Smallsound Bigsound mini

tome.talevski · Post by **tome.talevski** » 14 Oct 2020, 19:31

Hello! Just built the ssbs mini and it’s sounds awesome ... ! .. except that I cant get the bias pot and trim to work properly. The lowest voltage on the 5457 drain I can muster is approx 5v via the trim.. the bias pot also only has a 2v range (for example.. max is 7v min is 5v). It’s making some difference although not what it’s supposed to do.. it feels like it achieves only half it’s goal across the full sweep... also, based on the tagboard thread, the bias pot range ought to be around 5v. ... so, any ideas as to why I can’t achieve a lower drain voltage?? Plus, how to broaden that pot range??

Cheers

deltafred · Post by **deltafred** » 14 Oct 2020, 19:55

Give this a read -

viewtopic.php?f=91&t=22602

tome.talevski · Post by **tome.talevski** » 14 Oct 2020, 20:47

deltafred wrote: ↑14 Oct 2020, 19:55 Give this a read -

viewtopic.php?f=91&t=22602

Thanks.. I typically do include all of that information. This build is about 90% correct except for the depth and range of the bias function so I thought there might be something obvious I overlooked. I’ll aim to provide more details.

deltafred · Post by **deltafred** » 14 Oct 2020, 21:55

Without a schematic, the layout, and clear photos of your build posting a few voltages is pretty meaningless.

If you want people to help you then you have to provide them with all relevant information, not expect them to go searching for it.

tome.talevski · Post by **tome.talevski** » 15 Oct 2020, 02:55

deltafred wrote: ↑14 Oct 2020, 21:55 Without a schematic, the layout, and clear photos of your build posting a few voltages is pretty meaningless.

If you want people to help you then you have to provide them with all relevant information, not expect them to go searching for it.

Ok.. so here’s some additional info

Like I said above… the circuit is working great except for the bias pot and trim. When the bias pot is fully CW/activated the signal moves in the direction of being starved/splattery however isn’t anywhere near that. Interestingly, I swapped the 2n5457 with a j201 just to see what happens and although it didn’t quite work out, the trim managed to sweep across from 1 to 9v and the bias pot range was 5v (and not the 2v which it is currently).

Image attachments below of the layout and of my build. (final pic is of the schematic - i used the layout guide to build)

2n5457

D: the drain range is 9v with the trim CCW and 7.19 with the trim CW. I set this at full CW.
S: 1.9
G: 0

J201 top of the layout

D: 8v with the trim CCW and 1.5v trim CW I set this at approx 2 to 3v.
S: 1.6v
G: 0

J201 bottom of the layout

D: 4.8V
S: .53
G: 0

2n5089

E: .95v
B: 1.5v
C: 8.9v

Diode
1n5817
Neg:8.9
Pos: 9.2

deltafred · Post by **deltafred** » 15 Oct 2020, 08:42

It looks to me like your batch of 2N5457 FETs don't have the required characteristics to allow them to be biased to the correct point. It does say on the schematic that they require selection but not for what criteria.

You could try increasing the source resistors to see if that gets you nearer to the correct bias point. If you have a breadboard build up a single FET stage and try different source resistors rather than risk damaging your Vero with swapping parts that are soldered in.

tome.talevski · Post by **tome.talevski** » 15 Oct 2020, 09:19

deltafred wrote: ↑15 Oct 2020, 08:42 It looks to me like your batch of 2N5457 FETs don't have the required characteristics to allow them to be biased to the correct point. It does say on the schematic that they require selection but not for what criteria.

You could try increasing the source resistors to see if that gets you nearer to the correct bias point. If you have a breadboard build up a single FET stage and try different source resistors rather than risk damaging your Vero with swapping parts that are soldered in.

Thanks for the suggestion.. are you referring to all the source resistors or just the 2n5457 1k resistor? I suspect just the one attached to the 2n5457... also, what increased value do you suggest I start with?

I also noticed the schematic also suggests increasing the value of the trimpot...?

Thanks again

deltafred · Post by **deltafred** » 15 Oct 2020, 11:16

Just try one for a start, which one is your choice, they all have adjustable drain resistors. so change one and see if you can get the voltage closer to what it should be. Start with 2.2k and see what results you get. If it goes too far the other way then try 1.8k or 1.5k.

I would leave the drain presets as they are until you see what effect changing the source resistor makes.

Edit -
"are you referring to all the source resistors or just the 2n5457 1k resistor?"

All the FETs are 2N5457 and all 3 source resistors are 1k so pick one, I would go for the middle one with the bias pot as it will be slightly easier to adjust.

tome.talevski · Post by **tome.talevski** » 15 Oct 2020, 20:35

deltafred wrote: ↑15 Oct 2020, 11:16 Just try one for a start, which one is your choice, they all have adjustable drain resistors. so change one and see if you can get the voltage closer to what it should be. Start with 2.2k and see what results you get. If it goes too far the other way then try 1.8k or 1.5k.

I would leave the drain presets as they are until you see what effect changing the source resistor makes.

Edit -
"are you referring to all the source resistors or just the 2n5457 1k resistor?"

All the FETs are 2N5457 and all 3 source resistors are 1k so pick one, I would go for the middle one with the bias pot as it will be slightly easier to adjust.

The schematic and the layout are actually different.. the layout is updated based on information and input from the builder.

The thread is here
viewtopic.php?t=27727

In the layout there is only one 2n5457 and it’s attached to a 10k bias trim and 10k bias pot. It also has a 1k source resistor. So i suspect I’ll aim to change that.. I do wonder though whether increasing the bias trim (50k?) would work?

tome.talevski · Post by **tome.talevski** » 16 Oct 2020, 03:09

Update: I replaced the 2n5457 1k source resistor with an 100ohms resistor and that seemed to do the trick! Thanks for pointing me in the right direction... much appreciated!

deltafred · Post by **deltafred** » 16 Oct 2020, 05:34

Glad to help.
I wasn't sure which way you needed to take the source resistor, my gut feeling was up which was as you discovered the wrong way!