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New PCB for shoebox size NAC preamp part III

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Stupid question I know!

Three of the resistors I had delivered from Farnell for the NERT, 13R, 200R and 910R are 0.6w and are somewhat smaller in physical size than the 100R 0.75 w. Is the lower power rating a problem with a NERT, and is it OK to 'mix and match'?

The Farnell 0.75w looks like something that, many years ago, I would have called 1/2w, and the 0.6w looks like 1/4w!

Bob
 
I have most of the parts to start a starfish build soon. I do however have a quandry or two...

Volume Pot
I have a new Alps ("Blue Beauty" I think) as per Jim's latest BOM but I also have one of the old Alps pots ("Black" I think) from a donor NAC42 that would probably come apart and clean up OK. I don't have the confidence to solder/de-solder these parts to try each out so would like some advice please.

Would it be better to use the new Alps Blue, clean up the old Alps Black or get hold of something like a Noble instead?

Selector Switch
I have no idea about selector switches either. I'll probably want 5 inputs available along the lines of: Phono, Tuner, CD, Tape 1, & Tape 2. It may be worth having output on Tape 1 & 2 as well - fixed output on one, variable (following volume pot) on the other if possible. My knowledge is very limited here.

Would I be correct in thinking I need either a 5 way 2 pole or a 5 way 4 pole switch?

Would a fixed output and a variable output be possible on Tape 1/2 outputs and if so how?

Are ELMA switches any good (expensive though!)?



Any advice or thoughts on Volume Pots & Selector Switches for the starfish would be appreciated.

Bob
 

For the selector switch I can highly recommend the Twisted Pear Darwin.

And Noble pots are a smidgeon better than either of the Alps you have.
 
neiljadman said:
For the selector switch I can highly recommend the Twisted Pear Darwin.

And Noble pots are a smidgeon better than either of the Alps you have.
Click to expand...

I've taken a brief look at the Darwin previously and in does look like a good solution. They (Twisted Pear Audio) always seem to be out of stock though! I can't find any details of board size etc. - do you know if it will fit into an NAC42 case with a starfish?

Where/how would fixed and variable outputs be taken from the starfish? I've seen it mentioned in this thread somewhere but it isn't clear to me how it's done.

Bob
 
bobertuk said:
...details of board size etc. - do you know if it will fit into an NAC42 case with a starfish?
Click to expand...
It's 4 x 1.5 inches and I couldn't get it to fit inside a shoebox amp - space is jsut to tight immediately behind the front fascia. It also needs two rotary controllers (supplied) for input and output selection.

bobertuk said:
Where/how would fixed and variable outputs be taken from the starfish? I've seen it mentioned in this thread somewhere but it isn't clear to me how it's done.
Click to expand...
Of course the Darwin does this for you. But otherwise, You take the fixed outputs off from between the source selector and the input pins to the board. But you'll need to buffer any fixed outputs with a small resistor (680R) to stop the load the preamp sees being affected.
 
The Darwin Input selector board paired well with my fish, especially when using leds as input indicators.

If you are looking for both a decent attenuator and input selector then
Jos Vaneijndhoven's remote controlled attenuator is excellent. It only has 4 inputs though so may not be suitable for you, however as a replacement for an Alps pot the improvement was considerable. Throw in a programmable remote control (yes it works with a Naim remote) and its a winner for me.

If TP have no stock I have that darwin board still hanging around. PM me if interested.

Nick
 
Anyone know where I can get hold of cut lengths of 19/0.2 PTFE covered wire in at least 3 colours in the UK? I want to use it in the starfish as it doesn't burn/char when soldered.

Tony has no more, places like farnell only sell it in reels (I only want 4 - 5m of each) and hifi collective only do it in 2 colours!

Any ideas?

Bob
 
Uzinusa said:
Hi Neil....are there any specific Nobel pots you can recommend please> Merry Christmas.
Andy
Click to expand...

Andy, my Starfish has a 50k Noble pot which I originaly bought for my 32.5 from Les. The only problem I have is that I need to do swap the 47uF feedback cap for 22uF, with a change to the series resistor to reduce the Starfish"s gain, hence increasing the useable sweep of the pot. But this is driven more by the sensitivity of my speakers.
 
