Kevin Gilmore

[Pars]

Opamps would normally be OP27s for this ( up to +/-22Vdc rails). The OPA445s are used in his higher powered amps on +/-30V rails.

 

[KatieandDad]

I've bought the OPA445s.

 

[KatieandDad]

Hmmm, the vendor supplied a schematic but it bears no resemblance to the PCB.

 

[Pars]

There are a number of versions of this (confusing).

How about this one? Also, in place of the THAT 340 you can use JFETs (J74/K170).

 

[KatieandDad]

There are a number of versions of this (confusing).

How about this one? Also, in place of the THAT 340 you can use JFETs (J74/K170).

There are only 2 x LEDs on this PCB.

 

[KatieandDad]

I've managed to buy nearly all the components from the silk screen legend and with help from you folk.

The only remaining mystery are the LEDs. I'm guessing RED but that is only a guess.

I've still not seen a schematic that looks anything like this PCB ?

 

[Nisbeth]

1.7V red LEDs is the standard. There was a recommended p/n from Mouser in one of the head-case threads but I can't find it at the moment.

EDIT. Found it. Mouser p/n: 859-LTL-307ELC

 

[KatieandDad]

Thanks Nisbeth, I guessed that it might be RED LEDs.

I bought all the resistors from the Vishay MRS25000C range.

The only value that isn't available in the E96 range is 680R, I'm using 681R (that'll be close enough and close 1% tolerance).

 

[KatieandDad]

It was mentioned that it is matching of the resistors that is important as this is a balanced design.

 

[KatieandDad]

Oddly, the 34K0 was difficult to find ?

 

[KatieandDad]

I've also got no idea of the power requirements of this board. We've ascertained that it is +/-20VDC. I've got a 250mA supply with 4700uF caps but is that enough ?

 

[KatieandDad]

The boards have finally arrived.

 

[KatieandDad]

Sorry, that should have had the images attached.

I've not seen any schematics that relate to this board.

 

[KatieandDad]

Reverse Engineering

I'm trying to reverse engineer these boards to work out a schematic.

So far I am confused.

The lower LED appears not to be biased ? I've checked my work and that is how that transistor is connected.

 

[KatieandDad]

I think the bottom of the lower 10K resistor needs to be connected to the base of the lower transistor - NOT the emitter.

 

[KatieandDad]

I wonder if the Chinese are selling some faulty boards ?

 

[penmarker]

Sometimes in the headcase forum they will share links of chinese sales posts (DIY boards and finished units) that use their old and/or faulty boards. That is a 2014 board, there are more recent and working iterations of bal/ubal to bal input boards.

 

[Nisbeth]

I think this might be what you need? The Multi Amp aka Dynalo Mk2 - Page 51 - Do It Yourself - www.Head-Case.org

 

[U230421]

The lower LED appears not to be biased? I've checked my work and that is how that transistor is connected.

The middle pin of the TO92 is not the base. CBE is common, BCE too, BEC and CEB more rare but seen.
In this setup the led is connected to the emitter the 10k-100R node is on the base.

 

[KatieandDad]

These are MPSA06/56 so EBC

The transistor types were taken from a different board so may be wrong.

The board still doesn't make any sense with 2SA970 / 2SC2240.