level?Would resistor on the ouputs help? Please help, 10's of hours work gone in and no use as it is. Gutted:bawling:

Any sujjestion greatfully recieved.

Phil
 
F

filholder

Member
Joined 2005
I am pretty sur it is wired correctly, I spent a long time rewiring the pot and checking the values. There was definitely 10k across the input and output going the nothing if the pot was turned full. Also the volume works but very steeply.

Very annoying all around, so much work this weekend and now the thing is bloody useless. I might have to take the whole thing apart again and rewire in the 100k pot which work perfectly. grrrrrr.
 
Nuuk

Nuuk

Member
Joined 2003
decdun.me.uk
Fil, it sounds like either the pot isn't wired correctly, or it has a fault. In fact from what you say, it sounds to me that the connection to ground is missing.

I think if you use the 100K pot you will find some problems as the circuit is designed around a 10K version. However, it may be OK on a temporary basis to see if the problem is with the pot.

The darkest hour is just before dawn! ;)
 
F

filholder

Member
Joined 2005
Hmm, ok for the pot i downloaded the .pdf sheet for alps before i put it in. For ground in has two connections one per channel, however i tie both these together and then connected the source ground coming into the unit and the 0v line of the buffer (the output of the unit)to this since this is the ground in the buffer. Does that sound right? cant think of another way of wiring it.

i am thinking maybe the pot is faulty, if i did stick the 100k back in i would short the input an out put with resistors like i did before to lower the value. That seemed to work ok, wish i did left it all alone now.

fil


fil
 
Nuuk

Nuuk

Member
Joined 2003
decdun.me.uk
Hmm, ok for the pot i downloaded the .pdf sheet for alps before i put it in. For ground in has two connections one per channel, however i tie both these together and then connected the source ground coming into the unit and the 0v line of the buffer (the output of the unit)to this since this is the ground in the buffer. Does that sound right? cant think of another way of wiring it.
Click to expand...

I use a signal ground star for each channel. For each channel, the ground tab of the input socket, the 0 volt connection of the pot(or attenuataor) and the 0 volt connection on the buffer all go to the signal ground star. :att'n:
 
F

filholder

Member
Joined 2005
Hi Nuuk,

Sorry i posted another topic on this, should have waited. In my pre the grounds on the both channels are commom since there is only ground line on the remote board and there is only one 0v line on the psu. Could i tie them all together to the 0v in point on the psu connection for both channels?

Also even is the earth layout i used was linear rather than star wouldnt the pot still work just that i would get a bit of noise.

The other thing i was going to ask was should the 0v line be connected to the chassis earth?
 
Nuuk

Nuuk

Member
Joined 2003
decdun.me.uk
In my pre the grounds on the both channels are commom since there is only ground line on the remote board and there is only one 0v line on the psu. Could i tie them all together to the 0v in point on the psu connection for both channels?
Click to expand...

Yes you can join the grounds of both channels. They will meet somewhere in the chain anyway but some of us like to keep them apart as much as possible. In your case, this isn't an option.

Also even is the earth layout i used was linear rather than star wouldnt the pot still work just that i would get a bit of noise.
Click to expand...

Yes, if the pot is wired correctly, it will work.

The other thing i was going to ask was should the 0v line be connected to the chassis earth?
Click to expand...

Are we talking about an integrated amp here, or a separate preamp? If a preamp, I don't connect anything to the case other than the earth wire on the mains lead. If an integrated, you can connect the power ground star(s) to the case via a resistor and cap in series.
 
F

filholder

Member
Joined 2005
Thanks a lot Nuuk,

Very helpful as always.

This is a seperate preamp and when it was passive it didnt have anything connected to the case. One thing i notice though now was that when i touch the pot spindle with my finger the amp hums, i guess this could suggest that there is an earthing issue since is wasnt present before.

Phil
 
Nuuk

Nuuk

Member
Joined 2003
decdun.me.uk
Nuuk, sorry to interject this slightly off topic question but regarding the transistor buffer for the gainclone, do you know anyone who built it with the MPSA18?
Click to expand...

Not off-topic if it's to do with the buffer! ;)

But the answer is no, I don't know what other people have used. It would be good if we could get feedback on different transistors though! :att'n:

If the MPSA18 is a signal transistor then I would say try it if they are not too expensive. :cool:
 
F

filholder

Member
Joined 2005
OK i have done a lot of thinking and worked out that what i must have done is by wiring the pot wrong, I have put a variable resistance in the curcuit but not sent any to ground. The resistance starts at 10 k and end at zero but because of the wiring the no gets set to ground, thus it does lower the volume but only a tiny amount. Face on looking at the pot I must have wired it G-O-I as opposed to I-G-O where G is ground I in Input and O is Output.

Feel pretty confided that I can fix it now.

Amazing what a quite day at work and a couple of hours thought can do.

edit:
Just to prove it, the resistance from Input to output and to ground the way I wired it would both be 10k each at miniumum volume, Thus as a voltage divider the ouput would be basicly be half volume, which is exactly what i got!!!

Phil
 
dsavitsk

dsavitsk

diyAudio Member
Joined 2005
diyaudio.ecp.cc
Reviving a long dead thread: I replaced the transistors in my buffer with all 2N5088's. I had been using the ones that Nuuk used. It sounds a little different than it did. I am not sure that it is better, probably not worse, but a tad different -- a bit more detail at the expense of some warmth. It is different enough that I think trying out different transistors is worth it.
 
M

MrGuitardeath

Banned
Joined 2005
Heeeey, here's a really simple fix and probably might just very well be the best.

Add a simple LM386 circuit post passive preamp. More than enough juice for what is needed and simple circuits to build. I've also noticed how dense, rich and tube like these little amplifier chips sound. I think their potential is still greatly untapped...

You've probably solved it by now but just a thought to throw out rather than read 5 pages of replys
 
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