Today's fun idea: for the symmetrical local-regged design, how about replacing the differential mode chokes and caps with 2 teddyregs i.e.

Twin Traco supply +/- 30V out, common mode filter, Teddyregs @ +/- 24V out feeding the local 317/337s @ 12V out.

Aim is to get the benefit of the Teddyregs without the hassle of removing the 14 x 317/337s and replacing each one with a Teddyreg.
 
Hopefully another starfish will soon live :D

I've just finished populating my starfish PCB with all it's resistors, capacitors and inductors - I'm just about to start with the transistors. Jim's BOM specifies BC560C tranies but the Farnell part number quoted is for the BC560. I have a supply of both but have no idea which to use. Does it matter? Is it better to use one rather than the other? What difference will it make?

I am also about to order a Noble Pot from HiFi Collective but the closest I can find to Jim's recommendation (20k) is 25k. I don't think it will be a problem but am I correct in that assumption? Will 25k change useable pot range or output levels or anything else?

Thanks for your help...

Bob
 
Hi Bob,

For BC560/BC560C - it makes little difference. The suffix specifies a gain band for the transistor - but this circuit is not really dependant on the gain of the transistors. If you have a stock of both, use the C version.

A 25K pot will be fine. It wont change the range or anything like that. Should be a drop in replacement.

Cheers,

Jim.
 
Hi Jim,

OK. I'll use the BC560C and the 25k noble pot.

Hopefully I should have it live (and hopefully working properly!) within the week.

If it's of any use, I tried several different diameters of enamelled copper wire to wind the inductor coils and the largest that I could easily use on the specified ferrite cores was 0.45mm. Any larger caused 'overcrowding' problems.

I've wound the coils as specified - am I right in assuming that it doesn't matter how they are inserted on the PCB, as long as they connect along the relavent PCB line. i.e. it doesn't matter if the coils turn clockwise or anti-clockwise?

Cheers,

Bob
 
bobertuk said:
I've wound the coils as specified - am I right in assuming that it doesn't matter how they are inserted on the PCB, as long as they connect along the relavent PCB line.
Click to expand...

Shouldnt make any difference for the differential ones. It's quite important that the common-mode one is constructed as per the diagram.

Jim.
 
Jiiim said:
Shouldnt make any difference for the differential ones. It's quite important that the common-mode one is constructed as per the diagram.

Jim.
Click to expand...

I've wound the two coils on the common-mode choke as per diagram but it can be inserted into the PCB in two different orientations. The diagram shows the coils having 'in' and 'out' - how do I determin which of the PCB connections are 'in' (or 'out'). Knowing that would make it easier. I'm sure it's easy anyway and I'm just being a dumbo :confused:

I know one coil has to be connected along the +ve PCB (silk screened) line, the other along the -ve PCB line.

Thanks

Bob
 
bobertuk said:
I've wound the two coils on the common-mode choke as per diagram but it can be inserted into the PCB in two different orientations. The diagram shows the coils having 'in' and 'out' - how do I determin which of the PCB connections are 'in' (or 'out'). Knowing that would make it easier. I'm sure it's easy anyway and I'm just being a dumbo :confused:

I know one coil has to be connected along the +ve PCB (silk screened) line, the other along the -ve PCB line.

Thanks

Bob
Click to expand...


As long as the coils are wound in the orientation shown on the diagram, the 'in' and 'out' are interchangable.

I think of 'in' as the side connected to the traco, and 'out' the side supplying the rest of the circuit - but in terms of the coil itself it doesnt matter.

Jim.
 
Jiiim said:
As long as the coils are wound in the orientation shown on the diagram, the 'in' and 'out' are interchangable.

I think of 'in' as the side connected to the traco, and 'out' the side supplying the rest of the circuit - but in terms of the coil itself it doesnt matter.

Jim.
Click to expand...

Ah... that's good. Looks like I've connected it up properly then. So it's all go ahead to put all the transistors in tomorrow then. Only the NERT to go after that and I'm going to have a go at making a mini PCB myself for a bit of fun. It'll probably be a bit of a botch but that's OK as long as it works.

Maybe it'll be singing by the end of the week. I'll let you know.

Thanks again

Bob
 
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